Physiologic basis for the maintenance of glomerulotubular balance in young growing rats

Glomerular actions of endothelin in vivo

In Munich-Wistar rats, a micropipette was inserted into a first-order branch of the left main renal artery and continuously infused with human/porcine endothelin (0.4 ng/min). Micropuncture measurements revealed substantial differences within the cortical microcirculation of the same left kidney: SNGFR was some 35% lower in glomeruli exposed to endothelin compared with non-endothelin-perfused glomeruli (P less than 0.005). Similarly, glomerular plasma flow rate was some 38% lower in the endothelin-exposed glomeruli (P less than 0.001). The hypoperfusion and hypofiltration in the endothelin-exposed glomeruli reflected an increase in resistances in the afferent and efferent arterioles. There was no difference in the value of the glomerular capillary ultrafiltration coefficient between the two populations of glomeruli. We also studied kidneys that underwent 25 min of renal artery clamping 48 h before study. Antiendothelin antibody infused into one of the branches of the main renal artery ameliorated the vasoconstriction characteristic of postischemic nephrons: within the cortical microcirculation, the SNGFR in glomeruli exposed to antiendothelin antibody was 27.0 +/- 3.1 nl/min as compared with 17.4 +/- 1.7 measured in glomeruli not perfused with the antibody (P less than 0.001). Similarly, glomerular plasma flow rate was higher in the glomeruli exposed to antiendothelin antibody (128.7 +/- 14.4 nl/min vs. 66.6 +/- 5.6, P less than 0.005). Resistances in both the afferent and efferent arterioles were substantially lower in the antibody-exposed glomeruli. It is, therefore, suggested that endothelin, presumably released from damaged endothelium, may play an important intermediary role in the hypoperfusion and hypofiltration observed in postischemic kidneys.

Role of peritubular capillary forces in the renal action of carbonic anhydrase inhibitor

Micropuncture study was performed in Munich-Wistar rats to assess peritubular capillary Starling forces in renal superficial cortex during suppression of proximal fluid reabsorption by carbonic anhydrase inhibitor. Administration of benzolamide (2 mg/kg/hr, i.v., Group 1, N = 7 rats) caused not only reduction in absolute rate of proximal fluid reabsorption (APR, from 26.7 +/- 4.0 nl/min to 17.7 +/- 3.6, P less than 0.001), but also an increase in peritubular transcapillary hydraulic-pressure difference (from 10.0 +/- 0.5 mm Hg to 15.2 +/- 0.5, P less than 0.001). In a separate group of seven rats (Group 2), these parameters did not change significantly without benzolamide treatment. In Group 1 rats, an attempt was made to nullify the benzolamide-induced reduction in the peritubular capillary net reabsorptive forces by infusing hyperoncotic high-hematocrit blood. Following this treatment, while benzolamide administration was continued, values for APR returned to levels (25.6 +/- 4.8 nl/min) nearly identical to those measured prior to benzolamide administration, in association with a rise in peritubular transcapillary oncotic pressure difference. A separate group of six rats treated in a fashion identical to that of Group 1 showed continued suppression of carbonic anhydrase activity following blood infusion as indicated by low levels of whole kidney bicarbonate reabsorption rate. Peritubular capillary reabsorption coefficient was calculated based on the measured values for Starling forces in Group 1 and were unaffected throughout the study. Continued benzolamide administration alone without the treatment of hyperoncotic blood did not change APR significantly (Group 3, N = 7 rats).(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrafiltration coefficient of isolated glomeruli of rats aged 4 days to maturation

In the developing rat, RBF and GFR increase in parallel. We have calculated filtration coefficient (Kf) and hydraulic conductivity (Lp) of the glomerular capillary using glomeruli isolated from Munich-Wistar rats aged 4 days to adult. Lp derived as the quotient of Kf and total basement membrane area (LpS) may represent a minimum estimate, while the quotient of Kf and the peripheral basement membrane area (LpP) may more accurately represent the value that is operative in vivo. Rats in Group 1 (age 4 to 9 days) had an average glomerular diameter of 87 +/- 3 microns, Kf of 1.7 +/- 0.3 nl/min . mm Hg and LpS and LpP, 1.9 +/- 0.2 and 4.4 +/- 0.6 microliter/min . mm Hg . cm2, respectively. Glomerular diameter and Kf were not significantly increased in rats of Group 2 (22 to 36 days), but LpS and LpP diminished to 1.5 +/- 0.1 and 2.6 +/- 0.2 microliter/min . mm Hg . cm2 because of an increase in basement membrane surface density. In older rats of Groups 3 (69 to 84 days) and 4 (about 120 days, wt 205 +/- 4 g), glomerular diameter and Kf increased progressively. Kf and diameter were greater in deep than in superficial glomeruli of Group 4. In superficial glomeruli of Group 4, LpS averaged 1.1 +/- 0.1 and LpP 2.0 +/- 0.1 microliter/min . mm Hg . cm2, and were significantly lower than the values for Group 2 but not different from those of deep glomeruli of Group 4. Relative erythrocyte ejection increased with increasing glomerular diameter, indicating decreasing capillary resistance. Our observations at consistent with the previous reports that renal perfusion, Kf, and filtration rate increase in parallel, but do not support the hypothesis that glomerular capillary hydraulic conductivity increases as the filtration barrier matures.