The effects of two calcium antagonists (nifedipine and verapamil) on renal function in sheep

The effects of the calcium antagonists, nifedipine and verapamil, were measured in five conscious Merino ewes at a dose rate of 0.5 microgram min-1 kg-1 for 60 min. Nifedipine caused a significant fall in mean arterial blood pressure (MABP) and central venous pressure (CVP) and an increase in heart rate (HR). There was no significant effect on renal vascular resistance (RVR) but, with the fall in MABP, renal plasma flow (RPF) decreased. There was a significant rise in glomerular filtration rate (GFR) and a small rise in filtration fraction (FF). Solute excretion, urine osmolality and solute-free water reabsorption (Tc, H2O) increased and urine flow (V) decreased. Plasma potassium (PK) and osmolality (Posm) decreased. Verapamil produced a similar fall in MABP and also had similar effects on the other parameters, except that in this case RVR was significantly reduced and there were significant increases in RPF and GFR. The rise in GFR which occurred regardless of the change in RPF, can account for the increase in solute excretion, although an effect of the calcium antagonists on tubular function cannot be excluded.