DIFFERENTIAL SECRETION OF CATECHOLAMINES IN RESPONSE TO PEPTIDERGIC AND CHOLINERGIC TRANSMITTERS IN RAT ADRENALS

1. Rat adrenal medulla is stimulated by cholinergic and peptidergic transmitters released from splanchnic nerves. The peptidergic transmitter has been identified as vasoactive intestinal polypeptide (VIP) and its contribution in comparison to that of acetylcholine (ACh) is more prominent at low neuronal activity. The purpose of this study is to determine if ACh and VIP cause differential secretion of adrenaline and noradrenaline and whether the differential secretion also occurs when splanchnic nerves are stimulated at different frequencies. 2. Perfusion of the left adrenal gland with Krebs solution for several hours did not change adrenaline and noradrenaline contents (15.2 mug and 3.5 mug, respectively) and their ratio (4.4) from those of the unperfused right adrenal medulla (15.2 mug, 3.3 mug and 4.8, respectively). 3. Perfusion with ACh (10 muM for 4 min) resulted in the secretion of 109 ng of catecholamines and the ratio of adrenaline to noradrenaline was 3.8. Although the secretion increased with increased concentrations of ACh (30 and 100 muM), the ratios remained between 3 and 4. 4. Perfusion with VIP (10 muM for 4 min) resulted in the secretion of 27 ng of catecholamines and the ratio of adrenaline to noradrenaline was 9.7. A higher concentration of VIP (20 muM for 4 min) resulted in the secretion of greater amounts of catecholamines (102 ng) without significantly altering the ratio of adrenaline to noradrenaline (10.9). 5. Perfusion with as low as 0.01 muM pituitary adenylate cyclase-activating polypeptide (PACAP) increased the secretion of catecholamines to 31 ng and the secretion increased in a dose-dependent manner up to 0.3 muM. The ratio of adrenaline to noradrenaline was 6.8 with 0.1 muM PACAP and remained almost unchanged throughout the concentration range. 6. Electrical field stimulation at 0.5 Hz for 300 s resulted in the secretion of 34 ng catecholamines, and the ratio of adrenaline to noradrenaline was 6.9. Delivering the same number of pulses at 1 Hz increased the secretion to 42 ng but reduced the ratio to 6.3. Increasing stimulation frequency to 10 Hz for 15 s or 30 s further increased the secretion to 66 ng and 180 ng, respectively, but significantly reduced the ratio to about 5. 7. Delivery of same number pulses at 3 Hz continuously or at 30 Hz in bursts caused a substantial reduction in catecholamine secretion (55 versus 35 ng) and in the ratio of adrenaline to noradrenaline (7.4 versus 5). 8. These results agree with the hypothesis that cholinergic and peptidergic transmitters control catecholamine secretion at different levels of neuronal activity and provide new evidence that ACh stimulates the secretion of noradrenaline and adrenaline whereas VIP and PACAP predominantly stimulate the secretion of adrenaline.