Prolonged central effects of quinpirole on cardiovascular regulation

Central cardiovascular effects of the dopamine D2 receptor agonist quinpirole were studied in conscious rats. The i.v. injection of 0.3 mg/kg of quinpirole in spontaneously hypertensive rats (SHR) caused a rapid but short-lasting increase in blood pressure. Heart rate showed little change. Pretreatment with the centrally acting selective dopamine D2 receptor antagonist raclopride, but not the D1 antagonist SCH23390, completely prevented the rise in blood pressure. A second injection of quinpirole, 30 min after the first injection, induced little change in blood pressure, although at 4 or 24 hr after quinpirole treatment, we observed partial and complete recovery of the pressor response, respectively. This pattern of desensitization was similar to that seen after administration of the dopamine D2 receptor agonists N-propylnorapomorphine (0.3 mg/kg) or quinelorane (0.1 mg/kg), and was similar in spontaneously hypertensive rats, Wistar Kyoto and Sprague-Dawley rats. At 30 min after treatment with quinpirole, the hypotension induced by i.v. injection of clonidine (0.01 mg/kg) or of 8-hydroxy-dipropylaminotetralin (0.1 mg/kg) was markedly reduced when compared to that in saline-pretreated spontaneously hypertensive rats, suggesting a prolonged effect of quinpirole at the level of sympathetic regulation. The rapid fall in blood pressure caused by i.v. injection of the ganglion blocker pentolinium (10 mg/kg) was slightly, but significantly enhanced by treatment with quinpirole, which suggests an overall prolonged increase in resting sympathetic vasomotor tone. This would be difficult to reconcile with an inhibition of the action of sympatholytic drugs, unless it is hypothesized that the increase in sympathetic vasomotor tone was differential between different sympathetic beds or different neuronal populations in the brain. This may prohibit any additional pressor responses and, through a central feedback mechanism, may inhibit the action of sympatholytic drugs. No evidence was found for lasting changes in circulating levels of vasopressin, angiotensin or atrial natriuretic factor, nor were there changes in hematocrit. Cardiac sympathetic tone appeared to be enhanced, although vagal tone was normal and no major changes in baroreflex sensitivity were observed.