Effects of rat pancreatic polypeptide on islet-cell secretion in the perfused rat pancreas

Alpha-cells of the endocrine pancreas: 35 years of research but the enigma remains

Glucagon, a hormone secreted from the alpha-cells of the endocrine pancreas, is critical for blood glucose homeostasis. It is the major counterpart to insulin and is released during hypoglycemia to induce hepatic glucose output. The control of glucagon secretion is multifactorial and involves direct effects of nutrients on alpha-cell stimulus-secretion coupling as well as paracrine regulation by insulin and zinc and other factors secreted from neighboring beta- and delta-cells within the islet of Langerhans. Glucagon secretion is also regulated by circulating hormones and the autonomic nervous system. In this review, we describe the components of the alpha-cell stimulus secretion coupling and how nutrient metabolism in the alpha-cell leads to changes in glucagon secretion. The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions. We also highlight the pathophysiology of the alpha-cell and how hyperglucagonemia represents an important component of the metabolic abnormalities associated with diabetes mellitus. Therapeutic inhibition of glucagon action in patients with type 2 diabetes remains an exciting prospect.

Pancreatic polypeptide, microinjected into the dorsal vagal complex, potentiates glucose-stimulated insulin secretion in the rat

Specific binding sites for circulating pancreatic polypeptide (PP) have been found within the dorsal vagal complex (DVC) in the caudal medulla oblongata. Therefore, the effects of rat PP on pancreatic hormone secretion upon its microinjection into the DVC in halothane-anesthetized rats at doses of 0.4-40 pmol were investigated. At this range of doses, the changes in plasma concentrations of insulin, glucagon and glucose over basal levels did not differ from those after vehicle microinjection. In a separate series of experiments, vehicle and PP at doses of 0.4 and 4 pmol were microinjected into the right DVC 40 min after the continuous infusion of D-glucose had been started. In animals receiving continuous infusion of D-glucose, PP microinjected into the DVC (4 pmol), resulted in markedly higher insulin levels at corresponding time points compared to those with vehicle microinjected into the DVC. These data indicate, for the first time, that microinjection of PP into the DVC may potentiate glucose-stimulated insulin secretion in halothane-anesthetized rats.

Pancreatic polypeptide and splanchnic blood flow in anesthetized rats

The aim of the study was to evaluate the effects of pancreatic polypeptide (PP) on splanchnic blood flow in anesthetized rats. For this purpose, either PP dissolved in saline (225, 450, or 900 pmol/kg body weight/h) or saline alone was infused at a rate of 2 ml/h for 30 min. Immediately after the infusion the blood flow to the whole pancreas, islets, duodenum, and colon was measured with a microsphere technique. The highest dose of PP caused a small decrease in mean arterial blood pressure. The two lowest doses decreased duodenal blood flow, but did not affect the blood perfusion to the other investigated organs. The highest dose of PP decreased the blood flow to the pancreas, islets, and duodenum, did not affect colonic blood flow. No change in the fraction of whole pancreatic blood flow diverted through the islets could be seen after PP administration. It is concluded that administration of high doses of PP induces a general splanchnic vasoconstriction.