Effects of calcium and Bay K-8644 on calcium currents in adrenal medullary chromaffin cells.

The kinetic and steady-state characteristics of calcium currents in cultured bovine adrenal chromaffin cells were analyzed by the patch-clamp technique. Whole cell inward Ca2+ currents, recorded in the presence of either 5.2 or 2.6 mM Ca2+ exhibited a single, noninactivating component. To analyze the effects of Ca2+ and Bay K-8644 on the kinetics of the Ca2+ currents, we used a modified version of the Hodgkin-Huxley empirical model. At physiological [Ca2+] (2.5 mM) the midpoint of the steady-state Ca2(+)-channel activation curve lay at -6.9 mV. Increasing the [Ca2+] to 5.2 mM shifted the midpoint by -4.3 mV along the voltage axis. At the midpoint, changes in potential of 7.8 mV (for 5.2 mM Ca2+) and 9.2 mV (for 2.5 mM Ca2+) induced an e-fold change in the activation of the current. Increasing [Ca2+]o from 2.5 to 5.2 mM induced a marked increase in the rate constant for turning on the Ca2+ permeability. Conductances were estimated from the slope of the linear part of the current-voltage relationships as 8.7 and 4.2 nS in the presence of 5.2 and 2.5 mM Ca2+, respectively. Incubation of the cells in the presence of Bay K-8644 at increasing concentrations from 0.001 to 0.1 microM increased the slope conductance from 4.2 to 9.6 nS. Further increases in the concentration of Bay K-8644 from 1 to 100 microM induced a marked reduction in the conductance to 1.1 nS. In the presence of Bay K-8644 (0.1 microM) the midpoint of the activation curve was shifted by 6.1 mV towards more negative potentials, i.e., from -6.9 to -13 mV. At the midpoint potential of -13 mV, a change in potential of 6.9 mV caused an e-fold change in Ca2+ permeability. The kinetic analysis showed that Bay K-8644 significantly reduced the size of the rate constant for turning off the Ca2+ permeability.