Cytosolic Ca2+ of excitable pituitary cells at resting potentials is controlled by steady state Ca2+ currents sensitive to dihydropyridines.

Abstract

Different elements of voltage-gated Ca2+ influx and their role for cytosolic-free Ca2+, [Ca2+]i, were studied in cells of the pituitary line GH3B6. Single cell monitoring of [Ca2+]i with the fluorescent probe indo-1, as well as fast on-line ratio [Ca2+]i imaging with fura-2, were combined with electrophysiological recordings using the perforated patch configuration of the patch clamp technique. [Ca2+]i signals are generated by Ca2+ influx both during action potentials and in between. Steady state Ca2+ influx at resting potentials (-60 to -40 mV) was voltage-gated and sensitive to the dihydropyridine antagonist PN 200-110. Ca2+ influx could be demonstrated in physiological conditions as inward Ca2+ currents of maximally 10 pA, which were triggered when stepping up the holding voltage beyond a narrow threshold around -50 mV. Inward currents were well correlated with [Ca2+]i elevations (R = -0.8; p < 0.001). It is concluded that dihydropyridine-sensitive, low threshold voltage-gated steady state Ca2+ currents, which may be tonically activated at resting potentials, provide for [Ca2+]i signaling in excitable endocrine cells in a mode that parallels the modulation of Ca2+ influx during action potentials.