The Journal of Neurophysiology Vol. 85 No. 1 January 2001, pp. 54-60
Copyright ©2001 by the American Physiological Society

Number of K_Ca Channels Underlying Spontaneous Miniature Outward Currents (SMOCs) in Mudpuppy Cardiac Neurons

Department of Anatomy and Neurobiology, University of Vermont College of Medicine, Burlington, Vermont 05405

Scornik, Fabiana S., Laura A. Merriam, and Rodney L. Parsons. Number of K_Ca Channels Underlying Spontaneous Miniature Outward Currents (SMOCs) in Mudpuppy Cardiac Neurons. J. Neurophysiol. 85: 54-60, 2001. Spontaneous miniature outward currents (SMOCs) in parasympathetic neurons from mudpuppy cardiac ganglia are caused by activation of TEA- and iberiotoxin-sensitive, Ca²⁺-dependent K⁺ (BK) channels. Previously we reported that SMOCs are activated by Ca²⁺-induced Ca²⁺ release (CICR) from caffeine- and ryanodine-sensitive intracellular Ca²⁺ stores. In the present study, we analyzed the single channel currents that contribute to SMOC generation in mudpuppy cardiac neurons. The slope conductance of BK channels, determined from the I-V relationship of single-channel currents recorded with cell-attached patches in physiological K⁺ concentrations, was 84 pS. The evidence supporting the identity of this channel as the channel involved in SMOC generation was its sensitivity to internal Ca²⁺, external TEA, and caffeine. In cell-attached patch recordings, 166 µM TEA applied in the pipette reduced single-channel current amplitude by 32%, and bath-applied caffeine increased BK channel activity. The ratio between the averaged SMOC amplitude and the single-channel current amplitude was used to estimate the average number of channels involved in SMOC generation. The estimated number of channels involved in generation of an averaged SMOC ranged from 18 to 23 channels. We also determined that the Po of the BK channels at the peak of a SMOC remains constant at voltages more positive than 20 mV, suggesting that the transient rise in intracellular Ca²⁺ from ryanodine-sensitive intracellular stores in the vicinity of the BK channel reached concentrations most likely exceeding 40 µM.