Localization of the nephron site of gentamicin-induced hypercalciuria in the rat: a micropuncture study

Abstract

In vivo renal micropuncture techniques were used to locate the nephron site of hypercalciuria induced by acute gentamicin infusion in anaesthetized Sprague Dawley rats. Three series of experiments were conducted. The effect of gentamicin on calcium reabsorption in the proximal tubule (Series I) and loop of Henle (Series II) was investigated using in vivo microperfusion whereas the effect on distal calcium handling (Series III) was studied using in vivo microinfusion. In all three experimental series, acute systemic gentamicin infusion at 0.28 mg kg(-1) min(-1) caused significant hypercalciuria within 30 min of commencing drug infusion. Gentamicin had no effect on the rates of urine flow or sodium excretion. Acute gentamicin infusion had no effect on unidirectional calcium reabsorption in the proximal tubule or loop of Henle despite a simultaneous and highly significant hypercalciuria at the whole kidney level. Net fluid reabsorption was also unaffected by the drug in these nephron segments. Acute gentamicin infusion significantly increased the urinary recovery of calcium following microinfusion into early distal tubules, whereas urinary calcium recovery was decreased after microinfusion into late distal tubules. We conclude that acute gentamicin-induced hypercalciuria is mediated by a decrease in calcium reabsorption in the early distal tubule. Thus, the acute hypercalciuric effect of gentamicin occurs at a different nephron site to the nephrotoxic effects associated with longer-term administration of the drug. It is, therefore, unlikely that gentamicin-induced hypercalciuria is involved in the pathogenesis of subsequent proximal tubular cell injury.

Figure 1

Experimental protocol. There were three urine collection periods (A–C) for proximal and loop microperfusion experiments but only two (A,B) for the distal microinfusion experiments. Micropuncture was performed during the last 2 h of the experimental period. Tubular transit times were assessed and plasma samples obtained at the times indicated by the arrows.

Figure 2

Proximal micoroperfusion technique. Superficial proximal tubules were microperfused at a rate of 25 nl min⁻¹ as illustrated for a period of 3–5 min. The tubule was then filled with a silicone rubber compound (Microfil) and a cast of the perfused segment was subsequently dissected from the kidney for determination of the length of tubule perfused.

Figure 3

Urinary recovery of ⁴⁵Ca from micropuncture kidney during microinfusion of early and late distal tubules in animals infused with saline (controls) or gentamicin (GEN) at 0.28 mg kg⁻¹ min⁻¹. (n)=number of tubules infused in each experimental group. ^***P=<0.001; ^**P=<0.01 comparing GEN with respective control group (Student's t-test).