Abstract
Previous studies have shown that catecholamine secretion from the adrenal medulla plays a critical role in chronic intermittent hypoxia (CIH)-induced alterations in cardiovascular function. In the present study we examined the cellular mechanisms associated with the effects of CIH on adrenal chromaffin cell catecholamine secretion. Experiments were performed on adult male mice (C57/BL6) that were exposed to 1-4 days of CIH or to normoxia. Perforated patch electrical capacitance recordings were performed on freshly prepared adrenal medullary slices that permit separating the chromaffin cell secretion from sympathetic input. CIH resulted in a significant increase in the readily releasable pool (RRP) of secretory granules, and decreased stimulus-evoked Ca(2+) influx. Continuous hypoxia (CH) either for 2.5 h (equivalent to hypoxic duration accumulated over 4 days of CIH) or for 4 days were ineffective in evoking changes in the RRP and Ca(2+) influx. CIH activated PKC in adrenal medullae as evidenced by increased phosphorylation of PKC at Thr(514) and PKC inhibitors prevented CIH-induced increases in the RRP and restored stimulus-evoked attenuation of Ca(2+) influx. CIH resulted in elevated thio-barbituric acid reactive substances (TBARSs, an index of oxidized proteins) and an antioxidant prevented CIH-induced changes in the RRP, suggesting the involvement of reactive oxygen species (ROS). These results demonstrate that CIH increases the RRP in adrenal chromaffin cells via ROS-mediated activation of PKC and suggest that CIH can directly affect the secretory capacity of chromaffin cells and contribute, in part, to elevated catecholamine levels.