STIMULATION OF LUMBAR SYMPATHETIC-NERVES EVOKES CONTRACTIONS OF CAT COLON CIRCULAR MUSCLE MEDIATED BY ATP AND NORADRENALINE

1 The action of the lumbar sympathetic nerves to cat colon was studied in vitro using isolated muscle strips with attached lumbar colonic nerves (LCN) orientated in the axis of circular muscle layer. Electrical stimulation of LCN caused frequency-dependent increases in resting tension and in amplitude of spontaneous contractions. Contractile responses were abolished by tetrodotoxin (3 muM) and by guanethidine (30 muM), indicating that they were neurogenic, involving the release of neurotransmitter from sympathetic fibres. 2 Propranolol (1-9 muM), a beta-adrenoceptor antagonist, caused a concentration-dependent potentiation of LCN-evoked contractile response. Propranolol (3 muM) potentiated contractile responses to exogenously applied noradrenaline but not to phenylephrine. 3 Phentolamine (1-9 mum), an alpha-adrenoceptor antagonist, and prazosin (1-9 muM), an alpha1-adrenoceptor antagonist, caused a concentration-dependent reduction of amplitude but did not abolish LCN-evoked contractile responses. Prazosin (3 muM) or phentolamine (3 muM) antagonized contractile responses to noradrenaline and phenylephrine. 4 Desensitization of purinoceptors with the P2x-receptor agonist, alpha,beta-methylene ATP, caused a decrease in amplitude of LCN-evoked contractile responses and abolished contractile responses to ATP. In muscle strips where alpha1-adrenoceptors were blocked with prazosin (3 muM) and P2-purinoceptors were desensitized with alpha,beta-methylene ATP, the amplitude of contractile responses was reduced by 82-100%. 5 The P2X-purinoceptor antagonists, arylazido amino propyl adenosine triphosphate (ANAPP3) and suramin, affected LCN-evoked contractile responses. ANAPP3 (50-100 muM) caused a concentration-dependent reduction in the amplitude of contractile response. Suramin (100 muM) caused a small reduction in amplitude of contractile responses but potentiated their amplitude at a concentration of 500 muM. 6 ANAPP3 (100 muM) irreversibly inhibited contractions to alpha,beta-methylene ATP or ATP. Suramin (100-500 muM) inhibited contractions to alpha,beta-methylene ATP (0.5-1 muM) or low concentrations of ATP (10-50 muM) but potentiated contractions at higher concentrations. ANAPP3 (100 muM) and suramin (100, 500 muM) had no affect on contractile responses to noradrenaline. 7 Clonidine (0.05-1 muM), a selective alpha2-adrenoceptor agonist, caused a concentration-dependent reduction in amplitude of LCN-evoked contractile responses, at 10 Hz, while yohimbine (0.1-1 muM), a selective alpha2-adrenoceptor antagonist, increased them. At 1 muM, both compounds affected LCN-evoked contractions at all frequencies. This suggests that prejunctional alpha2-receptors are involved in autoinhibition at sympathetic terminals. 8 In summary, LCN-evoked contractile responses involve the corelease of noradrenaline and ATP or a related purine nucleotide from sympathetic fibres. It is likely that the neurogenic responses are mediated through excitatory postjunctional alpha1-adrenoceptors, excitatory suramin-sensitive and suramin-insensitive P2X-purinoceptors and inhibitory beta-adrenoceptors. Also, autoinhibitory prejunctional alpha2-adrenoceptors regulate the LCN excitatory pathway to cat colon circular muscle.