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 cytosoli c Ca2+ ([Ca2+]i) in individual pituitary melanotrophs and how this is affected by physiological ligands that inhibit or stimulate melanotrop h secretion. Melanotrophs were dispersed from neurointermediate lobes of Xenopus laevis adapted to a black background, and [Ca2+]i was measu red with fura-2. In basal (unstimulated) conditions, repetitive transi ent 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 sever al minutes, during which [Ca 2+]i rose several-fold. Pulsing was promp tly and reversibly arrested by the secreto-inhibitory transmitters, do pamine, neuropeptide-Y (NPY), and gamma-aminobutyric acid (GABA), and also by quinpirole, muscimol, and baclofen. Pertussis toxin eliminated the responses to dopamine, NPY, and GABA(B) receptor activation, but spared responses to GABA(A) receptor activation. The responses to the physiological inhibitors and to the Ca channel blocker Ni were close t o all or nothing; a mere doubling of an ineffective concentration comm only sufficed to arrest pulsing. Submaximal responses, seen over a nar row concentration range, involved a slowing of the pulsing. Cells not pulsing spontaneously were responsive to dopamine, GABA, and NPY and p ulsed in response to the secretagogues CRF and TRH. They are suggested to be melanotrophs not actively secreting. The behavior of [Ca2+]i pa rallels secretory activity and strengthens the view that spontaneous s ecretion is driven by spontaneous influx of Ca ions and that secreto-i nhibitory transmitters act by suppressing this influx and lowering [Ca 2+]i. Cytosolic Ca pulsing may provide an efficient means of sustainin g a high rate of spontaneous secretion.