The long term objective of our study is to elucidate the mechanism of insulin action at the molecular level. Chronically, insulin promotes cell growth; acutely, it controls energy metabolism mainly by regulating fat and carbohydrate metabolism by three types of reactions. In type A, insulin regulates glycogen synthase and other enzymes by promoting dephosphorylation of the enzyme proteins. In type B insulin stimulates cAMP phosphodiesterase (phosphodiesterase) by an ATP-dependent reaction. In type C, insulin increases the cellular glucose transport by facilitating membrane. However, it is yet to be ascertained how insulin exerts these effects on metabolic activities. The purpose of the studies proposed herein is to fill the gap in our knowledge between the insulin receptor and the insulin-sensitive enzymes, such as glycogen synthase and phosphodiesterase, especially the latter in rat adipocytes.
The specific aims of the proposed work are two-fold. First, to determine the significance and effects of internalization of insulin-receptor complex on a) the physiologic actions of insulin, (b) the termination of insulin effects, and (c) the binding of insulin to low-ATP adipocytes. Second to study the regulatory mechanisms of insulin-sensitive enzymes by (a) examining the potential effects of Ca2+, (b) characterizing deactivation of insulin-stimulated phosphodiesterase, and (c) attempting to reconstitute cAMP-dependent stimulation of phosphodiesterase. The first three projects are based on the finding that extracellular ATP is an inhibitor of endocytosis. Project 2-a is based on the observation that the effect of Ca2+ may be positively determined in cells that are treated with A-23187 and EDTA. Project 2-b is based on the discovery that dithiothretol rapidly deactivates phosphodiesterase without modifying its catalytic domain. Project 2-c is based on the fact that phosphodiesterase can be activated by either insulin or epinephrine.
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