CALCIUM-TRANSPORT IN THE PROXIMAL CONVOLUTED TUBULE AND LOOP OF HENLE OF RATS MADE DIABETIC WITH STREPTOZOTOCIN

In-vivo microperfusion was used to localize the reabsorptive defect responsible for the hypercalciuria of diabetes mellitus and to investigate possible causative factors. Unidirectional proximal calcium absorption was not significantly different in rats made diabetic with streptozotocin compared with controls, providing evidence against the involvement of this nephron segment in the phenomenon. Calcium absorption by the loop of Henle, was however, significantly (P < 0.01) lower in diabetic animals (32.1 +/- 1.2 vs 40.4 +/- 0.6 pmol/min). Based on our knowledge of calcium movements within the loop, it is likely that the reabsorptive defect resides within the thick ascending limb. The calcium lesion was found to be independent of acute changes in intraluminal glucose concentration and could not be corrected by acute insulin treatment. The study also provides new information on the relationship between intratubular glucose and fluid movements in the rat nephron. In diabetic rats a proximal perfusate containing 30 mmol glucose/1 resulted in fluid absorption comparable with that seen in control rats perfused with 5 mmol glucose/1. However, intraluminal glucose had a stimulatory effect on fluid absorption in the loop of Henle of diabetic rats (10.7 +/- 0.5 vs 7.9 +/- 0.4 nl/min; P < 0.01).