We have discovered a relationship between activating mutations of a G protein related kinase (GRK) and regulation of renal D1 receptor function that is important in the pathogenesis of human essential hypertension. A nephron segment-specific defective coupling between the dopamine DIA receptor and the G protein/effector enzyme complex may be a cause of the renal sodium retention in spontaneously hypertensive rats (SHR). The decreased ability of exogenous and renal endogenous dopamine to inhibit sodium transport in renal proximal tubules co-segregates with hypertension in F2 crosses of SHR and its normotensive control, the Wistar-Kyoto (WKY) rat. Similar defects were found in the Dahl salt-sensitive rat and more importantly, in humans with essential hypertension. Thus, cultures of renal proximal tubule cells from hypertensive humans have a defective coupling of a renal D1-like receptor to adenylyl cyclase (AC), similar to the coupling defect found in hypertensive rodents. These in vitro data are in agreement with in vivo studies demonstrating a defective D1-like agonist inhibition of renal proximal tubular reabsorption in subjects with salt sensitive hypertension. The uncoupling of a renal D1-like receptor from the G protein/effector enzyme complex is not due to homologous or heterologous desensitization, receptor down-regulation, G protein or effector enzyme """"""""defects"""""""" or a mutation in the primary sequence of the D1-like receptors. Rather, the uncoupling of the D1-like receptor is due to a ligand-independent hyper-phosphorylation of the D1 receptor (the major D1-like receptor in the kidney) due to homozygous mutations of a GRK isoform with limited organ and nephron expression, GRK4. The major objective of this proposal is to determine the mechanism(s) by which mutations of GRK4 impair the function of the D1 receptor. The secondary objective is to determine the frequency of GRK4 mutations/polymorphisms in human essential hypertension.
The specific aims are: 1. To determine the mechanism by which mutations of GRK4 impair D1 receptor signal transduction and regulation of sodium transport. 2. To determine the specificity of the uncoupling effect of GRK4 on the D1 receptor in essential hypertension, i.e., does GRK4 selectively phosphorylate and/or desensitize the D1 receptor? 3. To determine the frequency of GRK4 mutations/polymorphisms in human essential hypertension (especially those resistant to the proximal tubular action of dopaminergic agonists). These studies will determine if activating mutations of GRK4 are important in the uncoupling of the D1 receptor from the G protein/AC complex in renal proximal tubule cells in hypertension.
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