Studies of single calcium channel currents in rat clonal pituitary cells

Single Ca channel currents in tissue cultured clonal cells (GH3) isolated from a rat anterior pituitary tumour were studied with a patch voltage-clamp electrode filled with 100 mM-BaCl2 solution. The open time histogram was reasonably well expressed by a single exponential function with a time constant, which was about 1 msec and almost voltage-independent in the range of membrane potential studied (-20 to +40 mV). The closed time histogram was expressed by a sum of two exponential functions, one with a relatively voltage-independent time constant of about 1 msec and the other with a substantially larger voltage-dependent time constant. The latency (the time from the onset of the voltage-clamp pulse to the first channel opening) histogram could be expressed as the difference of two exponential functions with time constants which appeared to be voltage-dependent. The tentative conclusion is that the channel enters the activated state with voltage-dependent and relatively slow kinetics, and then flickers between open and closed states with substantially faster and relatively voltage-independent kinetics. The amplitude of the single channel current was similar to that estimated from noise analysis of the whole cell current (about -0.7 pA at 0 mV in 100 mM-BaCl2 solution); the apparent slope conductance was about 7-8 pS.