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80. The regulation of K+ metabolism.

Domoki Ferenc2019 March 4.

K+-homeostasis: quantitative data

� Total body K+ content� Distribution of body K+ between the

intra- and extracellular compartments� Normal value of extracellular (plasma)

K+, definitions of hypo- hyperkalemia� Daily uptake/excretion of K+

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Total body K+ and its distribution, normal values

� The body contains app. 6.1 mol = 240 g K+

� 98% of K+-s are found INTRACELLULAR

� normal serum K+=4 mmol/L (3.5-5.2 mmol/L)

� Hyperkalemia >5.5 mmol/L (>6.5 mM mmol/L)

� Hypokalaemia <3.5 mmol/L

Changes in EC K+ concentration elicit symtoms that can be explained due to altered resting membrane potential

� Hyperkalemia reduces EKhypokalemia reduces K+-permeability, they both induce DEPOLARIZÁTION

� Affects chiefly EXCITABLE tissues

� Symptoms of Hypo/hyperkalemia can be similar: life-threatening ARRHYTHMIAS, fatigue, muscle weakness, cramps, constipation, numbness/tingling

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Daily potassium metabolism:

� Absorption from GIS is NOT controlled� Absorbed K+ will be uptaken into the IC space

where from it is continously replenishing the EC K+ reduced by urine excretion

� K+ metabolism is controlled through the regulation of EXCRETION

Uptake:

Food/drinks: 50-100 mmol/day

total: 50-100 mmol/day

Excretion:

urine: 45-90 mmol/daystool, sweat: 10 mmol/day

total: 50-100 mmol/day

K+ absorption and IC uptake

� K+ absorption: in small intestine mainly by passive paracellular transport

� Absorbed K+ stimulates INSULIN-secretion, insulin in turn stimulates K+

uptake of target cells. In addition to insulin T4/T3 and β-adrenergic agonists possess similar effects.

ICU therapy of hyperkalemia (>6.5 mmol/L) is based on these principles:1. iv Calcium-gluconate (10%, 10 ml)2. 10 unit insulin + 50ml 50% glucose3. Salbutamol 10 mg

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secretion

Overview of renal K+-transport

The renal „handling” of K+

� Freely filtered, thus the filtered amount: GFRxPK=180 L/day x 4 mmol/L= 720 mmol/day

� 90% of filtered amount (~650 mmol/day) is automatically reabsorbed until the distal nephron

� In the collecting duct segment of the outer medulla, tubular secretion of K+ greatly exceeding the filtered amount is possible, BUT further net absorption is also a possibility

� Excreted amount can vary between 3-200%-of filtered amount ~20-1400 mmol/day, ensuring large safety margin to prevent hyperkalemia

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Mechanisms of K+

secretion/reabsorption in the outer medullary segment of the collecting duct

� K+ secretion in principal cells: primary active transport through the basolateral Na+/K+ pump, passive diffusion through luminal ROMK K+-channels (controlled by aldosterone).

� K+ reabsorption in A-type intercalated cells: primary active transport through luminal H+/K+

pump, passive diffusion through basolateral K+-channels. (controlled directly by EC K+)

The VITAL regulator of renal K+ excretion: Aldosterone!

Plasma K+ ↑

Adrenal cortexzona glomerulosa

Aldosterone-secretion ↑

Renal K+ excretion ↑ Aldosterone controls gene expression (Na/K pump, ENaC, ROMK) by binding to the mineralcorticoid receptor (MR). Specificity is ensured by unique pre-receptor mechanism, as the enzyme 11βHSDH inactivates cortisol!

Total aldosterone deficiency causes lethal hyperkalemia!

-

+

+

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Medical Physiology aspects

� EC pH and K+ are inversely coupled to each other:acidosis ↔ hyperkalemia, alkalosis ↔ hypokalemia. Causes: EC-IC K+ shift, and in the collecting duct H+–secretion is coupled to K+-reabsorption.

� Diuretics acting BEFORE the collecting duct INCREASE luminal flow in the collecting duct, therefore, they also INCREASE K+-secretion –hypokalemia develops. These diuretics require dietary potassium supplement. Diuretics acting on the collecting duct (ENaC inhibitor amilorid, and aldosterone-antagonist spironolaktone) are, however, „K+-sparing” diuretics.

81. The regulation of Ca2+ and phosphate metabolism.

Domoki Ferenc2019 March 4.

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Ca2+-homeostasis: quantitative data

� Total body Ca2+ content� Extracellular (plasma) Ca2+ normal

values, definitions of hypo- and hypercalcemia

� Extracellular (plasma) anorganic phosphate normal values

� Daily Ca2+ metabolism

Body calcium compartments

999g (25 mol)ECF:1g (0,025 mol)

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Normal plasma Ca2+ and Pi levels

� Total Ca2+: 2.1-2.6 mmol/L (50% ionized, 40% protein-bound, 10% complex salts)ionized (free) Ca2+: 1,16-1,32 mmol/Lbiologically active and under homeostatic control!!!

� Plasma phosphate conc. (HPO42-/H2PO4

-

; Pi): 0.8-1.2 mmol/l (50% ionized, 40% complex salts, 10% protein-bound)

�

Medical Physiology: hypocalcemia elicits (potentially) lethal tetany

� Ionized Ca2+ plays roles in many physiologic processes, but neuromuscular excitability is MOST sensitive to hypocalcaemia.

� Tetany means uncontrolled muscular spasms that can incolve laryngeal and respiratory muscles causing death!

� Latent tetany: no spontaneous spasms yet, but enhanced excitability can be shown by special tests

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The activation threshold of the voltage-gated Na+ channels is strongly dependent on the EC Ca2+ concentration

Mild hypoxia induced by blood pressure cuff inflation triggersCARPOPEDAL spasm: muscle cramps in the forearm/hand muscles

Trousseau’s sign Chvostek’s sign

Clinical signs of latent hypocalcaemia/tetany

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� the EC concentrations of calcium and anorganic phosphate are close to their solubility limit -further increases might evoke precipitation of insoluble Ca2+-salts in the soft tissues (e.g. kidney -nephrocalcinosis)

� In the bone physiological precipatition (mineralization) occurs under controlled conditions

Medical Physiology: hypercalcemia can elicit uncontrolled deposition of inorganic salts

Calcium-balance

� The balance betwen the daily amount of absorbed (GIS) and excreted (kidney) calcium

� positive calcium balance: until the end of net bone formation

� negative calcium balance: due to osteoporosis (especially in postmenopausal women, peak bone mass is 30% less)

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RDA: 1g/day (25 mmol/day), net absorption: 200 mg/day (5 mmol/day)

Filtered: 180 L/day x 1.4 mmol/L = 270 mmol/day, reabsorbed: 265 mmol/day

excretion: 200 mg/day (5 mmol/day, 2% of filtered amount)

Age-dependent changes in Ca2+-metabolism

35-40y

Positive balance Negative balance

+Increased Ca2+ demand: pregnancy, lactation (nursing),growth,

(women)

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Regulation of EC ionized Ca2+ and Piconcentrations

� EXCLUSIVELY endocrine regulation� calciotrop hormones:

parathormone (parathyroid gland chief cells) VITAL HORMONE!1,25 dihydroxy cholecalciferol(calcitriol) (D-vitamin-hormone, kidney)calcitonin (parafollicular C-cells (clear cells) of the thyroid gland)

Ca2+

PTH

calcitriol

bone

kidney

GIS

Inhibition ↑

Stimulation ↑

Overview of homeostatic endocrine control of EC Ca2+ concentration

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Parathormon, PTH

� Source: parathyroid glands, chief cells

� 84 amino acid peptide hormone

� half life in circulation ~5 min

� related molecules: PTHrP, (paraneoplastic)

Thyroid gland

Parathyroid

glands

Regulation of PTH secretion

� Ca2+ inhibits PTH through stimulation of cell membrane Ca2+ -sensor receptors (7TM/G-protein)

� Calcitriol also inhibits PTH (genomial effect-suppresses PTH gene expression)

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Effect of PTH: to prevent hypocalcaemia

� 7TM receptor Gs/cAMP or Gq/PLC/IP3stimulation

� bone: promoting osteolysis (complex effect, direct on osteocytes, indirect on osteoclasts)

� kidney: in the proximal tubular cells inhibits phosphate reabsorption, and stimulates calcitriol-synthesis; in the distal tubule increases calcium reabsorption

Calcitriol (D-hormone)

� Endogenous substance, we need exogenous supply (RDA: 600 IU) only because of limited exposure to UV light

� synthesis is stimulated by PTH, hypocalcemia, hypophosphatemia

� utilizes intracellular receptors (VDR) to regulate gene expression

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Pro-hormones: vitaminsD2- ergocalciferol (plants)D3- cholecalciferol (animals)

The regulated synthesizing enzyme:1α-hydroxylase, renal proximal tubule epithelial cells

Effects of calcitriol

� GIS: Stimulates intestinal calcium AND phosphate absorption

� bone: direct effect: osteolysis, indirect effect: promotes mineralization

� kidney: inhibits the expression of 1-alpha hidroxylase (negative feed-back)

� parathyroid gland: inhibits the synthesis of PTH (negative feed-back)

� Non-specific anti-proliferative and anti–inflammatory effects

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Medical Physiology (historical):D-hypovitaminosis: rickets (rachitis)

(Paris 1900)

Vitamin D-substitution: first 2 years, vitamin D fortified milk products, oily fish, vegetable oils, margarines, dietary supplements: D-hypervitaminosis!!!

Calcitonin

� Produced by the parafollicular C (clear) cells of the thyroid gland

� 32 amino acid, related peptide is CGRP (neurons) � Elevated calcium levels STIMULATES its release

(same calcium sensor receptor is coupled to a different transduction mechanism)

� major effect is the inhibition of bone resorption (osteoclasts) to decrease plasma calcium

� physiologic significance is uncertain – no symptoms of deficiency - growth, lactation?

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Ca2+

PTH

calcitriol

bone

kidney

GIS

Inhibition ↑

Stimulation ↑

Overview of homeostatic endocrine control of EC Ca2+ concentration

Stimulation of intestinal Ca2+ absorption by calcitriol

TRPV5/6: Transient Receptor Potential Receptor Vanilloid type 5/6

NCX1: Na+-Ca2+ Exchanger type 1; PMCA1b: Plasma Membrane Ca2+ pump type 1b

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GI handling of calcium

� Passive paracellular absorption (~50%)� transcellular active absorption via

specific membrane and intracellular proteins (TRPV channels, calbindins, PMCA and NCX transporters), regulated by calcitriol

� Net absorption is ~20% (max. 30%) of dietary intake

Ca2+

PTH

calcitriol

bone

kidney

GIS

Inhibition ↑

Stimulation ↑

Overview of homeostatic endocrine control of EC Ca2+ concentration

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Renal calcium handling� The free, ionized Ca2+ is

filtered, the daily filtered load is 240-270 mmol/day (~10 g/day)

� 90% reabsorbed in the proximal tubule and the Henle’s loop, passive paracellular transport

� 5-10% reabsorbed in the distal convoluted tubule, active transcellular and PTH regulated

� Excretion: 1-3% of filtered amount: 2,5-7,5 mmol/day

Active Ca2+ reabsorption in the distal convoluted tubule

� Free diffusion through luminal Ca2+ channels (ECaC)

� Primary or secondary active transport (shown) through the basolateral membrane

� This mechanism is ACTIVATED by PTH!

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� Ca2+ activate Ca2+

sensor receptors that inhibit Ca2+-reabsorption

� A kind of „autoregulation”

Direct effect of EC Ca2+ on Ca2+

reabsorption

Renal handling of inorganic phosphate (Pi)

� Free Pi is freely filtered (220-240 mmol/day)

� 85-95% is reabsorbed in proximal convoluted segment

� Glucose-type reabsorption: secondary active transport with Na+ (Tmax!)

� PTH inhibits phosphate reabsorption!

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Ca2+

PTH

calcitriol

bone

kidney

GIS

Inhibition ↑

Stimulation ↑

Overview of homeostatic endocrine control of EC Ca2+ concentration

Bone Physiology

Functions: Mineral reservoire (Ca, P, Mg, F) – can

mobilize Ca2+ an Pi

Provides mechanical stability – constantly adjusting to mechanical forces – bone remodeling, complete turnover in 3 years!

Encases bone marrow (haematopoesis)

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trabecular

Peak bone mass – bone remodeling

35-40y

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Age-related reductions in the bone densitiy of women

Bone compartments

� inorganic phase (70%) Ca10 (PO4)6 (OH)2hydroxyapatite crystals (pH-dependent)amorphous Ca-phosphate crystals-mechanical strength, rigidity

� organic matrix (22%) osteoid, mainly Type- 1 collagen, other structural proteins(polysulphated proteoglycans, osteocalcinetc.) – flexibility, endurance (analogy –reinforced concrete!)

� Cells: osteoblasts and osteocytes(mesenchymal origin) - networkosteoclasts: haemopoetic origin

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Osteoblasts: produce the osteoid proteins: collagen, osteocalcin etc; alkaline phosphatase – pirophosphatase: incrases Pi promoting mineralizationOsteocytes- maintain bone interstitiumOsteoclasts- multinucleated macrophage-like cells: secrete acid and proteolytic enzymes to dissolve bone

Bone remodeling cycle: resorption – new osteoid matrix -mineralization

� 20% of bone at any instant, once in 3 years

� Dependent/stimulated on/by compression/traction forces on the bone

� orchestrated by cytokines (TNF, IL-s), growth factors (IGF-s, TGF-beta), modulated by hormones (sexual hormones, glucocorticoids and calciotropic hormones)

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Important regulators of osteoclast activaton� RANK: Receptor for the Activation of Nuclear

factor Kappa-B (present on the earlyosteoclasts)

� RANK-ligand (RANKL): present in the osteoblast membranes (production stimulated by PTH, calcitriol, also cytokins prostaglandins and cortisol

� Osteoprotegerin (OPG): soluble decoy protein that INHIBIT RANKL-RANK binding produced by osteoblasts (production inhibited by PTH, stimulated by estrogen

PTH and calcitriol stimulate bone resorption through RANKL-RANK signaling

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Hormonal background of bone remodeling

Pathological changes of the bone tissue

Osteomalacia: bone softening: the ratio of mineral to organic material is reduced – bone mineralization is affected. vitamin D deficiency in adults, parathormon overproduction

Osteopenia – Osteoporosis: bones are fragile and brittle, loss of both mineral and organic materials(reduction of bone mass)lack of estrogen (postmenopausal women), Cushing syndrome, hyperthyreosis