Passive and active electrical properties of isolated rat adrenal glomerulosa and fasciculata cells were studied by intracellular voltage-recording and constant current stimulation. The average resting membrane potential was —78.9 ± 4.2 mV for glomerulosa cells and —77.8 ± 5.0 mV for fasciculata cells. The response of the membrane potential to changes in external K+ concentration was stable and reversible for changes up to 28 mM and was independent of external Cl^-. The relationship between membrane potential and the log of external K+ concentration was linear between 4 and 28 mM, and the membrane potential could be predicted by a simplified form of the constant field equation with a [K]_i of 138.5 mM and a P_NA/P_K of 0.015 for glomerulosa cells and a [K]_i of 112.4 mM and a P_NA/P_K of 0.011 for fasciculata cells.

Under current clamp conditions, both cells demonstrated a nonlinear relationship between membrane voltage and applied current for depolarizing current steps. Depolarizing current pulses elicited a regenerative response and were followed by a rectifying steady state potential. The maximum rate of rise and the peak amplitude of the regenerative response were increased by prior hyperpolarization below the resting membrane potential and decreased by depolarization. The regenerative response was unaffected by the removal of Na⁺. Elevated Ca²⁺ concentrations increased the rate of rise, peak amplitide, and rate of fall, but decreased the duration of the regenerative response. The regenerative response was maintained upon replacement of Ca²⁺ with Sr⁺²+ or Ba²⁺, but was inhibited by Mn²⁺ or Co²⁺. Regenerative responses elicited in both glomerulosa and fasciculata cells exhibited similar characteristics. The results suggest the ionic mechanism underlying the regenerative response to be a voltagedependent Ca²⁺ conductance.

Both adrenal glomerulosa and fasciculata cells demonstrate electrical properties in common with other excitable cells. They are good K⁺ sensors with regard to their membrane potential, approaching the maximum sensitivity expected for a membrane exclusively permeable to K⁺. In addition, the Ca²⁺ regenerative response, which has been identified in both adrenal glomerulosa and fasciculata cells, may be involved in secretagogue stimulation of steroidogenesis. (Endocrinology120: 903–914, 1987)