Effects of hypoproteinemia on the flow and composition of lymph in awake sheep.
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Animals↗
Biomedical subjects
E M Renkin
Publications and source records attributed to E M Renkin.
Limits to steady-state lymph flow rates derived from plasma-to-tissue uptake measurements.
Upper and lower limits to basal lymph flow rates per unit tissue mass calculated from measurements of initial tissue uptake rates of 131-I albumin and steady-state lymph/plasma or interstitial fluid/plasma concentration ratios of endogenous albumin are compared with published measurements of lymph flow. The upper limit corresponds to purely convective transport of albumin, the lower to purely diffusive transport. Of a total of nine feasible comparisons, five show good agreement defined by the measured value falling at or between the calculated limits. These include two direct comparisons in the same or closely similar animal species and experimental conditions, and three cross-specific comparisons. Disagreement, with measured lymph flows much higher than the estimated upper limit, was observed in three comparisons, one of which was direct and conspecific and two that were cross-specific. In one instance, also cross-specific, reported lymph flows were below our minimal estimate. We suggest that estimates of lymph flow derived from tissue uptake measurements may be useful in setting limits to lymph turnover of fluid and plasma proteins in tissues and organs from which it is not feasible to collect lymph. They may also provide a means of evaluating experimentally induced disturbances of lymph drainage and over- or underestimation of the tissue mass from which lymph is collected, these being serious experimental problems associated with lymphatic cannulation and lymph collection. If both direct and indirect measurements can be made in the same tissue or organ under the same conditions, the relative contributions of convective and dissipative (diffusion and vesicular) transport processes to protein extravasation can be evaluated from the relation of the directly measured flow to the two calculated limits. These requirements were met for only two tissues among those surveyed: rat tail skin and rabbit leg muscles. For rat tail skin, our results suggest that approximately half the albumin transport is convective and half dissipative, and for rabbit muscle, about two-thirds is convective and one-third dissipative.
Albumins↗
Interstitial exclusion of IgG in rat tissues estimated by continuous infusion.
Interstitial exclusion, defined as the fraction of interstitial fluid volume inaccessible to a solute, was evaluated for immunoglobulin G (IgG) in selected tissues of rats by a method previously applied to serum albumin (29). IgG distribution volumes were also measured for intestine. 125I-labeled rat IgG was infused for 5 or 7 days (n = 4 rats each) with an implanted osmotic pump (Alzet). At the termination of infusion, the rat was anesthetized, nephrectomized, and injected with 51Cr-labeled EDTA (4 h) to label total extracellular fluid volume and 131I-labeled bovine IgG (5 min) to label plasma volume. Samples of skin, muscle, and tendon were assayed for total and extractable tracer activity. Interstitial fluid from these tissues was sampled postmortem with nylon wicks for assay of 125I-labeled IgG and endogenous albumin and IgG. Exclusion of IgG was calculated from the difference between extravascular 125I-labeled IgG and 51Cr-labeled EDTA distribution volumes. In contrast to our previous experience with tracer albumin, 125I-labeled IgG was not fully extractable from minced skin, muscle, or tendon by isotonic saline; only 71-83% was recovered under conditions that eluted 92-96% of tracer albumin and 94-99% of tracer EDTA. We conclude that approximately 20% of extravascular 125I-labeled IgG in these tissues is sequestered or bound in the interstitium. Calculation of IgG fractional exclusion from extractable tracer yielded the following values (means +/- SE, n = 8 rats): leg muscles 0.37 +/- 0.09, leg skin 0.44 +/- 0.03, back skin 0.36 +/- 0.04, tail skin 0.40 +/- 0.08, and tail tendon 0.55 +/- 0.04.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Renal cortical interstitium and fluid absorption by peritubular capillaries.
Every minute, the cortical peritubular capillaries in a 1-g rat kidney take up more than 0.5 ml tubular reabsorbate. Studies of renal lymph and measurements of pressure in capillaries (Pc) and interstitium (Pi) indicate that normally the protein colloid osmotic pressure of peritubular capillary plasma (COPp) provides the necessary absorptive force, keeping Pi at 2-4 mmHg, i.e., 8-10 mmHg lower than Pc. At reduced COPp, continued delivery of fluid from the tubules automatically raises Pi to maintain capillary fluid uptake. The transient Pi response to sudden exposure of the kidney to subatmospheric pressure shows that such adjustment of forces may take place in only 5 s. Most remarkable, adjustment of forces may take place in only 5 s. Most remarkable, reabsorption continues during protein-free perfusion of the isolated rat kidney, apparently effected by a Pi exceeding Pc. A relative retardation of interstitial uptake of ferritin from plasma in this case suggests fluid reabsorption through both small and large pores in the capillary wall. Collapse of the capillaries is presumably prevented by tight tethering to the capillary wall, giving the narrow interstitium a very low compliance.
Animals↗
Blood-tissue transport of exogenous albumin and immunoglobulin G in genetically analbuminemic rats.
Tracer uptake studies were carried out in adult female Nagase (NA) strain analbuminemic rats [derived from Sprague-Dawley (SD) stock] and in adult female SD controls to determine the extent to which capillary permeability to plasma proteins is altered in the absence of endogenous albumin. Accessory measurements (arterial pressure, central venous pressure, plasma and interstitial fluid protein concentrations and oncotic pressures, plasma volume, and interstitial fluid volume) confirm the report of Joles et al. [Am. J. Physiol. 257 (Renal Fluid Electrolyte Physiol. 26): F23-F28, 1989] that shows elevated plasma volumes, normal interstitial fluid volumes, nearly normal plasma oncotic pressures (due to elevated globulin concentrations), and lower interstitial fluid oncotic pressures. In skin, skeletal muscles, and heart muscle, clearances of exogenous heterologous (bovine) albumin were 20-40% higher in NA than in SD controls. In intestine, albumin clearances were 20-30% lower. In NA rats blood-to-tissue clearances of heterologous (bovine) immunoglobulin G in skin and heart were higher and in the intestine they were lower than in SD controls; however, clearances in skeletal muscles were not elevated. The differences between NA and SD are small compared with the large increases in macromolecular permeabilities reported by others for organs and single microvessels perfused with albumin-free fluids.(ABSTRACT TRUNCATED AT 250 WORDS)
Albumins↗
Plasma volume expansion with colloids increases blood-tissue albumin transport.
Extravasation of plasma proteins is increased after volume expansion with whole blood or plasma. To investigate the mechanisms responsible for this phenomenon, we measured extravascular accumulation of exogenous 131I-labeled bovine serum albumin in several tissues and organs of anesthetized rats. Plasma volume was increased acutely by infusion of isoncotic albumin or polyvinylpyrrolidone, with or without subsequent infusion of a 1:10 dilution of the colloid to induce blood-to-tissue fluid movement. Controls were given only a slow sustaining infusion of saline. The amounts of fluid and plasma protein lost from the circulation were followed simultaneously by two methods: 1) material balance in the whole animal, and 2) changes in 131I-labeled albumin uptake (VA) and water content (VW) in the individual tissues. Plasma volume expansion of 80-90% increased plasma protein extravasation in the whole rat by an average of 2.7-fold over a 30-min period. Of the protein extravasated, 42% entered the abdominal cavity. The rest was distributed in the interstitial compartment of various tissues and organs. Tracer albumin accumulation (averaged over 30 min) was increased 38-82% in skin and paw, 40-59% in skeletal muscles, 131% in hearts, and 167-230% in different parts of the intestine. Increased convective transport does not appear to be a major factor. There was little or no relation of albumin transport increase to the magnitude or direction of net fluid transfer. Coupling of albumin transport to volume flow was not greater than previously reported for saline infusion or venous congestion. Convective redistribution (convective transport without net fluid transfer, "volume recirculation") is estimated to increase albumin transport no more than 10% under the conditions of our experiments. The greater part of the increase is thus dissipative, i.e., attributable to increased diffusion or increased vesicular exchange. Control of dissipative transport of albumin may play an important role in regulating plasma volume.
Animals↗
Interstitial exclusion of albumin in rat tissues measured by a continuous infusion method.
Steady-state 125I-labeled rat serum albumin (125I-labeled RSA) concentration in plasma was maintained by intravenous infusion of tracer for 72-168 h with an implanted osmotic pump. At the end of the infusion period, the rat was anesthetized and nephrectomized, and extracellular fluid was equilibrated with intravenous 51Cr-labeled EDTA for 4 h. Five minutes before final plasma and tissue sampling, 131I-labeled bovine serum albumin (131I-labeled BSA) was injected intravenously as a plasma volume marker. Samples of skin, muscle, tendon, and intestine were assayed for all three tracers. Apparent distribution volumes were calculated as tissue tracer content/plasma tracer concentration. Interstitial fluid volume (Vi) was calculated as V51Cr-EDTA-V131I-BSA. Steady-state extravascular distribution of 125I-labeled RSA as plasma equivalent volume (Va,p) was calculated as V125I-RSA-V131I-BSA. Steady-state interstitial fluid concentrations of 125I-labeled RSA in skin, muscles, and tendon were measured with nylon wicks implanted postmortem, and steady-state interstitial albumin distribution volumes were recalculated as wick-fluid equivalent volumes (Va,w). Relative albumin exclusion fraction (Ve/Vi) was calculated as 1-Va,w/Vi. For skin and muscle, steady-state 125I-labeled RSA tissue concentrations were reached at 72 h. Ve/Vi for albumin averaged 26% in hindlimb muscle, 41% in hindlimb skin, 30% in back skin, 39% in tail skin, and 54% in tail tendon. For muscle, Ve/Vi corresponds to expectation if all tissue collagen and hyaluronan is dispersed in the interstitium. However, for skin and tendon, albumin exclusion is considerably lower than expected on this basis, suggesting that much of their collagen is organized into dense bundles of fibers containing no fluid accessible to 51Cr-labeled EDTA or 125I-labeled RSA.
Animals↗
Tissue-specific effects of physiological ANP infusion on blood-tissue albumin transport.
Blood-tissue transport of 131I-labeled bovine serum albumin (BSA) during intravenous infusion of synthetic atrial natriuretic peptide (ANP) was examined in anesthetized male Wistar rats. Plasma volumes were maintained at pre-ANP levels by infusion of 2% BSA in lactated Ringer solution (LR) to minimize compensatory responses to ANP-induced hypovolemia. 131I-BSA clearance was measured over 30 min, and 125I-BSA was injected terminally to correct for intravascular volume. Thirty-minute infusion of 20 ng.kg-1.min-1 ANP resulted in a tissue-selective increase in 131I-BSA clearance in jejunum and colon compared with controls given LR only. Smaller but significant increases in tracer clearance also were observed in fat, kidney, left ventricle, and skeletal muscle exposed to 400 ng.kg-1.min-1 ANP. The observed elevation in tracer albumin extravasation was not associated with any measurable increase in tissue extravascular water content. Furthermore, it was shown that coupling of 131I-BSA transport to filtration induced by hindlimb venous congestion was similar in control and ANP-treated rats. In a second series of experiments, plasma ANP levels were determined after 30-min ANP infusions from 0 to 180 ng.kg-1.min-1. Significant linear associations between physiological ANP levels (62-578 pg/ml) and 131I-BSA clearance were demonstrable for small intestine, colon, fat, kidney, and skeletal muscle but not for skin, heart, diaphragm, and lung. We conclude that raising plasma ANP by infusion of the synthetic peptide results in a filtration-independent, tissue-selective increase in albumin transport. Tissue uptake of albumin is a potential mechanism for extrarenal fluid shift during circulatory volume overload.
Animals↗
Cellular and intercellular transport pathways in exchange vessels.
The endothelium of lung alveolar capillaries is of the continuous type, that of airway exchange vessels (capillaries and pericytic venules) includes both continuous and fenestrated types. Water and small lipophilic solutes penetrate via the endothelial cells (cell membrane pathway) as well as through intercellular junctions. Hydrophilic solutes are limited to junctional pathways and cytoplasmic vesicles. Permeation of hydrophilic solutes is progressively restricted with increasing molecular size, as by a sieve, with many openings 8 nm and a few 40 to 60 nm wide. In response to local tissue injury or to certain chemical mediators, larger junctional pathways may be opened, greatly increasing permeability to large molecules. Both alveolar capillaries and airway exchange vessels exhibit this response, but the effective stimuli may differ (e.g., alveolar capillaries are insensitive to histamine and bradykinin). Hydrophilic solutes are transported by diffusion, convection, and vesicular exchange (transcytosis). For small ions and molecules (radii < 2 nm), diffusion is the dominant transport mode; contributions of convection and transcytosis are negligibly small. Because diffusion decreases with increasing molecular size, all three mechanisms may contribute substantially to transport of large molecules (radii > 2 nm). Fenestrated endothelia have higher hydraulic conductivities and are more permeable to small ions and molecules than are continuous endothelia. However, their permeabilities to plasma proteins are about the same. Lung alveolar capillary endothelium has lower hydraulic conductivity and lower solute permeabilities than do other continuous endothelia (heart, skeletal muscle). Airway exchange vessel endothelium has about the same permeability to serum albumin as alveolar capillary endothelium.
Albumins↗
Sampling interstitial fluid from rat skeletal muscles by intermuscular wicks.
A modification of the implanted wick method (K. Aukland and H. O. Fadnes. Acta Physiol. Scand. 88: 350-358, 1973) was devised to sample interstitial fluid from rat muscles. Dry nylon wicks were inserted postmortem into intermuscular spaces between leg muscles by means of a plastic catheter, which was subsequently withdrawn. Inserting the wicks postmortem avoids contaminating wick fluid with proteins extravasated as a result of local inflammatory reactions; placing them intermuscularly avoids contamination by fluid and proteins from damaged muscle cells. Wick fluid protein concentrations (mg/ml) averaged 24.1 +/- 1.1 and 28.5 +/- 1.5 (means +/- SE) in medial and lateral hindlimbs muscles, respectively. The corresponding albumin concentrations were 13.0 +/- 0.7 and 13.9 +/- 0.7 mg/ml. Total protein and albumin concentrations in plasma were 54.1 +/- 0.8 and 22.5 +/- 0.3 mg/ml. Electrophoresis of wick fluid showed a pattern of peaks similar to that of plasma, with albumin relatively high and larger molecules relatively low. Proteins from muscle cells were not detected. Isotope studies (125I-labeled albumin, 51Cr-EDTA) showed that less than 2% of the albumin in wick fluid came directly from plasma and that wick fluid was not concentrated by cell swelling postmortem. Wick fluid from intermuscular wicks implanted in anesthetized rats in vivo had nearly the same total protein concentration as fluid from postmortem wicks, but albumin-to-globulin (A/G) ratios were slightly lower (1.22 +/- 0.07 vs. 1.53 +/- 0.21 measured by gel electrophoresis), and more significantly, nearly 50% of the albumin leaked to wick fluid from plasma as a result of wick implantation.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Influence of saline infusion on blood-tissue albumin transport.
Anesthetized rats were infused with lactated Ringer solution (LR) at constant rate for 30 or 60 min; delivered volume loads ranged from 0.03 to 0.08 ml/g body wt. Controls were given only a sustaining infusion of saline at 0.002 ml.g-1.h-1. Only 7-14% of the LR remained in the plasma at the end of the infusion; 76-88% entered the interstitial compartment, and 7-17% was excreted. The amount of plasma protein lost from the circulation with the extravasated fluid was studied simultaneously by two methods: 1) material balance in the whole animal and 2) changes in 131I-labeled albumin uptake (VA) and water content (VW) in individual tissues. The extravasation of 0.03-0.06 ml fluid/g body wt (75-160% initial plasma volume) did not significantly increase plasma protein extravasation in the whole rat. Nearly all of the sampled tissues of LR-infused rats had higher VW than controls. Tissue VA tended to increase with VW, but the regression slopes (delta VA/delta VW), a measure of the tracer albumin concentration of capillary filtrate relative to plasma, were low; skin, 0.006; paw, 0.018; skeletal muscles, 0.007; heart, 0.057; jejunum, 0.095; ileum, 0.045; cecum, 0.026; and colon, 0.027. These ratios are consistent with the very small loss of total plasma protein observed and attest to high solvent-drag reflection coefficients (sigma approximately equal to 1 - delta VA/delta VW): greater than 0.98 in capillaries of skeletal muscles, skin, and paw and 0.91-0.97 in heart and intestine.
Animals↗
Albumin extravasation rates in tissues of anesthetized and unanesthetized rats.
Bovine serum albumin (BSA) labeled with 131I was injected intravenously in chronically prepared, unanesthetized rats and into pentobarbital-anesthetized rats that had received 2 ml 5% BSA to help sustain plasma volume. Initial uptake rates (clearances) in skin, skeletal muscles, diaphragm, and heart (left ventricle) were measured over 1 h. BSA labeled with 125I was injected terminally to correct for intravascular 131I-BSA. Observed clearances were in the following order in both groups of animals: heart much greater than diaphragm approximately equal to skin greater than resting skeletal muscles. Differences between unanesthetized and anesthetized animals were small and inconsistently directed. Our results suggest that the lower albumin clearances reported in the literature for anesthetized rats are not the result of their immobility or any direct effect of anesthesia on albumin transport in these tissues. The lower transport rates appear to result indirectly from changes produced by anesthesia and/or surgery in controllable parameters such as plasma volume and intravascular protein mass.
Anesthesia, General↗
Coupling of albumin flux to volume flow in skin and muscles of anesthetized rats.
Bovine serum albumin (BSA) labeled with 131I or 125I was injected intravenously in pentobarbital sodium-anesthetized rats, and tracer clearances into leg skin and muscles were measured over 30, 60, and 120 min. BSA labeled with the alternate tracer was used as vascular volume reference. Two minutes before injection of the tracer, a ligature was tied around one femoral vein to occlude outflow partially and raise capillary pressure in that leg. The unoccluded leg served as control. Skin and muscles of the occluded leg had variably and substantially higher water contents (delta W) than paired control tissues and slightly but consistently increased albumin clearances (CA). The delta CA/delta W, equivalent to the albumin concentration of capillary filtrate relative to plasma determined by linear regression, were as follows: leg skin 0.004 (95% confidence limits -0.001 to +0.009), muscle biceps femoris 0.005 (0.001-0.010), muscle gastrocnemius 0.011 (0.004-0.019), muscle tibialis anterior 0.016 (0.012-0.021). All these values are significantly less than 0.10, which corresponds to a reflection coefficient for serum albumin (sigma A) of 0.90. Convective coupling of albumin flux to volume flux in skin and muscles of intact, anesthetized rats is low, with sigma AS in the range 0.98 to greater than 0.99.
Animals↗
Wick sampling of interstitial fluid in rat skin: further analysis and modifications of the method.
UNLABELLED: We compared modifications of the wick technique for analysis of interstitial fluid in rat subcutis. Nylon wicks were implanted for 60 min in back skin of rats after anesthesia with pentobarbital or after sacrifice by potassium chloride injection. Wicks were implanted dry or loaded with saline or varied dilutions of rat serum. Implantation of dry wicks and wicks loaded with diluted serum in living, anesthetized animals produced similar results; the protein concentration of wick fluid averaged about 60% that of the plasma protein concentration. The saline loaded wicks produced wick fluid with a lower protein concentration, average about 45% that of plasma protein concentration. The lower concentrations apparently resulted from simple dilution. Wick fluid sampled from dead animals had similar total protein concentrations, but in the dead animals there was a lower concentration of the large plasma proteins and a relatively higher concentration of the smaller proteins. CONCLUSIONS: Wick implantation in living animals causes a transitory inflammatory reaction and a decrease in the size selectivity of macromolecular sieving, but local osmotic forces bring about a concentration equilibrium with undisturbed interstitium. Implantation of dry wicks in subcutis either in vivo or post mortem provides a simple, direct method for sampling the total protein concentration and colloid osmotic pressure of interstitial fluid. Implantation of dry wicks postmortem permits measurement of individual component protein concentrations and evaluation of molecular selectivity between plasma and interstitium.
Animals↗
Some consequences of capillary permeability to macromolecules: Starling's hypothesis reconsidered.
Starling's hypothesis ascribes fluid movements across capillary walls to the interaction of hydrostatic and colloid osmotic forces. For 90 years it has been recognized as the basis of plasma-to-interstitial fluid balance. Its original statement was based on the notion of capillary impermeability to plasma proteins. However, as knowledge of transcapillary exchange of plasma proteins developed, its formulation was progressively modified to allow for protein transport and for interaction of protein transport with volume flow. The most important aspects of the conceptual evolution of Starling's hypothesis are reviewed in the text of this lecture.
Capillary Permeability↗
History of Physiology at University of California, Davis.
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Animal Husbandry↗
Flow and distribution of India ink in microvessels of the frog.
Velocities of an India ink front and of RBCs moving ahead of it were studied by intravital microscopy in capillaries of frog mesentery and skeletal muscle. Measurements were made during microperfusion of single vessels or groups of connected vessels in mesentery, and following intravenous ink injection in both tissues. The presence of ink did not appear to interfere with microvascular flow or vasomotion during the period of observation. On the average, the ink spearhead moved only slightly faster than the RBCs. There was substantial variation in relative velocities of RBCs in the same vessel and in the relative velocities of ink front and RBCs. The time course of ink filling showed substantial heterogeneity of flow in mesentery, more nearly uniform flow in skeletal muscle. Comparison of the measured velocity ratios of ink to RBCs with published observations on relative velocities of RBC to blood suggest that the advancing, apparently parabolic front of ink moves at less than twice the mean blood velocity. This is due in small part to diffusive dispersion of the ink particles in the laminar flow gradient, but more largely to stochastic dispersion of the front by interaction with RBCs and by displacement at branches.
Animals↗
B. W. Zweifach Award lecture. Regulation of the microcirculation.
Microcirculatory blood flow and transport are controlled to meet local and systemic demands for material exchange and body fluid balance. Control mechanisms act through effectors (smooth muscle cells) at many sites within the microvascular bed. Responses at different sites are not uniform, resulting in a broadly heterogeneous distribution of pressures and flows which is constantly changing. Simplified, uniform models of microvascular networks have made it possible to identify the principles governing blood circulation and blood-tissue transport. However, knowledge of how these principles are integrated at the microcirculatory level requires the variability and heterogeneity to be taken into account. Indeed, there is much reason to believe that heterogeneity is an important part of microcirculatory control.
Arterioles↗