Different alpha-adrenoceptor subtypes mediate constriction of arterioles and venules

Recent studies indicate that rat vascular smooth muscle can express mRNAs for the alpha 1A-(alpha 1c-), alpha 1B-, alpha 1D-, alpha 2B-, and alpha 2D-adrenoceptors (ARs). The present study sought to determine which subtypes mediate constriction of resistance and capacitance vessels in rat cremaster skeletal muscle using videomicroscopy. Arterioles (125 microns internal diameter) were isolated, cannulated, and pressurized in a tissue bath. Vascularly "isolated" first-order venules (211 microns) were studied in situ in the cremaster muscle maintained in a tissue bath. Concentration-response curves for stimulation of alpha 1-ARs (norepinephrine plus 1 microM rauwolscine) and alpha 2-ARs (UK-14,304 plus 10 microM 5-methyl-urapidil) were obtained in the presence or absence of alpha-AR subtype-selective antagonists. Chloroethylclonidine (35 and 70 microM), which is most potent against alpha 1B-ARs, had no effect on alpha 1-constriction of arterioles but decreased venule alpha 1-sensitivity (50% effective concentration) by 13-fold (P < 0.01). The alpha 1A- and alpha 1D-selective AR antagonists WB-4101 (50 nM) and 5-MU (350 nM) had no effect on venule alpha 1-sensitivity but reduced arteriole alpha 1-AR sensitivity by 35-fold (P < 0.001) and 6-fold (P < 0.01), respectively. The alpha 1D-antagonist BMY-7378 (350 nM) decreased arteriole sensitivity by fourfold (P < 0.01) but had no effect on venule sensitivity. The alpha 2D-antagonist BRL-44408 (1 microM) caused a ninefold (P < 0.01) decrease in arteriole sensitivity to UK-14,304, whereas the alpha 2B-antagonist ARC-239 (1 microM) had no effect. BRL-44408 also caused a fourfold decrease (P < 0.01) in arteriole sensitivity to oxymetazoline, an agonist selective for alpha 2D- over alpha 2B-ARs. Venule sensitivity to UK-14,304 was reduced fivefold by BRL-44408 (P < 0.05), whereas ARC-239 had no effect. alpha 2 Subtype-selective antagonists inhibited UK-14,304 constriction of venules, with 50% inhibitory concentration values for BRL-44408 (alpha 2D-selective) of 6.04 +/- 0.07, and for the alpha 2B-selective antagonists ARC-239 of 4.79 +/- 0.20, spiroxatrine of 4.91 +/- 0.07, and SK&F-104856 of 5.06 +/- 0.10. These results suggest that constriction of rat skeletal muscle arterioles is mediated predominantly by an alpha 1D-like receptor and the alpha 2D-AR, whereas constriction of venules is dominated by the alpha 1B- and alpha 2D-adrenergic receptor subtypes.