Effects of verapamil and nifedipine on systemic hemodynamics in spontaneously hypertensive rats

Verapamil (1 mg/kg, i.v.) and nifedipine (0.3 mg/kg, i.v.) were tested at equi-antihypertensive doses for systemic hemodynamic responses in conscious spontaneously hypertensive rats (SHR) using the Fick method. Systemic hemodynamic effects of these agents were also evaluated in areflexic, spinal cord-transected and vagotomized SHR using the electromagnetic flowmetry technique. Both verapamil and nifedipine lowered mean arterial pressure (MAP:verapamil = -24%; nifedipine = -28%) in conscious SHR by decreasing total peripheral resistance (TPR:verapamil = -48%; nifedipine = -59%) with a concomitant rise in cardiac output (CO: verapamil = 48%; nifedipine = 86%) and stroke volume (SV:verapamil = 54%; nifedipine = 65%), but verapamil prevented tachycardia, whereas nifedipine increased heart rate (HR:13%). Verapamil and nifedipine also altered systemic hemodynamics in the areflexic SHR; verapamil reduced MAP (-31%) by reducing CO (-18%) with associated bradycardia (-25% HR), whereas nifedipine also lowered MAP (-21%) by decreasing TPR (-18%) without changes in CO and HR. It is concluded that, firstly, the antihypertensive action of verapamil and nifedipine in conscious SHR is due to systemic vasodilation that is associated with reflexly increased CO; secondly, that verapamil has a direct negative chronotropic effect, but nifedipine appears to be devoid of such an effect, and finally that the ability of verapamil to decrease TPR may depend upon resting sympathetic tone.