1. The action of muscarine on membrane currents and cytosolic calcium (Ca2+) of dissociated rat adrenal chromaffin cells was investigated using standard whole-cell voltage-clamp techniques and microfluorimetry of unclamped single cells. 2. In cells held at a constant holding potential negative to - 40 mV, brief (5-10 s) applications of muscarine produced a transient activation of outward current. The activation of this current by muscarine also occurs in the presence of 5 mM-Co2+. 3. The outward current activated by muscarine at holding potentials negative to about - 40 mV is blocked over 90 % by either 200-mu-M-curare or 200 nM-apamin. One millimolar TEA produces variable blocking effects at such potentials. 4. The outward current activated by muscarine is transient even in the continuing presence of muscarine. Complete recovery between pairs of muscarine applications occurs over a 1-2 min period. If sufficient time was allowed for recovery between muscarine applications, the muscarine-activated outward current could be reliably elicited in dialysed cells for periods of 20-30 min. 5. Voltage ramps were used to examine effects of muscarine on currents over a range of membrane potentials. Over all potentials, muscarine activates a relatively voltage-independent component which is blocked almost completely by 200 nM-apamin and by 200-mu-M-curare. At potentials negative to about - 40 mV, the apamin- and curare-sensitive current accounts for virtually all muscarine-activated current. This current appears to correspond to a Ca2+-activated, voltage-independent current found in these cells. Effects of muscarine on currents activated at potentials positive to 0 mV are complex. At potentials above 0 mV, muscarine can produce either an activation or an inhibition of outward current. The outward current activated at positive potentials was primarily voltage dependent and blocked by 1 mM-TEA. However, in some cells activation of voltage-dependent current was not observed and, in such cases, muscarine produced an inactivation of the voltage-dependent component of current. The inactivation of outward current could also be observed in the presence of 5 mM-Co2+ indicating that the inactivation does not occur secondarily to an effect of muscarine on Ca2+ current. The possibility is discussed that the inactivation of outward current occurs as a result of intrinsic inactivation properties of the voltage-dependent Ca2+-dependent K+ current. According to this hypothesis. the extent to which inactivation of voltage-dependent outward current is observed depends on the magnitude of the muscarine-induced cytosolic Ca2+ elevation and the level of depolarization of the cell. 6. Muscarine application to single Fura-2-loaded chromaffin cells produced a transient increase in cytosolic Ca2+. The muscarine-induced increase in Ca(i)2+ also occurred following removal of Ca2+ from the extracellular saline or following addition of 2 mM-cobalt. 7. The results are consistent with the view that a muscarine-triggered release of Ca2+ from intracellular stores can activate two components of Ca2+-dependent K+ current. Furthermore, some mechanism occurs by which the voltage-dependent component of Ca2+-dependent current can undergo inactivation during the muscarine-induced elevation of Ca2+.
