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

Summary 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 2.6mm 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⁺] (2.5mm) the midpoint of the steady-state Ca²⁺-channel activation curve lay at –6.9 mV. Increasing the [Ca²⁺] to 5.2mm shifted the midpoint by –4.3 mV along the voltage axis. At the midpoint, changes in potential of 7.8 mV (for 5.2mm Ca²⁺) and 9.2 mV (for 2.5mm Ca²⁺) induced ane-fold change in the activation of the current. Increasing [Ca²⁺]₀ from 2.5 to 5.2mm 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 currentvoltage relationships as 8.7 and 4.2 nS in the presence of 5.2 and 2.5mm Ca²⁺, respectively. Incubation of the cells in the presence of Bay K-8644 at increasing concentrations from 0.001 to 0.1 m increased the slope conductance from 4.2 to 9.6 nS. Further increases in the concentration of Bay K-8644 from 1 to 100 m induced a marked reduction in the conductance to 1.1 nS. In the presence of Bay K-8644 (0.1 m) 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 ane-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.