PHARMACOLOGICAL, HEMODYNAMIC AND AUTONOMIC NERVOUS-SYSTEM MECHANISMS RESPONSIBLE FOR THE BLOOD-PRESSURE AND HEART-RATE LOWERING EFFECTS OF PERGOLIDE IN RATS

  • 1 January 1984
    • journal article
    • research article
    • Vol. 228  (3) , 779-791
Abstract
In conscious spontaneously hypertensive rats, pergolide (50.0 .mu.g/kg s.c.) produced a sustained decrease in tail artery pressure which was blocked by haloperidol (1.0 mg/kg s.c.) pretreatment. In anesthetized spontaneously hypertensive rats this effect was accompanied by a fall in total peripheral resistance inasmuch as pergolide did not significantly change cardiac output. In anesthetized normotensive rats, pergolid (30.0 .mu.g/kg i.v.) also lowered blood pressure. This effect was not sigificantly modified by adrenalectomy, methysergide, idazoxan (.alpha.2 adrenoceptor antagonist), vagotomy alone or plus ligation of carotid arteries or plus atenolol, but was entirely prevented by domperidone or sulpiride pretreatment and was reverted to a pressor response (due to stimulation of .alpha.-adrenoceptors and 5-hydroxytryptamine receptors) by blockade of ganglionic transmission with chlorisondamine. Pergolide given either i.v. or into the cisterna magna or the lateral cerebral ventricle produced changes in blood pressure of the same magnitude. In intact or adrenalectomized rats, i.v. pergolide significantly lowered plasma norepinephrine concentration. In saline but not sulpride-pretreated pithed rats, pergolide reduced the pressor responses and the accompanying increases in plasma norepinephrine evoked by electrical stimulation of the spinal cord. Pergolide failed to modify the vascular reactivity to several pressor agents and lacked .beta.2 and DA-1 dopamine receptor agonist properties. The decrease in blood pressure produced by pergolide can evidently be accounted for by an inhibition of sympathetic tone resulting from stimulation of peripheral neuronal dopamine receptors. A possible central contribution remains to be substantiated. The pronounced bradycardia produced by pergolide(30.0 .mu.g/kg i.v.) in anesthetized intact rats was partly reduced by vagotomy, methylatropine, domperidone, sulpiride, idazoxan, phentolamine or atenolol. The effects of pergolide in vagotomized rats were further diminished by domperidone but they were blocked by the combination of phentolamine or idazoxan plus domperidone. In rats pretreated with atenolol or in rats with the cervical section of spinal cord and the low level of heart rate increased with an isoprenaline infusion, the decrease in heart rate produced by pergolide was abolished by domperidone, methylatropine or idazoxan. In pithed rats, pergolide changed neither the base-line heart rate nor the tachycardia to exogenous norepinephrine nor the bradycardia evoked by carbachol or electrical stimulation of the peripheral cervical vagus. Pergolide (30.0 .mu.g/kg i.v.), like clonidine (5.0 .mu.g/kg i.v.), inhibited by over 50% the 100 beats/min tachycardia produced by sustained electrical stimulation of the spinal cord. This effect was entirely antagonized by phentolamine or idazoxan but only slightly by sulpiride or domperidone. The fall in heart rate produced by pergolide is evidently the manifestation of an increase in parasympathetic drive and a decrease in sympathetic tone to the sinoatrial pacemaker. The former effect is initiated by the stimulation of central dopamine receptors which appear to be linked to parasympathetic centers vis .alpha.2-adrenoceptors. The inhibition of sympathetic tone is mediated partly by the stimulation of central dopamine receptors and partly by activation of cardiac prejunctional .alpha.2 adrenoceptors.