Autonomic Regulation of Islet Hormone Secretion--implications for Health and Disease

The pancreatic islets are richly innervated by parasympathetic, sympathetic and sensory nerves. Several different neurotransmitters are stored within the terminals of these nerves, both the classical neurotransmitters, acetylcholine and noradrenaline, and several neuropeptides. The neuropeptides, vasoactive intestinal polypeptide, pituitary adenlyate cyclase activating polypeptide and gastrin releasing peptide are constituents of the parasympathetic nerves, whereas the neuropeptides galanin and neuropeptide Y are localised to sympathetic nerve terminals. Furthermore, the neuropeptide calcitonin gene-related peptide is localised to sensory nerves and cholecystokinin is also an islet neuropeptide, although the nature of the cholecystokinin nerves is not established. Stimulation of the autonomic nerves and treatment with neurotransmitters affect islet hormone secretion. Thus, insulin secretion is stimulated by parasympathetic nerves or their neurotransmitters and inhibited by sympathetic nerves or their neurotransmitters. The islet autonomic nerves seem to be of physiological importance in mediating the cephalic phase of insulin secretion, in synchronising the islets to function as a unit allowing oscillations of islet hormone secretion, and in optimising islet hormone secretion during metabolic stress, e.g. hypoglycaemia and neuroglycopenia. The autonomic nerves could also be involved in the islet adaptation to insulin resistance with possible implication for the development of glucose intolerance and Type II (non-insulin-dependent) diabetes mellitus. It is concluded that islet innervation, through the contribution of all branches of the autonomic nerves and several different neurotransmitters is of importance both for the physiology and pathophysiology of the islets.