Characterization of action potential waveform-evoked L-type calcium currents in pituitary GH(3) cells

The response of the L-type Ca2+ current (I-CaL) in pituitary GH(3) cells to variations in the action potential (AP) waveform was examined using the wh ole-cell configuration of the patch-clamp technique. I-Ca,I-L evoked during an AP waveform exhibited an early and a late component. The early componen t occurred on the rising phase of the AP; the late component coincided with the falling phase. Prolonging the falling phase of the AP increased the Ca 2+ Charge carried by I-Ca,I-L, although the amplitude of the late ICa,L was reduced. Prolonging the peak voltage of the AP waveform, however, increase d the amplitude of the late component. I-Ca,I-L. inactivated during a train of AP waveforms. When Ba2+ was used as the charge carrier, current inactiv ation during a train of APs decreased. Likewise, I-Ca,I-L evoked by the AP templates with irregular bursting pattern was inactivated. When the repetit ive firing of APs with depolarizing potentials was replayed to cells, Ca2entry was not only spread over the entire AP, but also occurred during the interspike voltage trajectory. After application of thyrotropin releasing h ort-none (TRH; 10 VM), IC,,,L in response to rectangular pulses was increas ed and the current/voltage relation shifted slightly to more negative value s. TRH (10 muM), thapsigargin (10 muM) or cyclopiazonic acid (30 muM) enhan ced the late component of the AP-evoked I-Ca,I-L. TRH also attenuated the i nactivation of IC,,,L during a train of APs. These results indicate that in pituitary GH3 cells, the time course and kinetics Of Ica,L during the AP w aveforms is distinct from that evoked by rectangular voltage clamp. Changes in the shape and firing pattern of APs in GH3 cells can modulate Ca2+ infl ux through L-type Ca2+ channels. Ca2+ release from internal stores may affe ct the magnitude of AP-evoked IC,,,L in these cells.