Micropuncture nephron

Micropuncture techniques have been applied to the direct investigation of the effect of diuretics on single nephron function. The observed changes in tubular fluid reabsorptive rates and electrolyte concentrations can be used to asses the mechanism of action. The... 

Both classical transport systems are located exclusively in the proximal tubule of the nephron. Several techniques such as visual observations, stop-flow experiments, tubular micropuncture, in vivo and in vitro tubular microperfusions have demonstrated this particular transport capacity of the proximal tubular segment of the nephron [19]. Secretion entails an increase of proximal cell concentration of transported substrates that may become higher than in interstitium, and in some case may result in nephrotoxicity [40-42]. 

Infusions of AmB, intravenously or into the renal artery, induce short-term reduction in renal blood flow (RBF) and GFR, and an increase in renal vascular resistance, in both rats and dogs [83-85]. The effects of short term infusions of AmB on the renal microcirculation in rats revealed that the single nephron GFR was decreased by 2 mechanisms (Table 1) 1) a decrease in single nephron plasma flow, due to vasoconstriction of the afferent and efferent arterioles, and 2) a reduction in the glomerular capillary ultrafiltration coefficient (Kf), an effect probably mediated by mesangial cell contraction [86]. Previous micropuncture studies demonstrated a similar vasoconstriction of the afferent arteriole but also an increased permeability of the tubular epithelium to inulin [75]. Thus, the reduction in GFR after acute AmB infusions can be attributed to contraction of afferent smooth muscle cells, efferent smooth muscle cells and glomerular mesangial cells, as well as increased tubular permeability with back-leak... 

More recent studies provided evidence against a role for TGF in acute AmB nephrotoxicity. In contrast to its inhibition of TGF activity, theophylline prevented the acute renal responses to AmB by a mechanism unrelated to adenosine receptor antagonism [102]. Furthermore, micropuncture studies revealed that the AmB-induced reduction in single nephron GFR was the same irrespective of whether TGF was active or interrupted (by measuring GFR from distal and proximal tubular collections, respectively) [96]. The latter study also showed that distal tubular chloride ion concentrations were not increased by AmB, which indicated that the signal for TGF was unchanged. 

The question, which parts of the nephron are responsible for the secretion or the reabsorption of urate cannot be easily solved by studies in man. Techniques giving direct access to renal tubules, such as tubular micropuncture and microperfusion (in vivo)... 

The technique of free flow micropuncture consists in the collection of fluid from different sites of the nephron. For urate, only data from superficial nephrons are available. 

The proximal and distal convoluted tubules of tliese nephrons reach the surface of the cortex. The pars recta of proximal tubules is not accessible. Progress in the micropuncture field was slow because a very sensitive method for the determination of urate was needed, the volume of tubular fluid collected being small (0.1 //liter) and the plasma urate concentration being low (table 1). Furthermore, all mammalian species have uricase activity... 

CO2 can be hydrated spontaneously, but in the tubular cell the conversion of CO2 into H2CO3 is catalyzed by an enzyme called carbonic anhydrase. Carbonic anhydrase activities are greater in the cortical portion of the kidney than in the medulla. This is in agreement with micropuncture and stop-flow studies, which suggest that the urine is acidified in the distal part of the nephron, possibly in the collecting ducts. Yet the proximal tubule may also participate in hydrogen secretion. 

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