In situ analysis of α-adrenoceptors on arteriolar and venular smooth muscle in rat skeletal muscle microcirculation

Abstract

The purpose of this study was to determine whether both α1- and α2-adrenergic receptors exist on vascular smooth muscle of microvessels and whether adrenergic constriction of anatomically distinct microvascular segments is differentially subserved by either receptor subtype. The cremaster skeletal muscle of anesthetized rats was acutely denervated and suspended in a Krebs bath containing cocaine, normetanephrine, and propranolol to block uptake1, and uptake2, and β-receptors, respectively. Intravital microscopy was used to study large distributing arterioles (mean diameter, 100 μm), small precapillary arterioles (25 μm), and capacitance venules (140 μm). Concentration-response (diameter change) curves were obtained for bath-added agonists norepinephrine (mixed α1/α2), phenylephrine (α1), and B-HT 933 (α2) in the absence or presence of antagonists prazosin (α1) and yohimbine (α2). Apparent pD2 (-log ED50) values for large arterioles and venules were, respectively, as follows: norepinephrine (7.41 and 7.15), phenylephrine (5.95 and 5.41), and B-HT 933 (5.05 and 5.06). Low concentrations of prazosin (10-8 M) and yohimbine (10-7 M) produced receptor subtype-selective antagonism and parallel, dextral displacement of norepinephrine curves for large arterioles and venules. The large arteriole pK(B) (-log K(B)) was 7.83 ± 0.65 for prazosin and 7.36 ± 0.46 for yohimbine. Higher concentrations of prazosin (10-7 and 3 x 10-7 M) and yohimbine (10-6 M) produced further dextral but nonparallel displacement of norepinephrine curves. In contrast, receptor subtype-selective concentrations of only yohimbine inhibited adrenergic constriction of small, precapillary arterioles; but prazosin had no effect at receptor subtype-selective concentrations. These data suggest that adrenergic regulation of large arterioles and venules in skeletal muscle uses both α1- and α2-adrenoceptors. Precapillary arterioles, however, may be subserved predominantly by α2-receptors.