The involvement of endogenous nitric oxide in vagal-cholinergic stimulation of exocrine and endocrine pancreas in dogs

Neural control of pancreatic peptide hormone secretion

Pancreatic peptide hormone secretion is inextricably linked to maintenance of normal levels of blood glucose. In animals and man, pancreatic peptide hormone secretion is controlled, at least in part, by input from parasympathetic (vagal) premotor neurons that are found principally in the dorsal motor nucleus of the vagus (DMV). Iatrogenic (insulin-induced) hypoglycaemia evokes a homeostatic response commonly referred to as the glucose counter-regulatory response. This homeostatic response is of particular importance in Type 1 diabetes in which episodes of hypoglycaemia are common, debilitating and lead to suboptimal control of blood glucose. Glucagon is the principal counterregulatory hormone but for reasons unknown, its secretion during insulin-induced hypoglycaemia is impaired. Pancreatic parasympathetic neurons are distinguishable electrophysiologically from those that control other (e.g. gastric) functions and are controlled by supramedullary inputs from hypothalamic structures such as the perifornical region. During hypoglycaemia, glucose-sensitive, GABAergic neurons in the ventromedial hypothalamus are inhibited leading to disinhibition of perifornical orexin neurons with projections to the DMV which, in turn, leads to increased secretion of glucagon.

Nitric oxide-endothelin-1 interaction in humans

Healthy men received NG-monomethyl-L-arginine (L-NMMA) intravenously to study cardiovascular and metabolic effects of nitric oxide synthase blockade and whether this alters the response to endothelin-1 (ET-1) infusion. Controls only received ET-1. L-NMMA effects were that heart rate (17%) cardiac output (17%), and splanchnic and renal blood flow (both 33%) fell promptly (all P < 0.01). Mean arterial blood pressure (6%), and systemic (28%) and pulmonary (40%) vascular resistance increased (P < 0.05 to 0.001). Arterial ET-1 levels (21%) increased due to a pulmonary net ET-1 release (P < 0.05 to 0.01). Splanchnic glucose output (SGO) fell (26%, P < 0.01). Arterial insulin and glucagon were unchanged. Subsequent ET-1 infusion caused no change in mean arterial pressure, heart rate, or cardiac output, as found in the present controls, or in splanchnic and renal blood flow or splanchnic glucose output as previously found with ET-1 (G. Ahlborg, E. Weitzberg, and J. M. Lundberg. J. Appl. Physiol. 79: 141-145, 1995). In conclusion, L-NMMA like ET-1, induces prolonged cardiovascular effects and suppresses SGO. L-NMMA causes pulmonary ET-1 release and blocks responses to ET-1 infusion. The results indicate that nitric oxide inhibits ET-1 production and thereby interacts with ET-1 regarding increase in vascular tone and reduction of SGO in humans.