Baroreflex control of heart rate after immediate and prolonged pressure reduction with urapidil in conscious normotensive and spontaneously hypertensive rats.

Baroreflex control of heart rate was studied in conscious normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats before and after acute and prolonged arterial pressure reduction with urapidil, a central and peripheral inhibitor of adrenergic function. While immediate treatment (1 mg/kg i.v.) caused significant reduction in mean arterial pressure (MAP) in both strains, prolonged treatment (20 mg/kg daily by gastric tube for 3 weeks) reduced MAP in the SH rats only. Following an i.v. bolus injection of norepinephrine (10 micrograms/kg) before each mode of treatment, and after immediate and prolonged treatment with urapidil, similar increases in MAP were observed in both strains, but only prolonged MAP reduction caused a significantly increased sensitivity of the reflex control of heart rate in SH rats (P less than 0.01). It is concluded that prolonged, but not immediate, reduction in MAP with urapidil was associated with a significant increase in the baroreflex sensitivity in the hypertensive strain. This may be related to the central effects of the drug.