Renal physiology最新文献

Lithium clearances were performed in conscious dogs on varying sodium intakes. Fractional lithium excretion (FE-Li) was 22.6 +/- 2.2% in sodium-replete dogs (fractional sodium excretion, FE-Na greater than 0.4%, n = 11) but only 6.9 +/- 2.3% in sodium-depleted dogs (FE-Na less than 0.4%, n = 25). In 4 markedly sodium-depleted dogs (FE-Na 0.03 +/- 0.02%) the distally acting diuretic amiloride caused an 8-fold increase in FE-Li (from 1.5 +/- 1.0 to 11.6 +/- 2.0%). Our data suggest that tubular reabsorption of lithium in sodium-depleted dogs is not restricted to the proximal tubules but also occurs in the distal nephron. Therefore the lithium clearance cannot be used as an index of sodium and filtrate delivery from the proximal tubules in sodium-depleted dogs.

The renal clearances (C) of p-aminohippurate (PAH) and tetraethylammonium (TEA) as markers for renal plasma flow (RPF) were compared in rats under different experimental conditions. CTEA and CPAH were of the same magnitude and varied synchronously during volume expansion and anesthesia. The renal extractions of the two substances were closely correlated and averaged 85.9% for PAH and 88.7% for TEA (p less than 0.01). High doses of furosemide (120 mg/kg as intravenous bolus) reversibly decreased the CPAH/CTEA ratio, suggesting that furosemide may compete for PAH secretion in the proximal tubule. A similar effect was observed after administration of probenecid. The results indicate that TEA is a more reliable indicator of RPF than PAH in studies involving accumulation of endogenous or exogenous organic anions.

In 22- to 26-day-old beagle puppies, just after superficial nephrons have begun to function, the single nephron glomerular filtration rate is extremely low (6.7 nl.min-1) approximating a tenth of the adult value (57 nl.min-1) even though blood pressure is as high as 90 mm Hg. Glomerular vascular resistance is almost six times the value found in the adult dog (1.8 vs. 0.32 mm Hg.ml-1.min), this increase being mainly due to a rise in afferent (80%) rather than in efferent (20%) resistance. The glomerulus, having only recently started to function, is characterized by filtration pressure equilibrium, a phenomenon atypical of superficial glomeruli in the adult dog. These findings favor the concept that a low glomerular blood flow rate might be the main cause of the very low SNGFR in the new glomeruli, although some contribution of an altered ultrafiltration coefficient cannot be excluded.

Thiazide diuretics increase salt and water excretion primarily by inhibiting mechanisms for electroneutral sodium and chloride transport by distal convoluted tubule cells. This might be termed the 'specific' effect of this class of diuretics and accounts for the 'chlorouretic' effectiveness of the drug. Secondary to this inhibition of sodium and chloride absorption, potassium secretion is stimulated most likely because of the resultant increase in distal tubule fluid flow rate, and calcium absorption is stimulated possibly via a decrease in distal convoluted tubule cell sodium activity and an increase in basolateral sodium/calcium exchange. To a varying degree, thiazides also inhibit carbonic anhydrase. This effect can contribute to the diuresis, but is largely buffered by the reserve transport capacity of the loop of Henle. To the extent that the effects of transport inhibition in the proximal tubule are transmitted to the distal tubule, tubuloglomerular feedback may be activated and effect a reduction in the glomerular filtration rate.

A method for microcatheterization of rat kidneys was developed. The catheter consists of 50-100 micron thick polyethylene tubes, and is inserted into the interlobular or arcuate arteries via a lumbar artery. The technique makes it possible to perfuse a small number of nephrons with very high doses of vaso active substances without affecting the nephrons in the vicinity or the central haemodynamics. By adding labelled inulin to the perfusate it is also possible to calculate the amount of drug reaching the nephron under study. It was shown that the catheterization and perfusion with Ringer solution per se have no effect on the nephron function. Perfusion with very high doses of acetylcholine was found to have a selective effect on the area perfused but caused no changes in central haemodynamics, or on nephrons in unperfused areas. It is concluded that the technique is suitable for use in studies concerning the handling of high doses of vasoactive substances by the nephron.

Loop diuretics such as furosemide, piretanide, and bumetanide bind reversibly to the Na+2Cl-K+ carrier. This transporter is responsible for the uptake of Cl- into the thick ascending limb segment. As a consequence, these compounds reduce or abolish NaCl reabsorption in this nephron segment and lead to a decreased interstitial hypertonicity and thus to a reduced water absorption. Apart from these profound effects, loop diuretics also inhibit Ca2+ and Mg2+ reabsorption in the thick ascending limb in a manner which is still not clear. They also increase urinary K+ excretion by an increase in distal tubule NaCl and water delivery and probably also by the reduction of K+ reabsorption which goes on under in vivo conditions in the thick ascending limb segment. Finally, by the reduction of the transport work in the thick ascending limb, these compounds reduce dramatically the substrate and oxygen dependence of this nephron segment.