Augmented bicarbonate reabsorption by both the proximal and distal nephron maintains chloride-deplete metabolic alkalosis in rats.

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Abstract
Whether augmented bicarbonate reabsorption by renal tubular epithelium contributes to the maintenance of chloride-deplete metabolic alkalosis is not clear. This study used free-flow micropuncture to investigate bicarbonate reabsorption by surface nephron segments in a rat model of diuretic-induced alkalosis compared to control. The proximal and distal nephron of the alkalotic animals had higher values for both delivered load to and absolute reabsorption from these segments. The proximal tubules of alkalotic and control animals had similar values for the slopes of the linear regression of delivered load vs. reabsorption and for the bicarbonate tubular fluid to plasma (TF/P) ratio at the late proximal tubule. By contrast, the corresponding analysis for the distal segment of alkalotic animals revealed a greater slope (0.98 vs. 0.81, P less than 0.003) and a smaller bicarbonate TF/P ratio at the late distal tubule (0.10 vs. 0.16, P less than 0.006). The data indicate that augmented bicarbonate reabsorption by both the proximal and distal nephron contributes to maintaining the alkalosis of this model. The data suggest primary stimulation of bicarbonate reabsorption in the distal nephron and load-dependent reabsorption in the proximal tubule.