AJP - Endocrinology and Metabolism, Vol 270, Issue 5 E793-E801, Copyright © 1996 by American Physiological Society
Long-term GABAA receptor activation increases [Ca2+]i in single lactotrophs
A. Lorsignol, A. Taupignon and B. Dufy
Laboratoire de Neurophysiologie, Centre National de la Recherche Scientifique Unite de Recherche Associee 1200, Universite de Bordeaux II, France.
Prolactin (PRL) release by pituitary lactotrophs is inhibited by
gamma-aminobutyric acid (GABA). We have investigated the effect of
long-lasting activation of GABAA receptors on membrane potential and
cytosolic free calcium concentration ([Ca2+]i) in single identified
lactotrophs. Membrane potential was recorded using the perforated
patch-clamp technique and [Ca2+]i using indo 1 as a fluorescent Ca2+ probe.
When cells were bathed in muscimol (10 microM) for 30 min, [Ca2+]i
unexpectedly increased in 53% of the lactotrophs. This was due to a Ca2+
influx, since it was inhibited by Ca(2+)-free extracellular medium or by
Ca2+ channel blockers such as the dihydropyridine PN 200-110. In cells
incubated in muscimol, wash of muscimol from the cell membrane potential
and reduced [Ca2+]i to the levels found in control cells. This effect was
mimicked by picrotoxin, a GABA-operated Cl- channel blocker, thus
supporting the involvement of a muscimol-induced Cl- flux. Conversely,
under similar experimental conditions, static assays of PRL release
revealed an inhibition of release by muscimol, unaffected by the
dihydropyridine PN 200-110. Our observations suggest that GABAA, receptors
may not regulate the process of exocytosis within the Ca(2+)- regulated
steps.
