Calcium Transport Mechanisms of PC12 Cells

Many studies of Ca(2+) signaling use PC12 cells, yet the balance of Ca(2+) clearance mechanisms in these cells is unknown. We used pharmacological inhibition of Ca(2+) transporters to characterize Ca(2+) clearance after depolarizations in both undifferentiated and nerve growth factor-differentiated PC12 cells. Sarco-endoplasmic reticulum Ca(2+) ATPase (SERCA), plasma membrane Ca(2+) ATPase (PMCA), and Na(+)/Ca(2+) exchanger (NCX) account for almost all Ca(2+) clearance in both cell states, with NCX and PMCA making the greatest contributions. Any contribution of mitochondrial uniporters is small. The ATP pool in differentiated cells was much more labile than that of undifferentiated cells in the presence of agents that dissipated mitochondrial proton gradients. Differentiated PC12 cells have a small component of Ca(2+) clearance possessing pharmacological characteristics consistent with secretory pathway Ca(2+) ATPase (SPCA), potentially residing on Golgi and/or secretory granules. Undifferentiated and differentiated cells are similar in overall Ca(2+) transport and in the small transport due to SERCA, but they differ in the fraction of transport by PMCA and NCX. Transport in neurites of differentiated PC12 cells was qualitatively similar to that in the somata, except that the ER stores in neurites sometimes released Ca(2+) instead of clearing it after depolarization. We formulated a mathematical model to simulate the observed Ca(2+) clearance and to describe the differences between these undifferentiated and NGF-differentiated states quantitatively. The model required a value for the endogenous Ca(2+) binding ratio of PC12 cell cytoplasm, which we measured to be 268 ± 85. Our results indicate that Ca(2+) transport in undifferentiated PC12 cells is quite unlike transport in adrenal chromaffin cells, for which they often are considered models. Transport in both cell states more closely resembles that of sympathetic neurons, for which differentiated PC12 cells often are considered models. Comparison with other cell types shows that different cells emphasize different Ca(2+) transport mechanisms.