Effect of prolonged treatment with adrenergic neuron blocking drugs on sympathoadrenal reactivity in rats

The effects of repeated high doses of the adrenergic neuron blocking drug guanethidine or a hexahydropyrazinoindole compound (2-guanyl-1,2,3,10,10a, hexahydro-1,2,a-pyrazinoindole, EMD 21192) (30 mg/kg i.p., 21.5 mg/kg i.p. respectively, equimolar doses) on sympathoadrenal activity were investigated in normotensive adult rats. During treatment for 5 weeks with either guanethidine or EMD 21192 the systemic blood pressure fell steadily. Noradrenaline content in the heart and vas deferens were decreased markedly by guanethidine and to a much less degree by EMD 21192. EMD 21192 markedly lowers the catecholamine content of the adrenal medulla, presumably as a result of inhibition of dopamine-beta-hydroxylase. The plasma catecholamine concentrations reflected the different sites of action of the drugs in the sympathoadrenal system, i.e. guanethidine mainly reduced circulating norepinephrine and dopamine-beta-hydroxylase by more than 50%, whereas EMD 21192 decreased considerably by the total catecholamines (mainly epinephrine) without altering significantly in the plasma norepinephrine. Disappearance or reduction of fluorescent nerve endings in the iris and the heart and a decrease of the intensity of fluorescence in chromaffin cells of the adrenal gland caused by the drugs were consistent with the biochemical alteration. Whereas the repeated doses of guanethidine caused degeneration of sympathetic nerves, destruction of adrenergic neurons was not found after prolonged treatment with EMD 21192.