Disparate effects of Ca channel blockade on afferent and efferent arteriolar responses to ANG II.

Previous reports have suggested that organic calcium antagonists only partially inhibit the renal hemodynamic actions of angiotensin II (ANG II). This study tested the hypothesis that the calcium antagonist-sensitive component of ANG II-induced vasoconstriction is localized at a preglomerular site. Videomicroscopic measurements of vascular dimension were performed on in vitro blood-perfused juxtamedullary nephrons from captopril-treated rats. Under control conditions, afferent and efferent arteriolar diameters averaged 23.0 +/- 1.6 and 21.2 +/- 2.2 microns, respectively. Topical application of 0.1 nM ANG II decreased the diameters of afferent (-17 +/- 2%) and efferent (-15 +/- 3%) arterioles. Both 50 microM verapamil and 10 microM diltiazem dilated afferent arterioles. Verapamil also elicited a modest efferent vasodilation. In the presence of either verapamil or diltiazem, the effect of ANG II to decrease efferent diameter was sustained (-15 +/- 4%); however, the effect of ANG II on afferent diameter was abolished (-1 +/- 1%). These observations document differential influences of calcium channel blockers on ANG II-mediated vasoconstriction and suggest that the pre- and postglomerular vasoconstrictor actions of ANG II may occur through different calcium entry or mobilization mechanisms.