Inhibition of Guinea-Pig Taenia Coli Mediated by eti-, f}2-Adrenoceptors and ATP-Receptor Activation

The actions of orand /8-adrenoceptor agonists, antagonists and ATP were investigated on the smooth muscle of the guinea-pig taenia coli. Electrical and mechanical activities were recorded by means of the sucrose gap method. Noradrenaline, phenylephrine and methoxamine hyperpolarized the smooth muscle membrane, decreased the membrane resistance, reduced the size of phasic contractions, decreased the smooth muscle tone and supressed the spontaneous and evoked spike activities. Clonidine and ephedrine were inactive. The effects of agonists were abolished by cti -adrenoceptor antagonists (carbidine and phenoxybenzamine) and ai.2-adrenoceptor antagonist (phentolamine) but were unchanged by cr2-adrenoceptor antagonist (yohimbine) and j8i.2-adrenoceptor antagonist (propranolol). Isoprenaline and salbutamol suppressed the spontaneous spike generation, reduced the smooth muscle tone, caused a small hyperpolarization of the membrane without any change in the membrane resistance. Tazolol was ineffective. The effects of isoprenaline were abolished by /Si,2-adrenoceptor antagonists (propranolol, metipranolol and pindolol) and only partially inhibited by a /Si-adrenoceptor antagonist (practolol). The actions of ATP were similar to those of a i-adrenoceptor agonists but they were not inhibited by aand ^-adrenoceptor antagonists. It is suggested that the aiand /^-adrenoceptors mediate the inhibitory action of adrenoceptor agonists in the taenia coli. The actions of ATP, which are very similar to those of cti -adrenoceptor agonists, are not mediated by adrenoceptors, but the sequence of events induced by ai-adrenoceptor and ATP-receptor activation could be very close.