Hyperinsulinemia has been linked to hypertension in both animals and humans. Furthermore, insulin has been shown to result in sodium retention by the kidney. In cell culture, insulin directly increases sodium transport capacity of the amiloride-sensitive sodium channel (ENaC), normally located in the renal connecting tubule and collecting duct. Inappropriate retention of sodium in the distal portion of the tubule could result in expanded extracellular fluid volume and hypertension. However, it is not clear what are direct versus indirect effects of hyperinsulinemia with regard to sodium balance. Furthermore, the impact of hyperinsulinemia on the expression and regulation of specific renal sodium transporters and/or channels has not been aqequately studied. Antibodies against many of these proteins have only recently become available. In this proposal, we plan to test the overall hypothesis that insulin and """"""""insulin sensitizing agents"""""""", such as PPAR-gamma agonists, will, as a whole, increase the protein abundances of several critical sodium transport proteins expressed in the kidney. We will examine both chronic and acute changes in the abundance and cellular location of these proteins by semi-quantitative immunoblotting and immunohistochemistry.
For specific aim 1, we plan to assess the direct effect of increased circulating insulin levels on the relative abundances of the two primary apical sodium transport proteins of the postmacula densa portion of the renal tubule: 1) the amiloride-sensitive epithelial sodium channel (ENaC); and 2) the thiazide-sensitive Na-CI cotransporter (NCC).
In specific aim 2, we will evaluate the correlation of changes in ENaC subunit and NCC protein abundances with changes in rat blood pressure, and sensitivity to transporter or channel selective diuretics, i.e., amiloride and polythiazide.
In specific aim 3, we will address candidate cellular mechanisms for the increase in ENaC subunit or NCC protein abundances observed with insulin infusion.
In specific aim 4 we will evaluate relative changes in cellular distribution of both NCC and ENaC subunits in response to acute and chronic insulin exposure. Finally, in specific aim 5, we will investigate the impact of dietary PPAR-gamma agonists on the regulation of all of the major renal sodium transporter proteins in normal rats and insulin resistant, obese Zucker rats. These studies will, hopefully, provide us with an enlightened understanding of the role of insulin in sodium balance.
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