Modulation of rat erg1, erg2, erg3 and HERG K⁺ currents by thyrotropin‐releasing hormone in anterior pituitary cells via the native signal cascade

Abstract

Native ether-à-go-go-related gene (erg) K + currents were first described in sinoatrial node cells as delayed rectifier K + current. This current exhibited clear inward rectification in the 'fully activated' current-voltage relationship (DiFrancesco et al. 1979) and this property was explained by a channel model with two independent gates: a fast inactivation gate and a slow activation gate (Shibasaki, 1987 1. The mechanism of thyrotropin-releasing hormone (TRH)-induced ether-à-go-go-related gene (erg) K + current modulation was investigated with the perforated-patch whole-cell technique in clonal somatomammotroph GH 3 /B 6 cells. These cells express a small endogenous erg current known to be reduced by TRH. GH 3 /B 6 cells were injected with cDNA coding for rat erg1, erg2, erg3 and HERG K + channels. The corresponding erg currents were isolated with the help of the specific erg channel blockers E-4031 and dofetilide and their biophysical properties were determined.2. TRH (1 µM) was able to significantly reduce the different erg currents. The voltage dependence of activation was shifted by 15 mV (erg1), 10 mV (erg2) and 6 mV (erg3) to more positive potentials without strongly affecting erg inactivation. TRH reduced the maximal available erg current amplitude by 12 % (erg1), 13 % (erg2) and 39 % (erg3) and accelerated the time course of erg1 and erg2 channel deactivation, whereas erg3 deactivation kinetics were not significantly altered. The effects of TRH on HERG currents did not differ from those on its rat homologue erg1. In addition, coinjection of rat MiRP1 with HERG cDNA did not influence the TRHinduced modulation of HERG channels.3. Rat erg1 currents recorded in the cell-attached configuration were reduced by application of TRH to the extra-patch membrane in the majority of the experiments, confirming the involvement of a diffusible second messenger.4. Application of the phorbol ester phorbol 12-myristate 13-acetate (PMA; 1 µM) shifted the voltage dependence of erg1 activation in the depolarizing direction, but it did not reduce the maximal current amplitude. The voltage shift could not be explained by a selective effect on protein kinase C (PKC) since the PKC inhibitor bisindolylmaleimide I did not block the effects of TRH and PMA on erg1. In addition, cholecystokinin, known to activate the phosphoinositol pathway similarly to TRH, did not significantly affect the erg1 current.5. Various agents interfering with different known TRH-elicited cellular responses were not able to completely mimic or inhibit the TRH effects on erg1. Tested substances included modulators of the cAMP-protein kinase A pathway, arachidonic acid, inhibitors of tyrosine kinase and mitogen-activated protein kinase, sodium nitroprusside and cytochalasin D.6. The results demonstrate that all three members of the erg channel subfamily are modulated by TRH in GH 3 /B 6 cells. In agreement with previous studies on the TRH-induced modulation of the endogenous erg current in prolactin-secreting anterior pituitary cells, the TRH effects on ov...

Ca2+-Mobilizing Endothelin-A Receptors Inhibit Voltage-Gated Ca2+ Influx through Gi/oSignaling Pathway in Pituitary Lactotrophs

“…The sustained membrane hyperpolarization and decrease in input resistance observed in ET-1-stimulated cells indicate that ET A receptors activate or augment a hyperpolarizing current rather than inhibit a depolarizing Pituitary lactotrophs express two channel subtypes that are sensitive to Cs ϩ at the concentrations used in our experiments, K ir and erg-like channels, but only the latter is also sensitive to E-4031 (Einhorn et al, 1991;Schafer et al, 1999;Schledermann et al, 2001). Initiation of [Ca 2ϩ ] i transients in quiescent lactotrophs and the increase in the frequency of [Ca 2ϩ ] i transients in spontaneously active cells after the addition of 5 mM Cs ϩ and 1 M E-4031 are consistent with the role of erg-like channels in controlling the firing frequency in unstimulated cells.…”

“…Two channels expressed in pituitary lactotrophs could mediate the ET-1-induced and Cs ϩ -sensitive hyperpolarizing current: K ir channels (Einhorn et al, 1991;Kuryshev et al, 1997) and/or ether-a go-go-related gene (erg)-like K ϩ channels (Schafer et al, 1999;Schledermann et al, 2001). Both channels are activated by hyperpolarizing pulses, exhibit inward rectification, and can be blocked by Cs ϩ .…”

Effects of Ranolazine, a Novel Anti-anginal Drug, on Ion Currents and Membrane Potential in Pituitary Tumor GH3 Cells and NG108-15 Neuronal Cells

“…However, it should be noted that the studies based on the usefulness of HERG current are limited owing to the non-native conditions (e.g., α-subunits) of the ion channel in Xenopus oocytes or in the host HEK293 cells. Moreover, GH 3 cells used in this study were reported to contain many different isoforms of endogenous erg channels (10,13). The issues of whether other isoforms of erg channels are subject to more sensitive to block by ranolazine remain to be clarified.…”

Species Diversity and Peptide Toxins Blocking Selectivity of Ether-à-go-go-Related Gene Subfamily K+ Channels in the Central Nervous System

“…1B). For rERG3, in the region from Ϫ40 to ϩ40 mV, a shift to the right has been reported (Schledermann et al, 2001;Sturm et al, 2005) with respect to rERG1 and rERG2. Our data, obtained with the use of a different method (see inset and Materials and Methods) in a much larger voltage range from Ϫ160 to ϩ20 mV, did not evidence this effect.…”