Abstract
We have shown previously that Ca2+-channel agonists, which open Ca2+ channels, inhibit parathyroid hormone (PTH) secretion from dispersed bovine parathyroid cells, whereas Ca2+-channel antagonists, which close Ca2+ channels, stimulate PTH release. We now have tested the effects of mouse antibodies specific for purified alpha subunits of rat skeletal muscle Ca2+-channel proteins on PTH secretion by bovine parathyroid cells in vitro. Mouse antisera (MC-2, MC-3, MC-4) blocked the secretion of PTH from parathyroid cells incubated with 0.5 mM Ca2+ ions. Affinity-purified MC-4 antibodies inhibited PTH release in a concentration-dependent manner. Incubation of parathyroid cells with pertussis toxin markedly reduced MC-4-dependent inhibition of PTH secretion. Parathyroid cell membrane proteins were fractionated by NaDodSO4/polyacrylamide gel electrophoresis under either reducing or nonreducing conditions and immunoblotted with MC-4 antiserum. Antibodies bound to one major band of protein with Mr approximately equal to 150,000. These results suggest that the antibodies bind to Ca2+-channel alpha subunits and act as agonists that open the channels and inhibit PTH release.