The correct option is Glucagon.
Explanation
The pancreas functions as a composite gland (heterocrine), acting as both an exocrine and an endocrine organ. The endocrine portion, known as the Islets of Langerhans, contains specific cell types (alpha, beta, and delta cells) that secrete hormones essential for the homeostasis of blood glucose levels.
Detailed Analysis:
- Glucagon is Correct: Glucagon is a peptide hormone secreted by the alpha cells of the Islets of Langerhans. It acts mainly on the liver cells (hepatocytes) to stimulate glycogenolysis (breakdown of glycogen into glucose) and gluconeogenesis (synthesis of glucose from non-carbohydrate sources). These processes release glucose into the bloodstream, resulting in an increase in blood sugar levels (hyperglycemia).
- Insulin is Incorrect: Insulin is secreted by the beta cells of the pancreas. It acts to lower blood glucose levels (hypoglycemia) by enhancing cellular glucose uptake and stimulating glycogenesis (conversion of glucose to glycogen). It acts antagonistically to glucagon.
- Somatostatin is Incorrect: Somatostatin is secreted by the delta cells of the pancreas. Its primary function is to inhibit the secretion of both glucagon and insulin, acting as a regulatory hormone for the other pancreatic secretions.
- Melatonin is Incorrect: Melatonin is secreted by the pineal gland in the brain, not the pancreas. It is responsible for regulating the diurnal (24-hour) rhythm of the body, such as the sleep-wake cycle.
Key Takeaway:
Glucagon is a hyperglycemic hormone that increases blood glucose levels, whereas Insulin is a hypoglycemic hormone that decreases blood glucose levels.