Adrenal catecholamine secretory responses in young and adult dogs

To elucidate the secretory sensitivity of the adrenal medulla in young animals, a comparative study of the adrenal secretory reactivity in young and adult dogs was made. In the isolated perfused adrenal gland, acetylcholine, nicotine, muscarine (10(-6)-10(-4) M) and high K+ (15-45 mM) enhanced catecholamine release. The release of catecholamines, expressed per weight of adrenal gland and caused by cholinergic secretagogues, was significantly higher in young dogs than in adult dogs. The proportion of noradrenaline to adrenaline plus noradrenaline in the tissue was higher in young dogs. The proportion of noradrenaline in the effluent was higher by increasing the concentrations of the stimulants, up to a similar level of the tissue noradrenaline/noradrenaline+adrenaline ratio. In in vivo studies, the increase in catecholamine concentration, especially of noradrenaline, in the plasma of femoral arteries, induced by intravenous injection of nicotine, was higher in young dogs than in adult dogs. We also measured the catecholamine-synthesizing enzymes activities in the adrenal medulla. Tyrosine hydroxylase and phenylethanolamine-N-methyltransferase activities were lower in young dogs, but there was no difference in dopamine-beta-hydroxylase activity between young and adult dogs. These results may suggest that young dogs are more sensitive to cholinergic stimulation in spite of their low catecholamine-synthesizing enzyme activity.