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

Abstract

The response of the L-type Ca2+ current (ICa,L) in pituitary GH3 cells to variations in the action potential (AP) waveform was examined using the whole-cell configuration of the patch-clamp technique. ICa,L evoked during an AP waveform exhibited an early and a late component. The early component 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 Ca2+ charge carried by ICa,L, although the amplitude of the late ICa,L was reduced. Prolonging the peak voltage of the AP waveform, however, increased the amplitude of the late component. ICa,L inactivated during a train of AP waveforms. When Ba2+ was used as the charge carrier, current inactivation during a train of APs decreased. Likewise, ICa,L evoked by the AP templates with irregular bursting pattern was inactivated. When the repetitive firing of APs with depolarizing potentials was replayed to cells, Ca2+ entry was not only spread over the entire AP, but also occurred during the interspike voltage trajectory. After application of thyrotropin releasing hormone (TRH; 10 microM), ICa,L in response to rectangular pulses was increased and the current/voltage relation shifted slightly to more negative values. TRH (10 microM), thapsigargin (10 microM) or cyclopiazonic acid (30 microM) enhanced the late component of the AP-evoked ICa,L. TRH also attenuated the inactivation of ICa,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 waveforms is distinct from that evoked by rectangular voltage clamp. Changes in the shape and firing pattern of APs in GH3 cells can modulate Ca2+ influx through L-type Ca2+ channels. Ca2+ release from internal stores may affect the magnitude of AP-evoked ICa,L in these cells.

Incidence of noncutaneous melanomas in the U.S.

“…Because the size and time course of I K(Ca) in response to change in waveforms of spontaneous action potentials was different from those during a rectangular voltage‐clamp pulse (Lo et al, 2001; Wu et al, 2003b), the effects of rottlerin on outward currents during a train of simulated action potentials generated from modeled GH 3 cells (Wu and Chang, 2006) was further investigated. In these experiments, simulated waveforms of spontaneous action potentials were digitally generated as voltage templates and re‐played to evoke ion currents in GH 3 cells (Lo et al, 2001). Because the currents in response to simulated action potential waveforms were sensitive to block by 1 µM paxilline, the results suggest that I K(Ca) may actually be K + flux through the BK Ca channel (Wu et al, 2003b).…”

“…This model consists of a Ca 2+ current, a delayed rectifier K + current, a Ca 2+ ‐activated K + current, an erg ( ether‐à‐go‐go ‐related)‐mediated K + , and an ATP‐sensitive K + current (Wu and Chang, 2006). Simulated action potentials used as voltage templates were re‐played to GH 3 cells through a digital‐analogue converter (Lo et al, 2001; Wu and Chang, 2006). Source file related to this modeled cell is available on‐line at http://senselab.med.yale.edu/senselab/modeldb.…”

Neurotensin modulates the amplitude and frequency of voltage‐activated Ca2+ currents in frog pituitary melanotrophs: implication of the inositol triphosphate/protein kinase C pathway

“…Recent studies performed on neurons (Stewart & Foehring, 2001) and endocrine cells (Mansvelder & Kits, 2000; Lo et al ., 2001) have clearly established that changes in the amplitude, duration or frequency of action potentials affect calcium conductances. Because f NT provokes, in frog melanotrophs, an increase in frequency and a decrease in the amplitude of action potentials (Belmeguenai et al ., 2000), we investigated whether the f NT‐evoked electrical activity induced variations in calcium influx through voltage‐activated calcium channels.…”

The Mechanism of Inhibitory Actions of S-Petasin, a Sesquiterpene ofPetasites formosanus, on L-Type Calcium Current in NG108-15 Neuronal Cells

“…Cells were bathed at room temperature (20 ± 25 8C) in normal Tyrode's solution that contained 1.8 mM CaCl 2 . Membrane potentials and currents were recorded in respective current-and voltage-clamp modes with patch electrodes in a whole-cell configuration of the patch-clamp technique [11], [16]. The pipettes were connected to the input stage of an RK-400 amplifier (Bio-Logic, Claix, France) or an Axopatch 200B amplifier (Axon Instruments, Union City, CA).…”

“…In some experiments, action potentials recorded from cells that were bathed in normal Tyrode's solution were re-played to elicit I Ca,L . Action potential waveform-evoked capacitative and leakage currents were determined by applying a negative going action potential waveform and subtracted from total membrane current to yield the voltage-dependent Ca 2+ current [16].…”