Glucose reabsorption in the rat kidney. Microperfusion studies

Intrahepatic bile ducts transport water in response to absorbed glucose

The physiological relevance of the absorption of glucose from bile by cholangiocytes remains unclear. The aim of this study was to test the hypothesis that absorbed glucose drives aquaporin (AQP)-mediated water transport by biliary epithelia and is thus involved in ductal bile formation. Glucose absorption and water transport by biliary epithelia were studied in vitro by microperfusing intrahepatic bile duct units (IBDUs) isolated from rat liver. In a separate set of in vivo experiments, bile flow and absorption of biliary glucose were measured after intraportal infusion of D-glucose or phlorizin. IBDUs absorbed D-glucose in a dose- and phlorizin-dependent manner with an absorption maximum of 92.8 +/- 6.2 pmol. min(-1). mm(-1). Absorption of D-glucose by microperfused IBDUs resulted in an increase of water absorption (J(v) = 3-10 nl. min(-1). mm(-1), P(f) = 40 x 10(-3) cm/sec). Glucose-driven water absorption by IBDUs was inhibited by HgCl(2), suggesting that water passively follows absorbed D-glucose mainly transcellularly via mercury-sensitive AQPs. In vivo studies showed that as the amount of absorbed biliary glucose increased after intraportal infusion of D-glucose, bile flow decreased. In contrast, as the absorption of biliary glucose decreased after phlorizin, bile flow increased. Results support the hypothesis that the physiological significance of the absorption of biliary glucose by cholangiocytes is likely related to regulation of ductal bile formation.

Dependence of renal protein reabsorption on glomerular filtration rate and infusion time

Egg-white-lysozyme was infused into rats intravenously to achieve plasma concentrations of lysozyme (PLy) in the range of 10-3,500 mg X l-1. Concentrations of inulin and lysozyme in plasma and urine samples were determined spectrophotometrically. The glomerular filtration rate (GFR) was measured as clearance of inulin. The lysozyme reabsorption rate (TLy) was calculated as the difference between the filtered load and excreted amount of lysozyme. TLy increased with increasing filtered loads of lysozyme up to 1,000 micrograms X min-1 whether the increase in load was caused by an increase in PLy or GFR. These TLy were not dependent upon lysozyme infusion time. At lysozyme loads above saturation of reabsorption (1,000 micrograms X min-1) TLy was dependent on GFR and on the passed time after onset of lysozyme infusion. The results of this study indicated that the transport rate of lysozyme is a function of PLy, GFR and duration of lysozyme delivery to tubular cells.

Glomerular tubular balance in preterm and fullterm infants

The development of glomerular and tubular function was studied in preterm and fullterm infants of varying gestational and postnatal age. The results indicate that glomerular functional development precedes tubular functional development until the 34th postmenstrual week. After the 34th week the tubular transport capacity seems to be more vulnerable than the glomerular filtration rate in states of disease. The release of a postulated renal vasoconstriction could account for the rapid changes in renal function after birth. The purpose of such a vasoconstriction could be to protect the tubules from an overload.

Water reabsorption capacity of the proximal convoluted tubule: a microperfusion study on rat kidney

1. The differences in the water reabsorption capacity observed from one proximal tubule to another were investigated in vivo by continuous microperfusion. 2. Two to seven loops were punctured along the same tubule. The [3H]inulin, 22Na, [14C]glucose, sodium, chloride and magnesium concentrations as well as the osmolality of the collected samples were studied as a function of the perfused length. 3. With Ringer bicarbonate solution perfused in Saclay Wistar rats, the water reabsorption capacity ranged from 0 to 3 nl . min-1 . mm-1 depending on the tubule. This reabsorption rate was closely correlated with the unidirectional reabsorption flux of sodium, and with the rise in tubular chloride and magnesium concentrations. 4. In Munich Wistar rats with glomeruli accessible at the kidney surface, tubule perfusion with a Ringer bicarbonate solution showed that the highest water reabsorption rates per mm of tubule were found for the perfusion sites closest to the glomerulus; water fluxes were also positively correlated with glucose transport. 5. In a second series of experiments on Saclay rats, perfusion of a Ringer solution containing a high chloride concentration (137 m-equiv/l.) was unable to increase the water reabsorption rate compared to the control perfusion; here again, water fluxes were positively correlated with glucose transport.

Dependence of water movement on sodium transport in kidney proximal tubule: a microperfusion study substituting lithium for sodium

The relationship between water and sodium movements through the mammalian proximal convoluted tubule was investigated by substituting lithium for sodium. Proximal convoluted rat Kidney tubules were perfused in vivo with a Ringer solution containing 107 meq/liter lithium and 42 meq/liter sodium. Several micropunctures were made along the same nephron, and [3H] inulin, [14C] glucose, 22Na, osmolality, Na, Mg and Cl were determined on each sample. Measurements of 22Na showed that sodium and lithium diffusion rates were practically identical throughout the entire epithelium. A one- for-one exchange of sodium for lithium induced a negative transepithelial net flux of Na from plasma to lumen. However, despite this negative flux, a positive net water movement was measured from lumen to plasma. This movement was proportional both to glucose reabsorption and to the rise in the chloride concentration, two mechanisms known to be dependent on the transcellular movement of sodium. It was therefore concluded that the net water flux was a function of the unidirectional transcellular net flux of Na. Rabbit proximal convoluted tubules were perfused in vitro with solution containing 75 meq/liter Li and 75 meq/liter Na on both the luminal and peritubular sides. Under these conditions, the water reabsorption rate dropped to half its control value. Water movement was therefore a function of the external sodium concentration, which in turn probably regulates the intracellular Na concentration.

The role of brush border enzymes in tubular reabsorption of disaccharides: a microperfusion study in rat kidney

Renal tubular reabsorption of maltose, sucrose and lactose were studied in vivo et situ by continuous microperfusion of single proximal convolutions of rat kidney. The 14C-label of maltose (2.5 mmol/l) was removed from the lumen of the proximal tubule at about the same rate as found for glucose. Maltose reabsorption was completely inhibited in presence of 30 mmol/l glucose or of 0.1 mmol/l phlorizin. Chemical analysis of the samples showed a complete conversion of maltose into glucose within a perfusion distance of 2 mm. It is concluded from these results that within the tubular lumen maltose is split very rapidly by a brush border glucosidase. The short half time of this process permits the breakdown product glucose to be almost completely reabsorbed subsequently within the proximal tubule. In contrast, sucrose and lactose were neither split nor reabsorbed by the tubule brush border.

Effect of (+)-catechin on renal and intestinal transport

The ability of dog renal cortex slices to accumulate beta-methyl-glucoside or glycine is enhanced by the flavonoid (+)-catechin at a concentration of 3.5 mM. This stimulatory effect is apparently due to a decreased rate of efflux of either substrate. On the other hand, the uptake of p-amino-hippuric acid and N1-methyl-nicotinamide is inhibited by (+)-catechin. The drug at the same concentration is without action on amino-acid transport by guinea-pig intestine in vitro.

Cycloleucine (1-amino-cyclopentane carboxylic acid): tubular reabsorption and inhibitory effect on amino acid transport in the rat kidney. (Microperfusion experiments)

Renal tubular reabsorption of cycloleucine (1-amino-cyclopentane carboxylic acid) was studied in vivo et situ by continuous microperfusion of single proximal tubules of the rat. The results show: a) cycloleucine is reabsorbed rapidly compared with other amino acids b) this reabsorption is saturable and can be inhibited by oligomycin c) cycloleucine inhibits tubular reabsorption of L-arginine, glycine, and of L-phenylalanine. Mutual reciprocal inhibition occurs only with L-phenylalanine (and perhaps also with glycine). A maximal possible permeability coefficient for cycloleucine (less than 6 times 10--5 cm times sec--1) was calculated. Assuming simple 2-parameter kinetics, Vmax and Km for tubular reabsorption of cycloleucine were estimated. It can be concluded from the present results that cycloleucine is reabsorbed by a mechanism that transports L-phenylalanine, but not by the system shared by dibasic amino acids.

Effect of extracellular fluid volume expansion on maximum glucose reabsorption rate and glomerular tubular balance in single rat nephrons

Extracellular fluid volume expansion with isotonic saline (7.5% of body weight) decreased maximum glucose reabsorption rate by rat kidneys at plasma glucose concentrations greater than 30 mM. Glucose reabsorption rate was 30.2 +/-1.6 (SE) mumoles/min.g kidney in nonexpanded rats; it was 18.4 +/-1.5 mumoles/min.g in volume-expanded rats. Glucose reabsorption determined by micropuncture was 92% complete at the end of accessible superficial proximal convolutions. Volume expansion resulted in a slight but statistically insignificant reduction of maximal glucose reabsorption rate in superficial nephrons from 786 +/-35 mumumoles/min.g kidney in nonexpanded rats to 720 +/-30 mumumoles/min.g in volume-expanded rats. Superficial nephron filtration rate was increased by volume expansion from 28.8 +/-1.2 nl/min.g to 36.6 +/-1.5 nl/min.g kidney. In nonexpanded rats, the ratio of glucose reabsorption to glomerular filtration (tmg/sgfr) was similar in superficial and juxtamedullary nephrons. In volume-expanded rats superficial nephron tmg/sgfr was greater than juxtamedullary nephron tmg/sgfr. Juxtamedullary nephron function was measured by puncturing loops of Henle in the exposed papillae of small rats. Volume expansion increased sgfr without much effect on tmg in superficial nephrons while it decreased tmg without much effect on sgfr in deep nephrons. Physical changes produced by volume expansion seem to exert their greatest effect on proximal tubular function in the inner cortex. The increase in heterogeneity of glomerular-tubular balance could account for increased splay of glucose titration curves previously reported to accompany volume expansion.