Comparative Physiology of Metabolic Responses to Neurohypophysial Hormones in Vertebrates

Abstract

The neurohypophysial (NHP) hormones in mammals are arginine vasopressin (AVP) (lysine vasopressin (LVP) in Suiformes) and oxytocin. Since these hormones have been implicated in the regulation of water balance and oxytocic functions in the body, metabolic responses to their action have to be considered in relation to problems associated with adaptive radiation and physiological evolution of vertebrates. Studies on species from different classes of vertebrates have shown that water deprivation, salt-loading and thermal stress could lead to the release of neurosecretory material (NSM) from the hypothalamo-neurohypophysial system (HNPS). Salt-loading has been shown to produce in rats significant decrease in both vasopressin and oxytocin in HNPS and also cause a fall in plasma free fatty acid (FFA) and a rise in plasma glucose. Injections of exogenous AVP and LVP decrease plasma FFA level in most of the mammals studied. AVP, LVP and oxytocin produce hyperglycemia in the mammals, birds and amphibians hitherto investigated. Arginine vasotocin (AVT) is shown to increase plasma FFA and glucose levels in the bird, fish and cyclostome. Possibly growth hormone (GH) is at least one of the extraneurohypophysial hormones through which the metabolic effects of AVT may be mediated. Thermal stress and dehydration in pigeons cause the release of NSM granules from the neurohypophysis and a rise in plasma levels of GH, cAMP and FFA with no significant change in glucose. Prostaglandin E, is found to increase plasma GH and FFA with a tendency toward hyperglycemia in the pigeon. Under heat stress, some reptiles, birds and mammals pant causing increased activity of the respiratory muscles. Increased activity can also cause rise in body temperature. Whether with exercise or increased body temperature the metabolic responses are indicated by increased mobilization and utilization of lipid which yields more energy and metabolic water than carbohydrate, thus sparing glucose for glycogen build-up in the muscles. In short-term muscular activity carbohydrate is the main fuel, whereas in long-term, fat becomes the preferred source of energy. In most mammals at least in in vivo experiments, AVP and LVP cause decrease in plasma FFA but increase in plasma glucose unlike AVT in birds as far as plasma FFA is concerned. This difference may be attributed to the presence of sweat glands in mammals which are thermoregulatory in function. The NHP hormones seem to have a regulatory role in bird migration. In certain birds there is found to be a build-up of NSM in HNPS towards migration and it is released before migration thereby possibly triggering a chain of regulatory and metabolic events. Melatonin which is known to release AVT has been shown to increase plasma levels of GH, FFA and glucose in the pigeon. The influence of environmental factors such as temperature and photoperiod in bird migration may be mediated through NHP and the pineal hormones.