Functional characterization of a1-adrenoceptor subtypes in human skeletal muscle resistance arteries

a1-adrenoceptor subtypes in human skeletal muscle resistance arteries were characterized using agonists noradrenaline (non-selective) and A61603 (a1A-selective), the antagonists prazosin (non-selective), 5-methyl-urapidil (a1A-selective) and BMY7378 (a1D-selective) and the alkylating agent chloroethylclonidine (preferential for a1B). Small arteries were obtained from the non-ischaemic skeletal muscle of limbs amputated for critical limb ischaemia and isometric tension recorded using wire myography. Prazosin antagonized responses to noradrenaline with a pA2 value of 9.18, consistent with the presence of a1-adrenoceptors, although the Schild slope (1.32) was significantly different from unity. 5-Methyl-urapidil competitively antagonized responses to noradrenaline with a pKB value of 8.48 and a Schild slope of 0.99, consistent with the presence of a1A-adrenoceptors. In the presence of 300 nM 5-methyl-urapidil, noradrenaline exhibited biphasic concentration response curves, indicating the presence of a minor population of a 5-methyl-urapidil-resistant subtype. Contractile responses to noradrenaline were not affected by 1 µM chloroethylclonidine suggesting the absence of a1B-adrenoceptors. Maximum responses to noradrenaline and A61603 were reduced to a similar extent by 10 µM chloroethylclonidine, suggesting an effect of chloroethylclonidine at a1A-adrenoceptors at the higher concentration. BMY7378 (10 and 100 nM) had no effect on responses to noradrenaline. BMY7378 (1 µM) poorly shifted the potency of noradrenaline giving a pA2 of 6.52. These results rule out the presence of the a1D-subtype. These results show that contractile responses to noradrenaline in human skeletal muscle resistance arteries are predominantly mediated by the a1A-adrenoceptor subtype with a minor population of an unknown a1-adrenoceptor subtype.