Dual pathways of calcium entry in spike and plateau phases of luteinizing hormone release from chicken pituitary cells: sequential activation of receptor-operated and voltage-sensitive calcium channels by gonadotropin-releasing hormone.
It has previously been shown that, in pituitary gonadotrope cells, the initial rise in cytosolic Ca2+ induced by GnRH is due to a Ca2+ mobilization from intracellular stores. This raises the possibility that the initial transient spike phase of LH release might be fully or partially independent of extracellular Ca2+. We have therefore characterized the extracellular Ca2+ requirements, and the sensitivity to Ca2+ channel blockers, of the spike and plateau phases of secretion separately. In the absence of extracellular Ca2+ the spike and plateau phases were inhibited by 65 +/- 4% and 106 +/- 3%, respectively. Both phases exhibited a similar dependence on concentration of extracellular Ca2+. However, voltage-sensitive Ca2+ channel blockers D600 and nifedipine had a negligible effect on the spike phase, while inhibiting the plateau phase by approximately 50%. In contrast, ruthenium red, Gd3+ ions, and Co2+ ions inhibited both spike and plateau phases to a similar extent as removal of extracellular Ca2+. A fraction (35 +/- 4%) of spike phase release was resistant to removal of extracellular Ca2+. This fraction was abolished after calcium depletion of the cells by preincubation with EGTA in the presence of calcium ionophore A23187, indicating that it depends on intracellular Ca2+ stores. Neither absence of extracellular Ca2+, nor the presence of ruthenium red or Gd3+ prevented mobilization of 45Ca2+ from intracellular stores by GnRH. We conclude that mobilization of intracellular stored Ca2+ is insufficient by itself to account for full spike phase LH release.(ABSTRACT TRUNCATED AT 250 WORDS)
Davidson
J S
JS
Department of Chemical Pathology, University of Cape Town Medical School Observatory, South Africa.
Wakefield
I K
IK
King
J A
JA
Mulligan
G P
GP
Millar
R P
RP
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D002412:Cations; D002645:Chickens; D003035:Cobalt / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D005682:Gadolinium / Q000494:pharmacology; D005711:Gallopamil / Q000494:pharmacology; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D007986:Luteinizing Hormone / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D012430:Ruthenium Red / Q000494:pharmacology
D002132:Calcium Radioisotopes; D002412:Cations; D007473:Ion Channels; D012430:Ruthenium Red; D007987:Gonadotropin-Releasing Hormone; D000001:Calcimycin; D005711:Gallopamil; D003035:Cobalt; D004533:Egtazic Acid; D007986:Luteinizing Hormone; D005682:Gadolinium; D009543:Nifedipine; D011188:Potassium; D002118:Calcium