Impact of gestational age on the catecholamine responses of the fetal sheep adrenal to cholinergic stimulation in vitro

We have used the retrograde perfused adrenal preparation to investigate the catecholamine responses of the fetal sheep adrenal to increasing doses of acetylcholine (ACh) and excess potassium. Adrenal glands were collected from fetal sheep between 84 and 99 days gestation (before innervation; n = 7), between 103 and 113 days gestation (during innervation; n = 8) and between 137 and 144 days (after innervation; n = 9). Whilst the basal output of noradrenaline (NA) did not change between 84 and 144 days gestation, there was a significant increase in the adrenal output of adrenaline (Adr) between 84 and 144 days. The NA response to submaximal doses of ACh (10-10(3) microM) was significantly greater (P < 0.05) between 84 and 99 days gestation (mean NA response: 49.6 +/- 11.4 nmol/(30 min)) than after 137 days gestation (mean NA response: 27.3 +/- 8.3 nmol/(30 min)). Similarly, when the NA response to 10-10(3) microM ACh was expressed as a proportion of the maximal NA response to 10(4) microM ACh, the proportional NA response was greater at 84-99 days (48.9 +/- 11.3% of maximal) than at 137-144 days gestation (27.2 +/- 8.3%). The adrenal NA and Adr responses to 10(3) microM ACh between 84 and 137 days were reduced by up to 99% after the addition of hexamethonium. It appears, therefore, that the fetal adrenal is responsive to ACh before the development of innervation of the gland. There is also a decrease in sensitivity of the NA-secreting cells to submaximal doses of ACh as gestation progresses, which is not associated with a decrease in the size of the releasable pool of NA in the fetal adrenal. We speculate, therefore, that innervation of the fetal adrenal may be associated with an increase in the excitation threshold of the NA cells to nicotinic stimulation.