Myo-inositol transport in plamsa membrane of rat kidney

Tubular reabsorption of myo-inositol vs. that of D-glucose in rat kidney in vivo et situ

Filtered myo-inositol, an important renal intracellular organic osmolyte, is almost completely reabsorbed. To examine tubule sites and specificity and, thus possible mechanism of this reabsorption, we microinfused myo-[(3)H]inositol or D-[(3)H]glucose into early proximal (EP), late proximal (LP), or early distal tubule sections of superficial nephrons and into long loops of Henle (LLH) of juxtamedullary nephrons and papillary vasa recta in rats in vivo et situ and determined urinary fractional recovery of the (3)H label compared with comicroinfused [(14)C]inulin. To determine the extent to which the proximal convoluted tubule (PCT) alone contributes to myo-inositol reabsorption, we also microperfused this tubule segment between EP and LP puncture sites. We examined specificity of reabsorptive carrier(s) by adding high concentrations of other polyols and monosaccharides to the infusate. The results show that >60% of the physiological glomerular load of myo-inositol can be reabsorbed in the PCT and >90% in the short loop of Henle (SLH) by a saturable, phloridzin-sensitive process. myo-Inositol can also be reabsorbed in the ascending limb of LLH and can move from papillary vasa recta blood into ipsilateral tubular structures. Essentially no reabsorption occurred in nephron segments beyond the SLH or in collecting ducts. Specificity studies indicate that reabsorption probably occurs via a luminal Na(+)-myo-inositol cotransporter.

Down's syndrome fibroblasts exhibit enhanced inositol uptake

The inositol metabolism of Down's syndrome (DS, trisomy 21) skin fibroblasts was examined. We report that DS cells accumulated [3H]inositol 2-3-fold faster than did other aneuploid or diploid controls. In contrast, trisomy 21 did not affect the uptake of choline, serine or glucose. Kinetic analysis demonstrated an increased maximal velocity of high-affinity, Na(+)-dependent, inositol transport, consistent with the expression of higher numbers of transporters by DS cells. Enhanced uptake was accompanied by a proportional increase in the incorporation of radiolabelled inositol into phospholipid. We suggest that an imbalance of inositol metabolism may contribute to plasma membrane abnormalities characteristic of DS cells.

Inositol metabolism during neuroblastoma B50 cell differentiation: effects of differentiating agents on inositol uptake

Inositol uptake was studied in the rat CNS neuroblastoma B50 cell line. Eadie-Hofstee analysis of the uptake pattern reveals two defined modes of inositol entry into the cell. The high-affinity uptake component requires the presence of extracellular sodium and is inhibited by phloridzin. Analysis of the uptake velocities of the high-affinity uptake component provided the following apparent kinetic parameters: Km = 13.7 microM and Vmax = 14.7 pmol/mg of protein/min (without correcting for residual diffusion) and Km = 12.9 microM and Vmax = 12.3 pmol/mg of protein/min (with correction). At physiological concentrations, the high-affinity transport process contributes approximately 70% to total uptake; the remainder is due to a low-affinity diffusion-like process. Uptake inhibition studies reveal that the uptake process is sensitive to ouabain, amiloride, and dichlorobenzamil inhibition but relatively insensitive to cytochalasin B or phloretin. When neuroblastoma B50 cells are induced to differentiate morphologically with high extracellular calcium or with dibutyryl cyclic AMP, a significant decrease in inositol uptake is observed. The dibutyryl cyclic AMP-mediated inhibition of uptake affects only the high-affinity uptake component and is noncompetitive in nature. The high extracellular calcium-mediated inhibition is less specific; it involves "disappearance" of the high-affinity process, some inhibition of the low-affinity process, and an increase of inositol efflux. The significance of these observations is discussed in the context of neuroblastoma B50 cell differentiation.

Mechanism of glucose-induced (Na+, K+)-ATPase inhibition in aortic wall of rabbits

Hyperglycaemia decreases (Na+, K+)-ATPase activity in specific tissues by a mechanism whose effects are prevented by aldose reductase inhibitors and by raising plasma myo-inositol. This mechanism was activated and studied in vitro in normal rabbit aortic intima-media. Raising medium glucose to 10 mmol/l for 60 min inhibited a major component of (Na+, K+)-ATPase-mediated 86Rb+/K+ uptake normally operative in resting aortic intima-media in medium containing normal plasma levels of glucose (5 mmol/l) and myo-inositol (70 mumol/l); 20 or 30 mmol/l glucose had no greater effect. This effect occurred under conditions in which the aortic intima-media's normal myo-inositol content is not detectably decreased. The inhibition was prevented by sorbinil (10 mumol/l) and by raising medium myo-inositol from 70 to 500 mumol/l, which had no effect on (Na+, K+)-ATPase activity when the medium glucose remained at 5 mmol/l. Raising medium glucose selectively inhibited a component of (Na+, K+)-ATPase activity that requires medium myo-inositol, because it is maintained by a regulatory system through rapid basal phosphatidylinositol turnover in a discrete pool, which is replenished by a fraction of basal de novo phosphatidylinositol synthesis that is selectively dependent on myo-inositol uptake. Medium myo-inositol at a normal plasma level became inadequate to maintain this fraction of basal de novo phosphatidylinositol synthesis [( 1,3-14C]glycerol incorporation) when the medium glucose was raised. When sorbinil was added raising medium glucose did not alter the ability of 70 mumol/l medium myoinositol to maintain the (Na+, K+)-ATPase activity that requires medium myo-inositol.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of inositol into the distal cauda epididymidis of the rat

The transport of [3H]myo-inositol into the lumen of the rat distal cauda epididymidis was studied by luminal perfusion in vitro. Entry was time and tubule length-dependent and saturable transport could be demonstrated with Vmax of 237 pmol/(30 min. cm) and K+ of 1 mM. Secretion of unlabeled inositol into the epididymal lumen was maintained for 5 hours at a concentration 6 times greater than that of the bathing solution. The turnover time of the epithelial pool of inositol was 4.5 hours, from which the intracellular concentration was estimated to be 26.6 mM. Transport was not reduced by metabolic inhibitors, and it was demonstrated that exchange diffusion across the basolateral membranes could drive the uptake in this region.

Estimation of free myo-inositol in milks of various species and its source in milk of rats (Rattus norvegicus)

Free myo-inositol content of milk of rats and other rodents fed standard laboratory chow is 80 mg/100 g as compared to about 4 mg/100 g for cow milk. Feeding experiments with graded amounts of myo-inositol demonstrated that the myo-inositol content of milk is strongly influenced by dietary intake. Radioactivity from either [carbon-14] glucose or [hydrogen-3] myo-inositol injected intraperitoneally was incorporated rapidly into milk myo-inositol. Thus, both mammary biosynthesis and active transport contribute to the high myo-inositol in rat milk, but the latter is quantitatively the more important.

The nutritional significance, metabolism, and function of myo-inositol and phosphatidylinositol in health and disease

Recent advances in nutritional and biochemical research have substantiated the importance of inositol as a dietary and cellular constituent. The processes involved in the metabolism of inositol and its derivatives in mammalian tissues have been characterized both in vivo and at the enzyme level. Biochemical functions elucidated for phosphatidylinositol in biological membranes include the mediation of cellular responses to external stimuli, nerve transmission, and the regulation of enzyme activity through specific interactions with various proteins. Inositol deficiency in animals has been shown to produce an accumulation of triglyceride in liver, intestinal lipodystrophy, and other abnormalities. The metabolic mechanisms giving rise to these latter phenomena have been extensively studied as a function of dietary inositol. Altered metabolism of inositol has been documented in patients with diabetes mellitus, chronic renal failure, galactosemia, and multiple sclerosis. A moderate increase in plasma and nerve inositol levels by dietary supplementation has been suggested as a means of treating diabetic neuropathy, although excessively high levels, such as are found in uremic patients, may be neurotoxic. A thorough consideration of the biochemical functions of inositol and a further characterization of various diseases with the aid of appropriate animal models may suggest a possible role for inositol and other dietary components in their prevention and treatment

scyllo-inositol and myo-inositol levels in tissues of the skate Raja erinacea

1. Many organs of the skate Raja erinacea have been found to contain scyllo-inositol levels that are much higher than myo-inositol, the opposite to that found in mammals. 2. Both inositols were found in all skate organs studied. 3. myo-Inosose-2 was found to accompany the inositols in many of these organs.

myo-Inositol binding and transport in brush border membranes of rat kidney

Using hypotonically treated brush border membranes, binding and transport of myo-inositol were examined. By hypotonic treatment, both total and non-specific uptake decreased significantly, but specific uptake was not affected. myo-Inositol release from membranes preloaded by incubation for 2 min was very rapid and about 98% of preloaded myo-inositol was released in 5 min of incubation. However, myo-inositol release from membranes preloaded by incubation for 20 min was fairly slow and 50% of myo-inositol remained in the membranes even after 10 min of incubation. Uptake of myo-inositol decreased by the increase of osmolarity in the medium. However, effect of osmolarity on the uptake was less significant when myo-inositol concentration was lower. Under conditions in which mainly binding occurred, myo-inositol binding to the membranes was measured. Two binding systems were demonstrated and high affinity site could bind 22 pmol/mg protein at most and the apparent Km value was 8.3 muM. Both binding and transport processes were dependent on Na+ and enhanced by Na+-gradient.