Before the preganglionic regulation of the adrenal medulla is established, hypoxia acts directly on the chromaffin cells to evoke the secretion of catecholamines. This direct action of hypoxia is suppressed by the gradual development of the preganglionic innervation and we have proposed that opioid peptides released from the adrenal splanchnic nerves may be responsible for this suppression. The effects of the specific opioid agonists DPDPE (δ-agonist), U-62066 (κ-agonist) and DALDA (μ-agonist) on the hypoxia-evoked response were investigated in both a whole-gland preparation and in isolated adrenal chromaffin cells using amperometry, whole-cell patch clamping and measurement of cytosolic [Ca2+]. The combined application of μ- and κ-type agonists abolished the hypoxia-evoked catecholamine secretion from whole perfused adrenal gland. In isolated chromaffin cells, μ- and κ-opioid agonists reduced the rise in [Ca2+]i that results from exposure to hypoxia. Both agonists decreased the voltage-dependent Ca2+ current in these cells. The μ-agonist increased the conductance through SK-type K+ channels and this action offset the decrease in K+ conductance produced by exposure to hypoxia. The κ-type agonist decreased the conductance through an action on BK-type K+ channels, a class of channels that are not involved in initiating the direct response to hypoxia. These data suggest that opioids, through their action on SK channels and voltage-dependent Ca2+ channels, may be responsible for the nerve-induced suppression of the hypoxic response of adrenal chromaffin cells and that these effects of endogenous opioids are mediated via μ- and κ-type receptors.
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