α1-Adrenoceptor subtypes in rat renal resistance vessels: in vivo and in vitro studies

Abstract

This study provides new information about the relative importance of different α1-adrenoceptors during norepinephrine (NE) activation in rat renal resistance vessels. In Sprague-Dawley rats, we measured renal blood flow (RBF) using electromagnetic flowmetry in vivo and the intracellular free calcium concentration ([Ca2+]i) utilizing ratiometric photometry of fura 2 fluorescence in isolated afferent arterioles. Renal arterial bolus injection of NE produced a transient 46% decrease in RBF. In microdissected afferent arterioles, NE (1 μM) elicited an immediate square-shaped increase in [Ca2+]i, from 90 to 175 nM ( P < 0.001). Chloroethylclonidine (CEC) (50 μM) had no chronic irreversible alkylating effect in vitro but exerted acute reversible blockade on norepinephrine (NE) responses both on [Ca2+]i in vitro and on RBF in vivo. The RBF response was attenuated by ∼50% by the putative α1A-adrenoceptor and α1D-adrenoceptor antagonists 5-methylurapidil (5-MU), and 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione dihydrochloride (BMY-7378) (12.5 and 62.5 μg/h), respectively. The in vitro [Ca2+]i response to NE was blocked ∼25% and 50% by 5-MU (100 nM and 1 μM). BMY-7378 (100 nM and 1 μM) attenuated the NE-induced response by ∼40% and 100%. The degree of inhibition in vitro was similar to the in vivo experiments. In conclusion, 5-MU and BMY-7378 attenuated the NE-induced responses, although relatively high concentrations were required, suggesting involvement of both the α1A-adrenoceptor and α1D-adrenoceptor. Participation of the α1B-adrenoceptor is less likely, as we found no evidence for CEC-induced alkylation.