Indications from Mn-quenching of Fura-2 fluorescence in melanotrophs that dopamine and baclofen close Ca channels that are spontaneously open but not those opened by high [K+]O; and that Cd preferentially blocks the latter.

In unstimulated melanotrophs, which secrete spontaneously, Mn caused a progressive quenching of Fura-2 fluorescence (F360) which was: (a) unaffected by tetrodotoxin to suppress spontaneous Na-action potentials; (b) slowed by lowering temperature to 23 degrees C; and (c) arrested by the Ca channel blocker, Ni. Mn quenching slowed on lowering [K+]O from 5 to 2 mM to hyperpolarize (indicating Mn entry through voltage-dependent channels) and accelerated on raising [K+]O to 50 or 100 mM to strongly depolarize (indicating recruitment of high threshold channels). The secreto-inhibitor, dopamine, arrested spontaneous Mn quenching and so, too, did the GABAB agonist, baclofen; and these effects like those of the two agonists on secretion and [Ca2+]i were blocked by the specific D2 and GABAB antagonists, sulpiride and CGP 35348, respectively, and were lost following exposure to pertussis toxin. By contrast, neither dopamine nor baclofen prevented Mn quenching in response to high K, although this was arrested by Ni. A second Ca channel blocker, Cd, in concentrations that inhibited the response to high K, failed to inhibit spontaneous entry of Mn. This preferential effect offers an explanation for observations made with Cd that have been interpreted as contrary to the notion of Ca-regulated secretion in the melanotroph. The results we have obtained are interpreted to mean that in the melanotroph secreting spontaneously some voltage-dependent Ca channels are in the open state; that this open state is not dependent on any Na spiking activity; and that these channels are preferentially closed by dopamine and baclofen which are without effect on Ca channels opened by strongly depolarizing concentrations of K.