Endocrine Pancreas: MRCOG Part 1 questions and key facts

During moderate-intensity exercise, the body's energy requirements change, leading to specific hormonal adaptations.

Working skeletal muscle takes up glucose through contraction-stimulated, insulin-independent translocation of GLUT4 transporters, so glucose utilisation rises steeply. Blood glucose nevertheless stays fairly constant during moderate exercise, because hepatic glucose output rises to match this uptake, and the islet hormones help drive that rise. Increased sympathetic activity inhibits the pancreatic beta cells, giving reduced insulin release; the lower insulin level removes its brake on hepatic glucose output and limits glucose uptake by inactive tissues. At the same time, sympathetic stimulation of the alpha cells gives elevated glucagon release. Glucagon acts primarily on the liver to promote glycogenolysis (breakdown of glycogen to glucose) and gluconeogenesis (production of glucose from non-carbohydrate sources), increasing glucose output into the bloodstream to maintain the supply to muscle and brain.

Elevated insulin release, Elevated glucagon release is wrong because a rise in insulin would oppose hepatic glucose output and promote storage at a time when glucose is needed for work. Elevated insulin release, Reduced glucagon release is the fed-state pattern, not the exercise response. Reduced insulin release, Reduced glucagon release gets insulin right but would remove a main hormonal stimulus to hepatic glucose production as muscle uptake rises. Minimal change in insulin release, Reduced glucagon release is wrong on both counts: insulin falls measurably during sustained exercise, and glucagon is essential to prevent hypoglycaemia during exercise.

The C-peptide level is the most useful test to assess endogenous insulin secretion because C-peptide is a direct by-product of insulin synthesis within the pancreatic beta cells.

C-peptide is produced in equimolar amounts to insulin when proinsulin is cleaved during insulin production. Measuring C-peptide provides a reliable indication of how much insulin the patient's pancreas is making, without interference from exogenously administered insulin. This distinction is crucial in managing diabetes, as it helps differentiate between type 1 (absolute insulin deficiency) and type 2 (insulin resistance with varying degrees of insulin deficiency) diabetes.

HbA1c measures average blood glucose levels over the preceding 2-3 months, providing a long-term picture of glycaemic control but not a direct measure of current insulin production.

Measuring proinsulin precursor levels is not a standard clinical test for evaluating endogenous insulin secretion. An oral glucose tolerance test (OGTT) assesses how the body handles glucose, helpful for diagnosing diabetes but not directly measuring endogenous insulin production.

Insulin measured in a 24-hour urine collection is not used: about half of secreted insulin is removed by the liver on first pass and the rest is cleared variably, so urinary insulin bears an inconsistent relation to secretion, and in an insulin-treated patient the assay cannot separate injected from endogenous insulin. Where a urine test is wanted, the urine C-peptide:creatinine ratio is used instead, because C-peptide escapes hepatic extraction and is excreted by the kidney at a steady rate.

During prolonged periods without food intake (fasting or starvation), the body undergoes significant metabolic adaptations to maintain energy supply, especially to the brain. The primary goal is to maintain blood glucose levels within a narrow range.

During fasting the hormonal profile is one of reduced insulin and elevated glucagon levels. Insulin, an anabolic hormone, is secreted in response to high blood glucose (after a meal). It promotes glucose uptake and storage (as glycogen) and inhibits glucose production. During fasting, blood glucose levels fall, leading to decreased insulin secretion. Glucagon, a catabolic hormone, is secreted by the pancreas in response to low blood glucose. It stimulates glycogenolysis (breakdown of glycogen to glucose) and gluconeogenesis (synthesis of glucose from non-carbohydrate sources like amino acids) in the liver, both of which increase blood glucose levels.

The other combinations do not fit the fasting state. Elevated insulin and diminished glucagon levels are characteristic of the fed state, when blood glucose levels are high. Reduced insulin and reduced glucagon levels are not a physiological fasting pattern: suppression of both hormones occurs with somatostatin excess, not as a response to falling blood glucose. Elevated insulin and elevated glucagon levels can occur together, but this pattern is usually associated with insulin resistance and type 2 diabetes. Elevated insulin and unchanged glucagon levels are not expected, because falling blood glucose suppresses insulin secretion.

In summary, a decrease in the insulin to glucagon ratio indicates an overall body shift to catabolism during nutritional absence.