Relative contribution of the Na+/Ca2+ exchanger, mitochondria and endoplasmic reticulum in the regulation of cytosolic Ca2+ and catecholamine secretion of bovine adrenal chromaffin cells

The relative importance of mitochondria, the Na+/Ca2+ exchanger (NCX) and the endoplasmic reticulum (ER) in the regulation of the cytosolic Ca2+ concentration ([Ca2+](i)) were examined in bovine chromaffin cells using fura-2 for average [Ca2+](i) and amperometry for secretory activity, which reflects the local Ca2+ concentration near the exocytotic sites. Chromaffin cells were stimulated by a high concentration of K+ when the three Ca2+ removal mechanisms were individually or simultaneously inhibited. When the mitochondrial Ca2+ uptake was inhibited, the [Ca2+](i) decayed at a significantly slower rate and the secretory activity was higher than the control cells. The NCX appears to function only in the initial phase of [Ca2+](i) decay and when the ER Ca2+ pump is blocked. Similarly, the ER had a significant effect on the [Ca2+](i) decay and on the secretion only when the NCX was blocked. Inhibition of ail three mechanisms reads to a substantial delay in [Ca2+](i) recovery and an increase in the secretion. The results suggest that the three mechanisms work together in the regulation of the Ca2+ near the Ca2+ channels and exocytotic sites and therefore modulate the secretory activity. When Ca2+ diffuses away from the exocytotic sites, the mitochondrial Ca2+ uptake becomes the dominant mechanism.