MUSCARINE INDUCES 2 DISTINCT CURRENT RESPONSES IN ADRENAL CHROMAFFIN CELLS OF THE GUINEA-PIG

Muscarine-induced membrane responses were studied in dissociated chromaffin cells of the guinea-pig adrenal medulla, using the whole-cell version of the patch-clamp techinque. Bath application of muscarine (1-10 mu-M) produced two distinct current responses at a holding potential of -40 mV. One is an inward current associated with an increase in current noise. This current response was sustained during stimulation and had a reversal potential of 4.5 +/- 3.4 mV (n = 6) with a negative slope conductance below about -30 mV in 12.5 mM K+ -containing perfusate. The other is a transient outward current. This was evoked at membrane potentials more positive than -60 mV and completely suppressed by addition of 2 mM TEA to the bath solution, suggesting a possible involvement of the Ca2+ -dependent K+ channel. Generation of the outward current response was suppressed for at least 60-90 s following 25 s muscarinic stimulation and was facilitated by activation of the nicotinic receptor. The maximum inward current seemed to be produced by 3 mu-M, whereas the threshold concentration required for generation of the outward current was somewhere between 3 and 10-mu-M. The outward current was evoked less often in cells treated with 2% collagenase for 1 h than in those treated with 0.2% for 30 min. The results suggest that guinea-pig chromaffin cells have two muscarinic receptors: one is coupled with a cation nonselective channel and the other may be related to a Ca2+ -dependent K+ channel.