Effects of thyroid and growth hormone deficiency, and food restriction on heart mass, with and without added stress (carboxyhemoglobinemia)

Effects of thyroid and growth hormone deficiency, and food restriction on heart mass, with and without added stress (carboxyhemoglobinemia)

DAV~D G. P ~ N P I E Y ' AND BERNII G. BAKT~IL:~ , .&?partmc>nf of Phvsic~kogy, Woyr.ee State LTniver,sir)i ScklooI of Meckicine, 4 0 E . Canfield, Drtroit, M j , ['.$.A. $8201

PENNEY, D. G., and B. G . BARTIIEL. 1985. Effects of thyroid and growth hormone deficiency, and food restriction on heart mass, with and without added stress (cartac~xyheme~g~~sbin1emi:i~. Can. J. Pkysiol. Pharmacol . $3: 642 - 648.

The roles of thyroid and growth horrntsne, and food restriction in maiaatenance of hear% mass arrd in carbon monoxide- stimulated cardiac growth were examined. First, thyroidectosnized and nornaal adult rraaIe rats inhaled up to 500 pprn CO in air for 42 days. Combined ventricular weigjats of thyroidectosnized rats inhaling CO and air were 12 and 23% smaller than predicted, respectively, while the combined ventricular weight of normal rats inhaling CCB was 29%) larger than predicted. Thyroidectorny increased the mass of the left vetatricle relative to the right ventricle; this was reversed by CO treatment. While the hematocrit incrcased in thyroidectomized-CO rats, it was lower than in norind-CO rats: likewise the hernatocrit of thyroidectornized-AIR rats was lower than that of normal ruts in air. Body weights of the thyroideetomized rats were 57% that of norma1s. As additional controls, two groups of normal rats (one AIR, one CO) were maintained at title same body weight as the thyroidectoinized rats, by adjusting food intake. Clornbined ventricular weight was less than predicted in AIR rats and failed to increase in CO animals, while henlatocrits averc the same as nornlals in air and ian CO. Serum thyroxine (T4) iind growth hormone levels assayed in tkyroidectomizcd rats were less than 15 and 25% c9f normal rats, respecti\~ely. Growth hormone levels were not altered by CO inhalation in thyroidecton~ized and iw normal rats. Levels of hc~th hormones were raorlrlal in food-restricted rats. While thyroidectonay produced cardiac atrophy. cardiac growth was stimulated by &'$ inhalation, although heart araass then only approached that of ~aormals in air. Food restriction also pmduced cardiac atrophy, but GO inhalation failed to stimulate heart growth.

PENNEY, D. G. , et B. G. BARTHEI.. 1985. Effects of thyroid and growth hormone deficiency, aiad food restrictioi~ on heart mass, with and without added stress (carboxyhernoglobinemia). Can. J . Physiol . Yharmacol. 63: 642 - 648.

On a examin6 les r6les de la thyroi'de, de l 'horm~ne de croissance et d'une restriction de nounitlare dans le maintien de Ba masse cardiaque et dans le dkveloppenaent du coeur sitirnule par de 190xyde dc carbone. D'abord, des rat nlales adultcs norrnaux et thyroldcctomisks inhalerent jusqu's 500 p p ~ n dc CO dans l'air pendant 42 jc~urs. Les poids ventriculaires combinks des rats thyroi'dectoaraisks inhalant hfu CO et de I'air (AIR) Ctaient, respcctiven~ent, 12 et 23% plus faibles q w prevus, alors yue le poids ventrlculaire cornbind des rats nalrmaux inhalant du CO Ctait 29% plus 6levC que prCvla. La thyrc~Ydectc~~n~ie augrnenta la lrlasse du ventricule gauche par rapport au ventricule droit, ce qu'un traitenaer~t au C 0 inversa. L'hkrnattxrite augrnenta chez Bes rats thyroi'dectsanisCs-CO, mais iB etait plus hible que chez les rats ms~n1aux--C0: similairement, I'hernatocrite des rats thyra,i'decton~PsCs-AIR h i t plus bible que celui des rats norrnaux-AIR. Les poicis C O B - ~ ~ ~ C I S des rats thyroi'dectornis@s correspondaient a 57% de ceux des normalax. On a maintenu d e w groupes de rats worrnaux ( t h o i n s supplCmcntaires) (un AIR, un CO) au rnhe poids corporel que celui des thyrimi'dectornisi.s, en ajtistant I'absorpticsn de nourriture. Le poids ventrictmlaire corlmbinC etait plus faible que prkvu dans le groupe AIR et nt: put augrnlenter dans le groupc C0 , alors que %es Mimatwrites Ctaient similaires ii ceux des g roups normaux air et Cbl. Les taux de thyroxine skrique (T4b et d'hornaone de croissarace 6valuks chez les rats thyroidcctasrnisis ktaient plus Faibles que ceux des nornaaux de 15 et de 25%, respectivemesat. L'inhalation dc CO n'altkra pas le Faux d9hormone de croissan~ce dcs rats thyroi'dectomis6s ni celrai des normatax. Les takax dcs dcux types d'hormnes 6taient wormaux chez les rats avec restriction de nouniture. ABc~rs que la thyroidectomie provoqua une atrophie cardiaque, I'inhalation de CO stirnula Ie d6velesppernent du coeur bien que la anasse cardiaqua: ne put que s'approcher de celie des normatax inhalant de %'air. La restriction de nourriture provoqraa aussi l'atrophie cardiaque, rnais I'inhaiatiasw de C 8 ne rkussit pas B stirnuler le dkiveloppement du coeur.

[Traduit par Be jijtrmal]

The heart normally displays a fixed relationship to body weight (BW) at a given BW (Styka and Penney 1978), and responds to chronic work over10ad by adding more myocardial constituents, hence mass. In previous studies of cardiomegaly (i.e., cardiac hypertrophy) using 500 ppna carbon monoxide (40% carboxyhennoglobh = COHb) inhalation as stressor, it was shown that heart weight increases graciually over 2-3 weeks in the rat to a level 35-40% above equal body weight contrc~ls (Penney , Dunhamm et al. 1974). This iancrcase occurs in both ventricles and in the atria (Penney, Benjamin et 31. 19741, and is maintained as long as CO inhalation continues. The increased heart mass is mot due to myclcardial edema, since protein content increases in parallel with fresh hear$ weight. Our studies (Penney et al. 1979; Kanten et al. 1983) suggest the initiating stimuli for CO-iaaduced cardiomegaly to be primarily

' A U ~ B P O T to whom all correspondence should be addressed.

increased stroke volume and tachycardia, raising cardiac output nearly proportionately to COHb saturation. Siwcc peripheral resistance and arterial blood pressure fall sharply at the same time, carboxyhe~noglobincmia is a hypotens~ve volaame overload stressor.

Numcrous studics point to a major role for thyroid horrnone in the maintenance of myocardial structure and function. For example, in the hyperthyroid or thyrotoxic statc, heart weight, and resting stroke volume, heart rate. and stroke work increase greatly above nomali (Beznak 196%). In the hypothyrr-~id state the opposite occurs: McQueen-Williams and Thornpson C 1940) noted that the heart underwent atrophy following thyroidectomy . More recently . Sharma and Baner.jee 4 1077) reported a 35% BOSS of hear? weight 9- 20 weeks aftcr thyroid gland removal in Spragrae-Dawley rats; however, Wobbcrecht et al. (1982) saw no significant change in the ria ti^^ of heart weight ~ C B body weight in Wistar rats made hypothyroid by administration of propylthicsuracil. Thus the role of thyroid

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PEMNEY 4MD BAR'TIQEB, 04 3

homone in maintaining normal heart rnass bears rein- vestigation. Severe food restriction produces reducticms in nsetabolic rate, body temperature, and thyroid hc~nnone level (Croxson and Ikbertson 1977). not unlike that which follows thyroidectomy .

When additional work load was applied using aortic constriction in thyroidectomized a n i ~ ~ ~ a l c . Beznak and Hajciu (1946) found an increase in heart mass. 'This contrasts with %sypophysecton.rlv., where heart growth failed to take place following imposition of pressure overlc~ading nusing the same pro- cedeire (Bez~aak 1952, 1955, 1956), unless crude growth hormone was also administered (Beznak 1952, 1968). While this point is disprnted (Lipana and Fanburg 1978; Tipton and Tcheng 197 H ), Bcznak's data suggest that growth hormone is essential for the compensatory response to pressure stress in the hypophysectomized state. Furthenmore, cardiac growth did not occur in these rats if highly purified growth hormone was given (Beznak 1956); some thyroxine niust also be present. While this may be the case, no data are available on the response of the hypothyroid heart t c ~ chronic vo8ur.rlie overload stress.

The aims of this study were fourfold. Using thyroidectomized rats and normal Iittermate controls, the B'oBlowi~ag were contrasted: (i) the effccts of norrnal and depressed thyroid and growth hormone lcvels in maintenance of heart mass; (ii) the limits placed on heart growth when s6innuPated by hypotensive- volume overload stress in the presence of' such depressed hormone leveis. Because caf similarities between thyroidectomy and starvation in terms of BW gain, it was deemed important to also investigate relative heart mass in the severely food- restricted rat: ( l i i ) in the presence of , and ( i v ) in the absence of added stress. Because the food-restricted rats were reduced to BWs comparable to thosc of the thyroidectomized rats. they served as a second '6control" group for comparison of heart mass. Interestingly. it was seen that while both thyrcpidectcpmy and food restriction result in cardiac atrophy, heart growth occurs with chronic carboxyhemoglobinemia in the former condition, while it fails to take plaie with such stress in the latter condition.

Methods and materials Expwi~zaental design

Adult male rats (Sprapue- Dawley . caesarian obtained-barrier sus- tained) were obtained from Charles River Breeding Laboratories (Wil- miragton, MA). Three exprirnents were pea-fornmed, the Birst two deal- ing pri~~xirily with heart mass in the thyroidectomized state in the presence and absence of added CO-stress. In experirnent I , littematc thyroidectorraized (surgery performed by Charles Rivcr Breeding Lab.- oratories) and norrnal rats were used. One-half of the normals and one-half of the thyroiidectnmized rats inhaled a mixture of air and 500 ppm CO for 42 days (Cob, while the other half remained in rclorlm air (AIM). ]in a second experinaent (experiment 2), an opportranity was provided to compare the responses of the %hyroidcctomizcd and normal rats to those of nutritional controls (food restriction). To ac- conaplish this. onme-half of the normal rat4 (CO and AIR) were given a daily food allowance just adequate to maintain the same BW as the thyroidectomi~ed rats. Carbom n~onc~xjide inhalation Iasted 4 4 days. Because the food-restricted rats proved to be inadequate controls for the thyroidectorraized rats and were interesting in their own right, a third expcrime~mt (experiment 3) was carried out as follows: one-half of a group of food-restricted rats sinmilar to those abovc, and one-half' of a group of ncprrnal rats inhaled 500 ppm C 0 for 49 days. The other half of each group remained in air.

Animal frenrment After arrival, the rats were observed for B week prior to the start of

each experiment to detect illness brought on by shipnaent. 'B'hey were

housed in solid botto~n, plastic shoe-box cages containilmg Beta- chip bedding. They were ~naintained at a ternperahire of 22-23OC. Lighting was artificial and was contrc)llcd on a 12-h day- 12-& night (0800-2000) cycle. Except for the food-restricted groups, treated and control animals were given Purina Rat Chow ad libitum. All animals had constant access to water. 'Fhyroidectomized rats received 1 % Ca2' as calcium lactate, and 0.02%~ N-6-propyithic>uracil in the drinking water to suppress any remaining thyroid function.

Inhalation of CO was carried out as described previously (Penney et al. 1980). In brief, this consisted of maintaining the rats 24 h per day, '9 days per wcek in the cages, enclosed within large plastic bags inflated by an air-CO mixture. Flow rate through each bag was approximately 25 L/min to ensure rapid rernoval sf waste gases. CO com~centratisn, lnonitored by a Becknaan rnodel LB-2 infrared gas analyzer, was recorded, the output signal in turn being utilized to maintain CO concentratic~n ?3% of the set value through an electronic feedback loop. 'Fhe control rats were maintained in close pn~ximity in ambient air. Those animals inhaling CO. in each case were exposed to a CQ concentration of 325 ppm for the first 2 days, 425 ppm CO for 5 days after that, and 500 pp~n CO thereafter.

Blood and rissue ac.quisitbon Hematocrit was obtained I day behre sacrifice using k~lood from the

tail. Each animal was taken from its "horne" cage and sacrificed within seconds by decapitation. The heart was renaovcd, washed in cold saline, dissected and weighed. Measured weights of left ventricle plus interventricular septum (LV +. S ) , right ventricle (RV), and cortlbined ventricle (LV + S f RV = 2V) were compared with values predicted on the basis of BW using rrlultiple exponential regression equations (Styka and Penney 3978). Combined right and left atrial weights were obtained only in expts. 2 and 3.

In expt. 2, blood was collected within 8 s of sacrifice from the severed neck and immediately cooleti to 0°C'. 'The red blood cells were separated by cerntrifugation in the cold armd the serum was assayed by radioimmunoassay for thyroid hormone ('T.,-Calbiuchern kit, CalbiocBmem-Behring: Corp., San Bdiego, CA) and growth hormone (Birge et al. 1967). Values were obtained from 2 animals in each group for thyroid hormone and up to 1 8 in each group for growth hormone.

Sfczfi~tic6~I cznulysis The values presented are means -t- standard cnor. Student's a-test

was used for determination csf statistical significance. Differences that resulted in probability values smaller than 0.05 were considered to be significant. Statistical analysis using the method of 1nultipIe analysis of variance was also used, giving sinallar results.

Because these studies involved three types of rats (i.e.. normal, thyroidectomized, and food restricted) and two treatment regimes (i.e., atrnospherlc air and atmospheric air containing 500 ppm CO), the respc~wses of each experimental groeip are presented individually below.

Carbo~henaogIobi~~en~ia Hematocrits of norrnaal CO-exposed rats increased sharply in

all three experiments relative to normal - AIR rats, presumably as the result of polycythemia (Tables 1 - 3). Combined ventricle weight and the ratio of 2V weight to BW increased significantly. The weights of LV -t S, WV, and 2V increased above those predicted on the bask of BW, indicating cardio- rnegaliy had occurred. The weight ratio caf WV to k V + S increased significantly only in expt. 3. Combined atrial weight and weight ratio of ccsrnbined atria to WW, measured only in expts. 2 and 3, were significantly increased. Thyroid hormone T4 armd growth hormone levels measured in expt. 2 were not altered by Hong-tern carboxyhemoglcabinemia (Table 4).

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CAN. S. PHYSIOL. PHARMACOL. VOL. 63, 1985

TABLE I . Experiment 1 . Body weight, blood data, and heart weight for adult rnale rats which had inhaled 500 ppm carbon monoxide (CO8 continuously for 42 days and their littermatc controls (AIR)

BW- Hct 2V 2V/BW RV/LV+S 96 above predicted

Group (g) (%) (mg) bng/g) LV+S WV 2V (mg/mgl

TX-AIR 241 -2" 43.93" 546" 2.27" -20.89" -3 1.94" -23.34" 0.2440" (9) 5 5 . 3 50.86 21 H 20 .04 -C 1.06 22.26 I+- 1.21 tO.0064

TX-CO 254.4"" 65.04"" 649"" 2-56"" - 10.99"" - 16.31** - 12.16"" 0.2660** (9) 5 5 . 9 20 .83 r 13 50.09 52.86 51 .98 52.56 k0.0052

ns +++ +++ ++ ++ +++ +++ + N-AIR 391.7 47.96 999 2.55 -0.67 0.3 1 -0.35 0.2749

(6) zk1'9.4 51.18 +-55 50.08 23.38 k3.85 53.38 +0.005B N-CO 466.4 68.67 1458 3.13 26.68 39.32 29.29 0.294 1

67) k17.0 ~ 0 . 6 3 3 1 20.12 53.03 26 , 16 53.61 k0.0079 + +++ +++ + 4- +++ +++ ++$ ns

NOTI:: TX, thyroidectornizd; N, norrnaI; Hct, hematmrit; ns, not significant. +, P < 0.05; + t, P .< 0.01; +++, P < [!.Mil comparing the treated and their respective control group. Nurmlkrs in parentheses indicate numbers of'anirnals used; values are means 2 standard error.

*$ <: 0.05 comparing TX/AIR and N!AYR. * *P -; 0.01 comparing TX/CO and N/CO.

TABLE 2. Experiment 2. Body weight, blood data, and heart weight of adult rats which had inhalcd 500 ppm carbon rnonoxide (CO) csntitauously for 44 days and their litternate controls (AHR)

--

BW % above predicted

Hct 2V 2V/BW RV/LV+S 2A 2A/BW Group b ~ ) 6%) (mg) (mg/g) Lb'+S RV 2 V ( m g / W ) (wag) (tng/g)

T% - AIR (12)

TX-CB (1 1)

Id- AIR (10)

N-CO ( 1 1 )

R - AIR (9)

R-CO 412)

NOTE: TX. thyroidectomized; N, normal; R, f w d restricted; ns, not significant. +. P <: 0.05; + f , P <: O.(!1; S + +, P < O.WL comparing the treated a n d their respective control group. Numbers in parentheses indicate numbers o f animals used; values are means 2 standard emr.

*$ < 0.01 comparing TX/AHR and N/AIR; **, P < 0.01 cumparing TX/CO and N!CO; t , P +: 0.05 comparing N/AIR and BPIA1I.a; $, P <: 0.001 comparing N/CO and R/C(4.

Thyroidec-tomy Body weight was much lower when comparison was made to

normal -AIR rats (Tables 1 and 2). Hematwrit was also significantly depressed. Combined ventricle weight was mush less than contro1 rats, and the ratio sf 2V weight to BW was somewhat depressed. Weights of LFr + S, RV, and 2V were much less than predicted on the basis of BW. indicating ventricular atrophy. This was particularly pronounced in RV. The weight ratio of WV to LV + S was significantly lower. The weight of the combined atria were much less than that of normal rats and the ratio sf combined atrial weight to BW was also somewhat depressed. Thyroid homowe T4 and growth hormone levels were low relative to euthyroid controIs (Table 4). The thy-

roidectomized rats appeared to have sraaaller frames, and relatively less body fat and skeletal muscle.

Thy roidectomy 4- carboxyhsrnogli~bi~~ern ku Body weight was similar to that of rats with thyroidectonny

alone, brat was much less than that of normal-CO rats (Tables B and 2). Hernatocrit was elevated by CO over that of rats with thyroidectomy alone, but was less than that of normal rats inhaling CO. Combined ventricle weight m d the ratio of 2V weight to BW were increased, but remained significantly depressed relative to that of normal-CC4 rats, at least in expt. I . On a prediction basis, weights of LV + S, RV, and 2V increased relative to that of rats with thyrc~idectomy alone, indi-

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PENNEY AND BARTHEL 645

TABLE 3. Experiment 3. Body weight, blood data, and heart weight of adult rats which had inhaled 500 ppm carbon monoxide (CO) coantinususiy for 49 days and their Bitternaate controls (AIR)

Group Hct (%I

above predicted RV/&V+S

LV 4- S WV 2V (~mg/m&)

N - AIR (8)

N-CO (9)

W - AIR (9)

R-CO (9)

660.2$ 223.4

HIS

NOTE N. normal, R, food restricted, ms, not ssgntflcant +, f' <. 0 05, t t t . f' c. (a 001 comparing tRe treated and their respwtlve ccmtrol group Kurnbers In papentheses ~ndscate nantben of anrmals used, values are means + standard error

* P K 0 05 cornparsrag N/AIW and M/AIW, **, P < 0 01 ccmparlng NIAIR dnd R/AIR, and * * * P L 0 001 cornpnrarag N:/AIW and K/AIR t P 0 05 comparing N l CO and R/CO, and 'F, P < 0 001 conlparlng N:CO and R/CO

cating cardiomegaly had occurred, but were less than that of normal -AIR rats, and were far less than that of normal-CQ rats. This was especially the case in RV, Moreover, the extent of relative increase in weights s f LV + S, MV, and 2V in the thyroidectomized rats treated with CO, as compared with normal rats treated with 60 , was less. 'The depressed weight ratio of WV to LV + S in thyroidectomized rats increased somewhat with 60 exposure in expt. I , but remained below that of nomal- AIR and normal-CO rats. In expt. 2, weight of the combined atria, and combined atrial weight to BW ratio im- creased, but were less than that of nonnal-CO rats, especially in expt. 2. Thyroid hormone T, and growth hormone levels were the same as with thyroidectomy alone (Table 4).

Food restrication Body weight was experimentally manipulated to approxi-

mately match that sf the thyrsidectomized rats in expts. 2 and 3, and thus was much less than that of the controls (Tables 2 and 3). Casual observation indicated that the food-restricted rats were more active than those free-fed. Upon sacrifice it was noted that these rats, although appearing to have nearly nomal appearance, had little body fat and greatly reduced. skeletal muscle mass. When comparison was made with the nomal- AIR rats, henaatocrit was unchanged or fell slightly, 2V weight on the other hand was depressed, while the ratio of 2V weight to BW was greater than that of normal- AIR rats. Weights of LV + S. RV, and 2V, however, were depressed when predicted on the basis of BW, indicating ventricular atrophy. The weight ratio of WV to LV -I- S was unchanged. Weights of the combined atria were less, while the ratio of comblned atrial weight to BW was greater in expt. 2. Thyroid hormone T4 and growth hormone Heveis were similar to those in nomnal-AIR rats (Table 4).

Food restriction + carbsxyhemogkot~inemiiz Body weight was experimentally maintained the same as in

the respective food-restricted controls (Tables 2 and 3). Mema- tocrit increased sharply and was the same or somewhat less than that of the nornaal-CO rats. In neither expts. 2 or 3 were there significant changes in 2V weight, the ratio of 2V weight to BW, nor in the weights sf LV + S, RV, or 2V on the basis of predictions from BW: thus indicating lack of cardiomegaly. Likewise, the weight ratio of WV to LV + S was unchanged.

TABLE 4. Experiment 2. Thyroid horrnoma: (T,) and growth hormone levels of adult rats which had inhaled 500 ppm carbon tnonsxide (C0) continucsusly for 44

days and their litternate controIs (AIR)

Group T4 (pg/dL) Growfh horrmaone (ng/mL)

TX-AlR 0.9 (2) [0.7, 1.21

TX-CO 0.7 (2) L0.5, 0.81

ns N - AIR 6.5 (2)

[6<5 , 6.51 N-CO 4.8 (2)

e4.4, 5.21 ns

R-AIR 5.3 (2) f4.7, 5.81

R-CO 5.4 (2) c4.6, 6.21

ns

- - - -

NOTE: TX, tkyroidectomized; N , normal: R, food restricted. Numbers in parentheses indicate nurllbers of animals used. Means plus individual values for T, are given in brackets. Values for growth horrnone are rneans t standard elTor.

The combined atrial weight and the ratio of combined atrial weight to BW increased in both experiments. Thyroid hormone T4 and growth hormone levels were not different from nomall - CB rats (Table 4).

Discussion The nomal rats inhaling 500 ppm 60 displayed the usual

cardiomegalic and polycythen-nic responses in all three experl- ments in this study, confirming our previous work (Penney, Benjamin et al. 19'74; Penney, Dunham et al. 1974: Penney et a$. 1984). On the other hand, the normal- AIR rats showed normal heart mass based on differences close to zero between measured and predicted heart weights (Styka and Penney 19'38). Thyroid hormone T4 levels in both the normal -AIR and CB-treated rats were similar to values obtained by R. T. Dowell and A. F. Martin (personal cornn-aunication).

Previous studies (Hervas et al. 1975) show that thy-

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646 CAN. J. PHYSHOL. PHAKMACOE. VOL. 63, I985

roidectomized rats to increase BW as cuthyroid rats do, in fact, growth nearly stops. Our results indicate that thyroidectorny , however, does not prevent stimulation of heart growth fojlowing imposit8oi.a of hypertensive volume over- loading. Neve~thcless, initial heart mass is less as a result of substantial atrophy, and the relative cxtenk of heart growth is less than in normal similarly stressed animals. Since both thyroid hormone and growth hormone were low, as previously reported in the thyroidectomizcd rat (Hervas et al. 1975). it is apparent that normal levels of neither hormone are required for such conanpensatory heart growth. While rt is reported (Tannenbaurra and Mar-tin 16376) that male rats have episodes of growth hormone secretion lasting approximately 3.3 h in which growth hormone levels may vary 200-fold, the variation in growth hormone we saw an-nong rats sacrificed over an 8-h period was much less. R/Boreover, thyroidectomy eliminates the bursts of growth hormone secretion, both during the day and at wight (Takeuchi et aI. 1978).

In the only other study using an analogous stressor (i.e., volume overload). Tipton and 'Tcheng ( I97 1 ) found significantly heavier hearts in hypophysectomized rats exercised by running for 10 weeks as coalapared with nontrained controls. Presumably growth hormone levels were very low in these animals, as well as thyroid gland activity and thyroxine levels, due to a lack of thyroid-stimulating hormone. It is likely, however, that growth hormone levels were higher in the present study than in that of 'Tipton and Tsheng (197 I ).

Lipana and Fanburg f 1970), using acute pressure over- loading, also demonstrated stimulation of heart growth in thc hypophysectomizcd rat when aortic constriction was suf- ficiently severe. They, as did 'Tipton and Tchewg (197 I), con- cluded that growth hormone is not essential for cardiac enlargement to occur in hypphysectomized rats. This contrasts with many studies by Beznak (1952. 1955, 1956, 1960) in which she found no compensatory heart growth in hypophy- sectornized rats no matter how manch the aorta was constricted, and with the more recent experiments of Powis f 1975) using nephrogenic hypertension as the stressor.

The cardiac atrophy following thyroidectonay, involved both the left and right ventricles and the atria, in contrast with that observed with f w d restriction. Cardiac atrophy has been reported in thyroidectomized rats by others (McQueen-Williams and Thompson 1940; Sharma and Banerjee 1977). It also occurs following hypophysectomy, but is supposedly not due to decreased food intake or the mere loss of BW (Beznak 1952). In both conditions it is probably directly related to the low level of thyroid hormonc, and indirectly related to the diminished metabolic demands on the heart. In this regard, cardiac output decreased while blood pressure and peripheral resistance remained constant in methylthiouracil-treated rats (Kapitola et al. B 974). Bradycardia (Sanderson 1976), depressed stroke volume (Selivonenko 11"%7), and decreased velocity of cardiac muscle shortening and rate of tension development (Buccino et a1 . 1967) are common findings in hypothyroidism. Moreover, Hoh et al. (1978) rcported a decrease in myofibrillar ATPase activity and a shift toward the V3 isomyosin form in the hypothyroid myocardium, changes also seen with aging and in cardiac failure.

Slightly depressed hematocrit in thyroidectomized rats compared with normals, both in air and in CO, has also been reported by Pankow et all. (1983). This probably reflects a subnormal rnetabcslic rate. Often, patients with hypothyroidism are moderately anemic, and respond to thyroid replacement

therapy (Chu et al. 1981); however, the presence of anemia may not be evident frona hemoglobin and hernaatcpcrit values, because both red cell mass and plasma volume are decreased (Das et al. 1975). Erythrocyte survival is usually normal, while bone m a m w erythropoietic activity is reduced.

la remains unclear why greater atrophy occu~red in the RV relative to the LV in the thyroidectomized rats. This ccmtrasts with hypsphysectomy, which reportedly results in an equal loss of weight in all parts c~f the heart (Beznak 1952). It has been suggested that the RV is exposed to a relatively greater work load per unit mass than the LV. This may be the reason, for example, why RV enlargeaslent in CO-exposed rats is greater than that of LV (Penney , Benjamin et al. 19774; Penney , Zak et al. 1983) in the absence of pulmonary hypertension: this was seen in the present study in both normal and thyroidectoanized rats. Anversa et al. f 1982) reported a sin~ilar ghcnomenon following a strenuous running program In rats. Thus as a corol- lary, when the heart experienced decreased work load, which probably occurred following thyroidectomy in the present study, KV mass declined the most.

The food-restricted rats, unlike animals subjected to nearly any other cardiovascaillar stress regimen used by this laboratory to date, failed to show significant cardiac growth during chronic carboxyhemoglobinenlia. There may be several pcrssi- ble reasons for this: (i) protein malnutrition, which provlded an amino acid pool sufficient for maintenance, but not for a response to stress; (ii) vastly decreased body fat and skeletal muscle and relatively more skeleton, requiring less blc~od flow and necessitating a nlinimall hemodynaanic response to CO (Kanten et al. 1983); and (iik) lower metabolic rate and body temperature, necessitating lower cardiac output. Although T4 levels were normal in the food-restricted rats, T4 is consistently found to be in the low normal range and serum triiodc~thyronine levels are depressed in anorcxia nervosa (Croxson and Ibbertson 1977). Similar findings are obtained in othcr maB- nourished states. Interestingly, growth hormone levels were normal in the food-restricted rats, as well as in the CO-exposed normal rats, although a nunaber of studies show that plasma growth hormone titer falls quickly in rats following acute stress (Kmlich et al. 1974).

Lack of significant heafl growth in thc food-restricted rats suggests that factors other than adequate semrn levels of thyroid and growth hormones limit compensatory ventricular growth. Like the thyroidectomized rats, ventricular atrophy was present with food restriction, but was equally distributed between the left and right ventricles. The latter was also seen by Beznak (1954) in rats starvled over 5 days, wlaile Spurgeon et al. (1983) repcsrt LV atrophy csnly in rats which underwent food restriction over a period of 2 years and more. On the other hand, in the present study the atria were normal when weight was related to body weight, and increased as a result of C64 exposure- This makes suspect the notion that decreased protein synthesis as a result of an inadequate supply of amino acids avas responsible for the lack of significant CO-induced ventricular enlargement. Atrial enlargement is a normal response daring long-term carboxyhernoglobine~nia in the rat (Penney, Benjamin et al. 1974). Whether the differential effects on atrial mass in the presence of thyroid hormone at normal (a. e. , food restriction) or subnolmal (thyroidectomy~ levels relate to the recent finding of lack of thyroid hormone modulation of atrial nayosin isoenzyrnes and ATPase activity (Bancrjee 1983), is unknown.

Nutritional constraints notwithstanding, the lack of stimu-

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PENNEY AND RAKTHEL 647

Iation of cardiac growth in the food-restricted rat model pro- vides evidence that development of polycythernia, hence increased blood viscosity, does not contribute to its induction, because both the food-restricted and free-fed rats underwent the same increase in hernatocrit. Other evidence strengthens this notion: cardionmegaly develops in the CO-exposed rat fetus, lacking polycythernia (Penney, Baylerian et a%. 1983); cardiomegaly is most severe in the CO-treated young rat during postnatal anemia (Pewney and Weeks 1949); and development of cardiomegaly in the adult inhaling CO, temporally Beads to development of polycythernia (Penney , Dunharn ct al. B 974). Moreover, the continued hypotension and depressed peripheral resistance observed even at high kesnatocrit after prolonged carboxyhemoglobinemia (Benney et al. 1979) argue against any significant role for polycythernia in the genesis of cardiomegaly in this rnodel. In contrast with our findings, Pankow et al. ( 1983) reported that hernatocrit increased to a greater extent in food-restricted rats treated with CQ.

The ratio of 2V weight to BW of the food-restricted-AIR rats was greater than that (1%' the normal- AIR rats. Oscai and HolBoszy (1970) also obsewcd this in a study involving weight reduction by food restriction in obese rats. While this seems inconsistent with ventricular atrophy, it is widely recognized that heart weight to BW ratios decrcahe with increasing body weight. making direct comparisons of such ratios between animals of disparate weights meaningless (Beznak 1954; Weroux and Gridgenaan 1958).

The fo'cBllowing conclusions naay be drawn from this study in the rat. ( i ) Substantial cardiac atrophy occurs as a result of thyrsidectomy, especially in the right ventricle. (i9) Thyroid- ectorny does not prevent stinlulation of heart growth following CO-induced hypotensive volume overloadi~~g; thus, normal levels of thyroid homone are unnecessary for it to occur. (kii) Because growth hormone levels also decline following thyroidectomy, normal levels of growth hormone are likewise not necessary for stimulation of heart growth. (iv) CO-induccd hyptensive volume overload stress doesn't alter either thyroid homone or growth hormone levels during cievclopment of cardioxmegaly. (v) Ventricular. but not atrial atrophy, is produced by severe food restriction. (\ti) CO-induced hypotcnsive volume overload stress fails to stimulate heart growth in food- restricted rats, even though thyroid horrnone and growth hormone levels are nomaal, suggesting that another factor or factors limit compensatory heart grtawth.

We wish to thank Dr. S. K. Banerjee (Pathology, Wayne State University) for performing the assays of thyroid hom~one and Dr. 9. @. Dunbar. Jr. (Physiolcdgy. Wayne State Univer- sity) for making the rneasurernents of growth hormone. We are also grateful to Drs. S. K. Bancrjee, J . C. Dunbar, J r - , and B. M. tawson (Physiology, Waync State University) for making constructive suggestions in the preparation of the manuscript.

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64 8 CAN. J . PHYSIOL. PHARMACOL. VOI,. 63, 1985

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Documents

Effects of thyroid and growth hormone deficiency, and food restriction on heart mass, with and without added stress (carboxyhemoglobinemia)