ABSTRACT
Insulin is a pleiotropic hormone that elicits a variety of responses, ranging in their time of appearance from early (within minutes) to intermediate (within 1 to 2 hr) to late (6 hr or longer). Among the early responses to insulin are stimulation of the rate of glucose transport and of Na+/K+-ATPase activity. Intermediate responses include stimulation of glycolytic flux and glycogen synthesis. Among the late responses are stimulation of cell growth and of progression through the cell cycle. Not all of the responses to insulin are expressed in all cell types. Thus, muscle and fat respond to insulin typically with an increase in glucose uptake, whereas glucose transport into liver is not affected by the hormone. The liver, however, does respond to insulin with an elevation in Na+/K+-pump activity. The reasons for the diverse hormonal response in different tissues are obscure, but it is speculated that different messenger signals mediate insulin action on the different responding pathways. The nature of the messenger signals remains largely unknown.
