EFFECTS OF MUSCARINE ON K+-CHANNEL CURRENTS IN THE C-CELLS OF BULLFROG SYMPATHETIC-GANGLION

The effects of muscarine on small, putative C-cells and large, putative B-cells dissociated from bullfrog paravertebral sympathetic ganglia were studied by whole cell and single channel recording techniques. The dominant action of muscarine was to activate an inwardly-rectifying K+ current (I-K(G)) in C-cells and to suppress M-current (I-M) in B-cells. However, both I-M and I-K(G) were affected by muscarine in 5 out of 78 putative C-cells and in 8 others only I-M was affected. By contrast, I-K(G) was only activated in 1 out of 105 B-cells. This predicts that the muscarinic slow IPSP, which can be evoked by preganglionic stimulation, occurs exclusively in C-cells. 6% of these cells could, however, generate a muscarinic slow EPSP in addition to a slow IPSP and 10% could generate a slow EPSP without a dow IPSP. The rectification associated with IK(G) was neither a direct consequence of the direction of movement of K+ ions nor a simple consequence of channel block by intracellular Mg2+ or Na+ ions. The fit of the activation curve by a Boltzmann equation suggests that the conductance underlying I-K(G) is controlled by a voltage-dependent gating charge (valency approximate to -2). Muscarine activated no new channels in outside-out or cell-attached patches but increased the opening probability of two types of K+ channels (unitary conductances approximate to 20 pS and approximate to 55 pS). The possible role of these channels in the generation of I-K(G) is discussed.