Smooth muscle α_1D‐adrenoreceptors mediate phenylephrine‐induced endothelial Ca²⁺ transients in hamster cremaster arterioles

Abstract

The α1‐adrenoreceptor (α1‐AR) agonist, phenylephrine (PE) induces Ca2+ transients in endothelial cells (EC) of arterioles, possibly due to movement of Ca2+ or IP3 from smooth muscle (SMC) into EC via gap junctions. However, the presence of α1‐AR on EC has not been excluded, and their identity in arterioles only has been inferred from pharmacology. Therefore, we determined which α1‐AR subtypes are expressed by SMC and EC, and which subtype mediates PE‐induced EC Ca2+ transients. Cell specific expression was assessed by real time RT‐PCR in samples of 50 cells from dissociated hamster cremaster arterioles using primers for α1A‐, α1B‐ and α1D‐AR with α‐SMC actin and eNOS as respective SMC and EC markers. SMC expressed α1A‐ and α1D‐AR, but not α1B‐AR, and no α1‐AR transcripts were detected in EC expressing eNOS. Western blots of vessel homogenates confirmed α1D‐AR expression, but surprisingly, did not detect α1A‐AR in up to 100 μg protein, despite positive results in heart. In cannulated vessels, the Ki for inhibition of PE‐induced constriction was 2.95 nM for the α1D‐AR antagonist, BMY 7378 and 4.11 nM for the α1A‐AR antagonist, 5‐methylurapadil, data consistent with PE acting through α1D‐AR. In vessels with Fura‐2‐loaded EC, PE‐induced constriction and EC Ca2+ transients were abolished by the α1‐AR antagonist, prazosin (30 nM). Thus, PE‐induced EC Ca2+ transients are mediated by SMC α1D‐AR. Supported by HL32469 to WFJ.