CALCIUM CHANNELS IN RAT MELANOTROPHS ARE PERMEABLE TO MANGANESE, COBALT, CADMIUM, AND LANTHANUM, BUT NOT TO NICKEL - EVIDENCE PROVIDED BY FLUORESCENCE CHANGES IN FURA-2-LOADED CELLS

Cobalt (Co), nickel (Ni), manganese (Mn), cadmium (Cd), and lanthanum (La) are commonly used as calcium (Ca) channel blockers, but some of them, besides reducing Ca entry, also traverse Ca channels and can exert effects intracellularly that confound interpretation of functional responses. Because of this and our need to use Ca channel blockers in an ongoing analysis of Ca channel activity in the regulation of the cytosolic free Ca concentration ([Ca2+]i) and secretion in melanotrophs, we assessed whether the cations mentioned enter these cells. This was done by incorporating the fluorescent probe fura-2 into isolated rat melanotrophs and monitoring fluorescence for changes that would signal the presence of the cations in the cytosol. In cell-free solution, where the probe and cations can interact freely, Mn, Co, and Ni all quench fluorescence, whereas Cd and La act in a Ca-like manner. When tested on fura-2-loaded melanotrophs in basal (unstimulated) conditions, Mn, Co, and Cd each yielded corresponding signals, thereby showing that they had penetrated the cells. By contrast, Ni caused no quenching of fluorescence even in melanotrophs exposed to 100 mM K+ to recruit additional Ca channels. Ni, therefore, did not penetrate the cells. However, as expected, Ni quenched fluorescence when given artificial access to the cytoplasm by ionomycin. Ni blocked spontaneous entry of Mn, Co, and Cd. It also lowered [Ca2+]i in unstimulated melanotrophs, consistent with blockade of spontaneous Ca entry. Like Ni, La lowered basal [Ca2+]i in unstimulated melanotrophs without penetrating the cells; however, unlike Ni, it penetrated when the melanotrophs were exposed to high potassium. We conclude that Ni is the most specific of the Ca channel blockers tested and that results obtained with Mn, Co, Cd, and La must be interpreted with reserve.