Kinetic properties of a slow apamin-insensitive Ca(2+)-activated K+ current in guinea pig vagal neurons.

1. The calcium-activated outward current (gKCa,2) following an action potential was recorded from neurons in the guinea pig dorsal motor nucleus of the vagus (DMV). gKCa,2 was activated after a delay after the action current and had a distinct rising phase in contrast to the apamin sensitive calcium-activated current in rat DMV neurons (gKCa,1). 2. The time course of gKCa,2 was well described by function of the form A*[exp(t/tau 1)-exp(t/tau 2)], where tau 1 = 1.42 +/- 0.05 (SE) s and tau 2 = 555 +/- 24 ms. 3. Increasing calcium influx by firing multiple action currents lead to an increase in the peak amplitude of gKCa,2 with no change in its kinetics. In cells loaded with low concentrations of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), gKCa,2 was smaller in amplitude but its time course was similar to that in cells not loaded with EGTA. 4. When preceded by a conditioning influx of calcium, the amplitude and time course of gKCa,2 was identical to one with no conditioning influx. These results show that, after influx of calcium, a potassium current with stereotyped amplitude and kinetics is generated. 5. These data are consistent with the idea that the source of calcium for activation of gKCa,2 is not the extracellular space.