Identification of delayed potassium and calcium currents in the rat sympathetic neurone under voltage clamp.

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Abstract
Post-ganglionic neurons of the isolated rat superior cervical ganglion were studied at 37.degree. C under 2 electrode voltage-clamp conditions. Membrane depolarization beyond -40 mV from holding levels between -50--100 mV produced a delayed outward current which exhibited no inactivation within this voltage range. The current is carried primarily by K+ ions and its instantaneous I-V relation is linear. The total outward current could be separated into 2 distinct components on the basis of ion-substituion experiments. A voltage-dependent component of the delayed current, termed IK(V), is activated by membrane depolarization beyond -40 mV when Ca2+ fluxes are selectively blocked by Cd2+ or in Ca2+-free solution. IK(V) develops following 1st-order kinetics and rises to a peak with a voltage-dependent delay (239 ms at -30 mV and 23 ms at +10 mV). GK(V) attains a saturating value of the order of 17 mS/cm2 at about +20 mV and can be described in terms of a simple Boltzmann distribution for a single gating particle with a valency equal to +2.cntdot.5. A 2nd component of the delayed outward current, termed IK(Ca), depends on Ca2+ entry for its activation and was isolated as difference current before and after block of Ca2+ movements across the membrane. IK(Ca) is larger and faster than IK(V): it is strictly related to Ca2+ influx and also depends on membrane potential depolarization. A distinct Ca2+ current, ICa, was recorded from the neuron exposed to Na+-free or tetrodotoxin solution. ICa was activated by membrane depolarization beyond -30 mV and reached a maximum value near 0 mV. Its activation agrees with 4th-order kinetics and becomes faster with increasing depolarization. The Ca2+ current developed with a voltage-dependent time to peak of 2.9-1.8 ms and thereafter completely inactivated. The relationship between ICa and IK(Ca) is discussed. The Ca2+-K+ repolarizing system is expected to be mainly associated with action potentials arising from a depolarized neuron, whereas the IA current dominates the repolarization mechanism at the normal membrane potential. The effect of muscarine was examined. Muscarine (10-50 .mu.M) produced a fall in conductance with a voltage dependence similar to that exhibited by GK(Ca) and was ineffective when removing extracellular Ca2+ or adding Cd2+. A partial suppression of ICa by muscarine is demonstrated. The decrease of the outward current magnitude in the presence of muscarine may be accounted for qualitatively by the reduction in ICa.