The ability of the fetal adrenal medulla to respond directly to hypoxaemia and secrete catecholamines before the development of a functional innervation of the gland is essential for intrauterine survival. The cellular mechanisms involved in this response to low PO2 are not known, although the presence of oxygen‐sensitive K+ channels in carotid body chemoreceptor cells and other sites suggests that these might underlie the chromaffin cell response. Whole‐cell patch‐clamp techniques have been used to study K+ currents during normoxia and hypoxia in chromaffin cells isolated from the adrenal glands of fetal sheep. Two types of chromaffin cells were observed, those with a fast inactivating K+ current and a larger capacitance and those with a delayed K+ current and smaller capacitance. No cell showed both types of current. The fast inactivating current showed voltage‐dependent inactivation and was blocked by 1 mM 4‐aminopyridine, characteristics of an IA‐type current. The delayed current had two components, a TEA‐sensitive, Ca2+‐dependent current and a component with the kinetic behaviour of a delayed rectifier. Both types of current were oxygen sensitive. The IA‐type current was reduced by 27.4 ± 3.2 % when the PO2 was reduced to about 15 mmHg. With the delayed current, hypoxia reduced the amplitude by 26.9 ± 2.4 %, largely by reduction of the Ca2+‐dependent component. In the presence of hypoxia, reduction in the amplitude of these oxygen‐sensitive K+ currents would increase the frequency and duration of action potentials, leading to increased activation of the L‐type Ca2+ channels, influx of Ca2+ and the subsequent secretion of catecholamines.

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.TheeffectsofthespecificopioidagonistsDPDPE(δ-agonist),U-62066(κ-agonist) and DALDA (µ-agonist) on the hypoxia-evoked response were investigated in both a wholegland preparation and in isolated adrenal chromaffin cells using amperometry, whole-cell patch clamping and measurement of cytosolic [Ca 2+ ]. 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 [Ca 2+ ] i that results from exposure to hypoxia. Both agonists decreased the voltage-dependent Ca 2+ 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 Ca 2+ 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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We have demonstrated that there are differential changes in the levels of tyrosine hydroxylase (TH), phenylethanolamine N-methyltransferase (PNMT), and proenkephalin A (Pro Enk A) mRNA in the fetal sheep adrenal during late gestation. Adrenal TH mRNA:18S rRNA ratios increased between gestational days 100 (0.98 ±0.13; n = 6) and 125 (1.40 ±0.15; n = 6) and then decreased, whereas adrenal PNMT mRNA:18S rRNA ratios increased regularly between gestational days 100 (0.08 ±0.01) and 146 (0.17 ±0.03). The ratio of adrenal Pro Enk A mRNA to lBS rRNA was higher at gestational day 125 (0.085 ±0.005) than at either 80-100 days (0.038 ±0.007) or 140-146 days of gestation (0.055 ± 0.013). In 12 ewes, the growth and development of the placenta were restricted (placental restriction group) from conception. The ratio of adrenal PNMT mRNA to 1 8S rRNA was significantly reduced in the placental restriction group of fetal sheep (0.003 ±0.002) compared with controls (0.011 ±0.002), and there was a significant correlation between the ratio of adrenal PNMT mRNA to 1 8S rRNA and the mean arterial P0 2 (r = 0.88, . (1991) Co-ordinate and differential regulation of proenkephalin A and PNMT mRNA expression in cultured bovine adrenal chromaffin cells: responses to cAMP elevation and phorbol esters. Mol. BrainRes. 9, 135-142. Wilburn L.A. and Jaffe R. B. (1988) Quantitative assessment ofthe ontogeny of met enkephalin, norepinephrine, and epinephrine in the human fetal adrenal medulla. Acta Endocrinol. (Copenh.) 118, 453-459.

1. We have investigated adrenal mRNA expression of the catecholamine synthetic enzymes tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) following acute hypoxia in fetal sheep before (< 105 days gestation, n = 20) and after (> 125 days gestation, n = 20) the development of adrenal innervation and following pretreatment with the nicotinic receptor anatgonist hexamethonium (n = 12).2. Total RNA was extracted from fetal adrenal glands collected at specific time points at 3-20 h after the onset of either hypoxia (~50% reduction in fetal arterial oxygen saturation (S O 2 ) for 30 min), or normoxia.3. Before 105 days, there was a decrease in adrenal TH mRNA expression at 20 h after hypoxia and adrenal TH mRNA expression was directly related to the changes in arterial P O 2 measured during normoxia and hypoxia. After 125 days, adrenal TH mRNA levels were suppressed for up to 12 h following hypoxia.4. In both age groups, adrenal PNMT mRNA expression increased at 3-5 h after hypoxia and was inversely related to the changes in fetal arterial P O 2 during normoxia or hypoxia.5. After 125 days, the administration of hexamethonium (25 mg kg _1 , I.V.) reduced TH mRNA but not PNMT mRNA expression after normoxia. After hexamethonium pretreatment, there was no significant change in either adrenal TH or PNMT mRNA expression following hypoxia.6. We conclude that acute hypoxia differentially regulates adrenal TH and PNMT mRNA expression in the fetal sheep both before and after the development of adrenal innervation. After the development of adrenal innervation, however, the effect of acute hypoxia upon adrenal TH and PNMT mRNA expression is dependent upon neurogenic input acting via nicotinic receptors.