The effects of beta adrenoceptor blocking agents on the membrane potential and spike generation in the smooth muscle of guinea-pig taenia coli.

1. The measurement of changes in spike generation and membrane resistance in the guinea-pig taenia coli, using the sucrose gap extracellular recording method, has been shown to be a useful way to demonstrate the intrinsic sympathomimetic activity of the beta adrenoceptor blocking agents.2. Pronethalol, INPEA, MJ 1999, MJ 1998 and dichloroisoprenaline abolish the spontaneous spike discharge and produce a hyperpolarization of the cell membrane. These sympathomimetic effects were blocked by a mixture of alpha (phentolamine) and beta (propranolol) adrenoceptor blocking agents.3. Propranolol reduced the frequency of the spontaneous spike potentials but did not raise the resting membrane potential of the taenia coli.4. When the metabolism of the taenia coli was depressed, all the beta adrenoceptor blocking agents produced a depolarization of the cell membrane and sometimes initiated spike potential activity. The depolarization of the cell membrane was antagonized by superfusing the taenia coli with Krebs solution containing phentolamine. The effects of the beta adrenoceptor blocking agents were not antagonized by the presence of atropine.5. Experiments performed on the reserpinized taenia coli preparation show that depletion of the stores of catecholamines did not modify the sympathomimetic actions of the beta adrenoceptor blocking agents on the normal or metabolically depressed taenia coli preparation. The sympathomimetic action of the beta blocking agents does not appear to be mediated through the release of catecholamines within the taenia coli.