Effects of hypoxia on stimulus-release coupling mechanisms in cultured bovine adrenal chromaffin cells

Summary

To clarify the effects of hypoxia on stimulus-release coupling, we have examined the effects of hypoxia on nicotine-induced catecholamine (noradrenaline and adrenaline) release from, and ²²Na⁺ influx, ⁴⁵Ca²⁺ influx and cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) in, cultured bovine adrenal chromaffin cells. Experiments were carried out in media pre-equilibrated with 21% O₂/79% N₂ (control) or with 0% O₂/100% N₂ (hypoxia). Cells were stimulated with either nicotine (activating nicotinic acetylcholine (ACh) receptors) or a high K⁺ concentration (55 mmol/1 KCI; directly activating voltage-dependent Ca²⁺ channels). Hypoxia reduced both nicotine- and high K⁺-induced catecholamine releases from the cells, but the reduction of the former (to about 30% of the control value) was more pronounced than that of the latter (to about 40% of the control value). Nicotine-induced ²²Na⁺ influx, which is considered to reflect the function of nicotinic ACh receptors, was inhibited by hypoxia. Both nicotine- and high K⁺-induced ⁴⁵Ca²⁺ influx into the cells were reduced by hypoxia, but the reduction of the former was more pronounced than that of the latter. Nicotine- and high K⁺-induced increases in [Ca²⁺]_i were reduced by hypoxia to about 30% and 40% of the control values, respectively. These results suggest that hypoxia reduces cation influxes (Na⁺ and Ca²⁺) through both the ligand-gated cation channels of the nicotinic ACh receptor and the voltage-dependent Ca²⁺ channels.