PROPERTIES OF CHANNELS MEDIATING THE APAMIN-INSENSITIVE AFTERHYPERPOLARIZATION IN VAGAL MOTONEURONS

1. Whole cell recordings were obtained from neurons of the dorsal moto r nucleus of the vagus in transverse slices of guinea pig medulla. Fro m a holding potential of -40 mV, short depolarizing voltage steps acti vated two calcium-dependent potassium currents, G(k(Ca),1) and G(k(Ca) ,2). 2. G(k(Ca),1) was completely blocked by apamin (100 nM). G(k(Ca), 2) was apamin insensitive, voltage independent, and reversed close to the potassium equilibrium potential. 3. Activation of G(k(Ca),2) was a ssociated with an increase in current variance. The channels underlyin g the slow component were analyzed by stationary and nonstationary flu ctuation analysis. Current variance was linearly related to mean curre nt for small current amplitudes but clearly deviated from linearity ne ar the peak of G(k(Ca),2). The predicted single channel conductance wa s 6.8 +/- 2.5 (SE) pS. Probability of channel opening rose to at most 0.68. The average number of available G(k(Ca),2) channels on vagal neu rons was 4,437 +/- 591.4. Power spectra were constructed from the peak current. Spectra were well fitted with a single Lorentzian with a cor ner frequency of 72 +/- 7 Hz. The mean burst duration of the channels was 3.8 +/- 0.5 ms. These results indicate that a new type of calcium- activated channel underlies G(k(Ca),2).