Effect of tetraethylammonium on an evoked spike potential in feline colonic muscle.

The purpose of these studies is to examine the contribution of calcium influx and potassium efflux to the amplitude and maximum rate of rise of an evoked spike potential in colonic circular smooth muscle. The double sucrose gap was used to record changes in the membrane potential after passing a constant depolarizing current. Tetraethylammonium (TEA) (5 mM) increased the amplitude of the evoked potential to 58 +/- 5 mV (P less than 0.001) from 32 +/- 4 mV in regular Krebs solution. The duration of the potential increased from 121 +/- 3 to 216 +/- 5 ms when a solution of 5 mM [TEA+]o bathed the tissue. The addition of [TEA+]o did not alter the dV/dtmax of the evoked spike. Other K+ channel blockers, cesium and 4-aminopyridine, did not alter the spike potential. Verapamil (10(-6) M) decreased the dV/dtmax of the evoked potential recorded from tissue bathed in 4.5 and 40 mM [K+]o. Verapamil also decreased the amplitude of the spike potential in tissue pretreated with 5 mM TEA. These studies suggest that the amplitude of an electrically evoked spike potential is dependent on Ca2+ influx in circular colonic muscle and an increase in K+ efflux, which occurs early after the current pulse, limits the amplitude of the evoked potential.