Glomerular filtration and tubular absorption of the basic polypeptide aprotinin.

The basic polypeptide aprotinin (Ap), mol. wt 6500, pI 10.5, is filtered in the glomeruli, virtually completely taken up by the proximal tubular cells and retained there for many hours. This process was studied in rats by determining the renal plasma clearance (CAp) as the amount of [125I]Ap accumulated in the kidney plus that excreted in the urine per unit of time divided by the integrated plasma concentration. In periods lasting 4-20 min after i.v. bolus injection or infusion to constant plasma concentration, CAp was 65% of glomerular filtration rate (GFR) estimated as kidney plus urinary clearance of [51Cr]EDTA (Ccr-EDTA). Less than 0.8% of the filtered Ap appeared in the urine. CAp varied inversely with plasma protein concentration in mg ml-1: CAp/Ccr-EDTA = 0.98-0.0058 x Ppr, corresponding to a glomerular Gibbs-Donnan distribution for a net molecular charge of +6, in agreement with the amino acid composition of Ap. CAp (kidney + urinary) was not altered by inhibiting tubular uptake of [125I]Ap by maleate or by exceeding the uptake capacity with large doses of unlabelled Ap. Neutralized Ap (malonylated) did not accumulate in the kidney, but showed a urinary clearance indistinguishable from that of [51Cr]EDTA. Both CAp and Ccr-EDTA were reduced to 0.04 ml min-1 when glomerular filtration pressure was lowered by ureteral stasis and increased Ppr (80-90 mg ml-1). These findings indicate: (1) no steric or charge restriction to filtration of Ap in the glomerular membrane, (2) the Gibbs-Donnan equilibrium should be considered when estimating glomerular sieving of charged polypeptides in intact animals (3) charge dependent tubular uptake, (4) little or no transtubular transport of intact Ap, (5) no appreciable tubular uptake of Ap from the peritubular side and (6) local renal accumulation of Ap in a period of up to 20 min may be used to estimate local glomerular filtration and/or local proximal tubular reabsorption rates. Model analysis based on the appearance of 125I in plasma, the time course of renal Ap content, and literature data on subcellular Ap distribution are consistent with two populations of endosomes, transporting Ap at widely different rates from the proximal tubular brush border to the lysosomes where breakdown occurs at a high rate.

Myogenic vasoconstriction in the rat kidney elicited by reducing perirenal pressure.

Autoregulation of renal blood flow is generally believed to result from tubuloglomerular feedback and/or a vascular myogenic mechanism, but there is no consensus on the relative importance of these mechanisms. We designed an experiment in which tubuloglomerular feedback would tend to oppose a myogenic response: the denervated kidney in situ was enclosed in an airtight chamber and exposed to a 35 mmHg subatmospheric pressure for 1 to 10 minutes. Renal blood flow recorded by an electromagnetic flowmeter fell by 33% in the course of a few seconds. Renal venous concentration of inulin showed no consistent change, indicating similar reduction in glomerular filtration rate. Since urine flow also fell, it is likely that the tubular flow rate was reduced. The kidney volume expanded by 10-20%, and subcapsular interstitial fluid pressure was reduced from 6.8 to -8.6 mmHg. Arterial pressure remained unchanged, while renal venous pressure inside the chamber fell from 9.4 to 5.8 mmHg. Normalization of perirenal pressure gave rapid normalization of all parameters. Elevation of ureteral pressure attenuated or even prevented the renal blood flow reduction. Renal decapsulation or sympathetic blockade by phentolamine, or infusion of furosemide or 0.9% NaCl to inactivate tubuloglomerular feedback, did not prevent the renal blood flow reduction. We interpret the results to indicate that myogenic vasoconstriction greatly overpowered TGF and even surpassed the constriction predicted by a mathematical model based on maintenance of the preglomerular wall tension as estimated from transmural pressure.

A multinephron model of renal blood flow autoregulation by tubuloglomerular feedback and myogenic response.

Tubuloglomerular feedback implies that a primary increase in arterial pressure, renal blood flow, glomerular filtration and increased flow rate in the distal tubule increase preglomerular resistance and thereby counteract the primary rise in glomerular filtration rate and renal blood flow. Tubuloglomerular feedback has therefore been assumed to play a role in renal autoregulation, i.e., the constancy of renal blood flow and glomerular filtration at varying arterial pressure. In evaluating this hypothesis, the numerous tubular and vascular mechanisms involved have called for mathematical models. Based on a single nephron model we have previously concluded that tubuloglomerular feedback can account for only a small part of blood flow autoregulation. We now present a more realistic multinephron model, consisting of one interlobular artery with an arbitrary number of evenly spaced afferent arterioles. Feedback from the distal tubule was simulated by letting glomerular blood flow exert a positive feedback on preglomerular resistance, in each case requiring compatibility with experimental open-loop responses in the most superficial nephron. The coupling together of 10 nephrons per se impairs autoregulation of renal blood flow compared to that of a single nephron model, but this effect is more than outweighed by greater control resistance in deep arterioles. Some further improvement was obtained by letting the contractile response spread from each afferent arteriole to the nearest interlobular artery segment. Even better autoregulation was provided by spreading of full strength contraction also to the nearest upstream or downstream afferent arteriole, and spread to both caused a renal blood flow autoregulation approaching experimental observations. However, when the spread effect was reduced to 25% of that in each stimulated afferent arteriole, more compatible with recent experimental observations, the autoregulation was greatly impaired. Some additional mechanism seems necessary, and we found that combined myogenic response in interlobular artery and tubuloglomerular feedback regulation of afferent arterioles can mimic experimental pressure-flow curves.

Renal autoregulation: models combining tubuloglomerular feedback and myogenic response.

As shown previously, autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR) at varying arterial pressure may result from a myogenic response (MR) acting to maintain wall tension in each preglomerular vessel segment. We now combine MR with tubuloglomerular feedback (TGF) responding to distal tubular flow rate. The model consists of preglomerular and postglomerular resistances, glomerular filtration, and a tubular system. TGF acting on preglomerular resistance with parameters that mimic responses to single nephron distal tubular flow rate in rats and dogs failed to account for the autoregulation of RBF and GFR observed experimentally. Good autoregulation was obtained by adding preglomerular MR. In this combination, TGF is activated mainly in the lower range of autoregulation. Addition of mechanisms that increase postglomerular resistance or increase the glomerular filtration coefficient at reduced arterial pressure impairs RBF autoregulation, whereas GFR autoregulation is only slightly improved. TGF regulation of pre- and postglomerular resistance in the same direction seems compatible with good autoregulation only when combined with a preglomerular myogenic mechanism.

A mathematical analysis of the myogenic hypothesis with special reference to autoregulation of renal blood flow.

To test the hypothesis that autoregulation of renal blood flow could result from myogenic regulation of arterial/arteriolar wall tension, we have explored a model based on the assumptions that (1) each preglomerular vessel segment reacts to a change in transmural pressure by altering its internal radius until the initial change in wall tension is reduced by a gain factor, (2) postglomerular structural resistance remains unchanged, (3) extravascular tissue pressure equals intrarenal venous pressure, and (4) the renal vascular system can be represented by one unbranched tube. General equations were obtained for flow and segmental radii and pressure as functions of aortic pressure. With a gain factor of 1 and a glomerular capillary pressure of 50% of aortic pressure under control conditions, the model predictions agree well with experimental data in dogs. Increasing aortic pressure from about 60% of control level causes only slight increase of blood flow. A rise in tissue pressure up to 40% of aortic pressure causes only moderate reduction. Changes in vessel radii begin in proximal vessel segments and spread distally toward glomerulus at increasing changes in aortic and tissue pressures from their control levels. Glomerular capillary pressure is autoregulated in proportion to blood flow. The degree of autoregulation is only moderately dependent on the gain factor: A moderate impairment caused by reducing the gain factor from 1 to 0.7 may be compensated by locating the myogenically responsive wall layer a distance 0.2 times the internal radius from the vessel lumen. "Superautoregulation," i.e., a rise in flow at reduced aortic pressure, is not possible. An upper limit of autoregulation is obtained only with the additional assumption of a fall in contractile force at extreme shortening of the muscle fibers. No definitive biological proof has yet been provided for a segmental wall tension-regulating mechanism in the preglomerular vessels, and obviously its existence cannot be proved by a mathematical model. However, if such a mechanism does exist, it can explain most of the renal resistance changes at varying arterial and intrarenal pressures, as well as the observed autoregulation of terminal interlobular arterial pressure.

Capillary transport of H2 gas generated locally in renal tissue.

Previous measurements by microspheres have shown a higher blood flow in outer cortex and a lower blood flow in inner cortex than found by diffusible tracers. During vasodilation microspheres have indicated a disproportionate increase in deep cortical blood flow, whereas diffusible tracer distributions remained unchanged. These discrepancies could possibly be explained by a variable net inward transport of diffusible tracers in postglomerular vessels, the transport existing in control, but disappearing during vasodilation. To test this hypothesis H2 gas was produced electrolytically for 1 s at a platinum electrode in midcortex and the resulting gas concentration curve measured polarographically at two electrodes placed above and below the source. Analysis of a mathematical model showed that the ratio of the curve maxima at the two electrodes (Cmo/Cmi) would best reveal a radial net transport. Average Cmo/Cmi at 25 positions in 7 clamped dog kidneys was close to unity, but rose to 1.24 at control flow. During acetylcholine infusion Cmo/Cmi rose to 1.68. Local washout rates at the two electrodes increased equally. Calculations indicated a small outwardly directed net transport in control (3 X 10(-4) cm/s), becoming slightly reinforced during vasodilation (5 X 10(-4) cm/s). Thus the control transport direction is opposite to the hypothesis, and the change during vasodilation was estimated to be too small to explain the disparity between diffusible tracer uptake and microsphere distribution in control. H2 concentration maximum was obtained earlier under control flow than in the clamped kidney, indicating an increase in apparent D of the gas in tissue from 3 X 10(-5) cm2/s to 5 X 10(-5) cm2/s, probably due to mixing of H2 gas in the capillary net work.