Metabolism of the 'organic osmolyte' glycerophosphorylcholine in isolated rat inner medullary collecting duct cells. II. Regulation by extracellular osmolality

In isolated inner medullary collecting duct (IMCD) cells requirements for the organic osmolyte glycerophosphorylcholine (GPC) vary with extracellular osmolality. To investigate mechanisms of osmotic adaptation GPC metabolism was studied under different osmotic conditions. In contrast to the GPC precursors choline and phosphatidylcholine (PC) cellular GPC was proportional to the osmolality. Hypotonic decrease in cellular GPC was mediated by fast initial release significantly exceeding the low hypertonic release. In long-term studies the total amount of GPC decreased significantly under hypotonic conditions but remained constant under hypertonic conditions resulting in a significant difference after 15 h. To investigate osmotic influences on GPC synthesis and GPC degradation studies with [methyl-3H]choline were performed. Pulse-chase experiments displayed no significant osmotic differences in PC synthesis or in PC degradation to GPC indicated by a similar specific activity of PC. This suggested that phospholipase A2 (PC degradation) was osmotically insensitive. A small and distinct metabolic PC pool may be responsible for high radioactive labeling of newly synthesized GPC which displayed a significantly higher specific activity under hypotonic conditions accompanied by a decrease in GPC amount. Therefore, a higher activity of glycerophosphorylcholine:choline phosphodiesterase (GPC:choline phosphodiesterase) (GPC degradation) under hypotonic conditions is proposed. Similar conclusions can be drawn from using phosphatidyl[methyl-3H]choline. As further evidence for osmotic regulation of GPC:choline phosphodiesterase the specific activity of choline displayed a significant hypotonic increase with chase time which may be equivalent to increased GPC degradation. Therefore, the in vitro experiments suggest that cellular GPC is regulated by an osmosensitive GPC:choline phosphodiesterase. Such a regulation also seems to be present during long-term in vivo experiments. No evidence was found for a genetic adaptation of GPC:choline phosphodiesterase in vivo.

Phosphorus-31 NMR study of the effects of UV on squirrel lenses

In a series of 31P NMR experiments the effects of near-UV radiation on the phosphorus metabolites in pigmented lenses from grey squirrels were studied; UV radiation ranged from 300 to 400 nm, peaking at 365 nm. Squirrels were killed, intact lenses removed, exposed to UV, put in an NMR perfusion cell, and 31P NMR spectra were recorded. In vitro exposure of squirrel lenses for periods ranging from 4 to 22 hr were made. ATP levels, as measured by NMR, decreased as the radiation dose increased. For a 22-hr exposure (121 J cm-2) ATP levels decreased by 67% relative to control values. At lower dose levels there was a correspondingly smaller decrease in ATP levels. Histological examination showed UV-irradiated lenses had disrupted epithelium, loss of attachment to cortical fiber cells and fluid accumulation under the capsule. It is clear that lenticular exposure to near-UV, predominantly at 365 nm (UV-A), does cause metabolic deterioration through degradation of normal ATP energy levels. The grey squirrel pigmented lens strongly absorbs near-UV radiation which enhances the observed metabolic changes and observed opacities. The results demonstrate the utility of using 31P NMR to evaluate metabolic changes associated with near-UV irradiation.

Metabolism of the 'organic osmolyte' glycerophosphorylcholine in isolated rat inner medullary collecting duct cells. I. Pathways for synthesis and degradation

In isolated inner medullary collecting duct (IMCD) cells the adaptation to changes in extracellular osmolarity involves alterations in intracellular content of organic osmolytes such as glycerophosphorylcholine (GPC), sorbitol and others. To elucidate the basis of such alterations, the metabolism of GPC in IMCD cells was investigated with the labeled GPC precursor [methyl-3H]choline. The lipids phosphatidylcholine (PC), lyso PC (LPC) and sphingomyelin (SM), as well as the non lipids phosphorylcholine (Pcholine), GPC and an unknown water-soluble compound could be identified as intermediates of choline metabolism. In pulse-chase experiments the radioactivity of PC expressed as specific activity was at a higher level than the other metabolites (> 10-fold after 1h). Extended chase incubations caused the specific activity of PC and LPC to decrease significantly. GPC was the only metabolite with a significant increase in specific activity under these conditions, suggesting that PC (via LPC) could be the precursor of GPC. In short-term pulse experiments the specific activity of PC and LPC was always significantly higher compared to the specific activity of GPC. Pulse chase incubations using phosphatidyl[methyl-3H]choline showed a significant decrease in specific activity of PC after 15 h accompanied by a significant increase in specific activity of LPC as well as GPC. Inhibition of the PC hydrolyzing enzyme phospholipase A2 revealed a significant increase in the specific activity of PC. For GPC, a significant decrease in the radioactive labeling could be detected. The total amount of PC decreased by 10% under these conditions whereas the amount of GPC decreased by 22% which was significantly higher because of GPC breakdown. GPC degradation was catalyzed by GPC: choline diesterase generating choline (and phosphoglycerol). Significant activity of GPC:phosphocholine diesterase could not be detected. Betaine synthesis from choline was also not present. The slowest, and probably rate-limiting reaction of GPC synthesis from choline may be the reaction of phosphocholine cytidylyltransferase generating CDP choline, since no radioactive CDP choline could be detected under any conditions. Thus, isolated IMCD cells possess the ability for the synthesis of GPC from choline via PC and LPC, as well as for the GPC degradation to choline (and phosphoglycerol). Significant experimental evidence for the occurrence of de-novo synthesis of GPC from choline or a precursor function of GPC for PC could not be detected. However, although the former possibility seems unlikely, a final proof is still lacking.

The effect of oestrogen ablation on the phospholipid metabolite content of primary and transplanted rat mammary tumours

The concentration of phospholipid metabolites was determined in chemical extracts from two types of rat mammary tumours and compared with proliferation data (S-phase fraction). One of the tumours was an oestrogen-sensitive transplanted tumour. In this tumour the concentration of phosphocholine (PC) and glycerophosphorylcholine (GPC) correlated strongly with the S-phase fraction but not with the number of cells actively synthesizing DNA. Oestrogen ablation resulted in tumour regression. Regressing tumours contained less PC and more GPC than those actively growing. The other tumour was induced in rats by intravenous administration of N-methyl N-nitrosourea. Phosphoethanolamine (PE), PC and GPC levels were not associated with the S-phase fraction in this tumour. Oestrogen ablation resulted in tumour regression. There was no significant difference between the regressing and growing tumours in PE, PC or GPC content.

Glycerophosphocholine release in human erythrocytes. 1H spin-echo and 31P-NMR evidence for lysophospholipase

The direct techniques of 1H spin-echo and 31P-NMR spectroscopy made it possible to monitor the release of glycerophosphocholine from lysophosphatidylcholine in lysates from human red blood cells. Thus, the existence of a lysophospholipase in human erythrocytes was confirmed using a new more direct method. No evidence for a phospholipase A2 activity in the haemolysates was found with the same approach; since this enzyme is present in leukocytes, the absence of activity helped verify the purity of the erythrocyte preparation. The lysophospholipase may constitute, with the earlier described glycerophosphocholine phosphodiesterase activity, a metabolic unit for the removal of haemolytic lysophosphatidylcholine which is formed in the erythrocyte membranes as well as taken up from the plasma.

Determinants of relative amounts of medullary organic osmolytes: effects of NaCl and urea differ

Sorbitol, inositol, betaine, taurine, and glycerophosphorylcholine (GPC) are organic osmolytes that accumulate in the renal inner medulla during antidiuresis. In the cultured cell model, high medium sodium increases all the cell osmolytes and high medium urea increases cell GPC and inositol. It has been difficult, however, to discriminate between the effects of sodium and urea on organic osmolytes in water-deprived animals. To determine the nature of the in vivo responses of osmolyte accumulation induced by extracellular sodium or urea, we measured the medullary organic osmolytes and tested the degree of their linear correlation with urine and tissue parameters in control, dehydrated, salt-loaded, and urea-loaded rats. All of the osmolytes except myo-inositol increased in salt-loaded rats. Betaine and sorbitol contents in dehydrated rats were less than in salt-loaded rats, but other osmolytes increased significantly. Conversely, in urea-loaded rats, only GPC increased significantly, whereas either no change occurred for other osmolytes or sometimes betaine and sorbitol levels decreased. These data suggest that high sodium increases all of the osmolytes except myoinositol, whereas high urea increases only GPC and may decrease the renal medullary contents of betaine and sorbitol. We also demonstrated, using linear regression analysis, that urea and electrolyte in urine as well as tissue correlate well with each osmolyte measured.

Organic osmolytes in human and other mammalian kidneys

Osmotically-active organic solutes, or osmolytes, have been found in high concentration in the renal inner medulla of a wide variety of mammalian species, but their existence in human kidneys has not yet been shown. The aim of this study was to demonstrate the presence of osmolytes in the human kidney. Human tissues were obtained from kidneys removed surgically for diseases which involved only one pole of the kidney; in most cases this was a tumor. Animal kidneys analyzed were from dogs, pigs and rabbits. Inner medulla and cortex tissue samples were analyzed and found to contain the organic osmolytes glycine betaine, myo-inositol, sorbitol and glycerophosphorylcholine. The levels were much higher in the medulla than in the cortex. Further dissection of the human kidneys showed that sorbitol, glycerophosphorylcholine and glycine betaine were maximally concentrated at the papillary tip, while myo-inositol was found in highest concentration at the papillary base. Osmolytes were in low concentrations or undetectable in rabbit skeletal muscle, ureter and bladder. The organic osmolytes detected are likely to be physiologically important in humans. Studies in other mammals can be used as models for the investigation of the osmolyte system in human kidney function.

Effects of dietary protein and salt on rat renal osmolytes: covariation in urea and GPC contents

Renal medullary cells contain high levels of (glycine) betaine, glycerophosphorylcholine (GPC), myo-inositol, and sorbitol. Two functions of these have been proposed: 1) that they are compatible osmolytes which regulate cell volume (against high external NaCl) without inhibiting proteins and 2) that methylamines (GPC and betaine) are counteracting osmolytes which stabilize proteins against perturbation from high renal urea. As a test of the latter, osmolyte contents in kidney medullas were measured in rats subjected to three types of dietary manipulation: 1) diets with protein and NaCl contents varied oppositely, 2) diets with a constant low NaCl and varied protein content, and 3) a low-calorie diet. With low-protein and low-calorie diets, only renal contents of urea, GPC, and inositol decreased; betaine and sorbitol contents increased such that contents of total nonurea organic osmolytes remained constant. With high-protein diets, only renal contents of sodium, urea, and GPC increased, with the latter giving total organic osmolytes a consistent correlation to sodium. Across all diets, the only consistent (linear) correlations were 1) between urea and GPC contents, supporting previous suggestions that GPC is the major counteractant to urea, and 2) between total organic osmolytes and sodium (but not urea) contents, as predicted by the compatible osmolytes hypothesis.

Renal sorbitol, myo-inositol and glycerophosphorylcholine in streptozotocin-diabetic rats

The polyols, sorbitol and myo-inositol, seem to be involved in the development of diabetic complications of different organs. High concentrations of both polyols were found in kidney medulla in addition to trimethylamines. To investigate the influence of diabetes mellitus on the regulation of both polyols and glycerophosphorylcholine in kidney, these osmolytes were quantitated enzymatically along the corticopapillary axis in untreated, streptozotocin-diabetic and insulin-treated streptozotocin-diabetic rats. In control animals three individual osmolyte patterns were found: a steep gradient of sorbitol in the papilla, increasing amounts of glycerophosphorylcholine from the outer medulla to the papilla, and nearly equal amounts of myo-inositol in the renal medulla, decreasing towards the cortex. Diabetic rats exhibit an up to fourfold increase of inner medullary sorbitol, whereas myo-inositol was only elevated in the outer medulla. Glycerophosphorylcholine was lowered in the papillary tip and elevated in the outer medulla and cortex. Insulin treatment reduced sorbitol to a concentration between those of diabetic and control rats, caused a restoration of glycerophosphorylcholine in the papillary tip and outer medulla to control values, and increased cortical myo-inositol. These data confirm previous in vitro data, which show that papillary sorbitol specifically increases in hyperglycaemic states, thereby counteracting the increased extracellular tonicity due to elevated tissue glucose concentrations. Imbalance of extra- vs intracellular osmolality during insulin treatment may be involved in the pathomechanism of renal papillary necrosis.

Effects of cholinergic agents on the metabolism of choline in muscle from Ascaris suum

The incorporation of [methyl-14C]choline into the choline-containing compounds of Ascaris suum muscle and the effects of acetylcholine and its agonists, carbachol and levamisole, on this incorporation were studied. Previous experiments reported a stimulation of phosphatidylcholine (lecithin) metabolism upon the administration of acetylcholine. Acetylcholine administered in vitro to A. suum muscle and body wall preparations resulted in a stimulation of phospholipase C activity that, in turn, produced an increased rate of hydrolysis of phosphatidylcholine to the corresponding diacylglyceride (DAG). The DAG, in turn, may act as a second messenger as it is required for the activation of an A. suum protein kinase C. Evidence presented here is in accordance with this hypothesis. The administration of cholinergics resulted in a stimulation of phosphatidylcholine turnover. Acetylcholine also stimulated isotope incorporation into glycerophosphorylcholine, presumably as a consequence of enhanced phospholipid turnover. These events appear to be associated with the ligand binding to the acetylcholine receptors of the A. suum muscle. Choline kinase activity is suggested in order to maintain the observed high ratio of phosphorylcholine to choline. Findings indicate that in the parasite's muscle phosphatidylcholine metabolism may be linked to receptor-dependent responses and subsequent signal transduction.

Release of choline by phospholipase D and a related phosphoric diester hydrolase in human erythrocytes. 1H spin-echo n.m.r. studies

A previously detected phosphatidylcholine-specific phospholipase D from lysates of human red blood cells has been further characterized by 1H spin-echo n.m.r. spectroscopy. A second choline-releasing enzymic activity was observed after addition of glycerophosphocholine. Both of these phosphoric diester hydrolase activities were activated to different extents by different concentrations of calcium ions. Differences between the two activities were also observed on inhibition by barium and phosphate ions. These distinct, choline-yielding, reactions which occur in the cytoplasm of red blood cells may be involved in the regulation of the levels of membrane phosphatidylcholine.

Evidence for a membrane defect in Alzheimer disease brain

To determine whether neurodegeneration in Alzheimer disease brain is associated with degradation of structural cell membrane molecules, we measured tissue levels of the major membrane phospholipids and their metabolites in three cortical areas from postmortem brains of Alzheimer disease patients and matched controls. Among phospholipids, there was a significant (P less than 0.05) decrease in phosphatidylcholine and phosphatidylethanolamine. There were significant (P less than 0.05) decreases in the initial phospholipid precursors choline and ethanolamine and increases in the phospholipid deacylation product glycerophosphocholine. The ratios of glycerophosphocholine to choline and glycerophosphoethanolamine to ethanolamine were significantly increased in all examined Alzheimer disease brain regions. The activity of the glycerophosphocholine-degrading enzyme glycerophosphocholine choline-phosphodiesterase was normal in Alzheimer disease brain. There was a near stoichiometric relationship between the decrease in phospholipids and the increase of phospholipid catabolites. These data are consistent with increased membrane phospholipid degradation in Alzheimer disease brain. Similar phospholipid abnormalities were not detected in brains of patients with Huntington disease, Parkinson disease, or Down syndrome. We conclude that the phospholipid abnormalities described here are not an epiphenomenon of neurodegeneration and that they may be specific for the pathomechanism of Alzheimer disease.

Induction of interleukin-6 release by interleukin-1 in rat anterior pituitary cells in vitro: evidence for an eicosanoid-dependent mechanism

We have reported previously that a subpopulation(s) of anterior pituitary cells released IL-6 and that this release was stimulated by interleukin-1 (IL-1) through a non-cAMP-dependent mechanism. We now report that IL-1 induces IL-6 release from anterior pituitary cells in an eicosanoid-dependent manner. Dispersed rat anterior pituitary cells were briefly prelabeled (2-3 h) with [3H]arachidonic acid (AA) to esterify the fatty acid within the lipid pool. Incubation of these prelabeled cells with 25 ng/ml IL-1 beta caused an increase only within 1-2 min in the amount of free [3H]AA detected in the extracts of the cells. During 15- to 30-min incubations, IL-1 beta (25 ng/ml) caused an increased accumulation of [3H]AA in the incubation medium which reached levels similar to those induced by 100 nM TRH. Perifused anterior pituitary cells responded to IL-1 beta (25 ng/ml) with a rapid (less than 2 min), biphasic, and reversible efflux of [3H]AA. The [3H]AA appears to have been derived from choline phospholipids, as formation of [3H]glycerophosphorylcholine was substantially increased by exposure of [3H]choline-prelabeled cells to either IL-1 alpha (171%) or IL-1 beta (236%); in addition, the complete deacylation of phosphatidylcholine suggests that other fatty acid species are liberated as a consequence of IL-1 receptor activation and, thus, may also contribute to the actions of IL-1 alpha and IL-1 beta. However, the levels of [3H]phosphorylcholine and [3H]choline were unchanged as well as those of catabolites of other lipid species. These data suggested an involvement of phospholipase-A2 (PLA2) in mediating the IL-1 induction of IL-6 release. Subsequently, we used inhibitors of the PLA2, cyclooxygenase, and lipoxygenase enzymes to investigate a possible role for the generation of AA and its subsequent enzymatic conversion in the signal transduction pathway activated by IL-1. The PLA2 inhibitor aristolochic acid (10 microM) blocked IL-1 beta-induced IL-6 release and the release of IL-6 caused by Pyrularia pubera thionin (5 micrograms/ml), a stimulator of PLA2 activity. The cyclooxygenase inhibitor indomethacin (10 microM) did not inhibit IL-1 beta-induced IL-6 release. In contrast, the general lipoxygenase inhibitor nordihydroguaiaretic acid (10 microM) and the more specific 5-lipoxygenase inhibitors AA861 and RHC5901 (both 10 microM) reduced basal and blocked IL-1 beta-induced IL-6-release.(ABSTRACT TRUNCATED AT 400 WORDS)

Alterations of phospholipid metabolites in postmortem brain from patients with Alzheimer's disease

To test the hypothesis that brain cell membranes degenerate in Alzheimer's disease (AD), we measured the levels of phospholipids, their water-soluble metabolites, and glycerophosphocholine (GPC) cholinephosphodiesterase activity in postmortem brain tissue from patients with AD and age-matched controls. We found significantly higher levels of the phospholipid catabolite GPC in AD brain. In contrast, choline and ethanolamine levels were significantly lower in AD, and phospholipid levels were slightly decreased. Furthermore, in AD the activity of the GPC-degrading enzyme GPC cholinephosphodiesterase was unaltered. Our results indicate that membrane phospholipid catabolism is increased in AD brain. Inasmuch as the tissue levels of initial phospholipid precursors were decreased, we suggest that phospholipid turnover is elevated in this neurodegenerative disease.

Osmotically regulated taurine content in rat renal inner medulla

During antidiuresis renal medullary cells were previously found to accumulate large amounts of organic osmolytes, namely sorbitol, myo-inositol, glycerophosphorylcholine (GPC), and betaine. Large quantities of amino acids are also present in the renal medulla, but it has been questionable whether the levels of medullary amino acids are osmotically regulated. Therefore we directly measured 26 different amino acids, as well as sorbitol, myo-inositol, GPC, and betaine alone the corticomedullary axis of rats that were either salt loaded, after infusion of hypertonic NaCl solution, or were hydrated, after infusion of hypotonic NaCl solution. The amounts of sorbitol, myo-inositol, GPC, and betaine are greater in the inner medullas of salt-loaded rats compared with hydrated rats. In addition, the amount of taurine is much greater in the inner medullas of salt-loaded rats. Aspartic acid also increases in salt-loaded rats but to a lesser extent. There are substantial gradients of taurine, aspartic acid, and some of 24 other measured amino acids along the corticomedullary axis. However, taurine and aspartic acid are the only measured amino acids that increase significantly during salt loading.

A highly sensitive chemiluminescent assay for glycerylphosphorylcholine in human seminal plasma

Glycerylphosphorylcholine (GPC), one of the major phosphorus-containing-choline compounds of seminal plasma, is secreted mainly by the epididymal epithelium under androgenic control. This study reports a new method that uses chemiluminescence to determine seminal GPC content, comparing it with a spectrophotometric technique. The results, obtained with both techniques studying 20 fertile patients (as control), 35 infertile patients with normospermia, 23 infertile patients with oligozoospermia and impaired motility and 9 patients with excretory azoospermia, demonstrate that the GPC chemiluminescent assay is more sensitive, simple and rapid than the spectrophotometric assay. Our data confirm that GPC may be used as a marker of vas deferens and ejaculatory duct perviousness, suggesting a possible role of this glycerophosphodiester in sperm motility.

Lipid metabolism in T47D human breast cancer cells: 31P and 13C-NMR studies of choline and ethanolamine uptake

31P and 13C-NMR were used to determine the kinetics of choline and ethanolamine incorporation in T47D clone 11 human breast cancer cells grown as small (150 microns) spheroids. Spheroids were perfused inside the spectrometer with 1,2-13C-labeled choline or 1,2-13C-labeled ethanolamine (0.028 mM) and the buildup of labeled phosphoryl-choline (PC) or phosphorylethanolamine (PE) was monitored. Alternatively the PC and GPC pools were prelabeled with 13C and the reduction of label was monitored. 31P spectra were recorded from which the overall energetic status as well as total pool sizes could be determined. The ATP content was 8 +/- 1 fmol/cell, and the total PC and PE pool sizes were 16 and 14 fmol/cell, respectively. PC either increased by 50% over 24 h or remained constant, while PE remained constant in medium without added ethanolamine but increased 2-fold within 30 h in medium containing ethanolamine, indicating a dependence on precursor concentration in the medium. The 31P and 13C data yielded similar kinetic results: the rate of the enzymes phosphocholine kinase and phosphoethanolamine kinase were both on the order of 1.0 fmol/cell per h, and the rate constants for CTP:phosphocholine cytidyltransferase and CTP:phosphoethanolamine kinase were 0.06 h-1 for both enzymes. The kinetics of choline incorporation did not alter in the presence of 0.028 mM ethanolamine indicating that they have non-competing pathways.

Evidence that hydrolysis of ethanolamine plasmalogens triggers synthesis of platelet-activating factor via a transacylation reaction

Addition of 1-O-alk-1'-enyl-2-lyso-sn-glycero-3-phosphoethanolamine (alkenyl-lyso-GPE) to human neutrophil membrane preparations containing 1-O-[3H]hexadecyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (1-O-[3H]alkyl-2-arachidonoyl-GPC) resulted in rapid deacylation of the 1-O-[3H]alkyl-2-arachidonoyl-GPC to 1-O-[3H]alkyl-2-lyso-GPC (lyso-platelet-activating factor, lyso-PAF). When acetyl-CoA was included in the incubation mixture, the [3H]lyso-PAF was converted to [3H]PAF. Studies of [3H]arachidonate-labeled neutrophils permeabilized with Staphlococcus aureus alpha-toxin revealed a major shift of labeled [3H]arachidonate from the choline to the ethanolamine-containing phosphoglycerides upon addition of alkenyl-lyso-GPE. The studies indicated that lyso-PAF is formed in the system by the transfer of arachidonate from 1-O-alkyl-2-arachidonoyl-GPC to the alkenyl-lyso-GPE by a CoA-independent transacylase reaction. Mass measurements revealed a rapid loss of arachidonate from 1-radyl-2-acyl-GPE and a concomitant increase in alkenyl-lyso-GPE upon stimulation of the neutrophils by ionophore A23187. Based on these and other findings, a pathway is proposed that may play a significant, if not obligatory, role in the synthesis of PAF in intact stimulated neutrophils. It has been widely accepted that phospholipase A2 acts directly on 1-O-alkyl-2-arachidonoyl-GPC as the first step in the synthesis of PAF via formation of lyso-PAF. In the proposed scheme, phospholipase A2, upon stimulation, acts rapidly on ethanolamine plasmalogen selectively releasing arachidonic acid and generating alkenyl-lyso-GPE. The CoA-independent transacylase then selectively transfers arachidonate from 1-radyl-2-arachidonoyl-GPC to the alkenyl-lyso-GPE generating lyso-PAF, which is then acetylated to form PAF. The interactions outlined can account for the synthesis of 1-acyl-2-acetyl-GPC, 1-O-alk-1'-enyl-2-acetyl-GPE, and eicosanoids, in parallel with PAF.

Renal medullary organic osmolytes

Sorbitol, inositol, GPC, and betaine are the predominant organic osmolytes in renal medullary cells. They protect the cells from harmful effects of the high interstitial NaCl and urea concentrations that occur normally in the renal medulla with operation of the urinary concentrating mechanism. Their levels correlate with extracellular NaCl concentration and, in the case of GPC, also with urea. Sorbitol is synthesized from glucose in a reaction catalyzed by aldose reductase. Inositol and betaine are transported into the cell. Glycerophosphorylcholine synthesis is dependent on choline. The transcription of aldose reductase and the transport of betaine and inositol are regulated, dependent on the degree of hypertonicity. Normal organic osmolyte regulation contributes to the survival and growth of medullary cells in their hyperosmolal environment, and defective regulation can damage them.

Evidence that the rate of phosphatidylcholine catabolism is regulated in cultured rat hepatocytes

The regulation of phosphatidylcholine (PC) catabolism has been studied in choline-deficient rat hepatocytes. Supplementation of choline-deficient hepatocytes, prelabeled with [3H]choline, with 100 microM choline increased the rate of PC catabolism by approx. 2-fold. The major product of PC degradation was glycerophosphocholine in both choline-deficient and choline-supplemented cells. Choline supplementation decreased the radioactivity recovered in lysoPC by 50%. This effect was accompanied by a 2-fold increase of labeled glycerophosphocholine. Comparable results were obtained when PC of the cells was prelabeled with [3H]methionine or [3H]glycerol. The activity of phospholipase A in cytosol, mitochondria and microsomes isolated from choline-deficient rat liver was similar to the activity in control liver, when determined with [3H]PC vesicles as the substrate. Measurement of the activity of phospholipase A with endogenously [3H]choline-labeled PC showed that the formation of lysoPC in mitochondria isolated form choline-supplemented cells was 40% lower than in choline-deficient cells. Alternatively, the formation of [3H]glycerophosphocholine and [3H]choline in microsomes from choline-supplemented cells was significantly higher (1.4-fold) than in microsomes from choline-deficient cells. These results suggest that the rate of PC catabolism is regulated in rat hepatocytes and that the concentration of PC might be an important regulatory factor.

Creatine phosphate as energy source in the cerulein-stimulated rat pancreas study by 31P nuclear magnetic resonance

Stimulation of the rat exocrine pancreas by cerulein induces a variety of cellular processes, some of which require the expenditure of energy. In this study, changes in the amounts of various energy metabolites, including creatine phosphate (PCr), ATP, and ADP were determined by high-resolution 31P NMR spectroscopy. The spectrum of a perchloric acid extract of pancreas from the 48 h fasted rat was taken as a reference for comparison of 31P NMR spectra recorded after stimulation by cerulein. The NMR results obtained from rat pancreas stimulated in vivo by cerulein (3, 5, 10, 20, 40 min) were compared to those determined by HPLC. We show that during hormonal stimulation, the relative concentrations of PCr in the pancreas of the fasted rat rise significantly (p less than 0.02), reach a maximum at 10 min, fall between the 10th and 20th min, and then return to the relatively low levels observed in controls. On the other hand, the relative concentrations of ATP fall during the first 10 min after stimulation by cerulein, then rise significantly between the 10th and 20th min, whereas the levels of ADP rise during the first 10 min and fall between the 10th and 20th min. The energy required for exocytosis was assumed to be supplied by ATP synthesized in acinar cells. The 31P NMR results indicated that this ATP was derived from phosphorylation of ADP by PCr, and that large amounts of PCr are synthesized during the first minutes after cerulein stimulation. In addition, a significant rise in glycerophosphocholine was observed after cerulein stimulation, which was attributed to an enhanced catabolism of membranes and an increase in phospholipid turnover. Injection of cerulein antagonists, such as asperlicin or lorglumide, inhibited the effects of cerulein stimulation on energy metabolites. Furthermore, no changes were observed after injection of secretin, a hormone that stimulates secretion of bicarbonate. However, the analog of cerulein, pentagastrin, produced the same effects as cerulein, although to a lesser extent.