Xiaoping Ruan

Regulation of angiotensin II receptor AT1 subtypes in renal afferent arterioles during chronic changes in sodium diet.

Studies determined the effects of chronic changes in sodium diet on the expression, regulation, and function of different angiotensin II (ANG II) receptor subtypes in renal resistance vessels. Rats were fed low- or high-sodium diets for 3 wk before study. Receptor function was assessed in vivo by measuring transient renal blood flow responses to bolus injections of ANG II (2 ng) into the renal artery. ANG II produced less pronounced renal vasoconstriction in rats fed a low- compared with high-sodium diet (16% vs. 56% decrease in renal blood flow, P < 0.001). After acute blockade of ANG II formation by iv enalaprilat injection in sodium-restricted animals, ANG II produced a 40% decrease in renal blood flow, a level between untreated dietary groups and less than high salt diet. Intrarenal administration of angiotensin II receptor type 1 (AT1) receptor antagonists losartan or EXP-3174 simultaneously with ANG II caused dose-dependent inhibition of ANG II responses. Based on maximum vasoconstriction normalized to 100% ANG II effect in each group, AT1 receptor antagonists produced the same degree of blockade in all groups, with an apparent maximum of 80-90%. In contrast, similar doses of the angiotensin II receptor type 2 (AT2) receptor ligand CGP-42112 had only a weak inhibitory effect. In vitro equilibrium-saturation 125I-ANG II binding studies on freshly isolated afferent arterioles indicated that ANG II receptor density was lower in the low- vs. high-sodium animals (157 vs. 298 fmol/mg, P < 0.04); affinity was similar (0.65 nM). Losartan and EXP-3174 displaced up to 80-90% of the ANG II binding; fractional displacement was similar in both diet groups. In contrast, the AT2 receptor analogues PD-123319 and CGP-42112 at concentrations < 10(-6) M had no effect on ANG II binding. RT-PCR assays revealed the expression of both angiotensin II receptor type 1A (AT(1A)) and angiotensin II receptor type 1B (AT(1B)) subtypes in freshly isolated afferent arterioles, while there was very little AT2 receptor expression. Total AT1 receptor mRNA expression was suppressed by low sodium intake to 66% of control levels, whereas it was increased to 132% of control by high-sodium diet, as indicated by ribonuclease protection assay. Receptor regulation was associated with parallel changes in AT(1A) and AT(1B) expression; the AT(1A)/AT(1B) ratio was stable at 3.7. We conclude that AT1 receptors are the predominant ANG II receptor type in renal resistance vessels of 7-wk-old rats. Chronic changes in sodium intake caused parallel regulation of expression and amount of receptor protein of the two AT1 receptor genes that modulate receptor function and altered reactivity of renal vessels to ANG II.

Nitric Oxide/cAMP Interactions in the Control of Rat Renal Vascular Resistance

This study aimed to characterize the interaction between nitric oxide (NO)- and cAMP-related pathways in the control of renal blood flow. Using the isolated perfused rat kidney model, we determined the effects of inhibition of NO formation by Nomega-nitro-L-arginine methyl ester (L-NAME; 1 mmol/L) and of NO administration by sodium nitroprusside (SNP, 10 micromol/L) on renal vascular resistance under conditions of elevated vascular cAMP levels. cAMP levels were increased either by adenylate cyclase activation via isoproterenol or by inhibition of cAMP phosphodiesterases (PDEs) 1, 3, and 4. We found that L-NAME markedly increased vascular resistance and that this effect was completely reversed by SNP. Both isoproterenol and inhibitors of the cAMP PDEs lowered basal vascular resistance. In the presence of isoproterenol (3 nmol/L) and inhibitors of PDE-1 [8-methoxymethyl-l-methyl-3-(2-methylpropyl)-xanthine; 8-MM-IBMX, 20 micromol/L] and PDE-4 (rolipram, 20 micromol/L), L-NAME again substantially increased vascular resistance, and this effect of L-NAME was completely reversed by SNP. In the presence of the PDE-3 inhibitors milrinone (20 micromol/L) and trequinsin (200 nmol/L), however, both L-NAME and SNP failed to exert any additional effects. Because PDE-3 is a cGMP-inhibited cAMP PDE and because the vasodilatory effect of SNP was abrogated by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) (20 micromol/L), our findings are compatible with the idea that an action of NO on PDE-3 could account for the vasodilatory properties of NO on the renal vasculature. Moreover, our findings suggest that PDE-3 activity is an important determinant of renal vascular resistance.

Impaired Prostaglandin E2/Prostaglandin I2 Receptor–Gs Protein Interactions in Isolated Renal Resistance Arterioles of Spontaneously Hypertensive Rats

Abstract —The protective effect of vasodilator agents linked to the cAMP pathway is less effective for buffering the vasoconstrictor effect of angiotensin II in young animals with genetic hypertension. To determine the underlying cellular mechanism, experiments were performed on freshly isolated preglomerular resistance arterioles obtained from kidneys of 7-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Specific high-affinity saturable binding of 3 H-prostaglandin (PG) E 2 revealed 1 receptor class in renal microvessels; PGE 2 receptor density was similar in SHR and WKY (106 versus 115 fmol/mg; P >0.8), as was receptor affinity (3.6 versus 3.5 nmol/L; P >0.7). Basal cAMP activity was similar in renal arterioles from SHR and WKY. A major finding was that PGE 2 , PGI 2 , and isoproterenol produced weaker stimulation of cAMP formation in arteriolar cells of SHR ( P s , G i , and G q ) in preglomerular arterioles. The relative amounts of discernible G-protein α-subunits in renal resistance vessels did not differ between SHR and WKY. These results extend previous in vivo studies of abnormal renal vascular reactivity in SHR and more directly localize defective coupling of the prostaglandin and β-adrenergic receptors to a stimulatory G protein and cAMP production in freshly isolated preglomerular arteriolar cells of young SHR. This dysfunction may be due to an abnormal interaction between prostaglandin receptors and G s protein that leads to inefficient coupling of initiating steps in the cAMP–protein kinase A cascade during the development of hypertension.