Insights into insulin and glucagon responses in fish

The focus of this review is the response of insulin and glucagon to various experimental and physiological conditions in fish. Circulating levels of insulin and glucagon have been analyzed in several fish species, principally salmonids. It is generally accepted that, in fish, the insulin secretion in response to amino acids is stronger than to glucose, although this information has been obtained mainly from carnivorous species. Studies of other species demonstrate that the alimentary pattern affect hormonal secretion and should be taken into account when the effects of secretagogues are analyzed. There are few studies of glucagon secretion, even in salmonids, although it is known that amino acids also stimulate its release. The level of this response and its relationship to insulin secretion depends upon the species of fish. We will discuss the significance of these observations, while also considering other important factors, including the influence of neuropeptides and interactions with other pancreatic and gastro-intestinal hormones. Finally, seasonal, temperature and reproductive stage effects will be discussed, all of which should be taken into account when attempting to understand the role of pancreatic hormones in fish. This short review will not cover the action of these hormones on their target tissues.