Steady-State Currents through Voltage-Dependent, Dihydropyridine-Sensitive Ca2+ Channels in GH3 Pituitary Cells

Ca²⁺ influx via voltage-dependent Ca²⁺ channels is known to be elicited during action potentials but possibly also occurs at the resting potential. The steady-state current through voltage-dependent Ca²⁺ channels and its role for the electrical activity was, therefore, investigated in pituitary GH₃ cells. Applying the recently developed `nystatin-modification' of the patch-clamp technique, most GH₃ cells (18 out of 23 cells) fired spontaneous action potentials from a baseline membrane potential of 43.7 ± 4.6 mV (mean ± s.d., n = 23). The frequency of action potentials was stimulated about twofold by Bay K 8644 (100 nM), a Ca²⁺-channel stimulator, and action potentials were completely suppressed by the Ca²⁺-channel blocker PN 200-110 (100 nM). Voltage clamping GH₃ cells at fixed potentials for several minutes and with 1 mM Ba²⁺ as divalent charge carrier, we observed steady-state Ca²⁺-channel currents that were dihydrophyridine-sensitive and displayed a U-shaped current-voltage relation. The results strongly suggest that the observed long lasting, dihydropyridine-sensitive Ca²⁺-channel currents provide a steady-state conductivity for Ca²⁺ at the resting potential and are essential for the generation of action potentials in GH₃ pituitary cells.