DIFFERENT ATYPICAL BETA-ADRENOCEPTORS MEDIATE ISOPRENALINE-INDUCED RELAXATION IN VASCULAR AND NONVASCULAR SMOOTH MUSCLES

Atypical beta-adrenoceptors mediating smooth muscle relaxation were co mpared in several rat tissues including the distal colon, fundic strip , thoracic aorta and common carotid artery. Isoprenaline, CGP 12177 an d BRL 37344 concentration-dependently relaxed longitudinal strips of t he distal colon and fundus precontracted with carbachol (10(-6)M) as w ell as ring segments of the aorta and carotid artery precontracted wit h noradrenaline (10(-7)M). The rank order of potency was isoprenaline = BRL 37344 > CGP 12177 in the distal colon, isoprenaline = CGP 12177 > BRL 37344 in the aorta and carotid artery segments and isoprenaline > BRL 37344 > CGP 12177 in the fundic strip. Pretreatment with BRL 373 44 induced a marked desensitization of the distal colon and fundic str ips but not the aorta and carotid artery to isoprenaline. In the fundu s and distal colon, pretreatment with CGP 12177 (10(-4)M) abolished th e effect of isoprenaline. Cyanopindolol (10(-6)M) shifted the isoprena line curve to the right, without reducing the maximum response, in the distal colon and fundic strip. - logK(B) values were 7.44 +/- 0.08 an d 7.53 +/- 0.10 in the distal colon and fundic strip respectively. The same concentration of cyanopindolol did not inhibit the relaxant effe ct of isoprenaline in the aorta and carotid artery segments. It was th erefore concluded that atypical beta-adrenoceptors in these preparatio ns were not identical, indicating heterogeneity of atypical beta-adren oceptors.