Calcium and magnesium transport along the inner medullary collecting duct of the rat.

We have measured calcium and magnesium transport along the inner medullary collecting duct (IMCD) in rats by the microcatheterization technique. The fraction of filtered divalent cation ((F/P)Ca,Mg/In) as a function of IMCD length was analyzed by linear regression. In eight control rats calcium reabsorption was demonstrated, inasmuch as there was a significant correlation and slope, P less than 0.001. (F/P)Ca/In decreased from 2.2 +/- 0.3% at the beginning of the IMCD to 0,84 +/- 0.1% in the urine. (F/P)Mg/In was not changed along the IMCD, After acute thyroparathyroidectomy (TPTX) (F/P)Ca/In increased to 4.6 +/- 0.4% at the beginning of the IMCD and 1.55 +/- 0.1% in the urine. The slope was not significantly different from the control group. (F/P)Mg/In was unchanged and was not different from the control group. Accordingly, net Ca reabsorption occurs along the IMCD and the fractional Ca reabsorption is not altered by TPTX. Net Mg transport along the IMCD was absent in both control and TPTX rats; The increased calciuresis after TPTX is not dependent on IMCD function nor is Mg excretion under the conditions studied.