RECTAL RESPONSE OF CARDIAC ORIGIN IN THE CAT - INVOLVEMENT OF NITRIC-OXIDE AND ACETYLCHOLINE

Local application of nicotine over the surface of the left ventricle a nd also occlusion of the left anterior descending coronary artery in t he lightly anaesthetised, open-chested, artificially ventilated cat re sulted a biphasic rectal movement - initial relaxation followed by sus tained contraction. However, distension of the atrial appendage did no t evoke any change in rectal motility, indicating the non-involvement of atrial volume receptors in initiating this rectal response of cardi ac origin. The relaxation phase of this response was not abolished by pretreatment with atropine or with phentolamine or propranolol but was abolished by the nitric oxide inhibitor, N-G-nitro-L-arginine (LNNA), and this blockade of the relaxation phase by LNNA was reversed by L-a rginine. The contraction phase, however, was abolished by atropine. Fr om these observations it is clear that the relaxation phase of the rec tal response to coronary occlusion or epicardial nicotine is mediated through neither cholinergic nor adrenergic pathways but through the re lease of nitric oxide whereas the contraction phase of such a cardio-r ectal response is mediated through the release of the neurotransmitter , acetylcholine.