Vol. 299, Issue 2, 729-734, November 2001

Functional Characterization of ₁-Adrenoceptor Subtypes in Human Subcutaneous Resistance Arteries

Yagna P. R. Jarajapu, Fiona Johnston, Colin Berry, Andrew Renwick, John C. McGrath, Allan MacDonald and Chris Hillier

Vascular Assessment Unit, School of Biological and Biomedical Sciences, Glasgow Caledonian University, Glasgow, Scotland, United Kingdom (Y.P.R.J., F.J., A.M., C.H.); Western Infirmary, Glasgow, Scotland, United Kingdom (C.B., A.R.); and Autonomic Physiology Unit, Institute of Biomedical and Life Sciences, Glasgow University, Glasgow, Scotland, United Kingdom (J.C.M.)

The functional characteristics of the ₁-adrenoceptor subtypes in human resistance arteries are still not clear. We recently reported that the _1A-adrenoceptor predominantly mediates contraction to norepinephrine in human skeletal muscle resistance arteries. In this study we extended these investigations to human subcutaneous resistance arteries. Arterial segments were isolated from the inguinal subcutaneous fat and mounted on a small vessel wire myograph. Potencies of agonists and antagonists were examined. N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl]methanesulphonamide (A-61603) was found to be 10- and 54-fold more potent than norepinephrine and phenylephrine, respectively. Brimonidine (UK 14304) evoked significantly smaller contractile responses than norepinephrine and phenylephrine, showing the presence of a small population of ₂-adrenoceptors in these arteries, and this was confirmed by the studies with selective ₁- and ₂-adrenoceptor antagonists prazosin and (8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13a-decahydro-3-methoxyl-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]-naphthyridine (RS 79948). Prazosin, 5-methyl-urapidil, and 2-[2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB 4101) shifted the potency of norepinephrine concentration dependently giving pA₂ values of 9.4, 8.9, and 10.1, respectively, showing the presence of the _1A-subtype in these arteries. Pretreatment with 1 and 10 µM chloroethylclonidine did not affect the potency of and maximum responses to norepinephrine, ruling out the presence of the _1B-subtype in these arteries. 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378, 10 and 100 nM) did not affect the potency of norepinephrine but a small shift was observed by 1 µM BMY 7378, giving a pK_B value of 7.1, much less than that reported for the _1D-subtype. These results suggest the predominant involvement of _1A-adrenoceptor in the contractile responses to norepinephrine in these arteries. The physiological role of this subtype in the maintenance of peripheral arterial resistance is yet to be confirmed.