The co-presence of Na⁺/Ca²⁺-K⁺ exchanger and Na⁺/Ca²⁺ exchanger in bovine adrenal chromaffin cells

We have previously shown that there is high Na⁺/Ca²⁺ exchange (NCX) activity in bovine adrenal chromaffin cells. In this study, by monitoring the [Ca²⁺]_i change in single cells and in a population of chromaffin cells, when the reverse mode of exchanger activity has been initiated, we have shown that the NCX activity is enhanced by K ⁺. The K⁺-enhanced activity accounted for a significant proportion of the Na⁺-dependent Ca²⁺ uptake activity in the chromaffin cells. The results support the hypothesis that both NCX and Na⁺/Ca²⁺-K⁺ exchanger (NCKX) are co-present in chromaffin cells. The expression of NCKX in chromaffin cells was further confirmed using PCR and northern blotting. In addition to the plasma membrane, the exchanger activity, measured by Na⁺-dependent ⁴⁵Ca²⁺ uptake, was also present in membrane isolated from the chromaffin granules enriched fraction and the mitochondria enriched fraction. The results support that both NCX and NCKX are present in bovine chromaffin cells and that the regulation of [Ca²⁺]_i is probably more efficient with the participation of NCKX.