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Biomedical subjects

F G Knox

Publications and source records attributed to F G Knox.

At least 199 records · Page 11Linked to original sources
PubMed1975

Measurement of renal interstitial fluid pressure with polyethylene matrix capsules.

The renal interstitial fluid pressure was measured in dogs from small chronically implanted solid capsules made of porous polyethylene matrix material (pore size approximately 60 mum). Tissue does not grow into the polyethylene and the matrix creates a permanent fluid-filled space in communication with the interstitium. The fluid pressure in the capsule matrix was measured from a tube leading from the capsule with an active servo-null pressure-measuring device. Renal interstitial fluid pressure measured from 28 hydropenic dogs averaged 6.1 plus or minus 0.51 mmHg (SE). Increasing oncotic pressure inside the capsule to 90 mmHg resulted in a small but significant increase in hydrostatic pressure inside the capsule of 3.3 plus or minus 0.46 mmHg. However, the hydrostatic pressure returned to control levels within 30 min. In vitro studies indicated that the pressure measured from the capsule was highly sensitive to change in surrounding hydrostatic pressure, but insensitive to change in surrounding oncotic pressure. Histological studies showed a small layer of connective tissue surrounding the capsule less than 100 mum thick in many areas. There was no evidence of inflammation or tissue rejection. It is concluded that the small chronically implanted polyethylene capsule is a reliable method for the measurement of renal interstitial fluid pressure.

Animals↗
PubMed1975

Regulation of glomerular filtration and proximal tubule reabsorption.

In this review current hypotheses for the regulation of filtration rate and the subsequent reabsorption of that filtrate by the proximal tubule are discussed. In the rat the filtration rate is highly plasma flow dependent because of the phenomenon of filtration equilibrium. On the other hand, in man and dog filtration rate is much less dependent on renal plasma flow. In these species glomerular capillary pressure rather than glomerular plasma flow may be the primary determinant of filtration. Three hypotheses for the autoregulation of glomerular filtration are discussed. Of these the myogenic theory, which holds that increases in perfusion pressure result in an intrinsic contraction of the smooth muscle in the walls of the afferent arterioles, remains the best, if not completely adequate, explanation for the phenomenon. The role of a macula densa feedback mechanism is controversial, although data are presented which strongly militate against this hypothesis. Similarly, the prostaglandins, although important in determining vascular resistance, probably do not mediate autoregulation. Two prevalent hypotheses for the regulation of fluid reabsorption by the proximal tubule, the physical factor hypothesis and the humoral hypothesis, are discussed. Data presented indicate that peritubule Starling forces have a marked effect on isotonic reabsorption by the proximal tubule in the presence of volume expansion and enhanced backleak into the proximal tubule lumen. However, in hydropenia and diminished backleak, changes in peritubule capillary Starling forces have little effect on proximal sodium reabsorption. Thus, the physical factor mechanism is a rather insensitive regulator of proximal reabsorption in the absence of volume expansion. Hormones known to regulate sodium transport per se, such as aldosterone, have no effect on sodium reabsorption by the proximal tubule. In contrast, those hormones primarily regulating anion reabsorption in the proximal tubule, such as parathyroid hormone, have significant effects on fluid reabsorption. Thus, the humoral regulation of anion reabsorption has secondary effects on proximal sodium reabsorption.

Adrenalectomy↗
PubMed1975

Effect of increased peritubule protein concentration on proximal tubule reabsorption in the presence and absence of extracellular volume expansion.

The effect of increased peritubule capillary oncotic pressure on sodium reabsorption by the proximal tubule of the dog was investistigated after extracellular volume expansion (ECVE) with Ringer's solution or during continued hydropenia. Control measurements were made after ECVE or during hydropenia and again during renal arterial infusion with hyperoncotic albumin solution. Absolute reabsorption by the proximal tubule was calculated from fractional reabsorption and single nephron filtration rates as determined by micropuncture. Direct measurements of efferent arteriole protein were used to determine efferent arteriolar oncotic pressure. Albumin infused into the renal artery after ECVE significantly increased efferent oncotic pressure by 17.6 plus or minus 5.3 mm Hg. Fractional and absolute reabsorption by the proximal tubule increased from 20 plus or minus 6 to 37 plus or minus 5% and from 22 plus or minus 6 to 36 plus or minus 7 nl/min, respectively. During hydropenia, the albumin infusion significantly increased efferent oncotic pressure by 15.0 plus or minus 4.4 mm Hg. However, in contrast to the effect seen during ECVE, neither fractional nor absolute reabsorption was changed, delta equals 0.3 plus or minus 1.5% and 3 plus or minus 5 nl/min, respectively. Single nephron filtration rates were not significantly different between the groups and were unchanged by the albumin infusion. Peritubule capillary hydrostatic pressures, measured with a null-servo device, were not changed by the albumin infusion in either group. Renal interstitial hydrostatic pressure, measured from chronically implanted polyethylene capsules, was decreased significantly from 7.2 plus or minus 0.9 to 3.4 plus or minus 0.6 mm Hg in the hydropenic group and from 0.6 plus or minus 0.6 to 4.8 plus or minus 0.7 mm Hg in the Ringer's expanded group. In the hydropenic group, the increase in efferent oncotic pressure was nearly compensated for by changes in interstitial forces so that the calculated net force for capillary uptake was almost unchanged, 17.8 mm Hg before vs. 21.4 mm Hg during the albumin infusion. The increased efferent oncotic pressure in the Ringer's expanded group was not compensated, so that the calculated net force for uptake was increased, 11.9 mm Hg before to 22.2 mm Hg during the albumin infusion. Thus, while the increase in efferent oncotic pressure during albumin infusion was not significantly different between the groups, absolute and fractional reabsorptions were increased only in the animals in which the extracellular volume was expanded. The results suggest that ECVE alters the effect of increased peritubule oncotic pressure on sodium reabsorption by the proximal tubule.

Albumins↗
PubMed1975

Effects of parathyroid hormone on renal function.

Parathyroid hormone has been recognized as an important factor in the regulation of renal function since the 1920s. Early investigations mainly characterized the role of the hormone in calcium and phosphorus metabolism. Subsequently, significant contributions have been made to the understanding of the influence of parathyroid hormone on acid-base balance and amino acid excretion. The following is a review of the effects of parathyroid hormone in the regulation of renal function with special emphasis on new developments.

Acid-Base Equilibrium↗
PubMed1974

Proximal tubule reabsorption after hyperoncotic albumin infusion. Role of parathyroid hormone and dissociation from plasma volume.

Preferential expansion of the plasma volume by infusion of salt-poor hyperoncotic albumin solution decreases sodium reabsorption by the proximal tubule. The present micropuncture studies test the thesis that albumin infusion depresses proximal reabsorption by an effect unrelated to expansion of the plasma volume, perhaps due to an effect of parathyroid hormone (PTH) on proximal sodium reabsorption. Infusion of salt-poor hyperoncotic albumin significantly decreased plasma ionized calcium, increased immunoreactive PTH (iPTH) in plasma, decreased sodium reabsorption by the proximal tubule, and increased phosphate clearance. In contrast, infusions of albumin, in which the ionized calcium was restored to normal plasma levels, had no significant effect on ionized calcium, iPTH, proximal reabsorption, or phosphate clearance in intact dogs. Similarly, in parathyroidectomized animals given a constant replacement infusion of PTH, albumin infusion had no significant effect on proximal reabsorption or phosphate clearance. Plasma volume was markedly expanded following albumin infusion in all groups of dogs. These findings (a) indicate that PTH plays a significant role in the decrease in sodium reabsorption by the renal proximal tubule after salt-poor hyperoncotic albumin infusion, and (b) dissociate preferential expansion of the plasma volume from decreases in sodium reabsorption by the proximal tubule.

Absorption↗
PubMed1973

Effect of volume expansion on sodium excretion in the presence and absence of increased delivery from superficial proximal tubules.

The role of the proximal tubule in the natriuresis after volume expansion was investigated by evaluating sodium excretion both in the presence and absence of increased delivery from the proximal tubule. Proximal delivery was calculated from fractional reabsorption in superficial proximal tubules determined by micropuncture and glomerular filtration rate of the micropunctured kidney. Infusion of Ringer's solution in six dogs increased delivery from the proximal tubule 4.7+/-1 ml/min (P < 0.01) and increased fractional sodium excretion 3.6+/-1.1% (P < 0.025). Infusion of hyperoncotic albumin into the renal artery during sustained volume expansion decreased delivery from the proximal tubule 6.5+/-0.9 ml/min (P < 0.01). Although proximal delivery was restored to below control levels, fractional sodium excretion was significantly increased 2.5+/-0.5% (P < 0.01) as compared with the hydropenic control period. Fractional phosphate excretion was increased 15.5+/-3.7% (P < 0.01) after Ringer's infusion and was decreased 10.5+/-1.6% (P < 0.005) after intrarenal albumin infusion, suggesting that changes in superficial nephron reabsorption were paralleled by changes in reabsorption in deeper nephrons. Similar results were found in six additional dogs in which other factors known to affect phosphate reabsorption were controlled; however, these studies cannot completely eliminate a role for deep nephrons in the natriuresis after intrarenal albumin infusion. Since 70% of the natriuresis after volume expansion was present without increased delivery from superficial proximal tubules, it is likely that increased delivery from the proximal tubule contributes a relatively minor fraction to the natriuresis of volume expansion.

Albumins↗