Spontaneous cytosolic calcium pulsing detected in Xenopus melanotrophs: modulation by secreto-inhibitory and stimulant ligands.

The purpose of the experiments was to examine the behavior of cytosolic Ca2+ ([Ca2+]i) in individual pituitary melanotrophs and how that is affected by physiological ligands that inhibit or stimulate melanotroph secretion. Melanotrophs were dispersed from neurointermediate lobes of Xenopus laevis adapted to a black background, and [Ca2+]i was measured with fura-2. In basal (unstimulated) conditions, repetitive transient elevations in [Ca2+]i, not hitherto observed in any melanotrophs, were detected in 73% of the cells. These cytosolic Ca pulses occurred at fairly regular intervals (1-10 min) and lasted from a half to several minutes, during which [Ca2+]i rose several-fold. Pulsing was promptly and reversibly arrested by the secreto-inhibitory transmitters, dopamine, neuropeptide-Y (NPY), and gamma-aminobutyric acid (GABA), and also by quinpirole, muscimol, and baclofen. Pertussis toxin eliminated the responses to dopamine, NPY, and GABAB receptor activation, but spared responses to GABAA receptor activation. The responses to the physiological inhibitors and to the Ca channel blocker Ni were close to all or nothing; a mere doubling of an ineffective concentration commonly sufficed to arrest pulsing. Submaximal responses, seen over a narrow concentration range, involved a slowing of the pulsing. Cells not pulsing spontaneously were responsive to dopamine, GABA, and NPY and pulsed in response to the secretagogues CRF and TRH. They are suggested to be melanotrophs not actively secreting. The behavior of [Ca2+]i parallels secretory activity and strengthens the view that spontaneous secretion is driven by spontaneous influx of Ca ions and that secreto-inhibitory transmitters act by suppressing this influx and lowering [Ca2+]i. Cytosolic Ca pulsing may provide an efficient means of sustaining a high rate of spontaneous secretion.