Effects of extracellular calcium on the subcellular translocation of bovine parathyroid PKC isozymes

The release of parathyroid hormone is regulated by the extracellular concentration of Ca2+ through a sensor(s) on the surface of the parathyroid cells, but few details are known on the further relay of the signal inside the cell. Activation of protein kinase C (PKC) isozymes is associated with their translocation from the cell soluble fraction to the particulate fraction of the cell. Therefore, identification of a subcellular localization of a PKC isozyme in parathyroid cells as a response to changes in extracellular Ca2+ should be an indication for its putative role in signal transduction coupled to the Ca2+ sensor. We have determined the subcellular localization of six PKC isozymes (alpha, betaI, betaII, epsilon, zeta, and iota) in nonstimulated parathyroid cells and in those treated with low (0.5 mM) and high (3.0 mM) extracellular Ca2+ by confocal microscopy. At the physiological concentration of serum Ca2+, all PKC isozymes studied were localized mainly to the cytosol, although to different extents. Low extracellular Ca2+ caused a redistribution of PKCalpha to the periphery of the cells. In contrast, PKCbetaI, -epsilon, -zeta, and -iota were translocated to the periphery of the cells at high extracellular Ca2+. These results indicate that PKCalpha, -betaI, -epsilon, -zeta, and -iota are involved in the response of parathyroid cells to changes in extracellular Ca2+.