Upregulation of V1 receptors in renal resistance vessels of rats developing genetic hypertension

Abstract

Previous studies have demonstrated that arginine vasopressin (AVP) produces exaggerated renal vasoconstriction in young spontaneously hypertensive rats (SHR) relative to normotensive rats. The exaggerated renal vascular reactivity does not appear to be due to a primary defect in postreceptor calcium signal transduction. Although the magnitudes of vascular responses differ, the relative proportions of calcium entry and mobilization pathways evoked by AVP in renal resistance vessels are similar in these rat strains. The purpose of the present study was to evaluate possible differences in V1 mRNA and receptor density and affinity in preglomerular resistance vessels (<50 μm) obtained from young Wistar-Kyoto (WKY) and SHR. Quantitative RT-PCR analysis revealed twofold greater expression of the V1areceptor gene in preglomerular arterioles of 7-wk-old SHR compared with WKY. In vitro radiolabeled ligand binding studies were performed under equilibrium conditions on preglomerular resistance arterioles freshly isolated from kidneys of 7-wk-old rats. The results indicate that AVP receptor density (Bmax) is two to three times greater in SHR than in WKY (248 ± 24 vs. 91 ± 11 fmol/mg protein, P < 0.001). The affinity does not differ between strains ( K d = 0.5 nM). Displacement studies yielded similar results for SHR and WKY. Unlabeled AVP completely displaced [3H]AVP binding, with an IC50 of 2.5 × 10− 10 M. Expression of AVP receptor types in afferent arterioles was evaluated using the V1 receptor agonist, [Phe2, Ile3,Org8]vasopressin, the V1 receptor antagonist, [d(CH2)5, Tyr(Me)2,Tyr(NH2)9]Arg8-vasopressin, and the V2 receptor agonist, desamino-[d-Arg8]vasopressin. Both the V1 agonist and antagonist displaced up to 90% of the AVP binding with IC50 values of 4 × 10− 8 and 8 × 10− 7 M, respectively. The V2receptor agonist was a weak inhibitor, displacing less than 15% of AVP binding at a high concentration of 10− 4M. These results demonstrate that virtually all AVP receptors in the preglomerular arterioles are of the V1 type. Collectively, our results provide evidence that the enhanced renal reactivity to AVP is mediated by a higher density of V1 receptors associated with increased gene expression in renal resistance vessels of SHR developing genetic hypertension.