Relationship between salt loading and distribution of nephron filtration rates in the dog

Autoregulation of superficial nephron function in the alloperfused dog kidney

Isolated dog kidneys were each pump-perfused by another dog during 4 experimental periods at perfusion pressures (PP) of 21, 17, 13, and 8 kPa, resp. (i.e. 160, 130, 94, and 60 mm Hg). At the 3 highest PP values, the total kidney renal blood flow (RBF) and glomerular filtration rate (GFR) were perfectly autoregulated while at the lowest value both values were significantly lowered. No significant difference was observed between the single nephron GFR (SNGFR) of periods 1 and 2; in period 3 (PP = 13 kPa) a lower value was observed (P less than 0.05). Free flow pressure in proximal convolution (FFP), stop-flow pressure (SFP), and peritubular capillary pressure (PCP) were not different in period 2 than in period 1, but were significantly lower in period 3 (P = 0.02--0.05). Effective filtration pressure (EFP) was the highest in period 1, decreasing significantly with decreasing PP. Filtration pressure equilibrium was observed in period 4 at PP 8 kPa. Total blood flow resistance (RT) fell with decreasing PP, the drop being due to a steep decline in afferent resistance (RA). Efferent resistance (RE) increased as PP decreased. Ultrafiltration coefficient (Kf) rose with declining PP both within and outside the autoregulatory range. The results indicate that the lower limit of autoregulation is higher in superficial nephrons than in the whole kidney.

Renal blood flow distribution at varying perfusion pressure in the alloperfused dog kidney

Tissue blood flow (TBF), its percent distribution and glomerular blood flow (GBF) were measured using labelled microspheres 15 micrometer in diameter (M) and chicken red blood cells (CRBC) at perfusion pressures (PP) of 17.3, 12.8 and 8.0 kPa (130, 95 and 60 mm Hg) in isolated alloperfused dog kidneys. Renal blood flow (RBF) was never interrupted during the isolation. Experiments with M showed a marked inequality of the tissue blood flow in different parts of the renal cortex at a constant PP of 17.3 kPa. TBF was highest in the outermost quarter and lowest in the juxtamedullary one. Using CRBC, a homogeneous TBF was observed in the outer 3/4 of the renal cortex with a lower flow in the innermost quarter. With M, a typical percent "redistribution" of TBF and GBF into the inner cortical regions was indicated during PP reduction. With CRBC, this phenomenon was observed only at PP below the range of RBF autoregulation (8.0 kPa) and was much less conspicuous than with M. The smaller size and higher elasticity of CRBC as compared with M, may result in a more realistic reflection of cortical blood flow distribution. The GBF of outermost superficial glomeruli decreases, even with CRBC, with each PP reduction, the difference exhibiting only a 5% significance level. The lower limit of BF autoregulation in these glomeruli seems to be somewhat higher than that of total RBF autoregulation.

Effects of glucagon on the renal hemodynamics of dogs

An intrarenal infusion of glucagon resulted in an increase of glomerular filtration rate (GFR) with the same order of magnitude of renal blood flow (RBF) but did not affect the intrarenal distribution of blood flow. A superimposition of acetylcholine and glucagon decreased GFR even though RBF increased significantly. These findings indicate that the effect of glucagon on GFR depended on the selective dilation of afferent arterioles without any change in permeability of glomerular capillaries and redistribution of filtration.

Renal intracortical blood flow distribution, function and sodium excretion in response to saline loading of anesthetized and unanesthetized dogs

In order to study the effect of anesthesia on the canine response to saline loading, experiments were performed on 10 dogs, first while awake and then during pentobarbital anesthesia. Individual kidney function and intrarenal blood flow response to saline loading (7.5% body weight) were measured in each condition and all data are reported as the average of a single kidney. CIN is considerably reduced under anesthesia (24.7 +/- 3.2 vs. 43.2 +/- 3.9 ml/min, P less than 0.01). A directionally similar reduction of PAH clearance was noted (89 +/- 17 vs. 122 +/- 13 ml/min). The natriuretic response to saline loading of the dogs reached 290 +/- 61 muEq/min while awake, but only 70 +/- 27 muEq/min while anesthetized. No measurable increase of CIN or CPAH occurred in response to saline loading either in the anesthetized or unanesthetized state. The natriuresis was entirely due to a rise of CNA/GFR in both circumstances. The change of CNA/GFR in response to saline load was also appreciably larger while awake (1.2 leads to 4.7% vs. 0.7 leads to 1.8%). Although the fraction of blood flow to the outermost quarter of the kidney was initially the same (31 +/- 3 vs. 29 +/- 3%) awake or anesthetized, the changes with saline loading were in the opposite direction and the values reached were significantly different (37 +/- 3, awake, vs. 27 +/- 3%, P less than 0.05). We conclude that while increased outer cortical blood flow is not necessary for natriuresis, it may occur during sodium loading and may facilitate sodium excretion.

Anatomical and functional heterogeneity of nephrons in the rabbit: microdissection studies and SNGFR measurements

The single nephron glomerular filtration rate (SNGFR) was determined in superficial (S) and juxtamedullary (JM) nephrons of 10 anesthetized rabbits by the 14C ferrocyanide infusion technique. The length of the proximal tubules and the volume of the glomeruli were also determined for the same nephrons. SNGFR was higher in JM than in S: 28.6 +/- 3.4 versus 22.6 +/- 3.0 nl/min, P less than 0.001. In JM nephrons, glomeruli were larger than in S: 1.3 +/- 0.1 versus 0.9 +/- 0.1 nl, P less than 0.001, whereas there was no difference between proximal tubule length in either category (s, 8.7 +/- 0.3 and JM, 8.9 +/- 0.5 mm). In 6 out of 8 animals, SNGFR was significantly correlated to glomerular volume. Lack of correlation was observed between SNGFR and length of proximal tubule in all animals but one. These results show that the rabbit, as well as small rodents and the dog, has a higher SNGFR in juxtamedullary than superficial glomeruli. Although this functional difference is not related to the length of the proximal tubule in each individual animal, the ratio between the mean SNGFR value and the mean length of the proximal tubule in superficial rabbit nephrons is similar to the ratio found in other species.

Plasma renin activity and renal sodium and water excretion following infusion of arachidonic acid in rats

Plasma renin activity (PRA) and urinary sodium and water excretion were measured following infusion of the prostaglandin (PG) precursor arachidonic acid (C20:4) in normal hydrated rats (saline i.v.:0.5 ml/hr/100 g body weight) and in rats with moderate volume expansion (saline i.v.:1.5 ml/hr/100 g body weight). Nonhypotensive doses of C20:4(40-70 mu/min/100 g b.w.) increased PRA in both normal and volume-expanded rats (p less than 0.05, and p less than 0.025, respectively). In volume-expanded rats, C20:4 was followed by reduced lrine volume (p less than 0.01) and sodium excretion (p less than 0.001) in comparison to volume-expanded control rats, whereas in normal hydrated rats these parameters remained unchanged after C20:4. The present results indicate that the C20:4-stimulated PG-synthetase system can increase PRA independently of the extracellular fluid volume. Furthermore, these results suggest a complex interrelationship between PG-synthetase action, activation of the renal renin-angiotensin system and urinary water and sodium excretion.