Effect of quinpirole, a specific dopamine DA2 receptor agonist on the sympathoadrenal system in dogs

The effects of quinpirole, a specific dopamine DA2 receptor agonist, were investigated on both cardiovascular responses in conscious dogs and catecholamine release from the adrenal medulla in anesthetized dogs. In conscious normal dogs, i.v. quinpirole (30 micrograms/kg) elicited a decrease in blood pressure and a marked increase in heart rate associated with a rise in plasma catecholamine levels. The increase in heart rate is due to both baroreflex and central mechanisms because a slight but significant positive chronotropic effect persists in sinoaortic denervated dogs (i.e., animals deprived of baroreflex pathways). The central origin of this excitatory effect was confirmed by two subsequent protocols: intracisterna magna injection of quinpirole (5 micrograms/kg) increased blood pressure, heart rate and plasma catecholamines; i.v. domperidone reversed the hypotensive effect of i.v. quinpirole into a pressor response. The rise in plasma catecholamines was associated with an increase in plasma vasopressin levels. In anesthetized dogs, i.v. quinpirole (10 micrograms/kg/min during 12 min), which also decreased blood pressure, failed to modify epinephrine (and norepinephrine) release from the adrenal medulla whatever the stimulation frequencies (1, 3 and 5 Hz) of the sectioned splanchnic nerve. Similar results were obtained with apomorphine (5 micrograms/kg/min during 12 min). These results show that two mechanisms are involved in the action of quinpirole: first, a peripheral depressor action (which elicits the decrease in blood pressure) and secondly, a central pressor component involving an increase in both sympathetic tone and vasopressin release. They also demonstrate clearly that peripheral DA2 receptors are not involved in the control of catecholamine release from the adrenal medulla under in vivo conditions.