CONTRACTIONS MEDIATED BY ALPHA(1)-ADRENOCEPTORS AND P-2-PURINOCEPTORS IN A CAT COLON CIRCULAR MUSCLE

1 The postjunctional excitatory and inhibitory effects of adrenoceptor and purinoceptor agonists and antagonists were studied in circular smooth muscle strips of cat colon. 2 In the presence of tetrodotoxin (0.5 mu M), noradrenaline caused contraction or relaxation of circular smooth muscle at resting tension or with raised tone, respectively. The noradrenaline-evoked contractions were potentiated and the noradrenaline-evoked relaxations were antagonized by propranolol (1 mu M), suggesting beta-adrenoceptor involvement. 3 At resting tension, noradrenaline, adrenaline and the selective alpha(1)-adrenoceptor agonist, phenylephrine, caused concentration-dependent contractile responses, with EC(50) values of 1.8 +/- 0.2 mu M, l.9 +/- 0.4 mu M and 4.3 +/- 1.7 mu M, respectively. The EC(50) values and the amplitude of maximal responses were not significantly different from one another. Clonidine (0.1-500 mu M), a selective alpha(2)-adrenoceptor agonist, was not effective. 4 Prazosin (0.1-9 mu M), competitively antagonized the contractile effects of noradrenaline with an estimated pA(2) value of 6.93 and a slope of 1.07 +/- 0.03. The K-b values, estimated from a single shift (O.1 mu M prazosin) of the concentration-response curves to noradrenaline, adrenaline and phenylephrine were 92.8 +/- 9.3 nM, 108.7 +/- 6.4 nM and 18.4 +/- 3.1 nM, respectively. 5 At resting tension, adenosine 5' triphosphate (ATP, 5-1000 mu M), alpha,beta-methylene adenosine 5'triphosphate (alpha,beta-MeATP, 0.05-50 mu M), beta,gamma-methylene adenosine 5'-triphosphate (beta,gamma-MeATP, 0.5-100 mu M); and 2-methylthioadenosine 5'-triphosphate (2-MeSATP, 1-500 mu M) caused concentration-dependent contractions with EC(50) values of 60.5 +/- 15.9 mu M, 0.7 +/- 0.1 mu M, 7.6 +/- 0.1 mu M and 25.3 +/- 12.8 mu M, respectively. The maximal responses to alpha,beta-MeATP and beta,gamma-MeATP were greater than maximal responses to 2-MeSATP and ATP. 6 Suramin (50-500 mu M), competitively antagonized the contractile responses of alpha,beta-MeATP with an estimated pA(2) value of 4.92 and a slope of 1.08 +/- 0.04. The K-b values, estimated from a single shift (100 mu M suramin) of the concentration-response curves to ATP, alpha,beta-MeATP, beta,gamma-MeATP and 2MeS-ATP were 52.3 +/- 20.2 mu M, 25.2 +/- 4.5 mu M, 21.7 +/- 11.0 mu M and 11.6 +/- 2.7 mu M, respectively. 7 At resting tension, reactive blue 2 (100 mu M), a selective antagonist of the P-2Y-purinoceptor, and 8-(p-sulphophenyl)-theophylline (8-SPT) (1 mu M), a selective antagonist of the P-1-purinoceptor, did not antagonize the contractile responses to alpha,beta-MeATP (0.5-5 mu M). Contractile responses to ATP (50-500 mu M) were not altered by 8-SPT (1 mu M) but were potentiated by reactive blue 2 (100 mu M). 8 With raised tone, ATP and 2-MeSATP caused a relaxant effect. This effect of ATP was not altered by either tetrodotoxin (TTX) (0.5 mu M) or suramin (100 mu M), but was antagonized by reactive blue 2 (100 mu M) and 8-SPT (1 mu M), suggesting that inhibitory P-2Y- and P-1-purinoceptors are involved. In contrast, alpha,beta-MeATP and beta,gamma-MeATP caused only contractions. This contractile effect of alpha,beta-MeATP was resistant to TTX (0.5 mu M) and antagonized by suramin (100 mu M). 9 In summary, cat colon circular muscle contains postjunctional alpha(1)-adrenoceptors and P-2X- purinoceptors which mediate contractions and P-2Y- and P-1-purinoceptors which mediate relaxation. Postjunctional alpha(2)-adrenoceptors appear not to be present.