Microinjection Studies on the Effect of Furosemide on the Rat Nephron

Abstract

The tubular site of furosemide (F) action was studied by the technique of diuretic microinjection (MIJ) into proximal tubules of the rat nephron. F was injected into the last proximal superficial loop of 51 proximal tubules, at the concentration of 3 · 10^-4 mol/l in an infusate that contained ¹⁴C-inulin. Collections were performed at the early distal tubule before and after MIJ. Single nephron filtration rate (SNGFR) remained unchanged, while the percent of filtered volume reabsorbed up to the site of collection was 85 ± 2 before, 79 ± 2% after MIJ, p < 0.005. The calculated concentration of F in the collected distal tubular fluid during the post-MIJ measurements averaged 3 · 10^-5 mol/l. This experiment was repeated by injecting F into the early proximal convolutions of 43 nephrons, while the collections were performed at the last proximal segments. In these studies of proximal volume absorption, SNGFR was 34 ± 3 before, 35 ± 4 nl/min after F (p > 0.6). The respective percent reabsorptions were 70 ± 3 and 73 ± 3% (p ± 0.3). In order to determine whether the technique per se was suitable to detect changes in reabsorption, the proximal MIJ study was repeated by using the carbonic anhydrase inhibitor dichlorphenamide 3 · 10^-5 mol/l in the microinjectate: while SNGFR remained unchanged, percent reabsorption fell from 63+5 to 45 ± 7% during injection of the diuretic, p < 0.03. We conclude that the technique is adequate to examine the effects of drugs, and that F does not reduce proximal volume absorption at concentrations of 3 · 10^-5 mol/l. The loop diuretic decreases distal volume absorption by abolishing the osmotic gradient between blood and tubular fluid along the early distal convoluted tubule.

Effect of Endothelin 1 on Proximal Reabsorption and Tubuloglomerular Feedback

“…The values obtained during MIJ had to be corrected for the dilution operated by the injected fluid using the formulas already published for previous MIJ studies [12].…”

“…More detailed descriptions of the techniques have been published elsewhere [12]. The experimental protocol required two samplings from the same nephrons:…”

“…Since the lower the MIJ rate, the higher is the validity of the technique, we preferred manual injections, since the micropumps could be unsteady at very low rates. Details on this manual microperfusion had been given previously [12].…”

“…For further details the reader is referred to previous publications [12]. The 3 H-methoxyinulin was the systemically infused tracer for GFR and SNGFR measurements.…”

Inhibition of COX-2 counteracts the effects of diuretics in rats

“…Since both inhibition of saluresis/diuresis and of the renin system depend on the dose of rofecoxib, these parameters could serve as possible markers for adjusting the therapeutic dose in patients with Bartter disease. action of prostaglandins in the distal nephron, and indicates that COX-2-derived prostanoids are not involved in the breakdown of the corticomedullary osmolality gradient induced by loop diuretics [28]. Considering the Bartter disease, this study aimed to determine the role of cyclooxygenase-2 in this context.…”