IN VIVO ESTIMATION OF CHANGES IN DISTAL TUBULE FLOW AND THEIR ROLE IN DEXAMETHASONE-INDUCED KALIURESIS IN CONTROL AND POTASSIUM-ADAPTED RATS

The aim of this study was to determine whether the kaliuresis associated with glucocorticoids is due to a direct tubular action or is secondary to effects of glucocorticoids on distal tubule flow. A whole kidney technique was used to avoid the problem, inherent in microperfusion and micropuncture studies, of deciding whether (all) the appropriate nephron segment(s) are being studied. The method used was to determine the best whole kidney measure of distal tubule flow (the independent variable) by correlating this with the dependent variable (potassium excretion, corrected for differences in plasma potassium, UkV/PIK+) in conscious intact and adrenalectomized control and potassium-adapted rats. After an intragastric potassium chloride load, the correlation of UkV/PIK+ with UkV + UNaV was better than with either UNaV or UV, as measures of distal tubule flow. From the relationship a measure of potassium excretion independent of distal tubule flow can thus be calculated as UkV/PIK+ divided by (UkV + UNaV), defined as UK#. Measurement of UK# clearly demonstrates decreases in potassium excretion with adrenalectomy and increases in potassium excretion with aldosterone and in the potassium-adapted rat, consistent with described changes in potassium secretion. In contrast, with dexamethasone treatment, whilst there was an increase in UkV and UkV/PIK+, there was no change in UK# either in the control of potassium-adapted rats. These results suggest that the kaliuretic effect of dexamethasone cannot be attributed to direct tubular effects of glucocorticoids but rather can be explained by its effect on distal tubule flow.