Micropuncture study of diuretic effects on sodium and calcium reabsorption in the dog nephron


between the clearance rates of sodium and calcium under a variety of diuretic conditions. The thiazide di-uretics act differently in dissociating the renal tubular reabsorption of sodium and calcium. This phenomenon has been further investigated using recollection micro-puncture and clearance techniques in a group of 14 dogs subjected to three consecutive experimental phases: expansion to 3 % of body weight (BWt) with Ringer's solution, chlorothiazide infusion at 20 mg/kg/h, and furosemide in a prime of 10 mg/kg and a 10 mg/kg/h infusion. Diuretic losses were balanced with infusion of equal volumes of Ringer's solution throughout the experiment. Chlorothiazide increased the fractional ex-cretion (FE) of sodium almost threefold awhile FEc. was not significantly altered. Furosemide increased FENa and FEc. to an approximately equal, and more marked, degree. This dissociation of sodium and calcium reabsorption after chlorothiazide was also evident in the superficial distal tubule, where (tubule fluid/plasma sodium) (TF/PN.) increased from 0.32 to 0.49 (P < 0.01) and TF/(ultrafiltrate)UFca was unchanged (0.35-0.31). Furosemide markedly reduced the trans-tubular concentration gradient for both sodium (0.86) and calcium (0.94). TF/Pinut.n decreased progressively from 3.79 to 2.78 to 2.33 in three phases. In the late proximal tubule, chlorothiazide induced a fall of TF/ PinulIn from 1.57 to 1.44 (P < 0.01), but the ratio TF/ UFc.: TF/PNa was unchanged. Furosemide had no sig-nificant proximal effect. It is concluded that acute ad-ministration of chlorothiazide reduces sodium reab-sorption in the distal nephron, presumably the cortical diluting segment, without affecting calcium reabsorption

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