Smooth muscle α 1D ‐adrenoreceptors mediate phenylephrine‐induced endothelial Ca 2+ transients in hamster cremaster arterioles

The α 1 ‐adrenoreceptor (α 1 ‐AR) agonist, phenylephrine (PE) induces Ca 2+ transients in endothelial cells (EC) of arterioles, possibly due to movement of Ca 2+ or IP 3 from smooth muscle (SMC) into EC via gap junctions. However, the presence of α 1 ‐AR on EC has not been excluded, and their identity in arterioles only has been inferred from pharmacology. Therefore, we determined which α 1 ‐AR subtypes are expressed by SMC and EC, and which subtype mediates PE‐induced EC Ca 2+ transients. Cell specific expression was assessed by real time RT‐PCR in samples of 50 cells from dissociated hamster cremaster arterioles using primers for α 1A ‐, α 1B ‐ and α 1D ‐AR with α‐SMC actin and eNOS as respective SMC and EC markers. SMC expressed α 1A ‐ and α 1D ‐AR, but not α 1B ‐AR, and no α 1 ‐AR transcripts were detected in EC expressing eNOS. Western blots of vessel homogenates confirmed α 1D ‐AR expression, but surprisingly, did not detect α 1A ‐AR in up to 100 μg protein, despite positive results in heart. In cannulated vessels, the K i for inhibition of PE‐induced constriction was 2.95 nM for the α 1D ‐AR antagonist, BMY 7378 and 4.11 nM for the α 1A ‐AR antagonist, 5‐methylurapadil, data consistent with PE acting through α 1D ‐AR. In vessels with Fura‐2‐loaded EC, PE‐induced constriction and EC Ca 2+ transients were abolished by the α 1 ‐AR antagonist, prazosin (30 nM). Thus, PE‐induced EC Ca 2+ transients are mediated by SMC α 1D ‐AR. Supported by HL32469 to WFJ.