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Lorsignol A, Taupignon A, Dufy B: Short applications of gamma-aminobutyric acid increase intracellular calcium concentrations in single identified rat lactotrophs. Neuroendocrinology. 1994 Oct;60(4):389-99.
We have investigated the direct effect of GABA receptor agonists on the cytosolic free calcium concentration ([Ca2+] i) and the membrane potential of rat lactotrophs in primary culture. [Ca2+] i was recorded in single identified lactotrophs by dual emission microspectrofluorimetry using indo-1 as intracellular fluorescent calcium probe. Whole cell and perforated patch-clamp were performed. A short application of GABA (10 (-5) M, 10 s) induced a marked transient [Ca2+] i increase in 66% of lactotrophs, which could be readily mimicked by muscimol (10 (-5) M). By contrast, neither L-homocarnosine (10 (-3) M) nor baclofen (10 (-5) M), a GABAB agonist, had any effect on [Ca2+] i. The GABA-induced [Ca2+] i increase was antagonized by picrotoxin (10 (-5) M), bicuculline methiodide (10 (-5) M) and strychnine (10 (-4) M), demonstrating GABAA receptor specificity. Furthermore, clonazepan (1.5 x 10 (-4) M) could potentiate the GABA effect on [Ca2+] i. The [Ca2+] i increase disappeared in the absence of Ca2+ in the extracellular medium or in the presence of Ca2+ channel blockers (cadmium, PN 200-110). GABA and muscimol depolarized the membrane potential with a concomitant fall in cell input resistance, thus suggesting, as in other cell types, the opening of receptor-operated chloride channels. When Ca2+ entry was prevented by the use of cadmium (500 x 10 (-6) M), GABA still elicited membrane depolarization but did not raise [Ca2+] i. Our results suggest that a short application of GABA leads to Ca2+ entry through voltage-gated Ca2+ channels in single lactotrophs. This Ca2+ influx is due to depolarization of the prolactin cell. |
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