Integration of cytoplasmic calcium and membrane potential oscillations maintains calcium signaling in pituitary gonadotrophs.

Pituitary gonadotrophs exhibit spontaneous low-amplitude fluctuations in cytoplasmic calcium concentration ([Ca2+]i) due to intermittent firing of nifedipine-sensitive action potentials. The hypothalamic neuropeptide, gonadotropin-releasing hormone, terminates such spontaneous [Ca2+]i transients and plasma-membrane electrical activity and initiates high-amplitude [Ca2+]i oscillations and concomitant oscillations in membrane potential (Vm). The onset of agonist-induced [Ca2+]i oscillations is not dependent on Vm or extracellular Ca2+ but is associated with plasma-membrane hyperpolarization interrupted by regular waves of depolarization with firing of action potentials at the peak of each wave. The Vm and Ca2+ oscillations are interdependent during continued gonadotropin-releasing hormone action (greater than 3-5 min), when sustained Ca2+ entry is necessary for the maintenance of [Ca2+]i spiking. The initial and sustained agonist-induced Ca2+ transients and Vm oscillations are abolished by blockade of endoplasmic reticulum Ca(2+)-ATPase, consistent with the role of Ca2+ re-uptake by internal stores in the oscillatory response during both phases. Such a pattern of synchronization of electrical activity and Ca2+ spiking in cells regulated by Ca(2+)-mobilizing receptors shows that the operation of the cytoplasmic oscillator can be integrated with a plasma-membrane oscillator to provide a long-lasting signal during sustained agonist stimulation.