The proposed studies seek to determine the function of, and interactions between distinct atrial natriuretic peptide (ANP) binding sites in glomerular cells, and to uncover potential abnormalities in ANP binding site regulation or function in genetically hypertensive rats. Based on preliminary evidence, occupancy of ANP-binding proteins in glomeruli by selective, biologically inactive ANP analogs that do not interact directly with ANP receptors, augments binding of biologically active ANP to guanylate-cyclase coupled receptors. It is therefore proposed that membrane- associated ANP binding proteins, distinct from ANP receptors, function to regulate the interaction of ANP with ANP receptors and thus may play a role in modifying the functional responsiveness of cells to ANP. By studying the modifications in cellular ANP uptake and degradation brought about by occupancy of the ANP binding protein, and by labeling of ANP binding sites in the presence and absence of selective ANP analogs, using affinity cross-linking techniques, the studies in this proposal seek to determine whether the ANP binding protein regulates the interaction of ANP with its receptors by degrading circulating ANP through cellular uptake, or by inducing conformational changes in ANP receptors. Also, measurement of ANP transport across glomerular and aortic endothelial cell monolayers is proposed, in order to assess whether the ANP binding protein mediates transcellular ANP transport in endothelial cells, a mechanism that may serve to deliver the hormone to cells immediately adjacent to the endothelial cell layer. As a functional correlate to the binding data, studies are proposed to delineate ANP effects on guanylate cyclase activity, mesangial cell sodium and calcium metabolism and intracellular PH regulation. Furthermore, as ANP alters cell sodium transport and inhibits angiotensin II-induced contraction of smooth muscle cells, and smooth muscle cell contraction is accompanied by augmented cell sodium uptake and by changes in intracellular PH and calcium, studies in this proposal seek to determine ANP- angiotensin II interactions in modifying these functions. Finally, it is proposed that a deficiency in ANP activity, potentially due to abnormal tissue ANP-responsiveness may produce a defect in renal salt excretion, and therefore result in hypertension. By determining the regulation of, and interaction between ANP binding proteins and ANP receptors in genetically hypertensive rats, and by measuring the physiological ANP responsiveness of cells derived from these rats, the studies in this proposal seek to uncover possible defects in the cellular mechanism of ANP action that may lead to hypertension. It is anticipated that the proposed studies will enhance our understanding of the cellular mechanisms underlying essential hypertension.