Evidence that myoelectric complexes in the isolated mouse colon may not be of myogenic origin

The hypothesis that spontaneous depolarisations (myoelectric complexes, MCs) can occur in the absence of neuronal activity, depending on the level of the membrane potential, was systematically studied. In control Krebs' solution, MCs were recorded approximately every 5 min and were abolished by tetrodotoxin (TTX, 1.6 microM). However, TTX also induced sustained membrane depolarisation (19 mV) in the circular muscle. To test whether MCs were blocked by the depolarisation induced by TTX, graded membrane repolarisations were generated, in the continuing presence of TTX, using sodium nitroprusside (SNP, 10 nM-1 microM). Under these conditions, MC activity was not restored. The addition of SNP (1 microM) to control preparations, in normal Krebs' solution, hyperpolarised the membrane of the circular muscle cells, but did not inhibit ongoing MC activity. It is suggested that the underlying mechanisms involved in MC generation are unlikely to be dependent upon the level of membrane potential in circular smooth muscle.