Mechanism of agonist-induced [Ca2+]i oscillations in pituitary gonadotrophs.
Gonadotropin-releasing hormone (GnRH) activates oscillatory Ca2+ signaling in pituitary gonadotrophs at a frequency (up to 25 min-1) that is dose-dependent and is determined by the degree of receptor-mediated inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) formation. Similar dose-dependent and frequency-modulated Ca2+ oscillations were elicited by intracellular administration of Ins(1,4,5)P3 and its nonhydrolyzable analogs, consistent with models in which Ins(1,4,5)P3 levels determine the frequency of Ca2+ oscillations but do not fluctuate in synchrony with [Ca2+]i. At constant agonist concentrations, Ca2+ spiking varied in amplitude, with a number of progressively larger transients before the onset of maximal oscillations, followed by a gradual decrease in spike amplitude that was accompanied by an increase in spiking frequency. The decline in the amplitude and increase in frequency of Ca2+ transients during stimulation by GnRH were not related to a decrease in the propagation of the Ca2+ signal within the cell but were associated with gradual depletion of the agonist-sensitive Ca2+ pool. Once initiated, the pattern of Ca2+ spiking was not altered by blockade of receptor occupancy, by inhibition of phospholipase C, or by reduction of extracellular [Ca2+]. Also, the endoplasmic reticulum (Ca2+)-ATPase blocker, thapsigargin, could substitute for Ins(1,4,5)P3 in initiating the oscillatory Ca2+ response. These findings indicate that although the Ins(1,4,5)P3 concentration determines the pattern of transients at the initiation of the oscillatory Ca2+ signal, maintenance of the signal does not require a sustained rise in Ins(1,4,5)P3. Since the frequency of Ca2+ oscillations is also influenced by depletion of luminal [Ca2+], it is possible that the Ins(1,4,5)P3-sensitive channels in the endoplasmic reticulum are tonically inhibited by high intraluminal Ca2+ levels and that Ins(1,4,5)P3 surmounts such inhibition by promoting Ca2+ discharge. When a critical level of Ca2+ discharge is attained, repetitive Ca2+ transients are generated by an autocatalytic mechanism in which a sustained rise in Ins(1,4,5)P3 is not an essential requirement.
Stojilković
S S
SS
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892.
Kukuljan
M
M
Tomić
M
M
Rojas
E
E
Catt
K J
KJ
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D004963:Estrenes / Q000494:pharmacology*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D009991:Oscillometry; D010052:Ovariectomy; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*; D011760:Pyrrolidinones / Q000494:pharmacology*; D051381:Rats; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D013997:Time Factors; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors
D004963:Estrenes; D011760:Pyrrolidinones; D013729:Terpenes; C060229:1-(6-((3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione; D007987:Gonadotropin-Releasing Hormone; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D010738:Type C Phospholipases; D000252:Calcium-Transporting ATPases; D002118:Calcium