DIFFERING ACTIONS OF NITROPRUSSIDE AND D-600 ON TENSION AND CA-45 FLUXES IN CANINE RENAL-ARTERIES

Abstract

Administration of nitroprusside (NP) subsequent to induction of contraction of canine renal arterial strips with norepinephrine (NE) yielded a dose-dependent relaxation and with 80 mM potassium (KCl) yielded only a small degree of relaxation. The effect elicited with 0.1 mM NP was significantly greater for responses obtained with NE (68%) than for those with KCl (12%). Responses of muscles pretreated for 5 min with NP to NE or KCl were reduced by 23 and 31%, respectively. The relaxant effect of NP persisted after incubation with D-600 [verapamil] and/or 30 min of a O-Ca plus 0.05 mM EDTA solution (low EDTA). Neither D-600 nor NP altered 45Ca uptake in the absence of stimulatory agents. Efflux of 45Ca (into low EDTA solution) was decreased by NP in a maintained manner but was unaffected by D-600. The NE-induced decrease in 45Ca efflux (but not that with KCl) was blocked by NP. Changes in 45Ca efflux elicited with KCl or NE were not affected by D-600. In contrast to D-600 (known to inhibit increased Ca2+ entry associated with some stimulatory conditions), NP acts in a manner relatively independent of uptake of extracellular Ca2+. The 2 vasodilators appear to affect Ca2+ utilization by different mechanisms and in an additive manner.