The cortical and medullary blood flow at different levels of renal nerve activity

The effect of (1) renal denervation and (2) stimulation of the renal nerve on the regional renal blood flow were determined by the Rb uptake method. Under control conditions the total renal blood flow was 3.64±0.09 ml·min^-1·g^-1 tissue increasing significantly (p<0.02) to 4.39±0.28 ml·min^-1·g^-1 after denervation. Upon stimulation of the peripheral portions of the sectioned renal nerves the blood flow decreased almost linearly with the frequency of stimulation reaching 0.99±0.24 ml·min^-1·g^-1 at 10 Hz. Utilizing the relation between blood flow and stimulation frequency the control blood flow correspond to a spontaneous activity of 1.5 Hz. As expected the cortical blood flow responded in the same way as for the total renal blood flow. In the renal medulla denervation gave a much more pronounced response where e.g. the inner medullary flow increased from 0.88±0.09 to 1.30±0.16 ml·min^-1·g^-1, i.e. a 50% increase (p<0.05). Stimulation with 2 Hz produced a steep fall in the blood flow, whereafter it decreased linearly with the stimulation frequency reaching 0.11 ml·min^-1·g^-1 at 10 Hz stimulation. This demonstrates again that the renal medulla is sensitive to renal nerve activity primarily in the low level range. It should be remarked, however, that the 86-Rb uptake method reflects the effective blood flow, which might differ from the blood flow in absolute terms. It is concluded that the renal nerve activity influences the blood flow of all regions of the kidney within the entire range 0–10 Hz. The renal medullary blood flow is affected presumably to a greater extent in the low level range around the basal tone. The sympathetic nerves might then also be important with respect to the urine concentration mechanism.