Renal blood flow, sodium excretion, and concentrating ability during saline infusion

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Abstract
During rapid infusion of isotonlc saline (1,500 ml in 30 min), p-aminohippurate clearance (CPAH) increased 20% and PAH extraction (EPAH) decreased from 83 to 71%. Estimated non-cortical plasma flow increased strikingly, from 49 to 129 ml/min. When the same volume of saline was given more slowly (1,500 ml in 2 hr.), CPAH increased 9% whereas Epah decreased only from 81 to 78%. Estimated noncortical plasma flow increased only slightly, from 48 to 65 ml/min. The differing responses of total and noncortical plasma flow to rapid and slow infusion seem most closely related to the rapidity of initial expansion, although the duration of saline infusion may also be a factor. Despite greater noncortical plasma flow, medullary sodium content and negative free-water clearance (TH2O) were higher during rapid saline infusions. This suggests that sodium transport in the loop of Henle was greater during rapid infusion. In another series of experiments, it was found that sodium excretion increases during slow saline infusion, despite reduction of total and noncortical plasma flow, as well as filtered sodium, by aortic clamping. These studies demonstrate that the natriuresis of saline loading is not caused by increased total or noncortical renal plasma flow.