Angiotensin II-induced changes in G-protein expression and resistance of renal microvessels in young genetically hypertensive rats

Altered regulation of cAMP may contribute to enhanced renal reactivity to a ngiotensin II (Ang II) in spontaneously hypertensive rats (SHR). Such a phe nomenon may occur in renal preglomerular arterioles and may involve changes in expression of GTP-binding regulatory proteins. We have examined the eff ects of Ang II on steady state levels of G(alphai-1,2), G(alphai-3), G(alph as) and G(alphaq) in preglomerular arterioles from young marginally hyperte nsive SHR and on mean arterial pressure (MAP), renal vascular resistance (R VR) and renal cAMP excretion (UcAMP.V). Young (5-6 week old) SHR and Wistar Kyoto (WKY) rats received Ang II (35 ng/kg/min, s.c.) or vehicle for 7 day s via osmotic minipumps. Urine was collected over the last 24 h. On day sev en, MAP and renal blood flow were measured in anesthetized rats and RVR was determined. Preglomerular arterioles were isolated by perfusing the kidney s with iron oxide and using a series of mechanical steps coupled with the u se of a magnet to retain iron-laden vessels. Membranes were prepared and th e expressions of G(alphai-1,2), G(alphai-3), G(alphas) and G(alphaq) were e valuated by Western immunoblotting. Baseline MAP (124 +/- 6 mmHg) was only marginally (p > 0.05) higher in SHR when compared with WKY rats (110 +/- 4 mmHg). RBF (3.04 +/- 0.16 mL/min) was significantly lower and RVR (41.10 +/ - 1.37 mmHg.min/mL) was significantly higher in SHR when compared to age-ma tched WKY rats (4.36 +/- 0.30 mL/min and 25.79 +/- 1.58 mmHg.min/mL, respec tively). Ang II significantly increased MAP in SHR (17 mmHg) but not in WKY rats. These increases in MAP were accompanied by significant increases in RVR in SHR (48% over control) but not in WKY rats. Compared to WKY rats, pr eglomerular arterioles from SHR exhibited significantly higher basal expres sion of G(alphai-1,2) (11- fold), G(alphai-3) (13- fold) and G(alphas) (3-f old). Chronic infusion of Ang II, however, downregulated the expression of G(alphas) (by 53%; p < 0.05), G(alphai-1,2) ( by 72%; p < 0.05) and G(alpha i-) (3) (by 35%; p > 0.05) in SHR preglomerular arterioles but significantl y upregulated the expression of these proteins in WKY by 3-, 8- and 15-fold , respectively. Basal levels of G(alphaq) were not different in preglomerul ar arterioles from the two strains but were downregulated by Ang II in both WKY (74% of basal) and SHR (52% of control). Baseline UcAMP.V was signific antly lower in SHR (31.22 +/- 6.51 nmol/24 h) compared with WKY rats (65.33 +/- 3.60 nmol/24 h). Chronic Ang II infusion significantly increased UcAMP .V in SHR as well as WKY rats. These data clearly demonstrate that expressi ons of G(i) isoforms as well as G(s) in renal microvessels are elevated dur ing early stages of hypertension and suggest that the elevated levels of G( i) proteins may be directly associated with a blunted adenylyl cyclase-cAMP cascade in the renal microvasculature. Furthermore, Ang II appears to dire ctly downregulate the expression of G(s) in young SHR but not in young WKY renal microvessels. Such diversity in its effect on G-protein expression ma y be important for enhanced renal sensitivity to Ang II in SHR.