Membrane potential regulates inositol 1,4,5-trisphosphate-controlled cytoplasmic Ca2+ oscillations in pituitary gonadotrophs.
The influence of membrane potential (Vm) on cytoplasmic calcium ([Ca2+]i) oscillations during the sustained extracellular Ca(2+)-dependent phase of the Ca2+ signaling response to gonadotropin-releasing hormone (GnRH) was analyzed in cultured pituitary gonadotrophs. In agonist- and inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3)-stimulated cells, sustained [Ca2+]i oscillations were extinguished by hyperpolarization after 3-15 min despite the availability of Ca2+ in the extracellular medium. Single depolarizing pulses transiently restored the amplitude of the sustained spiking in a dihydropyridine- and extracellular Ca(2+)-sensitive manner. The responses to depolarization showed a marked dependence on Vm that was correlated with the steady-state inward Ca2+ current. In addition, repetitive application of brief depolarizing pulses modulated the frequency of agonist- and Ins(1,4,5)P3-controlled spiking; depolarization pulses at frequencies lower than the intrinsic rate of episodic Ca2+ release triggered large transients between the autonomous spikes, whereas higher frequencies of depolarizing pulses overcame the original Ca2+ spiking frequency. These extrinsically driven and extracellular Ca(2+)-dependent oscillations were sensitive to the Ca(2+)-ATPase blocker, thapsigargin, but not to ryanodine. On the other hand, spontaneous firing and application of depolarizing pulses to nonstimulated cells failed to induce thapsigargin-sensitive oscillations. These findings demonstrate that the pattern of Ca2+ signaling in gonadotrophs does not depend exclusively on the Ins(1,4,5)P3 concentration, but also on the excitable status of the cell. Such modulation of the Ins(1,4,5)P3-controlled Ca2+ signaling system by changes in Vm could provide a mechanism for the integration of multiple inputs that utilize diverse signal transduction pathways.
Kukuljan
M
M
Laboratory of Cell Biology and Genetics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Rojas
E
E
Catt
K J
KJ
Stojilkovic
S S
SS
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D004558:Electric Stimulation; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D007700:Kinetics; D008564:Membrane Potentials / Q000502:physiology*; D009991:Oscillometry; D010052:Ovariectomy; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D051381:Rats; D013997:Time Factors
D007987:Gonadotropin-Releasing Hormone; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium