Stimulation of single L-type calcium channels in rat pituitary GH3 cells by thyrotropin-releasing hormone (original) (raw)
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Pflugers Archiv-european Journal of Physiology, 1995
Treatment of GH3 cells with either hypothalamic peptide thyrotropin-releasing hormone (TRH), the endomembrane Ca2+-ATPase inhibitor thapsigargin or the Ca2+ ionophore ionomycin mobilized, with different kinetics, essentially all of the Ca2+ pool from the intracellular Ca2+ stores. Any of the above-described treatments induced a sustained increase in intracellular Ca2+ concentration ([Ca2+]i), which was dependent on extracellular Ca2+ and was prevented by Ni2+ but not by dihydropyridines (DHPs), suggesting that it was due to capacitative Ca2+ entry via activation of a plasma membrane pathway which opened upon the emptying of the intracellular Ca2+ stores. The increase of the plasma membrane permeability to Ca2+ correlated negatively with the filling degree of the intracellular Ca2+ stores and was reversed by refilling of the stores. The mechanism of capacitative Ca2+ entry into GH3 cells differed from similar mechanisms described in several types of blood cells in that the pathway was poorly permeable to Mn2+ and not sensitive to cytochrome P450 inhibitors. In GH3 cells, TRH induced a transient [Ca2+]i increase due to Ca2+ release from the stores (phase 1) followed by a sustained [Ca2+]i increase due to Ca2+ entry (phase 2). At the single-cell level, phase 2 was composed of a DHP-insensitive sustained [Ca2+]i increase, due to activation of capacitative Ca2+ entry, superimposed upon which DHP-sensitive [Ca2+]i oscillations took place. The two components of the TRH-induced Ca2+ entry differed also in that [Ca2+]i oscillations remained for several minutes after TRH removal, whereas the sustained [Ca2+]i increase dropped quickly to prestimulatory levels, following the same time course as the refilling of the stores. The drop was prevented when the refilling was inhibited by thapsigargin. It is concluded that, even though the mechanisms of capacitative Ca2+ entry may show differences from cell to cell, it is also present and may contribute to the regulation of physiological functions in excitable cells such as GH3. There, capacitative Ca2+ entry cooperates with voltage-gated Ca2+ channels to generate the [Ca2+]i increase seen during phase 2 of TRH action. This contribution of capacitative Ca2+ entry may be relevant to the enhancement of prolactin secretion induced by TRH.
Dual modulation of K channels by thyrotropin-releasing hormone in clonal pituitary cells
Proceedings of the National Academy of Sciences, 1985
Transmembrane electrical activity in pituitary tumor cells can be altered by substances that either stimulate or inhibit their secretory activity. Using patch recording techniques, we have measured the resting membrane potentials, action potentials, transmembrane macroscopic ionic currents, and single Ca2+-activated K channel currents of GH3 and GH4/C1 rat pituitary tumor cells in response to thyrotropin-releasing hormone (TRH). TRH, which stimulates prolactin secretion, causes a transient hyperpolarization of the membrane potential followed by a period of elevated action potential frequency. In single cells voltage clamped and internally dialyzed with solutions containing K+, TRH application results in a transient increase in Ca2+-activated K currents and a more protracted decrease in voltage-dependent K currents. However, in cells internally dialyzed with K+-free solutions, TRH produces no changes in inward Ca2+ or Ba2+ currents through voltage-dependent Ca channels. The time cour...
Proceedings of the National Academy of Sciences, 1990
The relationships between the activation status of voltage-sensitive Ca2+ channels and secretory responses were analyzed in perfused rat gonadotrophs during stimulation by high extracellular K+ concentration ([K+]e) or the physiological agonist, gonadotropin-releasing hormone (GnRH). Increase of [K+]e to 50 mM evokes an on-off secretory response, with a rapid rise in luteinizing hormone (LH) secretion to a peak at 35 sec (on response) followed by an exponential decrease to the steady-state level. Cessation of K+ stimulation elicits a transient (off) response followed by an exponential decrease to the basal level. The LH response to high [K+]e is nifedipine-sensitive and its amplitude depends on membrane potential. There is a close relationship between the LH secretory response to high [K+]e and the amplitude of the inward Ca2+ current measured at 100 msec in whole-cell patch clamp experiments. In addition, the profile of the LH secretory response is similar to that of the response o...
Electrical Activity and Calcium Channels in Neuroendocrine Cells
Annals of the New York Academy of Sciences, 1994
MATHIAS JOHN," ERNST-OTTO RIECKEN," potentials, whereas Ca2+ channel blockers suppress the spiking. Consistent with this, inhibitory hormones, such as somatostatin, inhibit not only Ca" influx through voltage-dependent CaZ+ channels and hormone secretion but also the spontaneous spiking a~t i v i t y .~ On the other hand, stimulatory hormones, such as TRH, not only stimulate voltage-dependent Caz+ channels (Hescheler et af., this volume) and hormone release but also the electrical activity of pituitary cells.
Cell Calcium, 1996
Spontaneous transients of [Ca*+], were recorded from single nonstimulated cells of a clonal pituitary cell line of cotticotropes, AtT-20/D16v. The spontaneous [Ca*+], transients were dependent on calcium entry from the extracellular solution because they were abolished both in the absence of extracellular calcium and with the addition of cobalt to the calcium-containing extracellular solution. Calcium entry occurred through voltage-gated (VGCC) L-type calcium channels because the [Ca*+], transients were blocked by L-type calcium channel antagonists, e.g. nifedipine, and were unaffected by the addition of tetrodotoxin. Bay K 8644 (1 ~.LM) induced transient increases in [Ca*+], which were also blocked reversibly by either the absence of extracellular calcium or the addition of an L-type calcium channel antagonist (e.g. nifedipine). The resting levels of [Ca'+], and the frequency, but not the amplitude or duration, of the spontaneous [Ca*+], transients increased as the concentration of extracellular calcium was elevated in concentrations ranging from 1.8-7.2 mM. Potassium depolarization reversibly elevated resting levels of [Ca*+], and initiated the spontaneous calcium transients. These results indicate that extracellular calcium modulates the frequency of spontaneous [Ca'+], transients in AtT-20 cells which are caused by the activation of L-type calcium channels by a spontaneous increase in the permeability of the cell membrane to calcium. Received
Regulation of L-Type Calcium Channels In Pituitary GH4C1 Cells by Depolarization
Journal of Biological Chemistry, 2001
The neurosecretory anterior pituitary GH 4 C 1 cells exhibit the high voltage-activated dihydropyridine-sensitive L-type and the low voltage-activated T-type calcium currents. The activity of L-type calcium channels is tightly coupled to secretion of prolactin and other hormones ...
Endocrinology, 2004
The GT1 cell has been widely used as a model cell to study cellular functions of GnRH neurons. Despite the importance of Ca 2؉ channels, little is known except for Land T-type Ca 2؉ channels in GT1 cells. Therefore, we studied the diversity of voltage-gated Ca 2؉ channels in GT1-7 cells with perforatedpatch clamp and RT-PCR. An R-type Ca 2؉ channel blocker, SNX-482, inhibited the Ca 2؉ currents by 75.6% in all cells examined (n ؍ 9). A T-type Ca 2؉ channel blocker, Ni 2؉ , inhibited the Ca 2؉ currents by 12.6% in all cells examined (n ؍ 9). An L-type Ca 2؉ channel blocker, nimodipine, inhibited the Ca 2؉ currents by 17.9% in five of 11 cells examined. When using Ba 2؉ as a charge carrier, another dihydropyridine antagonist, nifedipine, clearly inhibited the currents by 12.1% in all cells examined (n ؍ 16). An N-type Ca 2؉ channel blocker,-cono-toxin-GVIA, inhibited the Ca 2؉ currents by 13.8% in three of 20 cells examined. A P/Q type Ca 2؉ channel blocker,-agatoxin-IVA, had no effect on the currents (n ؍ 9). RT-PCR revealed that GT1-7 cells expressed the ␣ 1B , ␣ 1D , ␣ 1E , and ␣ 1H subunit mRNA. Furthermore, SNX-482 and nifedipine inhibited the high K ؉-induced increase in the intracellular Ca 2؉ concentration and GnRH release.-Conotoxin-GVIA and-agatoxin-IVA had no effect. These results suggest that GT1-7 cells express R-, L-, N-, and T-type voltage-gated Ca 2؉ channels; the R-type was a major current component, and the L-, N-, and T-types were minor ones. The R-and L-type Ca 2؉ channels play a critical role in the regulation of Ca 2؉-dependent GnRH release. (Endocrinology 145: 2375-2383, 2004)
Immunology & Cell Biology, 2000
Influx of Ca 2+ via Ca 2+ channels is the major step triggering exocytosis of pituitary somatotropes to release growth hormone (GH). Voltage-gated Ca 2+ and K + channels, the primary determinants of the influx of Ca 2+ , are regulated by GH-releasing hormone (GHRH) through G-protein-coupled intracellular signalling systems. Using whole-cell patch-clamp techniques, the changes of the Ca 2+ and K + currents in primary cultured ovine and human somatotropes were recorded. Growth hormone-releasing hormone (10 nmol/L) increased both Land T-type voltage-gated Ca 2+ currents. Inhibition of the cAMP/protein kinase A (PKA) pathway by either Rp-cAMP or H 89 blocked this increase in both Land T-type Ca 2+ currents. Growth hormone-releasing hormone also decreased voltage-gated transient (I A) and delayed rectified (I K) K + currents. Protein kinase C (PKC) inhibitors, such as calphostin C, chelerythrine or downregulation of PKC, blocked the effect of GHRH on K + currents, whereas an acute activation of PKC by phorbol 12,13-dibutyrate (1 µmol/L) mimicked the effect of GHRH. Intracellular dialysis of a specific PKC inhibitor (PKC 19-36) also prevented the reduction in K + currents by GHRH. It is therefore concluded that GHRH increases voltage-gated Ca 2+ currents via cAMP/PKA, but decreases voltage-gated K + currents via the PKC signalling system. The GHRH-induced alteration of Ca 2+ and K + currents augments the influx of Ca 2+ , leading to an increase in [Ca 2+ ]i and the GH secretion.
Facilitation of Ca2+Action Potential Frequency by a Small G Protein Rab3A in Rat Pituitary GH3Cells
Biochemical and Biophysical Research Communications, 1997
tion of the activated TRH receptor with heterotrimeric GH 3 pituitary cells have high tendency to exhibit GTP-binding proteins (G-proteins). Although the PI spontaneous Ca 2/ action potentials and their fresignaling pathway in GH cells has been well elucidated, quency (Ca 2/ APF) is increased by treatment with thydirect electrophysiological evidence concerning the sigrotropin-releasing hormone (TRH). Although spontanal transduction mechanism (s) that links TRH recepneous Ca 2/ firing was thought to be significant for the tor activation to the increase in Ca 2/ APF remains to induction of oscillations in cytosolic Ca 2/ concentrabe provided. Previous studies have suggested that actition ([Ca 2/ ] i ), little attempt to elucidate the mechanism vation of PKC (1,5,6) is involved in the TRH-induced has been done so far. We demonstrate here that increase of Ca 2/ APF which is implicated to be caused spontaneous Ca 2/ APF in GH 3 cells was increased 1.5by suppression of the K / currents in GH 3 cells(2,3,7,8).