Glutamate increases cytosolic calcium in GH3 pituitary cells acting via a high-affinity glutamate transporter.
Hormone secretion by GH3 pituitary cells is regulated by oscillations of the cytosolic Ca2+ concentration ([Ca2+]i), which are driven by electrical activity and modulated by hypothalamic releasing factors. We find that micromolar concentrations of L-glutamate and other acidic amino acids, but not selective excitatory amino acid receptor agonists, increase [Ca2+]i in GH3 cells. Activation by glutamate is blocked by dihydropyridines or removal of extracellular Ca2+ or Na+, but not by tetrodotoxin or excitatory amino acid receptor antagonists. Glutamate also accelerated the entry of Mn2+ used as a Ca2+ surrogate for Ca2+ channels. L-Glutamate and other acidic amino acids were taken up into GH3 cells by an Na(+)-dependent high-affinity transporter. The half-maximal effect of glutamate on [Ca2+]i was reached at concentrations similar to the Km for the glutamate transporter. Moreover, only those amino acids taken up through this transporter were able to increase [Ca2+]i. We propose that electrogenic entry of Na(+)-glutamate depolarizes the plasma membrane, thus causing an increase of action potentials firing and Ca2+ entry through voltage-gated channels. Our results suggest that glutamate may cooperate to the modulation of pituitary hormone secretion by an unconventional mechanism involving a high-affinity glutamate transporter rather than excitatory amino acid receptors.
Villalobos
C
C
Departamento de Bioquímica y Biología Molecular y Fisiología, Facultad de Medicina, Universidad de Valladolid, Spain.
García-Sancho
J
J
D018528:ATP-Binding Cassette Transporters / Q000037:antagonists & inhibitors / Q000378:metabolism*; D027322:Amino Acid Transport System X-AG; D000600:Amino Acids, Dicarboxylic / Q000378:metabolism; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism*; D004095:Dihydropyridines / Q000494:pharmacology; D008564:Membrane Potentials; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D017459:Receptors, Amino Acid / Q000378:metabolism; D012970:Sodium Glutamate / Q000378:metabolism*
D018528:ATP-Binding Cassette Transporters; D027322:Amino Acid Transport System X-AG; D000600:Amino Acids, Dicarboxylic; D004095:Dihydropyridines; D017459:Receptors, Amino Acid; D002118:Calcium; D012970:Sodium Glutamate