Hypoxia‐induced catecholamine secretion in isolated newborn rat adrenal chromaffin cells is mimicked by inhibition of mitochondrial respiration

Abstract

In newborn mammals, systemic hypoxia provokes catecholamine secretion from the adrenal medulla. In contrast to adults, this release is independent of sympathetic innervation. We have studied the cellular processes involved in hypoxia‐induced catecholamine secretion, employing fluorimetric techniques to measure changes in [Ca2+]i, NADH and mitochondrial potential, and voltammetric techniques to record changes in PO2 and catecholamine secretion. In adrenal chromaffin cells freshly dissociated from newborn rats, severe hypoxia increased [Ca2+]i and secretion of catecholamines, indicating that the response of the newborn adrenal medulla to hypoxia is an intrinsic property of these cells. Discrete quantal secretory events were identifiable, suggesting an exocytotic mechanism of secretion. Hypoxia‐induced secretion was only seen when PO2fell below 5 mmHg, similar to the threshold arterial PO2 reported to stimulate release in vivo. Such oxygen tensions also inhibited mitochondrial metabolism, shown by an increase in NADH autofluorescence. We therefore explored the involvement of mitochondria in oxygen sensing. Inhibition of mitochondrial respiration either by CN− at complex IV or by rotenone at complex I mimicked severe hypoxia, reversibly increasing both [Ca2+]i and catecholamine secretion. The CN−‐induced depolarization of the mitochondrial inner membrane potential preceded the increase in [Ca2]i by ∼6 s. The effects of severe hypoxia and CN− on [Ca2+]i and catecholamine secretion were not additive, suggesting a common mechanism. Chemical anoxia failed to increase [Ca2+]i in a significant proportion of cells dissociated from 2‐ to 4‐week‐old rats. Thus, the sensitivity to hypoxia is specific to adrenal chromaffin cells dissociated from newborn rats. These data indicate that hypoxia‐induced catecholamine secretion in the newborn adrenal medulla is mediated by reversible inhibition of mitochondrial respiration, leading to an increase in [Ca2+]i and catecholamine secretion.

Rat Adrenal Chromaffin Cells Are Neonatal CO2Sensors

“…These effects are independent of extracellular acidification but are accompanied by a drop in intracellular pH. Neonatal chromaffin cells were known to be O 2 sensors (Seidler and Slotkin, 1985;MochizukiOda et al, 1997;Mojet et al, 1997;Thompson et al, 1997;Rychkov et al, 1998), but, to our knowledge, the direct involvement of these cells in CO 2 homeostasis have not yet been reported.…”

“…In fact, a non-neurogenic secretory response to hypoxia has been extensively studied in adrenal glands of newborn rats Slotkin, 1985, 1986). Neonatal chromaffin cells have been shown to be O 2 sensors that release CATs in response to local decreases of O 2 tension (Mochizuki-Oda et al, 1997;Mojet et al, 1997;Thompson et al, 1997;García-Fernández et al, 2001). Similar to glomus cells of the carotid body (for review, see Ló pez-Barneo et al., 2001), chromaffin cells contain O 2 -sensitive K ϩ channels (Thompson et al, 1997;Rychkov et al, 1998;Keating et al, 2001).…”

Change in the Membranous Lipid Composition Accelerates Lipid Peroxidation in Young Rat Hearts Subjected to 2 Weeks of Hypoxia Followed by Hyperoxia

“…This finding is similar to previous reports of the fatty acid composition of cardiac phospholipids in membrane lipids in adolescents animals exposed to high doses of catecholamines, 19 aortic constriction 20 or intermittent high altitude hypoxia. 21 Although exposure to chronic hypoxia is initially associated with an increase in adrenergic activity, 22,23 adrenoceptor blockade does not attenuate the changes in the fatty acid composition of phospholipids. 20 In our preliminary study, the fatty acid composition of hypoxic rat hearts followed by 2 weeks ofnormoxia was not remarkably different in the comparison with that of normoxic rat hearts (data not shown).…”

Acid‐evoked quantal catecholamine secretion from rat phaeochromocytoma cells and its interaction with hypoxia‐evoked secretion

“…All acquisition was performed using a Digidata 1200 interface and Fetchex software from the pCLAMP 6.0.3 suite (Axon Instruments). The same equipment was also used to monitor POµ levels in the recording chamber, except that the polarity of the microelectrode was reversed to −800 mV (see Mojet et al 1997). The time course of the fall in PO 2 in the recording chamber was highly reproducible for any given degree of hypoxia.…”