Insulin secretion and resistance (MORE than you ever wanted to know) Tisha Joy August 14, 2013

Insulin secretion and resistance(MORE than you ever wanted to know)

Tisha Joy

August 14, 2013

Objectives

• Briefly describe insulin secretion and resistance in the context of:– Pathophysiology– Clinical relevance– Assays

Pre-talk Question 1

1. What is this structure?

Pre-talk question 2

2. Which of the following is correct?

A. Insulin consists of 2 chains connected by disulfide bonds

B. Proinsulin levels are low in insulinoma

C. Insulin has minimal first-pass hepatic clearance

D. C-peptide is derived from cleavage of preproinsulin

Pre-talk Question 3

3. All of the following are correct EXCEPT:

A. Insulin secretion is normally characterized by 2 phases

B. There is loss of the 1st phase of insulin secretion with diabetes

C. Pulsatility of insulin secretion is maintained with diabetes

D. There is a hyperbolic relationship between insulin secretion and insulin resistance

Pre-question 4

4. Which of the following is CORRECT?A. Development of type 2 diabetes is due to insulin resistanceB. Loss of 30% of beta-cell mass leads to diabetes C. Non-diabetic individuals never exhibit insulin resistanceD. Insulin resistance occasionally is an adaptive response

Pre-talk Question 5

5. What is the gold standard for determining insulin resistance?

A. Insulin tolerance test

B. Hyperinsulinemic euglycemic clamp

C. Minimal model of FSIVGTT

D. HOMA-IR

Background

• Beta cells are found in islets of Langerhans, which are variable in size

• Normal ~106 islets; total weight 1-2 g; constitute 1-2% of the pancreatic mass

www.uptodate.com

Image from http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/pancreas/anatomy.html

Background

• Beta-cells (70%) – insulin

• Alpha cells – glucagon

• Delta cells – somatostatin

• PP cells – pancreatic polypeptide

www.uptodate.com

Image from http://www.abpischools.org.uk

β cells α cells

Insulin

• 51 aa molecule

• In portal circulation, insulin concentration is ~200-500pM in fasting state and ~1000-5000 pM in fed state.

• Approximately 50-80% of insulin is cleared on 1st pass through the liver.

• Estimated basal secretion is 24 units/24 h

• T1/2 =10 minSchofield CJ and Sutherland C Diabetic Medicine 2012;29:972-979

Caumo A, Lizio L AJP Endo Metab 2004;287:E371-E385

Insulin synthesis

www.uptodate.com

Ribosome

Golgi

Secretory granules

Insulin synthesis and secretion

Adapted from http://www.igis.com

Insulin

Schofield CJ and Sutherland C Diabetic Medicine 2012;29:972-979

Glucokinase

Insulin secretion

Adapted from Rorsman P et al Physiology 2000;15:72-77

Caumo A and Luzi L Am J Physiol End Metab 2004;287:E371-E385

• 1st phase important in shutting down hepatic glucose production

• The magnitude of the 1st phase is dependent on the pancreas’ past history of glucose exposure

Why important?

PreformedDe novo

Multiple players in insulin secretion

Gut

• Incretins

• Glucose

• Neuropeptide Y

• Parasympathetic nervous system

+

• Activation of K ATP channels

• Autocrine

+

Glucagon

-

+

Insulin secretion

Insulin secretion

Adapted from Rorsman P et al Physiology 2000;15:72-77

Insulin secretion

• Individually, β-cells all have differing pulse times (2-5 min), but when formed into aggregates, the pulse rhythm develops into ~4 min cycles

• So, the integration of all islets is required to ensure the pancreas releases insulin in a regulated fashion – the integration is mediated by the autonomic nervous system


Insulin secretion

• Why is pulsatility important?– Maintains the responsiveness of beta-cells to

glucose and insulin by preventing the desensitization of the signalling cascade


Insulin resistance

• Decreased tissue effect of insulin per unit delivered

• Hepatic vs. peripheral

• In people WITHOUT diabetes or obesity, are there times that they can transiently develop insulin resistance?

Insulin resistance

Tsatsoulis A et al Metabolism 2013;62(5):622-633

“Normal” conditions of IR


Sample conditions associated with insulin resistance

• Obesity (apple vs. pear)

• DM2

• PCOS

• Lipodystrophies [familial, acquired (HIV)]

Morelet et al JCEM 2006;91:2689-2695

Control

BMI 22.8

Control

BMI 34.8

HIV

BMI 28.3

Hegele R, Joy T, et al J Lip Res 2007;48:1433-1444

Differences in adipose distribution

Differences in adipose distribution

Hegele R, Joy T, et al J Lip Res 2007;48:1433-1444

Control

BMI 34.8

Control

BMI 22.8

HIV

BMI 28.3

Muscle

Insulin resistance


Insulin secretion and resistance are intertwined

Kahn SE et al Diabetes 1993;42(11):1663-1672

http://www.medscape.org/viewarticle/561655


Butler AE et a Diabetes 2003:52:102-110


Brunzell JD et al JCEM 1976;42:222-229


6.4 – 8.3

8.3 – 19.4

5.6 – 6.4

• Alterations in insulin secretion in T2DM–Absent 1st-phase insulin

response–Reduced 2nd-phase

response–Larger proportion of

secreted insulin is basal insulin

–Post-meal secretory pulses are smaller in amplitude and cycles are shorter and more irregular (less coupling to glucose)

Williams Textbook of Endocrinology, Edition 11.

DM2

Clinical relevance of insulin synthesis

Diagram representing the human preproinsulin molecule showing location of mutations causing ND. The amino acid residues in the signal peptide are indicated in green, the B chain in red, the C-peptide in orange, and the A chain in blue. The dashed circles indicate the basic residues that are the cleavage site for conversion from proinsulin to insulin. The mutations are noted in black circles together with location in the B or A chain.

Stoy J et al PNAS 2007;104(38):15040-15044

http://www.pnas.org/content/104/38/15040/F2.large.jpg

Clinical relevance of insulin secretion

Bell GI et al Nature 2001;44:788-791

Glucokinase

Assays of insulin secretion and insulin resistance

How do you know you have enough insulin or ineffective

insulin action?

Assays/Measurements

• Clinical Methods– Fasting blood glucose– Oral Glucose Tolerance Test

• Research Methods– Plasma insulin concentration – C-peptide concentration– HOMA/QUICKI– Hyperinsulinemic euglycemic insulin clamp– Minimal model analysis of FSIVGTT– Insulin suppression test – Insulin tolerance test– Intravenous Glucose Tolerance Test– Arginine stimulation

Measures of insulin secretion and beta-cell mass

• Fasting blood glucose• Fasting insulin• OGTT• C-peptide• HOMA %β• Intravenous GTT• Arginine stimulation• ?PET scan

Fasting plasma glucose

• Simple test• Assumes that glucose, insulin, and hepatic

glucose production are in steady state• Very insensitive• Need to lose more than 40-60% of beta-cell

mass before blood sugars increase in humans; in rats, need more than 70% of beta-cell mass loss.

• Clinical use – one of the diagnostic criteria for diabetes

Fasting insulin concentration

• Simple – just a blood test• But, peripheral insulin levels are based on

secretion, degradation, and distribution….• Influenced by BMI• Does not distinguish between endogenous

and exogenous insulin• So, measuring plasma insulin alone is not

useful as a measure of insulin secretion (see later)

Oral glucose tolerance test

• 75 grams of oral glucose given; measurements of fasting glucose usually only at 0 and 120 min

• False negative – take a walk/exercise during the 2 hours (unusual); not take all of the drink (makes some nauseated)

• False positive – low CHO load prior to test• Can be affected by gastric emptying, gastric bypass• Clinical relevance: used as diagnostic test for diabetes –

0 min 7.0 or higher mmol/L120 min 11.1 or higher mmol/L

• Stumvoll et al. show derived equations using data from OGTT has reasonable accuracy in predicting 1st phase & 2nd phase insulin release Diabetes Care, 2000; 23: 295

C-peptide

• Secreted in equimolar concentration to insulin

• Can use a technique of C-peptide deconvolution to estimate C-peptide secretion (Eaton-Polonsky approach) – removes the substance kinetics aspect

• T1/2 = 35 minutes

HOMA %β

• Calculation based on fasting insulin and glucose levels

• = 20 x insulin (mU/L) %

glucose (mmol/L) – 3.5

• Simple calculation

• Caveat – uses steady state conditions to derive a dynamic situation (r=0.64 with IVGTT)

Matthews DR et al Diabetologia 1985;28(7):412-419

Measures of insulin resistance

• Direct– Hyperinsulinemic euglycemic clamp– Insulin suppression test

• Indirect– Minimal model of FSIVGTT

• Simple surrogates– HOMA– QUICKI

Hyperinsulinemic euglycemic clamp

http://diabetesmanager.pbworks.com/w/page/17680151/Assessing%20Insulin%20Sensitivity%20and%20Resistance%20in%20Humans


• Insulin IV run at constant rate of 5-120 mU/m2/min

• BG monitored q5-10 min until steady state achieved

• D20 infusion – rate adjusted to keep BGs in normal range

• K infusion to avoid hypokalemia• When steady state = HGP is suppressed. Thus

glucose infusion rate = glucose disposal rate• More insulin resistant means less glucose

infused



• Gold standard

• It directly measures whole body glucose disposal at a given level of insulinemia under steady-state conditions

• Disadvantages:– Cumbersome, labor-intensive, expensive– Feasibility limited (n should be small)– Time consuming


Insulin suppression test

• After overnight fast, infusion of somatostatin (250 μg/h) or octreotide (25 μg bolus followed by 0.5 μg/min) given to suppress endogenous secretion of insulin and glucagon.

• Simultaneously, insulin (25 mU/m2/min) and glucose (240 mg/m2/min) are infused into same antecubital vein

• In contralateral arm, blood samples for glucose and insulin taken q30 min for 2.5 h and then q10 min from 150-180 min (latter is steady-state)

• SSPG (steady state plasma glucose) will be higher in IR patients


Insulin suppression test

• Highly reproducible

• Less labor intensive and technically demanding compared to clamp but still requires more than one person and is cumbersome

• Feasibility limited


Indirect – Minimal model of FSIVGTT

• After overnight fast, IV bolus glucose (0.3 g/kg body weight) is infused over 2 min.

• IV insulin (4 mU/kg/min) is infused over 5 min beginning 20 min AFTER the IV glucose bolus.

• Frequent blood samples for plasma glucose and insulin over the next 180 min

• Values entered into a computer model (MINMOD)


• Slightly less labor-intensive since there are no infusions requiring constant adjustment

• Equations also can be used to calculate beta cell function (unlike the prior tests)

• Not as accurate as clamp or ISTT

• Can be done in larger n, potentially

Indirect – Minimal model of FSIVGTT


HOMA

• HOMA - IR = homeostasis model of assessment for insulin resistance

• = Glucose (mmol/L) x Insulin (mU/L) 22.5• Normal HOMA IR is 1• R=0.88 with hyperinsulinemic euglycemic

clamp• Simple, quick, can be done in large

populations, relatively inexpensive


QUICKI

• Quantitative insulin sensitivity check index (QUICKI)

• 1 =

[log(fasting insulin) + log (fasting glucose)]

• Correlates better with clamp data over a wide range of IR better than minimal model or HOMA IR

• Simple, validated, easy, quick, inexpensive


Post-talk Question 1

1. What is this structure?

Post-talk question 2

2. Which of the following is correct?

A. Insulin consists of 2 chains connected by disulfide bonds

B. Proinsulin levels are low in insulinoma

C. Insulin has minimal first-pass hepatic clearance

D. C-peptide is derived from cleavage of preproinsulin


3. All of the following are correct EXCEPT:

A. Insulin secretion is normally characterized by 2 phases

B. There is loss of the 1st phase of insulin secretion with diabetes

C. Pulsatility of insulin secretion is maintained with diabetes

D. There is a hyperbolic relationship between insulin secretion and insulin resistance

Post-question 4

4. Which of the following is CORRECT?A. Development of type 2 diabetes is due to insulin resistanceB. Loss of 30% of beta-cell mass leads to diabetes C. Non-diabetic individuals never exhibit insulin resistanceD. Insulin resistance occasionally is an adaptive response


5. What is the gold standard for determining insulin resistance?

A. Insulin tolerance test

B. Hyperinsulinemic euglycemic clamp

C. Minimal model of FSIVGTT

D. HOMA-IR

Thank you

Dodson and Steiner Curr Op Struc Biol 1998;8:189-194

Documents

Insulin secretion and resistance (MORE than you ever wanted to know) Tisha Joy August 14, 2013