AFLUID May 45/5

Salomonsson, Max, and William J. Arendshorst. Calcium recruitment in renal vasculature: NE effects on blood flow and cytosolic calcium concentration. Am. J. Physiol. 276 (Renal Physiol. 45): F700–F710, 1999.—This study provides new information about the relative importance of Ca21 mobilization and entry in the renal vascular response to adrenoceptor activation. We measured renal blood flow (RBF) in Sprague-Dawley rats in vivo using electromagnetic flowmetry. We measured intracellular free Ca21 concentration ([Ca]i) in isolated afferent arterioles utilizing ratiometric photometry of fura-2 fluorescence. Renal arterial injection of NE produced a transient decrease in RBF. The response was attenuated, in a dose-dependent manner, up to ,50% by nifedipine, an antagonist of L-type Ca21 entry channels. Inhibition of Ca21 mobilization by 3,4,5-trimethoxybenzoic acid-8-(diethylamino)octyl ester (TMB-8) inhibited the renal vascular effects of NE in a dose-dependent manner, with maximal blockade of ,80%. No additional attenuation was observed when nifedipine and TMB-8 were administered together. In microdissected afferent arterioles, norepinephrine (NE; 1026 M) elicited an immediate square-shaped increase in [Ca]i, from 110 to 240 nM. This in vitro response was blocked by nifedipine (1026 M) and TMB-8 (1025 M) to a degree similar to that of the in vivo experiments. A nominally calcium-free solution blocked 80–90% of the [Ca]i response to NE. The increased [Ca]i elicited by depolarization with medium containing 50 mM KCl was totally blocked by nifedipine. In contrast, TMB-8 had no effect. Our results indicate that both Ca21 entry and mobilization play important roles in the renal vascular Ca21 and contractile response to adrenoceptor activation. The entry and mobilization mechanisms activated by NE may interact. That a calcium-free solution caused a larger inhibition of the NE effects on afferent arterioles than nifedipine suggests more than one Ca21 entry pathway.