Research reportRegulation of Ca2+-activated nonselective cationic currents in rat pituitary GH3 cells: involvement in L-type Ca2+ current
Introduction
Ca2+-activated nonselective cationic currents (ICAN) are recognized in a wide variety of cell types, including cardiac myocytes, thyroid follicular cells, neurons, neutrophils, pancreatic duct cells and pituitary cells 1, 5, 6, 10, 11, 13, 17, 19, 20. The activation of these channels may: (1) maintain the cell at depolarized state, and hence affect the firing pattern of neuroendocrine cells 1, 3, 10, (2) induce an after-depolarization following an action potential [6], or (3) affect the action potential configuration and consequently alter Ca2+ influx [20]. In rat lactotrophs like GH3 cells, repetitive firing in the membrane potential, as found in neurons, has been observed and was thought to induce spontaneous release of prolactin 2, 8. It has been found that the firing pattern in GH3 cells is potential and calcium dependent 8, 15. Furthermore, the change in Na+ conductance may affect spontaneous depolarization in pituitary cells 1, 15. However, it is unclear whether the alterations in electrical activity of GH3 cells related to ICAN which can be elicited by depolarizing pulses that activate Ca2+ current play an important role in regulating hormonal secretion.
The purpose of the present study was to clarify the characteristics and physiological significance of ICAN in GH3 cells with the aid of the whole-cell clamp techniques. The present study clearly demonstrates that in these cells, the Ca2+-activated inward current is elicited if Ca2+ entry is provoked through voltage-dependent L-type Ca2+ channels by applying the depolarizing pulses. Moreover, the change in the pattern of Ca2+ spikes caused by the activation of ICAN which is elicited by depolarizing pulses that readily activate Ca2+ current is believed to play an important role in regulating hormonal secretion in GH3 cells.
Section snippets
Cell culture
The clonal strain GH3 cell line, originally derived from a rat anterior pituitary adenoma, was obtained from American Type Culture Collection (ATCC; Rockville, MD) [16]. GH3 cells were grown in monolayer culture in 50 ml plastic culture flasks in a humidified environment of 5% CO2/95% air at 37°C. Cells were maintained at a density of 106/ml in 5 ml Ham's F-12 nutrient media (Gibco, Grand Island, NY) supplemented with 15% heat-inactivated horse serum, 2.5% fetal calf serum, and 2 mM l-glutamine
Characterization of L-type Ca2+ currents and Ca2+-activated currents in GH3 cells
It has been shown that GH3 cells have at least two types of the voltage-dependent Ca2+ channels, one of which is high threshold, dihydropyridine-sensitive, and classified as L-type [14]. The present experiments were conducted in cells bathed in normal Tyrode's solution containing 1.8 mM CaCl2, tetrodotoxin (1 μM) and tetraethylammonium chloride (10 mM). As shown in Fig. 1, when pipette solution contained Cs+ ions without addition of EGTA, the cell was held at −50 mV, and the depolarizing
Discussion
The major findings of the present study are (1) Ca2+-activated nonselective cationic current (ICAN) which was activated by Ca2+ influx due to the voltage-dependent L-type Ca2+ currents (ICa,L) was identified in anterior pituitary GH3 cells, and (2) the regulation of ICAN may play an essential role in maintaining the depolarizing state and thus determining firing properties of these cells in response to depolarizing stimuli, particularly under the conditions in which K+ channels are blocked.
Acknowledgements
This work was partly supported by grants from National Science Council (NSC-87-2341-B075B-013) and Veterans General Hospital-Kaohsiung (VGHKS-87-52 and VGHNSU-87-06), Taiwan, ROC.
References (20)
- et al.
Nonselective cation channel on pancreatic duct cells
Biochim. Biophys. Acta
(1990) - et al.
Uncoupling of calcium mobilization and entry pathways in endothelin-stimulated pituitary lactotrophs
J. Biol. Chem.
(1997) - et al.
Calcium-activated cation channel in rat thyroid follicular cells
Biochim. Biophys. Acta
(1985) - et al.
Calcium-activated nonspecific cation channels
Trends Neurosci.
(1988) - et al.
Ca2+-dependent and Ca2+-independent mechanisms modulate whole-cell cationic currents in human neutrophils
Biochem. Biophys. Res. Comm.
(1992) Clonal strains of hormone-producing pituitary cells
Methods Enzymol.
(1979)- et al.
Ionic mechanisms of tetrandrine in cultured rat aortic smooth muscle cells
Eur. J. Pharmacol.
(1997) - et al.
Action potentials and membrane ion channels in clonal anterior pituitary cells
Proc. Natl. Acad. Sci.
(1983) - et al.
Alterations in the frequency and shape of Ca2+ fluctuations in GH4Cl cells induced by thyrotropin-releasing hormone and Bay K 8644
Biochem. J.
(1995) - et al.
A long-lasting calcium-activated nonselective cationic current is generated by synaptic stimulation or exogenous activation of group I metabotropic glutamate receptors in CA1 pyramidal neurons
J. Neurosci.
(1997)
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