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...

In rat lactotrophs from primary culture an inward-rectifying K¤ current is present which is characterized by sustained inward currents at membrane potentials more positive than −50 mV and by transient K¤ currents at more negative membrane potentials, when measured in high KCl external solution (Corrette et al. 1996). This current is similar to the inward-rectifying K¤ current (IK,IR) in clonal rat pituitary cells (GH×ÏBÜ cells;Bauer et al. 1990). However, in contrast to GH×ÏBÜ cells, the time course of current decay of the inwardrectifying K¤ current in most native lactotrophs exhibited a considerable slowly deactivating current component at negative membrane potentials. Characteristically, the contribution of this slow inward current component varied from cell to cell. A quantitative analysis of this new property of the inward-rectifying K¤ current was not possible because it could not be isolated from the other K¤ currents present in these cells (Corrette et al. 1996).The characteristic gating properties of IK,IR in GH×ÏB6 cells and its selective block by E_4031 and other class III antiarrhythmics (Weinsberg et al. 1997) indicated that it is mediated by K¤ channels expressed by the rat homologue of the human ether-à-go-go-related gene (erg; Sanguinetti et al. 1995;Trudeau et al. 1995). Further experiments provided evidence that the erg-mediated current is indeed a direct correlate of IK,IR in GH×ÏBÜ cells . The pharmacological isolation of the inward-rectifying K¤ current as the E_4031-sensitive current allowed measurement of this current in 5 mÒ external K¤ and demonstrated that in GH×ÏBÜ cells it provides an outward current with a maximum amplitude near −40 mV. This outward current was inhibited by thyrotrophin-releasing hormone (TRH) and this effect is involved in the TRH-induced depolarization characterizing the second phase of the TRH response in GH×ÏBÜ cells (Bauer, 1998 1. The ether-à-go-go-related gene (erg)-like K¤ current in rat lactotrophs from primary culture was characterized and compared with that in clonal rat pituitary cells (GH×ÏBÜ). The class III antiarrhythmic E_4031 known to block specifically erg K¤ channels was used to isolate the erg-like current as the E_4031-sensitive current. The experiments were performed in 150 mÒ K¤ external solution using the patch-clamp technique. 2. The erg-like K¤ current elicited with hyperpolarizing pulses negative to −100 mV consisted of a fast and a pronounced slowly deactivating current component. The contribution of the slow component to the total current amplitude was potential dependent and varied from cell to cell. At −100 mV it ranged from 50 to 85 % and at −140 mV from 21 to 45%. 3. The potential-dependent channel availability curves determined with 2 s prepulses were fitted with the sum of two Boltzmann functions. The function related to the slowly deactivating component of the erg-like current was shifted by more than 40 mV to more negative membrane potentials compared with that of the fast component. 4. In contrast to that of native lactotrophs st...