Changes in Renal Blood Flow and Possibly the Intrarenal Distribution of Blood during the Natriuresis Accompanying Saline Loading in the Dog*

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Abstract
The effects of isotonic expansion of the extracellular volume on filtered and excreted Na, renal blood flow, and extraction ratios of p-aminohippurate (PAH), Diodrast, and Hippuran were studied in anesthetized dogs. During saline loading renal blood flow increased and the extraction ratio of PAH (EpAH) decreased. In some experiments spontaneous changes in the net tubular reabsorption of Na were observed during the course of saline loading. Changes in net tubular reabsorption of Na during the natriuresis of saline loading could be demonstrated also by unilateral reduction of renal blood flow. Decreased reabsorption of Na was always accompanied by decreased EpAH and increased renal blood flow, whereas increased reabsorption was accompanied by increased EPAH and decreased blood flow. The fall in EPAH during saline loading did not appear to be due to diminished ability to transport the substance, and it is suggested that the changes in the extraction ratio may relate in part to changes in noncortical plasma flow, which could include medullary blood flow. A causal relationship may exist between these hemodynamic changes and the associated changes in Na reabsorption. These observations are consistent with the suggestion that diminished net tubular reabsorption of Na during saline loading may relate in part to an increased medullary blood flow.