We performed the present studies to determine whether a proximal renal tubular dopamine D1-like receptor defect exists in human essential hypertension. Twenty-four subjects were studied (13 normotensive and 11 hypertensive) in a randomized, double-blind, vehicle-controlled study using fenoldopam, a selective D1-like receptor agonist. Subjects were studied in sodium metabolic balance at 300 mEq/d, after which the salt sensitivity of their blood pressure was determined. Fenoldopam at peak doses of 0.1 to 0.2 microgram/kg per minute decreased mean arterial pressure in hypertensive subjects but did not change mean pressure in normotensive subjects. Fenoldopam increased renal plasma flow to a greater extent in hypertensive than normotensive subjects. Fenoldopam increased both urinary and fractional sodium excretions in the hypertensive and normotensive groups. In normotensive but not hypertensive subjects, fenoldopam increased the fractional excretion of lithium and distal sodium delivery. In contrast, both distal fractional sodium reabsorption and sodium-potassium exchange fell significantly in hypertensive subjects. We conclude that human essential hypertension is associated with a reduction in the proximal tubular response to D1-like receptor stimulation compared with normotensive subjects. Hypertensive subjects appear to have a compensatory upregulation of renal vascular and distal tubular D1-like receptor function that offsets the proximal tubular defect, resulting in an enhanced natriuretic response to D1-like receptor stimulation.

The dopamine D3 receptor subtype was identified in rat kidney using both light microscopic immunohistochemistry and electron microscopic immunocytochemistry. Antipeptide polyclonal antisera were directed to both extracellular and intracellular regions of the native D3 receptor. Selectivity of the antipeptide antisera was validated by their ability to recognize native receptor protein expressed in permanently transfected mouse LTK- cells or Spodoptera fragiperda (Sf9) cell membranes. Light microscopic immunohistochemical staining for the D3 receptor was observed only in the cortex. Specific staining was present in proximal and distal tubules, cortical collecting ducts, glomeruli, and renal vasculature. Immunostaining was observed predominantly in the apical portion of both the proximal and distal tubules. Renal arterial staining was prominent in the medial and adventitial layers. Electron microscopic immunocytochemistry revealed immunogold particles in arteriolar smooth muscle cells of the renal vasculature. In proximal and distal tubules and cortical collecting duct, immunogold staining was localized to apical portions of tubule cells. D3 receptor immunogold staining in the glomeruli was clearly present in podocytes. Western blot analysis demonstrated a single D3 receptor band in infected Sf9 cell membranes, in transfected LTK- cells, and in kidney and brain but not in noninfected Sf9 cell membranes or in D2 or D3 receptor transfected or nontransfected LTK- cells. The use of receptor subtype-selective antibodies allows for the tissue localization of specific dopamine receptors that are not distinguished by current pharmacological or ligand-binding technology. The rat kidney expresses the D3 receptor at sites previously deemed to have D2-like receptors.

The dopamine D1A receptor subtype was identified in rat kidney with both light microscopic immunohistochemistry and electron microscopic immunocytochemistry. Antipeptide polyclonal antisera were directed to both extracellular and intracellular regions of the native receptor. The use of such receptor-subtype-selective antibodies allows for the identification of specific dopamine receptor subtype clones that are not distinguished by current pharmacological or receptor-ligand binding technology. Selectivity of the antipeptide antisera was validated by their ability to recognize native receptor protein expressed in permanently transfected mouse LTK- cells. In the rat kidney, D1A receptor protein was localized to the juxtaglomerular apparatus (JGA), proximal tubule, distal tubule, cortical collecting duct, and renal vasculature. In the JGA, the receptor was predominantly located in the arteriolar smooth muscle layer within cytoplasmic granules previously shown to contain renin. In the proximal tubules, staining was localized both on the brush-border and basolateral membranes. The D1A receptor, which is present in the central nervous system, is now identified in the rat kidney at those sites previously labeled as DA1 receptor sites on the basis of pharmacological binding studies. These results suggest that at least some of the renal dopamine DA1 receptors correspond structurally to the central dopamine D1A receptor.