Analysis of the dynamics of renal vascular resistance and urine flow rate in the cat following electrical stimulation of the renal nerves

In ten sino-aortic denervated, vagotomized and anaesthetized cats, renal efferent nerves were stimulated for 30 s with trains of constant current pulses at frequencies in the range 5-30 Hz. The arterial pressure, heart rate, urine flow rate (electronic drop counter) and renal blood flow (electromagnetic technique) were recorded. Subsequent computer processing gave the true means of renal artery pressure (MRAP) and renal blood flow (MRBF) and hence the renal vascular resistance (MRVR), over each cardiac cycle. Recovery of MRVR after the end of stimulation exhibited two distinct time constants. The fast component had a time constant of 2.03 ± 0.26 s and represented 60.2 ± 1.71% of the recovery. The time constant of the slower component was 14.1 ± 1.9 s and represented 36.0 ± 1.6% of the recovery. The relationship between MRVR and stimulus frequency was sigmoidal with maximum sensitivity at stimulus frequencies of 12.6 ± 0.76 Hz. Changes in urine flow rate, in contrast, followed a hyperbolic function with maximum response sensitivity occurring at very low stimulus frequencies. Changes in urine flow rate were 50% complete at stimulus frequencies of 5 Hz. Identification of two distinct components in the relaxation phase of renal vascular resistance leads to a reasonable hypothesis that 60% of total renal vascular resistance may lie proximal to the glomerulus, whereas 36% may be accounted for by the efferent arterioles.