Relation between active sodium transport and distance along the proximal convolutions of rat nephrons: evidence for homogeneity of sodium transport

Digital image capture and analysis for split-drop micropuncture

A method for automated image capture and analysis of shrinking drop sequences is described. The procedure can be managed by a single operator and allows estimates of proximal tubule volume flux to be calculated during the experiment. Speed of analysis is increased considerably (mean time 1.5 min per sequence) compared with previous methods. The potential exists for measurement of droplets in tubules that follow a tortuous path in the horizontal plane and further increases in capture and analysis rate would allow estimation of volume flux under non-steady state conditions. Film processing is eliminated and the removal of operator errors, bias and fatigue associated with manual measurement, coupled with the greatly reduced time required for analysis of sequences makes the shrinking drop method much more reliable and attractive.

Atrial natriuretic peptide inhibits angiotensin-stimulated proximal tubular sodium and water reabsorption

The discovery that atrial extracts have potent diuretic and natriuretic properties revealed a possible endocrine function of the heart in the regulation of extracellular fluid volume. Since that first report intense research activity has been directed towards determining the mechanism of action of the active atrial natriuretic polypeptides (ANP) found in these extracts. Despite these efforts it remains controversial whether the renal actions of ANP are exerted solely on the process of glomerular filtration or involve additional direct actions on tubular transport. We have investigated the possibility that atrial natriuretic polypeptides may induce natriuresis by suppression of proximal tubular sodium and water reabsorption. Using shrinking split-drop micropuncture combined with simultaneous capillary perfusion in anaesthetized rats we report that 20 nanomolar alpha-rANP (the main component of ANP in rat plasma) added to the peritubular fluid had no direct effect on proximal fluid uptake whereas picomolar angiotensin II had a marked stimulatory action as reported. The stimulatory effect of angiotensin II on fluid reabsorption was inhibited by peritubular ANP at physiological concentrations and abolished by higher concentrations of ANP. Thus at physiological concentrations ANP acts within the kidney to decrease proximal reabsorption by inhibition of angiotensin-stimulated sodium and water transport.

Modulation of proximal tubular reabsorption by angiotensin II

In shrinking-drop micropuncture studies in anaesthetized rats proximal tubular fluid reabsorption (JVa) decreased by 36% following intravenous infusion of enalapril. In a separate group of rats enalapril reduced fractional lithium clearance indicating decreased proximal fluid reabsorption. Following enalapril, GFR rose by 46% but absolute proximal reabsorption rose by 22% indicating 48% effectiveness of proximal glomerulo-tubular balance (GTB). Since renal blood flow and glomerular filtration rate (GFR) increased in parallel and arterial pressure fell, fluid uptake and proximal GTB were unlikely to have been decreased by peritubular physical forces. In anaesthetized rats proximal fluid reabsorption and proximal GTB are modulated by endogenous AII through direct stimulation of proximal tubular transport.

Sodium transport inhibitor in proximal tubular urine during acute volume expansion

Harvested proximal tubular fluid from mannitolsaline expanded rats caused a 50% inhibition of transepithelial sodium concentration difference when compared to an artificially prepared test solution used in the same and nonexpanded animals. Because of the methodology employed, none of the usual factors known to affect sodium reabsorption by the kidney could have been responsible for these changes. The factor responsible acts from the luminal side, is an inhibitor and has a short duration of action.

Dependency of proximal tubular fluid transport on the load of glomerular filtrate

In hydropenic rats, the reabsorption of glomerular filtrate by the proximal convoluted tubules was measured before and after reduction of its intratubular flow rate. Three different protocols were used. (1) In 26 tubules (14 rats), nephron glomerular filtration rate (SNGFR) was varied from 37.2 +/- 7.3 to 20.4 +/- 7.1 nl/min by microperfusing their loops of Henle at 0 to 5 nl/min and 40 nl/min, respectively. This 43% reduction of SNGFR was followed by a 36.0 +/- 23.3% reduction of volume reabsorption rate (P less than 0.001). Between both parameters a linear regression line can be calculated, which is given by y = 0.92 chi + 0.0017. (2) In 17 tubules (14 rats), SNGFR was altered again by feedback from 46.0 +/- 9.7 to 28.8 +/- 9.3 nl/min. The volume resorption from the first half of the proximal convoluted tubule was compared with the reabsorption in its late proximal segments, which were microperfused with proximal tubular fluid at a rate of 20 nl/min. The 36.8% reduction of SNGFR was followed by only a 28.2% reduction of volume reabsorption rate in the first half of the tubule. In the microperfused segments, however, reabsorption remained unaltered. (3) In 29 tubules (21 rats), at the midpoint of proximal convolutions, some of the tubule fluid was removed by a suction pump, and volume reabsorption rate in the late segments was compared with that in the early parts of this tubule, when SNGFR remained stable. The reduction of intratubular flow from 27.7 +/- 8.5 to 14.7 +/- 5.8 nl/min, which is 53% of control, were followed by a reduction of volume reabsorption rate in the late segment to 60.6% control. Between both parameters a regression line was calculated, which is given by y = 0.76 chi +/- 0.01. We conclude that the rate of volume reabsorption by the proximal tubule depends on its intratubular load of glomerular filtrate and, further, that this dependency accounts predominantly for the maintenance of glomerular tubular balance under conditions of hydropenia.

Effect of parathyroid hormone and cyclic adenosine 3',5'-monophosphate on isotonic fluid reabsorption: polarity of proximal tubular cells

Isotomic fluid reabsorption (JV) of rat renal proximal tubules was examined by the shrinking droplet method in combination with simulatneous perfusion of blood capillaries. Sensitivity of JV measurement was improved by using each punctured tubule for control measurements: 1) Parathyroid hormoen (PTH) on the contraluminal cell side reduced JV in a dose-response behavior. The maximal inhibition was achieved at a PTH concentration of 10(-5) M, the half maximal inhibition at a concentration of 3 X 10(-9) M. PTH on the luminal cell side had a small inhibitory effect. 2) Cyclic AMP inhibited JV preferentially when applied to the luminal cell side. On the luminal cell side, both cyclic AMP and dibutyryl cyclic AMP inhibited JV in a similar dose-dependent behavior. Concentrations of both nucleotides as low as 10(-10) M had a definite inhibitory effect. Tested at a high concentration, N6-butyryl cyclic AMP was almost as effective as cyclic AMP. Deoxy cyclic AMP, 5' AMP, cyclic guanosine monophosphate (cyclic GMP), dibutyryl cyclic GMP had no effect. ATP inhibited JV to a very small extent. 3) The reduction of JV after administration of PTH and dibutyryl cyclic AMP was not additive. The similar inhibitory effect of PTH at the contraluminal cell face and of cyclic AMP at the luminal cell face suggests the following sequence of events in the mediation of the action of PTH: 1) activation of adenylate cyclase by PTH in the contraluminal cell membrane, and 2) action of the generated cyclic AMP on the luminal cell membrane. The interaction of cyclic AMP and the luminal cell membrane is initiated at the luminal cell surface.

Dose-dependent stimulation and inhibition of proximal tubular sodium reabsorption by angiotensin II in the rat kidney

The effect of val5-angiotensin II on steady-state sodium concentration gradients (deltacNa) was studied in rat proximal tubules by stationary micro-perfusion combined with perfusion of the peritubular capillaries. Angiotensin added to the peritubular perfusion fluid had a biphasic action with stimulation of sodium reabsorption at low doses (10(-12)-10(-10)M) and inhibition at high doses (3 X 10(-7) - 3 X 10(-6)M). Stimulation of transport was also observed with intraluminal angiotensin but only at a dose of 10(-9)M. Transepithelial potential difference was calculated from the steady-state chloride distribution; no significant change was observed at low (10(-11)M) or high (10(-6)M) concentrations and a direct action on sodium transport is postulated. This biphasic effect is discussed in relation to the responses of the intact kidney to intra-renal infusion of angiotensin, and to the control of tubulo-glomerular feed-back.

Kinetics of active sodium transport in rat proximal tubules and its variation by cardiac glycosides at zero net volume and ion fluxes. Evidence for a multisite sodium transport system

1. Transepithelial Na concentration difference, deltaCNa, across proximal tubules of rat kidney was measured at varying intraluminal Na concentrations (CNainfinity) under conditions of zero net volume and Na flux. Simultaneous stopped-flow intratubular and artificial peritubular capillary perfusion techniques were used together with intratubular raffinose to achieve zero net fluxes. Under these conditions in rat proximal tubules, deltaCNa represents active transport, JactNa, factored by permeability, PNa, plus an electrical factor depending on transepithelial potential difference. 2. The relationship between CNainfinity and deltaCNa appeared sigmoidal with saturation being reached when intratubular Na was above 80 m-mole/kg. In the presence of ouabain (10(-2)M) and scilliroside (10(-3)M) the relationship remained the same. The maximum deltaCNa was reduced by approximately 50% by cardiac glycoside inhibition whereas the half-saturation constant was essentially unchanged. These changes from the control represent simple non-competitive inhibition by the cardiac glycosides. 3. Absence of potential difference (p.d.) measurements precludes exact description of the relation between true active transport and substrate concentration but much evidence indicates that the apparently sigmoid relation in the presence and absence of cardiac glycoside inhibition, would be retained if correction of deltaCNa values were possible. Such results could then be explained if there are at least three or more sites for Na on the pump system, of which at least two are not cardiac glycoside sensitive. They would also unequivocally exclude the presence of a single-site single-pump system or the simple algebraic addition of two such units since the kinetic curves for both would be hyperbolic rather than sigmoidal.

Microperfusion study of the kinetics of reabsorption of cycloleucine in early and late segments of the proximal convolution of the rat nephron

The proximal tubular reabsorptive capacity for the non-metabolizable amino acid, cycloleucine, was studied in the rat nephron by stationary microperfusion. Tubular reabsorptive rates were greatest near the glomerulus and declined progressively along the convolution. A kinetic analysis of cycloleucine reabsorption in terms of luminal concentration revealed that this reduced transport rate was associated with an increase in the half-saturation constant of the kinetic curve, rather than a decrease in the maximum transport capacity. Since our previous findings with the metabolizable amino acid, -L-histidine, were identical we can conclude that this decline in reabsorption of neutral amino acids as a function of distance along the convolution is an intrinsic property of the transport system and is not related to tubule cell amino acid metabolism. The transport curves for cycloleucine absorption did not give a simple Michaelis-Menten relation but rather followed a course suggesting that more than one transport system might be involved.