A paired tracer microinjection technique designed for assessment of single-nephron glucose-calcium interactions in the anesthetized rat.

The first part of this study evaluates a new paired microinjection technique for studying single-nephron permeability (in this case to calcium) following injection of 5-10 nL of a Ringer solution into a superficial proximal tubule. The mean difference in fractional 45Ca recovery from two identical microinjections into the same nephron site was 2.2 +/- 0.2% for 89 paired microinjections. Individual nephrons therefore normally show differences in calcium permeability with time. However, moment-to-moment variations in ion transport in any one nephron are in a random direction; differences cancel one another out if enough experiments are performed. The technique thus appears well suited to studies where comparisons are made between the acute nephron responses to two test solutions. It specifically overcomes problems of nephron heterogeneity seen in some other micropuncture techniques. The second part of this study uses the new technique to investigate the effects of a raised intratubular D-glucose concentration on single-nephron calcium transport. Urinary 45Ca recoveries from late proximal microinjections were significantly higher when D- (as opposed to L-) glucose was included in the injectate (6.87 +/- 0.88 vs. 5.24 +/- 0.50%; p < .02). The ability of D-glucose to depress tubular calcium reabsorption at distal nephron sites may contribute to the observed hypercalciuria following systemic D-glucose loading. It may also be relevant to the acute renal failure accompanying renal stone disease, where a relationship between hypercalciuria, urolithiasis, and the consumption of refined carbohydrates has been proposed.