Non-cholinergic nervous control of catecholamine secretion from perfused bovine adrenal glands

1. Field stimulation of adrenal nerves was used to study nervous control of adrenal catecholamine secretion in isolated, retrogradely perfused, bovine adrenal glands. 2. Secretion of both adrenaline and noradrenaline was maximal at 10 Hz. Secretion at 2 Hz was < 10% of maximum. Stimulating with trains of pulses at ten times the average frequency for 1 s out of every 10 s gave 2-fold greater secretion at 2 Hz average frequency, similar release at 5 Hz, and only half the secretion at 10 Hz, compared to continuous stimulation at the average frequency. 3. At 10 Hz, adrenaline and noradrenaline secretion was virtually abolished by tetrodotoxin (1 microM), but was only reduced by 75% by prolonged perfusion with a combination of mecamylamine (5 microM) and atropine (1 microM). Mecamylamine and atropine completely abolished the secretory response to 2 Hz stimulation. Tetrodotoxin had no significant effect on secretion induced by perfusing glands with nicotine (5 microM), while mecamylamine abolished this response. Mecamylamine and atropine had no effect on secretion induced by K+ depolarization. 4. The secretion of adrenaline and noradrenaline induced by 10 Hz stimulation was not inhibited by naloxone at either 1 or 30 microM. 5. The results suggest that bovine adrenal chromaffin cells, like those in the rat, receive a significant non-cholinergic secretomotor innervation. In contrast to the rat, however, the non-cholinergic component in the bovine adrenal is negligible at low-frequency nerve stimulation and substantial at higher frequencies, and is not antagonized by naloxone. The identity of the non-cholinergic transmitter remains to be determined.