Relative contribution of the Na(+)/Ca(2+) exchanger, mitochondria and endoplasmic reticulum in the regulation of cytosolic Ca(2+) and catecholamine secretion of bovine adrenal chromaffin cells

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