Tight mitochondrial control of calcium and exocytotic signals in chromaffin cells at embryonic life

Calcium buffering by mitochondria plays a relevant physiological function in the regulation of Ca2+ and exocytotic signals in mature chromaffin cells (CCs) from various adult mammals. Whether a similar or different role of mitochondrial Ca2+ buffering is present in immature CCs at early life has not been explored. Here we present a comparative study in rat embryonic CCs and rat mother CCs, of various physiological parameters that are known to be affected by mitochondrial Ca2+ buffering during cell activation. We found that the clearance of cytosolic Ca2+ transients ([Ca2+]c) elicited by high K+ was 7-fold faster in embryo CCs compared to mother CCs. This strongly suggests that at embryonic life, the mitochondria play a more significant role in the clearance of [Ca2+]c loads compared to adult life. Consistent with this view are the following results concerning the transient suppression of mitochondrial Ca2+ buffering by protonophore FCCP, in embryonic CCs compared to mother CCs: (i) faster and greater inactivation of inward calcium currents, (ii) higher K+-elicited [Ca2+]c transients with 25-fold faster clearance, (iii) higher increase of basal catecholamine release and (iv) higher potentiation of K+-evoked secretion. These pronounced differences could be explained by two additional features (embryo versus mother CCs): (a) slower recovery of mitochondrial resting membrane potential after the application of a transient FCCP pulse and (b) greater relative density of the mitochondria in the cytosol. This tighter control by the mitochondria of Ca2+ and exocytotic signals may be relevant to secure a healthy catecholamine secretory response at early life.