1. Experiments were conducted in two-kidney, one-clip renal vascular hypertensive rats (GHR) to assess the responses of each kidney to acute treatment with the antihypertensive calcium channel blocking agent verapamil in the presence and in the absence of converting enzyme inhibitor (CEI). One group of GHR (0.2 mm inner diam. clip 3 weeks before study) were examined during a control period, and during a second period of infusion of verapamil (600 μg h−1 kg−1). A second group of GHR were examined during a control period, during CEI (teprotide, 3 mg h−1 kg−1) infusion and during a third period of verapamil (600 μg h−1 kg−1) infusion superimposed on CEI infusion.

2. Although systemic blood pressure (BP) decreased from 175 ± 4 to 149 ± 5 mmHg (mean ± SEM) in response to verapamil alone, renal blood flow for non-clipped kidneys increased from 5.9 ± 0.4 to 6.5 ± 0.3 ml/min, indicating a 30% reduction of renal vascular resistance (P values 0.01; n = 9). Glomerular filtration rate (GFR) for non-clipped kidneys (n = 24) increased from 0.91 ± 0.09 to 1.47 ± 0.14 ml/min and filtration fraction increased from 0.32 ± 0.04 to 0.47 ± 0.03 (P values 0.05). Urine flow rate and absolute and fractional sodium excretion for non-clipped kidneys increased. GFR for clipped kidneys decreased during verapamil.

3. Treatment with CEI alone resulted in nearly identical responses of BP and function of the non-clipped kidney, except filtration fraction was unchanged. The addition of verapamil to ongoing converting enzyme blockade tended to augment the increased GFR of the non-clipped kidney.

4. Plasma renin activity (PRA) increased from 30 ± 3 to 59 ± 7 ng of angiotensin (ANG) I h−1 ml−1 with verapamil alone, a significantly larger increment than the increase of PRA from 27 ± 5 to 39 ± 9 ng of ANG I h−1 ml−1 in GHR subjected to comparable blood pressure reduction by mechanical aortic constriction.

5. Verapamil resulted in many similar effects on renal function to those observed during blockade of converting enzyme. The increased filtration fraction observed in response to verapamil may be the result of vasodilatation of the afferent arteriole or of an increase in the glomerular ultrafiltration coefficient.

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