Voltage-activated currents through calcium channels in normal bovine lactotrophs

The properties of whole cell Ba2+ currents were studied in immunocytochemically identified, normal bovine lactotrophs using the patch clamp technique. In the current clamp mode, current-induced and spontaneous Ba2+ action potentials were recorded. These were of longer duration and showed less inactivation with stimulation frequency when compared with Na+ action potentials. Under voltage clamp, isolated Ba2+ currents had an activation threshold of about -35 mV and peak value at -15 mV to +20 mV. Inactivation of the current to a potential-dependent, non-zero steady-state level indicated the presence of one rapidly and one slowly inactivating component to the current. These two components were also distinguished by: (1) the voltage dependence of the inactivation time constant of the current, (2) the differential frequency-dependent inactivation of the peak and steady-state currents, and (3) the presence of two half-inactivation potentials for the current. Analysis of the ensemble current variance of the non-inactivating component gave a single-channel amplitude of 0.19 pA at 0 mV and a slope conductance of 3 pS. Fluctuation analysis of the voltage-activated Ba2+ current noise revealed two time constants, one which was voltage dependent and the other was independent of potential. The contribution of these two currents to Ca2+-dependent hormone secretion remains to be clarified.