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Biomedical subjects

L G Navar

Publications and source records attributed to L G Navar.

At least 199 records · Page 11Linked to original sources
PubMed1977

Relationship between colloid osmotic pressure and plasma protein concentration in the dog.

This study was done to establish the correct relationship between protein concentration and plasma colloid osmotic pressure in the dog and to determine the possible influence of the relative albumin and globulin content (A:G ratio). Plasma samples from dogs, rats, and humans were evaluated for total protein concentration, globulin concentration, and colloid osmotic pressure. Samples were concentrated and diluted by ultrafiltration to provide a range of total protein concentrations from 1 to 12 g/dl. Rat and human plasma samples had A:G ratios of 1.4 and 2.1, respectively, and the relationship between protein concentration and colloid osmotic pressure was in agreement with the Landis-Pappenheimer equation. In contrast, dog plasma samples consistently exhibited lower colloid osmotic pressures for any given protein concentration. Two forms of empirical equations were derived to relate these parameters in the dog. Dog plasma samples had higher concentrations of globulin and the A:G ratio averaged 0.59 +/- 0.35 SD. There was a significant relationship between the A:G ratio and the plasma colloid osmotic pressure. Analysis of the possible effect of this altered relationship on glomerular filtration dynamics predicted that efferent plasma colloid osmotic pressure was not specifically affected and was dependent only on the filtration fraction and the plasma colloid osmotic pressure.

Animals↗
PubMed1977

Autoregulation of nephron filtration rate in the dog assessed by indicator-dilution technique.

Micropuncture studies were conducted in anesthetized dogs to evaluate single nephron glomerular filtrate rate (SNGFR) and SNGFR autoregulation when assessed by means of an indicator-dilution technique (SNGFRID), which does not require interruption of distal volume delivery. In 18 dogs, control renal arterial pressure was 124 +/- 11 mmHg (SD), renal blood flow (RBF) was 185 +/- 58 ml/min, and whole kidney GRF was 37 +/- 7 ml/min. SNGFRID averaged 57 +/- 15 nl/min and was significantly lower than SNGFR determined on the basis of total collections (SNGFRTC) from proximal tubules (75 +/- 17 nl/min). However, SNGFRID was not significantly different from the overall average SNGFR of 62 +/- 12 nl/min estimated from whole kidney GFR and the total number of glomeruli (613,000 +/- 74,000). In 10 animals, renal arterial pressure was reduced to an average of 94 +/- 8 mmHg; whole kidney autoregulation was highly efficient and average RBF and GRF remained within 1 and 3% of their control values. Likewise, SNGFRID was not significantly altered at 52 +/- 17 and 52 +/- 16 nl/min. In contrast, SNGFRTC decreased significantly from 72 +/- 17 to 51 +/- 13 nl/min. These results indicate that the indicator-dilution technique provides a reliable assessment of SNGFR and allows the manifestation of single nephron autoregulatory behavior. They provide further support for the hypothesis that maintenance of distal volume delivery may be a necessary aspect of the autoregulation phenomenon.

Animals↗
PubMed1975

Absence of estimated glomerular pressure autoregulation during interrupted distal delivery.

Micropuncture studies in dogs have suggested that a distal tubule-to-afferent arteriole feedback system may participate in the autoregulation mechanism at the single-nephron level. To evaluate the effect of interrupted distal delivery on glomerular capillary pressure (GP) and its autoregulation, the proximal tubule was blocked with oil and maximal stop-flow pressure was measured with a micropressure servo-null system. The GP was estimated from the sum of stop-flow pressure and the plasma colloid osmotic pressure (membrane oncometer). In 18 dogs given a mild mannitol load, average +/- SD control arterial pressure was 118 +/- 16 mmHg, proximal tubule pressure was 24 +/- 5 mmHg, and estimated GP averaged 70 +/- 10 mmHg. There was a highly significant relationship between estimated GP and arterial blood pressure. Similar results were obtained in hydropenic dogs. In response to decreases in renal arterial pressure in individual dogs, stop-flow pressure and estimated GP failed to exhibit autoregulation although autoregulation of renal blood flow, GFR, and proximal tubule pressure was observed over an arterial pressure range of 150-95 mmHg. These results indicate that interruption of normal distal delivery by proximal tubule blockage interferes with the ability of the nephron to autoregulate glomerular pressure. They provide further evidence in support of the concept that a distal tubular feedback mechanism participates, at least in part, in the autoregulatory control of glomerular pressure.

Animals↗
PubMed1974

Distal tubular feedback in the autoregulation of single nephron glomerular filtration rate.

Renal clearance and recollection micro-puncture experiments were conducted to evaluate the possible role of a distal tubular feedback mechanism in the phenomenon of renal autoregulation in dogs. Single nephron glomerular filtration rate (SNGFR) was measured from collection sites in both the proximal (proximal SNGFR) and distal tubules (distal SNGFR). Single nephron autoregulatory behavior was assessed by evaluating the response of SNGFR to a reduction in renal arterial pressure imposed by means of an aortic constrictor. Whole kidney function was evaluated by parallel measurements of renal blood flow and inulin clearance. Whole kidney autoregulation was observed when renal arterial pressure was decreased from 141 +/- 3 (SE) mm Hg to 101 +/- 2 mm Hg; renal blood flow and GFR were not significantly altered from control values of 3.76 +/- 0.2 ml/min.g and 0.69 +/- 0.04 ml/min.g kidney weight, respectively. In 11 autoregulating preparations, proximal transit time was likewise unchanged from the control value of 26 +/- 2 s, indirectly suggesting that the superficial nephrons also participated in the autoregulatory response. However, proximal SNGFR decreased significantly from 88 +/- 7 nl/min to 66 +/- 6 nl/min, a reduction which was proportional to the decrease in arterial pressure. In 14 dogs in which both proximal SNGFR and distal SNGFR were measured at control blood pressure (136 +/- 5 mm Hg), distal SNGFR was 47 +/- 4 nl/min, a value significantly lower than that for proximal SNGFR (79 +/- 6 nl/min). In contrast to the results based upon proximal collections, distal SNGFR was not significantly altered following aortic constriction (44 +/- 5 nl/min vs. 47 +/- 5 nl/min) therefore exhibiting autoregulation in association with that observed for the whole kidney. These experiments indicate that though superficial nephrons do possess autoregulatory capability, interruption of distal delivery due to complete collection from the proximal tubule interferes with that nephron's ability to manifest an autoregulatory response. They support the concept that a feedback mechanism, related to some function of distal delivery, is of significance in the intrinsic regulation of SNGFR. The data further suggest that quantitative estimates of SNGFR based on complete proximal collections may not be representative of those throughout the superficial cortex of the dog, at least in certain experimental circumstances.

Animals↗