SINGLE-CHANNEL PROPERTIES OF HIGH-VOLTAGE-ACTIVATED AND LOW-VOLTAGE-ACTIVATED CALCIUM CHANNELS IN RAT PITUITARY MELANOTROPIC CELLS

1. Single-channel properties of voltage-dependent calcium channels wer e investigated in rat melanotropes in short-term primary culture. Unit ary currents were resolved using the cell-attached configuration. 2. D epolarizations higher than -50 mV activated a population of 8.1-pS cal cium channels [low-voltage activated (LVA)]. The LVA channel ensembles displayed a monoexponential time course of inactivation and a sigmoid al time course of activation fitted best by an m(2)h Hodgkin-Huxley-ty pe model. Microscopic kinetic analysis suggested that at least one ope n state, two closed states, and one inactivated state are involved in channel gating. 3. At potentials positive to -20 mV a second class of calcium channels was activated with a conductance of 24.7 pS [high-vol tage activated (HVA)]. HVA channels display different gating modes. Ga ting with high open probability( mode 2) and low open probability (mod e 1) as well as blank traces (mode 0) are observed. The HVA channels w ere heterogeneous with respect to their inactivation properties. Ensem bles that decayed entirely during a 300-ms test pulse as well as nonde caying ensembles were observed. Both HVA channel subtypes displayed si gmoidal activation, which was fitted by an m(2) model. Microscopic kin etic analysis suggested that at least one open state and two closed st ates are involved in mode two gating of both HVA channel subtypes. 4. Depolarizing prepulses did not recruit or facilitate calcium channel a ctivity in response to a test pulse, but inactivating HVA channel acti vity was strongly reduced. Depolarizing prepulses (+50 mV) did not aff ect the probability of opening of the noninactivating HVA channel. 5. The voltage dependence and kinetics of the LVA as well as both HVA cha nnels are in good agreement with previously published data on the prop erties of the various calcium current components derived from whole-ce ll recordings of rat melanotropes. The data suggest that a T-type as w ell as two L-type channels (an inactivating and noninactivating channe l) underlie the calcium current in these cells.