CYTOSOLIC CA2-CELLS AT RESTING POTENTIALS IS CONTROLLED BY STEADY-STATE CA2+ CURRENTS SENSITIVE TO DIHYDROPYRIDINES( OF EXCITABLE PITUITARY)

Different elements of voltage-gated Ca2+ influx and their role for cyt osolic-free Ca2+, [Ca2+](i), were studied in cells of the pituitary li ne GH3B6. Single cell monitoring of [Ca2+](i) with the fluorescent pro be indo-1, as well as fast on-line ratio [Ca2+](i) imaging with fura-2 , were combined with electrophysiological recordings using the perfora ted patch configuration of the patch clamp technique. [Ca2+](i) signal s are generated by Ca2+ influx both during action potentials and in be tween. Steady state Ca2+ influx at resting potentials (-60 to -40 mV) was voltage-gated and sensitive to the dihydropyridine antagonist PN 2 00-110. 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 m V. 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 toni cally 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.