Amino acid loss during volume regulatory decrease in cultured chick heart cells

The contribution of newly synthesized cholesterol to bile salt synthesis in rats quantified by mass isotopomer distribution analysis

A new stable isotope procedure has been developed and validated in rats, applying [1-(13)C]acetate infusion to quantify the production of bile salts from de novo synthesized cholesterol making use of the mass isotopomer distribution analysis (MIDA) principle. Ions (m/z) 458-461, 370-373 and 285-288 were monitored by GC/MS (EI-mode) for the methyl trimethylsilylether derivatives of cholate, chenodeoxycholate and beta-muricholate, respectively. Rats with intact exteriorized enterohepatic circulation and rats with chronic bile diversion were infused with [1-(13)C]acetate for up to 14 h. After 10 h of infusion the enterohepatic circulation of the intact group was interrupted to deplete the existing bile salt pool (acute bile diversion). The fractions of biliary cholesterol and individual bile salts derived from newly synthesized cholesterol were determined by MIDA at t=14 h. In rats with acute bile diversion, these fractions were 20, 25, 27 and 23% for biliary cholesterol, cholate, chenodeoxycholate and beta-muricholate, respectively. After bile diversion for 8 days to induce hepatic cholesterol and bile salt synthesis, these fractions increased significantly to 32, 47, 41 and 47%, respectively. Calculated enrichments of the acetyl-CoA precursor pools were similar for all bile salts and biliary cholesterol within the two rat groups. However, chronic enterohepatic interruption decreased the acetyl-CoA pool size almost two-fold. We conclude that MIDA is a validated new stable isotope technique for studying the synthetic pathway from acetyl-CoA to bile salts. This technique provides an important new tool for studying bile salt metabolism in humans using stable isotopes.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Changes in cell volume induced by ion channel flux in guinea-pig cardiac myocytes

1. The cell width of guinea-pig ventricular myocytes was measured using an optic device during patch-clamp experiments and the relationship between the ion channel flux and changes in cell volume was examined. 2. On superfusing myocytes with 50, 70, 150 and 200% osmotic solutions, the relative cell width changed to 121.1 (n = 4), 110.8 (n = 27), 87.1 (n = 6) and 82.6% (n = 6) of control, respectively. Changes in cell length were less than 2% in these test solutions. 3. The application of 300 nmol/L isoprenaline to myocytes swollen in the 70% hypotonic solution induced a decrease in cell width from 111.2 to 106.2% (n = 13). The application of isoprenaline in the isotonic solution also induced a decrease in cell width to 96.5% in eight of 13 cells. A membrane depolarization of 2-4 mV accompanied the isoprenaline-induced decrease in volume. In the remaining five cells, neither an obvious isoprenaline-induced decrease in volume nor membrane depolarization was observed. Under ruptured whole-cell voltage clamp conditions, the activation of inward isoprenaline-induced Cl- current decreased cell width. 4. Cell width was seen to either decrease or increase when a large outward or inward K+ current, respectively, was induced by shifting the holding potential or by applying 200 mumol/L pinacidil. Under gramicidin-perforated whole-cell clamp conditions, the cell width did not change, even when a large inward K+ current was induced. 5. When the test solution was applied to half of an elongated myocyte by using a micropipette, the cell width increased or decreased in the part exposed to the hypotonic or hypertonic test solutions, respectively. In contrast, in the other half of the elongated myocyte, the cell width responded in the opposite direction. 6. It is concluded that a continuous ionic flux through ion channels is capable of inducing changes in cell volume by generating a localized osmotic gradient across the cardiac sarcolemma.

The pravastatin-induced decrease of biliary cholesterol secretion is not directly related to an inhibition of cholesterol synthesis in humans

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors have been reported to suppress biliary cholesterol secretion and saturation. It remains unproven whether this is mediated by inhibition of cholesterol synthesis. Therefore, the effect of a long-term administration of pravastatin on cholesterogenesis and on biliary lipid secretion was investigated in seven healthy volunteers. Placebo or 40 mg of pravastatin were taken daily at bedtime for 5 weeks using a double-blind crossover design. During the last week, 12 hours after the last drug intake 0.04 mmol [1-13C]acetate/kg. h and 0.5 g polyethylene glycol 4,000/h were infused intraduodenally for 15 hours. Plasma and duodenal bile samples were collected hourly. Thereafter, the decay of [13C]labeled plasma cholesterol was measured during the following 3 days. The fractional and absolute syntheses of plasma and biliary cholesterol were determined by gas chromatography mass spectrometry using mass isotopomer distribution analysis. At the end of the pravastatin period plasma total and low-density lipoprotein (LDL) cholesterol had decreased by 20% and 24%, respectively. Similarly, pravastatin suppressed biliary secretion rates of cholesterol, total bile acids and phospholipids (P <.05) by 46%, 36%, and 51%. As a consequence, cholesterol saturation index remained unchanged. However, fractional syntheses of cholesterol were comparable (P >.05) on placebo compared with pravastatin with 3.1% versus 4.0% in plasma and 4.3% versus 5.2% in bile after 15 hours, respectively. The mean absolute synthesis rates amounted to 0.3 mg/kg/h on placebo versus 0.4 on pravastatin (P >. 05). In conclusion, the pravastatin-induced reduction of biliary cholesterol secretion is not directly related to an inhibition of cholesterol synthesis.

Quantitative determination of free intracellular amino acids in single human polymorphonuclear leucocytes. Recent developments in sample preparation and high-performance liquid chromatography

The described procedure allows quantitative, highly precise and reproducible analysis of free amino acid concentrations in single polymorphonuclear leucocytes (PMLs). This method is superior to previously described procedures with regard to sample size, PML separation, sample preparation and stability, as well as the chosen fluorescence high-performance liquid chromatography procedure, and can satisfy the high demands for ultra-sensitive and comprehensive amino acid analysis, especially for the continuous surveillance of severe diseases and organ dysfunction.

Chloride channel inhibition blocks the protection of ischemic preconditioning and hypo-osmotic stress in rabbit ventricular myocardium

The objective of this study was to examine the role of chloride (Cl-) channels in the myocardial protection of ischemic preconditioning (IP). Isolated rabbit ventricular myocytes were preconditioned with 10-minute simulated ischemia (SI) and 20-minute simulated reperfusion (SR) or not preconditioned (control). The myocytes then received 180-minute SI or 45-minute SI/120-minute SR. Indanyloxyacetic acid 94 (IAA-94, 10 micromol/L) or 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 1 micromol/L) was administered before IP or before SI or SI/SR to inhibit Cl- channels. Electrophysiological studies indicate that these drugs, at the concentrations used, selectively abolished Cl- currents activated under hypo-osmotic conditions (215 versus 290 mOsm). IP significantly (P<0.001) reduced the percentage of dead myocytes after 60-minute (30.8+/-1.3%, mean+/-SEM), 90-minute (35.3+/-1.3%), and 120-minute (39.2+/-1.7%) SI compared with controls (44.7+/-1.6%, 54.5+/-1.3%, and 58.9+/-1.8%, respectively) and after 45-minute SI/120-minute SR (36.3+/-0.6%) compared with control (56.6+/-2.2%). Hypo-osmotic stress also produced protection similar to IP. IAA-94 or NPPB abolished the protection of both IP and hypo-osmotic stress. In buffer-perfused rabbit hearts preconditioned with three 5-minute ischemia/10-minute reperfusion cycles given before the 40-minute long ischemia and 60-minute reperfusion, IP significantly (P<0.0001) reduced infarct size (IP+vehicle, 4.7+/-0.9%, versus control+vehicle, 26.6+/-3.3%; mean+/-SEM). Again, IAA-94 or NPPB abolished the protection of IP. Our results implicate Cl- channels in the IP protection of the myocardium against ischemic/reperfusion injury and demonstrate that hypo-osmotic stress is capable of preconditioning cardiomyocytes.

Osmotic shock: modulation of contractile function, pHi, and ischemic damage in perfused guinea pig heart

To determine the contribution of changes in extracellular osmolarity to ischemic injury, isolated guinea pig hearts were perfused with hyposmotic (220 mosM) or hyperosmotic (380 mosM) buffer. 31P NMR spectroscopy was used to follow changes in intracellular pH (pHi) and energetics. Hyposmotic buffer decreased myocardial developed pressure by 30 +/- 2% and pHi by 0.02 +/- 0.01 unit, whereas hyperosmotic buffer increased myocardial developed pressure by 34 +/- 1% and pHi by 0.14 +/- 0.01 unit. All hearts recovered to control values on restoration of isosmotic (300 mosM) buffer. The hyperosmolar-induced intracellular alkalosis and developed pressure increase were not prevented by inhibition of Na+/H+ exchange with use of 1 microM HOE-642 but were abolished with use of bicarbonate-free buffers. After 20 min of total global ischemia, hearts perfused with hyposmotic buffer showed significantly greater recoveries of developed pressure, phosphocreatine, and ATP than control hearts, but hearts perfused with hyperosmotic buffer did not recover after ischemia. In conclusion, buffer osmolarities between 220 and 380 mosM alter myocardial pHi and developed pressure but are not deleterious during perfusion. However, buffer osmolarity significantly alters the extent of myocardial ischemic injury.

Effects of arginine vasopressin on cell volume regulation in brain astrocyte in culture

Astrocytes initially swell when exposed to hypotonic medium but rapidly return to normal volume by the process of regulatory volume decrease (RVD). The role that arginine vasopressin (AVP) plays in hypotonically mediated RVD in astrocytes is unknown. This study was therefore designed to determine whether AVP might play a role in astrocyte RVD. With the use of 3-O-[3H]methyl-D-glucose to determine water space, AVP treatment resulted in significantly increased 3-O-methyl-D-glucose water space within 30 s of hypotonic exposure (P = 0.0001) and remained significantly elevated above baseline (1. 75 microliter/mg protein) at 5 min (P < 0.021). In contrast, in untreated cells, complete RVD was achieved by 5 min. At 30 s, cell volume with AVP treatment was 37% greater than in cells that received no treatment (2.9 vs. 2.26 microliter/mg protein, respectively; P < 0.006). The rate of cell volume increase (dV/dt) over 30 s was highly significant (0.038 vs. 0.019 microliter. mg protein-1. s-1 in the AVP-treated vs. untreated group; P = 0.0004 by regression analysis). Additionally, the rate of cell volume decrease over the next 4.5 min was also significantly greater with vasopressin treatment (-dV/dt = 0.0027 vs. 0.0013 microliter. mg protein-1. s-1; P = 0.0306). The effect of AVP was concentration dependent with EC50 = 3.5 nM. To determine whether AVP action was receptor mediated, we performed RVD studies in the presence of the V1-receptor antagonists benzamil and ethylisopropryl amiloride and the V2-receptor agonist 1-desamino-8-D-arginine vasopressin (DDAVP). Both V1-receptor antagonists significantly inhibited AVP-mediated volume increase by 40-47% (P < 0.005), whereas DDAVP had no stimulatory effects above control. Taken together, these data suggest that AVP treatment of brain astrocytes in culture appears to increase 3-O-methyl-D-glucose water space during RVD through V1 receptor-mediated mechanisms. The significance of these findings is presently unclear.

Shape and size changes induced by taurine depletion in neonatal cardiomyocytes

Taurine is a very important organic osmolyte in most adult cells. Because of this property it has been proposed that large changes in the intracellular content of taurine can osmotically stress the cell, causing changes in its size and shape. This hypothesis was examined by measuring cell dimensions of taurine deficient cardiomyocytes using confocal microscopy. Incubation of isolated neonatal rat myocytes with medium containing 5 mM beta-alanine led to a 55% decrease in intracellular taurine content. Associated with the loss of taurine was a reduction in cell size. Two factors contributed to the change in cell size. First, there was a shift in cell shape, favoring the smaller of the two cellular configurations commonly found in the myocyte cell culture. Second, the size of the polyhedral configuration was reduced after beta-alanine treatment. These same two events also contributed to size reduction in cardiomyocytes incubated with medium containing 30 mM mannitol. Nonetheless, some qualitative differences exist between cells osmotically stressed by increasing the osmolality of the incubation medium and decreasing intracellular osmolality. The results support a role for taurine in the regulation of osmotic balance in the neonatal cardiomyocyte.

Protein kinase inhibitors attenuate cardiac swelling-induced amino acid release in the rat

Rat Langendorff heart preparations have been used to study the efflux of cardiac amino acids into coronary artery perfusates during brief (5-min) periods of exposure to hyposmotic stress (70 mM NaCl). Coronary flow rates, heart rates and intra-aortic pressures were recorded. Amino acid levels were measured by high-performance liquid chromatography. Hyposmotic stress caused marked percentage increases in taurine, glutamate and aspartate levels in the coronary perfusate, with smaller increases in phosphoethanolamine, glycine and alanine and non-significant increases in serine and glutamine. Amino acid levels declined during reperfusion with isosmotic Krebs-Henseleit bicarbonate buffer. Inhibition of protein kinase C with chelerythrine chloride (5 microM) depressed the osmotically-induced release of aspartate, glutamate, taurine and glycine. The protein tyrosine kinase inhibitor, genistein, reduced the anisosmotic efflux of aspartate, glutamate, taurine and phosphoethanolamine. Lavendustin A, another inhibitor of tyrosine kinase, depressed the osmotically evoked release of aspartate, glutamate and taurine. These studies demonstrate the involvement of protein kinase C and tyrosine kinases in the efflux of amino acids from the osmotically challenged rat heart and imply that these enzymes are involved in the mechanisms responsible for volume regulation by cardiac cells.

Cardiac cell volume: crystal clear or murky waters? A comparison with other cell types

The osmolarity of bodily fluids is strictly controlled so that most cells do not experience changes in osmotic pressure under normal conditions, but osmotic changes can occur in pathological states such as ischemia, septic shock, and diabetic coma. The primary effect of a change in osmolarity is to acutely alter cell volume. If the osmolarity around a cell is decreased, the cell swells, and if increased, it shrinks. In order to tolerate changes in osmolarity, cells have evolved volume regulatory mechanisms activated by osmotic challenge to normalise cell volume and maintain normal function. In the heart, osmotic stress is encountered during a period of myocardial ischemia when metabolites such as lactate accumulate intracellularly and to a certain degree extracellularly, and cause cell swelling. This swelling may be exacerbated further on reperfusion when the hyperosmotic extracellular milieu is replaced by normosmotic blood. In this review, we describe the theory and mechanisms of volume regulation, and draw on findings in extracardiac tissues, such as kidney, whose responses to osmotic change are well characterised. We then describe cell volume regulation in the heart, with particular emphasis on the effect of myocardial ischemia. Finally, we describe the consequences of osmotic cell swelling for the cell and for the heart, and discuss the implications for antiarrhythmic drug efficacy. Using computer modelling, we have summated the changes induced by cell swelling, and predict that swelling will shorten the action potential. This finding indicates that cell swelling is an important component of the response to ischemia, a component modulating the excitability of the heart.

Amino acid release during volume regulation by cardiac cells: cellular mechanisms

Mechanisms of amino acid efflux during volume regulation in hypoosmotically treated isolated rat hearts were studied by collecting the coronary artery perfusate and analysis by high pressure liquid chromatography. Hypoosmotic stress resulted in marked percentage increases in perfusate taurine, aspartate and glutamate levels, smaller increases in phosphoethanolamine, glycine and alanine and non-significant increases in serine and glutamine. Amino acid levels declined during reperfusion with isosmotic perfusate. The anion channel blocker 4-acetamido-4-isothiocyanostilbene-2:2'-disulfonic acid (SITS, 500 microM) significantly reduced hypoosmotic release of taurine, aspartate, glutamate and glycine. Furosemide reduced hypoosmotically-evoked releases of taurine, glycine, alanine and phosphoethanolamine. The polyunsaturated amino acids, arachidonic and linoleic also reduced amino acid efflux. Phospholipase A2 inhibition with 7,7-dimethyleicosadienoic acid (DEDA, 2 microM) reduced osmotically-evoked releases of taurine, aspartate and glutamate. 4-Bromophenacyl bromide (1 microM) inhibited osmotically-evoked release of glutamate and glycine. Combined applications of SITS + DEDA markedly reduced osmotically evoked release of all eight amino acids. Glutamate and aspartate effluxes were not inhibited by the glutamate transport inhibitor dihydrokainic acid (1 mM). These results indicate that the hypoosmotic stress, by inducing cell swelling, can initiate an amino acid efflux as part of a regulatory volume decrease. An opening of anion-permeant channels and phospholipase activation appear to be involved in the regulatory volume decrease phenomenon.

The contribution of newly synthesized cholesterol to biliary cholesterol in healthy humans

Hypersecretion of biliary cholesterol appears to be the key defect in the pathogenesis of cholesterol gallstones, and this may be due to an enhanced synthesis of cholesterol. To measure fractional syntheses of biliary and plasma cholesterol, five male and 3 female healthy humans with an intact enterohepatic circulation were infused intravenously with [1-13C]acetate for 15 h. Samples of duodenal bile and blood were taken hourly and an enteral formula diet was given. Free cholesterol mass distribution was analyzed by gas chromatography mass spectrometry. The Mass Isotopomer Distribution Analysis (MIDA) technique allowed to calculate fractional synthesis. After 6 hours of infusion, the [13C]label of the cytosolic acetate pool reached a plateau of approximately 12%. Individual fractional cholesterol synthesis is plasma and bile correlated significantly (6-15 h) and amounted to 4.2% and 5.3% after 15 h, respectively. It may be concluded from this study, that newly synthesized cholesterol is secreted into bile to a higher extent than into plasma.

Effect of taurine on angiotensin II-induced hypertrophy of neonatal rat cardiac cells

The effect of taurine on angiotensin II-induced hypertrophy of cultured neonatal rat heart cells (myocytes and nonmyocytes) was examined. Angiotensin II (1-100 nM) alone caused an increase in the rate of protein synthesis of myocytes without changing the rate of DNA synthesis and cell number. It mediated increases in DNA synthesis and cell number in nonmyocytes. Furthermore, at the lower concentration of 1 nM, it induced c-fos and c-jun expression in both cultured myocytes and nonmyocytes. Exposure of the cells to taurine (20 mM) in the absence of angiotensin II had no effect on either hyperplastic growth or immediate early response gene expression by the two types of cultured cardiac cells. However, myocytes pretreated for 24 h with 20 mM taurine exhibited reduced responsiveness to angiotensin II (1 nM), resulting in lower levels of angiotensin II-mediated stimulation in protein synthesis, and immediate early response gene expression was attenuated. Similarly, taurine treatment of nonmyocytes reduced the degree of hyperplastic growth (DNA synthesis and cell number) and immediate early response gene expression stimulated by angiotensin II. Finally, taurine partially prevented the increase in intracellular free calcium [Ca2+]i mediated by angiotensin II in cardiac cells. Our results indicate that taurine is an effective inhibitor of angiotensin II action.

Hyperosmotic stress up-regulates amino acid transport in vascular endothelial cells

Cultured vascular endothelial cells take up L-proline by sodium-dependent transport. Cells incubated in medium made hyperosmotic by addition of sucrose showed a dose-dependent increase in Na+/proline cotransport. Studies with alpha-(methylamino)isobutyric acid revealed that the up-regulation was specific for amino acid transport system A. Up-regulation was blocked by actinomycin D and cycloheximide, indicating roles for gene transcription and protein synthesis. Up-regulation was maximum after five to six hours of hyperosmotic treatment, but returned to control levels when osmotic stress was maintained for 24 hours. The decline at 24 hours was accompanied by a significant increase in Na+/gamma-aminobutyric acid cotransport. The activity of this system, which also transports betaine, remained unchanged after just five hours of hyperosmotic stress. Inclusion of betaine in the hyperosmotic medium reduced up-regulation of system A. Na/Pi cotransport also was up-regulated by five hours of hyperosmotic stress. Up-regulation of system A, but not Na/Pi cotransport, was detected in isolated membrane fractions indicating that increased activity of this membrane transport system may be one mechanism by which vascular endothelial cells accumulate amino acids. The amino acids may act as organic osmolytes to help maintain normal cell volume during the early phase of hyperosmotic stress.

Membrane currents underlying the modified electrical activity of guinea-pig ventricular myocytes exposed to hyperosmotic solution

1. Guinea-pig ventricular myocytes were superfused with hyperosmotic (sucrose) Tyrode solution (1.2-2.8 times (T) normal osmolality) for up to 40 min. Action potentials were recorded with microelectrodes, and membrane currents with the perforated- or ruptured-patch technique. 2. Hyperosmotic treatment for 20 min shrunk cell volume and hyperpolarized the membrane. Moderate (1.2-1.5 T) treatment caused biphasic changes in action potential configuration (rapid minor shortening quickly followed by lengthening to a stable 110% control duration). Severe (2.2-2.8 T) treatment caused triphasic changes (marked early shortening, strong rebound lengthening and subsequent pronounced shortening). At peak lengthening (6-10 min) action potentials (165% control duration) had a hump near -30 mV and slowed terminal repolarization. 3. In accordance with previous studies, hyperosmotic solution inhibited the delayed rectifier K+ current, and enhanced the outward Na(+)-Ca2+ exchange current (INaCa) at plateau potentials. A novel finding was that hyperosmolality reduced the amplitude of L-type Ca2+ current (ICa,L) and slowed its rate of inactivation. Experiments on myocytes loaded with indo-1 suggest that the reduction in ICa,L is due to a rapid elevation of [Ca2+]i. 4. When impaled myocytes were preloaded with EGTA, severe hyperosmotic treatment induced a rapid monotonic shortening of the action potential to a stable 20% of control duration. Addition of external K+ quickly nulled the hyperpolarization and slowly lengthened the action potential. 5. The results suggest that modified electrical activity in osmotically shrunken myocytes is primarily caused by increases in [K+]i, [Na+]i and [Ca2+]i: (i) elevated [K+]i hyperpolarizes the membrane (which may contribute to increased [Na+]i); (ii) elevated [Na+.]i shortens all phases of the action potential (increased outward-directed INaCa); and (iii) elevated [Ca2+]i has antagonistic plateau shortening (inhibition of inward ICa,L) and plateau lengthening (reduced outward INaCa) influences, as well as a strong subplateau lengthening effect (enhanced inward INaCa).

Taurine depletion, a novel mechanism for cardioprotection from regional ischemia

Three processes that have been implicated in ischemic injury are impaired Ca2+ movement, altered osmoregulation, and membrane remodeling. Because the amino acid, taurine, affects all three processes, it seemed logical that changes in the myocardial content of taurine might affect ischemic injury. To test this hypothesis, infarct size and areas at risk were compared in isolated hearts from control and taurine-depleted rats after a 45-min ligation of the left anterior descending coronary artery and 2 h of reperfusion. Hearts of rats treated for 4 wk with the taurine inhibitor, beta-alanine, exhibited a 57% reduction in the infarct size-to-risk area ratio. The degree of cardioprotection was found to correlate (r = 0.85) with the extent of taurine depletion, the latter dependent on the length of beta-alanine feeding. When the taurine-depleted rats were fed taurine, myocardial taurine levels were restored and the cardioprotection was lost. However, addition of neither beta-alanine (3%) nor taurine (20 mM) to the perfusion medium altered infarct size. We conclude that taurine depletion renders the heart resistant to injury caused by regional ischemia.

F-actin modulates swelling-activated chloride current in cultured chick cardiac myocytes

The integrity of F-actin and its association with the activation of a Cl- current (I(Cl)) in cultured chick cardiac myocytes subjected to hyposmotic challenge were monitored by whole cell patch clamp and fluorescence confocal microscopy. Disruption of F-actin by 25 microM cytochalasin B augmented hyposmotic cell swelling by 51% (from a relative volume of 1.54 +/- 0.10 in control to 2.33 +/- 0.21), whereas stabilization of F-actin by 20 microM phalloidin attenuated swelling by 15% (relative volume of 1.31 +/- 0.05). Trace fluorochrome-labeled (fluorescein isothiocyanate or tetramethylrhodamine isothiocyanate) phalloidin revealed an intact F-actin conformation in control cells under hyposmotic conditions despite the considerable changes in cell volume. Sarcoplasmic F-actin was very disorganized and occurred only randomly beneath the sarcolemma in cells treated with cytochalasin B, whereas no changes in F-actin distribution occurred under either isosmotic or hyposmotic conditions in cells treated with phalloidin. Swelling-activated I(Cl) (68.0 +/- 6.0 pA/pF at +60 mV) was suppressed by both cytochalasin B (22.7 +/- 5.1 pA/pF) and phalloidin (22.5 +/- 3.5 pA/pF). On the basis of these results, we suggest that swelling of cardiac myocytes initiates dynamic changes in the cytoarchitecture of F-actin, which may be involved in the volume transduction processes associated with activation of I(Cl).

Swelling-activated Gd3+-sensitive cation current and cell volume regulation in rabbit ventricular myocytes

The role of swelling-activated currents in cell volume regulation is unclear. Currents elicited by swelling rabbit ventricular myocytes in solutions with 0.6-0.9x normal osmolarity were studied using amphotericin perforated patch clamp techniques, and cell volume was examined concurrently by digital video microscopy. Graded swelling caused graded activation of an inwardly rectifying, time-independent cation current (ICir,swell) that was reversibly blocked by Gd3+, but ICir,swell was not detected in isotonic or hypertonic media. This current was not related to IK1 because it was insensitive to Ba2+. The PK/PNa ratio for ICir,swell was 5.9 +/- 0.3, implying that inward current is largely Na+ under physiological conditions. Increasing bath K+ increased gCir,swell but decreased rectification. Gd3+ block was fitted with a K0.5 of 1.7 +/- 0.3 microM and Hill coefficient, n, of 1.7 +/- 0.4. Exposure to Gd3+ also reduced hypotonic swelling by up to approximately 30%, and block of current preceded the volume change by approximately 1 min. Gd3+-induced cell shrinkage was proportional to ICir,swell when ICir,swell was varied by graded swelling or Gd3+ concentration and was voltage dependent, reflecting the voltage dependence of ICir,swell. Integrating the blocked ion flux and calculating the resulting change in osmolarity suggested that ICir,swell was sufficient to explain the majority of the volume change at -80 mV. In addition, swelling activated an outwardly rectifying Cl- current, ICl,swell. This current was absent after Cl- replacement, reversed at ECl, and was blocked by 1 mM 9-anthracene carboxylic acid. Block of ICl,swell provoked a 28% increase in swelling in hypotonic media. Thus, both cation and anion swelling-activated currents modulated the volume of ventricular myocytes. Besides its effects on cell volume, ICir,swell is expected to cause diastolic depolarization. Activation of ICir, swell also is likely to affect contraction and other physiological processes in myocytes.

Regulatory volume decrease of cardiac myocytes induced by beta-adrenergic activation of the Cl- channel in guinea pig

A new method was developed to automatically measure the thickness of a single ventricular myocyte of guinea-pig heart. A fine marker was attached on the cell's upper surface and changes in its vertical position were measured by focusing it under the microscope. When the osmolarity of the bath solution was varied, the cell thickness reached a new steady level without any obvious regulatory volume change within the period of observation up to 15 min. The cell thickness was 7.8 +/- 0.2 microns (n = 94) in the control Tyrode solution and was varied to 130.4 +/- 3.1% (n = 10), 119.1 +/- 1.1% (n = 50), 87.2 +/- 1.9% (n = 9), and 75.6 +/- 3.2% (n = 5) of control at 50, 70, 130, and 200% osmolarity, respectively. The application of a Cl- channel blocker, 500 microM anthracene-9-carboxylic acid (9AC) did not modify these osmotic volume changes. We discovered that the application of isoprenaline induced a regulatory volume decrease (RVD) in cells inflated by hypotonic solutions. This isoprenaline-induced RVD was inhibited by antagonizing beta-adrenergic stimulation with acetylcholine. The isoprenaline-induced RVD was mimicked by the external application of 8-bromoadenosine 3':5'-cyclic monophosphate. The RVD was inhibited by blocking the cAMP-dependent Cl- channel (ICl, rAMP) with 9AC but was insensitive to 4,4'-diisothiocyanostilbene-2,2'-dissulphonate (DIDS). Taken together these data suggest an involvement of ICl, cAMP activation in the RVD. Whole cell voltage clamp experiments revealed activation of ICl, cAMP by isoprenaline under the comparable conditions. The cardiac cell volume may be regulated by the autonomic nervous activity.

Hypotonic stress induces translocation of the osmolyte channel protein pICln in embryonic skate (Raja eglanteria) heart

Volume expansion of cardiac cells from a wide variety of species stimulates the efflux of the beta-amino acid taurine through an osmolyte channel. Previous studies have suggested that the osmolyte channel in epithelial cells is a swelling-activated anion channel (pICln). In skate heart, a 37-kDa protein is present which is recognized by a specific antibody to a protein characterized in MDCK cells as pICln. This protein is present predominantly in the cytosol (only 10% in the membrane fraction) of heart incubated under isotonic conditions. After transfer to hypotonic medium (one-half osmolarity), the distribution of this protein is markedly altered and significant amounts of the protein are found in the membrane fraction. After hypotonic exposure, the amount of the protein in the membrane fraction rises to 38 +/- 11% (range 18-53, n = 3). The translocation to the membrane fraction suggests that this protein may play a role in the taurine efflux in this tissue stimulated by hypotonic stress.

Applications of mass isotopomer analysis to nutrition research

Investigations into regulating metabolic pathways with stable isotopes have, over the past decade, undergone major development with the use of nuclear magnetic resonance and mass spectrometry in studying labeling patterns of newly synthesized biomolecules. In this review, we concentrate on investigations of mass isotopomer distribution (MID) measured by mass spectrometry. We review the applications of MID to analytical problems, in particular the possibility of amplifying the measurement of low isotopic enrichments by incorporating multiple molecules or atoms of a primary analyte into the molecule of a secondary analyte, the MID of which is assayed. We also review new information on the regulation of intermediary metabolism gathered from the analysis of MID patterns of synthesized compounds. Lastly, we review the applications of MID to the synthesis of polymeric molecules, with emphasis on the validity of these techniques. A number of these techniques are applicable to investigations of nutrient metabolism in health and disease.

A mass isotopomer study of urea and glutamine synthesis from 15N-labeled ammonia in the perfused rat liver

This study examines the incorporation of 15N from 15NH4Cl into urea and glutamine, predicts the pattern of isotopomers produced as a function of the 15N enrichment of the relevant precursor pools, and presents a means of determining the isotopic enrichment of these pools. Rat livers were perfused, in the nonrecirculating mode, with 0.3 mM 15NH4Cl, and the isotopomers of urea and of glutamine produced were determined by gas chromatography-mass spectrometry methodology. Three different nitrogen mass isotopomers of urea were found, containing no, one, or two atoms of 15N. Four glutamine isotopomers were found, containing no 15N, one atom of 15N in either the amino or amide position, or two 15N atoms. A mathematical relationship was deduced that predicts that the relative proportions of the urea isotopomers depends not only on the relative enrichment of 15N in the two precursor pools of urea nitrogen (mitochondrial ammonia and cytoplasmic aspartate) but on their absolute enrichment. This relationship was validated in experiments in which the isotopic enrichment of the substrate, 15NH4Cl, was varied. The proportions of the urea isotopomers produced can be predicted if one knows the 15N enrichment in the two precursor pools. We found that when the 15N enrichment of citrulline and aspartate in the perfusate were used as proxies for that in the mitochondrial ammonia and cytoplasmic aspartate pools we could accurately predict the relative proportion of the three isotopomers. The production of the four nitrogen isotopomers of glutamine could be used to determine the 15N enrichment in the two precursor pools of glutamine nitrogen, the cytoplasmic ammonia and glutamate pools of the perivenous hepatocytes.

Cell distension-induced increase of the delayed rectifier K+ current in guinea pig ventricular myocytes

Single ventricular myocytes of guinea pig heart were distended by applying a positive pressure of 5 to 20 mm Hg in the pipette during the whole-cell voltage clamp. The amplitude of delayed rectifier K+ current (I(K)) was increased by approximately 1.5 times, whereas the inward rectifier K+ current was scarcely affected. The increase of I(K) was reversible by applying a negative pressure of -10 to -30 mm Hg accompanied by shrinkage of the inflated cell. This response of I(K) was largely attributed to the E-4031-insensitive component of I(K). The fully activated current amplitude, measured using long-lasting depolarizing pulses (> 30 seconds) to +60 mV, was increased by the cell distension. The activation time course of I(K) during the long pulse consisted of more than three exponential components, and the slowest time constant was decreased by the distension from control 20.2 +/- 7.7 seconds (n=4) to 7.6 +/- 1.6 seconds (n=5). We failed to detect an involvement of microtubules or microfilaments, protein kinase C, and Ca2+ in the inflation-mediated increase of I(K).

Synthesis of fat in response to alterations in diet: insights from new stable isotope methodologies

Synthesis of fatty acids, or de novo lipogenesis (DNL), is an intensively researched metabolic pathway whose functional significance and metabolic role have nevertheless remained uncertain. Methodologic problems that limited previous investigations of DNL in vivo and recent methodologic advances that address these problems are discussed here. In particular, deuterated water incorporation and mass isotopomer distribution analysis techniques are described. Recent experimental results in humans based on these techniques are reviewed, emphasizing dietary and hormonal factors that modulate DNL and quantitative significance of DNL under various conditions, including carbohydrate overfeeding. The somewhat surprising finding that DNL appears not to be a quantitatively major pathway even under conditions of surplus carbohydrate energy intake, at least in normal adults on typical Western diets, is discussed in depth. Nutritional and metabolic implications of these results are also noted, and some speculations on possible functional roles of DNL in normal physiology and disease states are presented in this context. In summary, methodologic advances have added to our understanding of DNL and its regulation, but many questions concerning quantitation and function remain unanswered.

Extracellular osmotic pressure modulates sodium-calcium exchange in isolated guinea-pig ventricular myocytes

1. The sensitivity of the cardiac Na(+)-Ca2+ exchange current to changes in osmotic pressure was investigated in guinea-pig ventricular myocytes, using the whole-cell patch-clamp technique. 2. A hyposmotic challenge applied by removal of sucrose from the standard bathing solution reduced exchanger current, measured as the Ni(2+)-sensitive component of whole-cell transsarcolemmal current. These changes were fully reversible. 3. No response of whole-cell current to hyposmosis was observed when Ca2+ was removed from the bathing solution by chelation with 1 mM EGTA. 4. Inclusion of 25 microM exchanger inhibitory peptide (XIP) in the pipette solution caused a marked reduction in the Ni(2+)-sensitive component of membrane current, but the percentage change in Ni(2+)-sensitive membrane slope conductance evoked by hyposmosis was the same as when XIP was omitted from the pipette solution. 5. Exposure of cells to hyperosmotic solutions produced variable responses. In a majority of cells, solutions 30% hyperosmotic compared with control evoked a persistent increase in exchanger current, whereas for solutions 50% hyperosmotic, a larger but transient increase in current was observed. 6. Over a wide range of osmolalities (50-130% of isosmotic) the changes in Ni(2+)-sensitive membrane slope conductance were linearly related to the changes in extracellular osmotic pressure. 7. We propose that one consequence of exposing ventricular myocytes to anisosmotic solutions is modulation of Na(+)-Ca2+ exchange current.

Cyclic AMP prevents activation of a swelling-induced chloride-sensitive conductance in chick heart cells

1. Changes in myocardial cell volume and whole-cell currents were measured simultaneously during hyposmotically induced cell swelling. In the conventional patch clamp configuration, hyposmotic challenge caused myocytes to swell continuously and was associated with the development of a sustained, swelling-induced chloride conductance (ICl). In contrast, perforated patch-clamped myocytes demonstrated regulatory volume decreases (RVD) during hyposmotic challenge, and ICl was not generated. 2. The swelling-induced ICl in conventionally patch-clamped myocytes was inhibited by application of forskolin (15 microM) and was prevented when the pipette filling solution contained cAMP (10 microM) and isobutylmethylxanthine (IBMX, 1 mM). ICl could also be prevented by inhibition of protein phosphatase activity, using okadaic acid (100 nM). Conversely, a swelling-induced current could be generated in myocytes under perforated patch clamp by inhibition of protein kinase A, using the antagonist Rp-cAMPS (10 microM). These data demonstrate that cAMP-dependent protein phosphorylation is both necessary and sufficient to prevent development of ICl during cell swelling. 3. Unlike other chloride currents described previously in heart muscle, generation of the novel swelling-induced ICl requires dephosphorylation of a cAMP-dependent protein phosphorylation site; hence it can be prevented by stimulation of cAMP-dependent protein phosphorylation or by inhibition of protein phosphatase activity.

Volume regulation in a toad epithelial cell line: role of coactivation of K+ and Cl- channels

1. We have measured changes in cell volume, membrane potential and ionic currents in distal nephron A6 cells following a challenge with hypotonic solutions (HTS). 2. The volume increase induced by HTS is compensated by a regulatory volume decrease (RVD), which is inhibited by both 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) and quinine. Quinine (500 microM) completely blocked RVD, whereas 100 microM NPPB delayed and attenuated RVD. 3. The resting potential in A6 cells was -52.3 +/- 4.8 mV (n = 53), and shifted to -35.1 +/- 2.2 mV (n = 33) during HTS. 4. Resting membrane current in A6 cells was 0.35 +/- 0.12 pA pF-1 at -80 mV and 0.51 +/- 0.16 pA pF-1 at +80 mV (n = 5). During cell swelling these values increased to 11.5 +/- 1.1 and 29.3 +/- 2.8 pA pF-1 (n = 29), respectively. 5. Quinine (500 microM) completely blocked the HTS-activated current at -15 mV, the reversal potential for Cl- currents, but exerted only a small block at -100 mV (K+ equilibrium potential). NPPB (100 microM) inhibited the current at both potentials almost to the same extent. The HTS-induced net current reversed at -41 +/- 2.5 mV (n = 15), which is close to the measured resting potential during HTS. 6. The quinine-insensitive current reversed near the Cl- equilibrium potential. The quinine-sensitive current reversed near the K+ equilibrium potential. The respective conductances activated by HTS at the zero-current potential were 2.1 +/- 0.7 nS for K+ and 5.2 +/- 1.3 nS for Cl- (n = 15). 7. Single channel analysis unveiled activation of at least two different channels during HTS. A 36 pS channel reversing at the Cl- equilibrium potential showed increased open probability at depolarized potentials. HTS also activated a K+ channel with a 29 pS conductance in high-K+ extracellular solutions (130 mM) or 12 pS in 2.5 mM K+. 8. This coactivation of K+ and Cl- channels shifts the membrane potential towards a value between EK and ECl (the reversal potentials for K+ and Cl-), where a net efflux of Cl- (Cl- inward current) and K+ (K+ outward current) under zero-current conditions occurs. Block of either the K+ or the Cl- conductance will shift the zero-current potential towards the equilibrium potential of the unblocked channel, preventing net efflux of osmolytes and RVD. This coactivation of K+ and Cl- currents causes a shift of osmolytes out of the cells, which almost completely accounts for the observed RVD.

Volume-sensitive taurine transport in bovine articular chondrocytes

1. The swelling of bovine articular chondrocytes isolated from, or in situ within, cartilage by hypotonic shock rapidly activated the efflux or influx of radiolabelled taurine, an amino acid involved in volume regulation in a range of other cell types. 2. When chondrocytes were isolated by the use of collagenase into media of 280 or 380 mosmol l-1, the activation of taurine efflux was at about the osmolarity of the isolating medium, but it was more marked for a given hypotonic shock in the cells isolated at the lower osmolarity. The volume of chondrocytes following isolation in these two osmolarities was the same, suggesting that the cells possess volume regulatory capacity. 3. In isolated chondrocytes, the induced pathway had some of the characteristics of a volume-activated channel, i.e. no transport saturation with increasing substrate concentration, and lack of trans acceleration. The pattern of inhibition of the volume-activated pathway by pharmacological blockers (e.g. pimozide, [(dihydro-indenyl)oxy]alkanoic acid (DIOA) and tamoxifen) differed from that described for a similar pathway in other cell types. 4. The transport of other potential osmolytes (uridine, sorbitol and inositol) was stimulated by cell swelling, independent of sodium and inhibited by pimozide with a selectivity ratio of taurine, 1.00; uridine, 0.84; sorbitol, 0.66; and inositol, 0.38. The selectivity of taurine: inositol was not altered at different cell volumes. 5. The intracellular taurine concentration of chondrocytes within cartilage was low (about 2 mmol (l cell water)-1) showing a negligible role for taurine as an osmolyte during recovery from cell swelling. The swelling-induced loss of taurine from chondrocytes in situ was largely inhibited by pimozide and other drugs, showing that despite the rigid nature of cartilage, the chondrocytes were osmotically sensitive within the extracellular matrix.

Swelling-induced anion and cation conductances in human epididymal cells

1. Activation of both anion and cation conductances was observed in primary cultured human epididymal cells during osmotic swelling under the patch-clamp whole-cell configuration. The swelling-induced anion conductance was 25.66 +/- 4.70 nS and the cation conductance was 7.35 +/- 1.40 nS. The permeability ratio of K+ to Cl- (PK/PCl) was calculated to be 0.40. Known anion or cation channel blockers could inhibit both conductances simultaneously. 2. When the major permeant ion species in the pipette and bath solution was Cl-, the mean conductance was found to be 17.06 +/- 1.8 nS, significantly smaller than that obtained in the presence of intracellular K+, 25.66 +/- 4.70 nS (P < 0.05). No significant current activation was observed when solutions containing only K+ as the permeant ion were used. 3. When the anionic amino acids glutamate and aspartate were used to replace extracellular Cl-, the permeability ratios were calculated to be PGlut/PCl = 0.20 and PAsp/PCl = 0.17. 4. The cation conductance was found to be non-selective since its permeability to other cations such as Na+ and choline, an organic compound highly concentrated in epididymal fluid, was similar to that of K+. 5. Regulatory volume decrease (RVD) was observed after initial osmotic swelling; this could be inhibited by either anion or cation channel blockers. 6. The results of this study suggest that both anion and cation conductances are activated during cellular swelling, and indicate the existence of an interdependent relationship between the swelling-induced cation and anion conductances. Both swelling-induced cation and anion conductances are involved in the volume regulatory process and may be responsible for transporting amino acids or organic compounds in human epididymal cells.

Responses of endothelial cells to hypotonic solutions: lack of regulatory volume decrease

Hypotonic stress (HTS) activates a Cl- current and releases intracellular Ca2+ in vascular endothelial cells, but there is no co-activation of K(+)-channels. The concomitant increase in cell volume, as assessed from the changes in endothelial cell thickness, is not followed by a regulatory volume decrease (RVD). This lack compensation of the volume increase may be explained by the absence of a concomitant activation of a K+ outward current, resulting in an insufficient efflux of osmolytes during HTS.

A chloride current associated with swelling of cultured chick heart cells

1. Cultured chick heart cells challenged by hyposmotic stress underwent regulatory volume decrease (RVD) that was attenuated by prior depletion of intracellular chloride. 2. During hyposmotic swelling, cell aggregates experienced an initial increase in spontaneous contractile activity followed by eventual quiescence. Conventional microelectrode studies revealed an underlying increase in spontaneous electrical activity, followed by a sustained depolarization beyond threshold. 3. Whole-cell patch clamp studies, with K+ currents blocked, indicated that exposure of cells to hyposmotic solution (NaCl reduction) resulted in a rapid osmotic swelling followed by a substantial increase in whole-cell conductance which persisted for the duration of hyposmotic exposure and was almost completely reversed on return to isosmotic bath solution. 4. For a variety of Cl- concentrations, the reversal potentials (Erev) of the measured swelling-activated current closely followed the calculated Cl- equilibrium potential (ECl) with a linear regression slope of 0.82. When estimated by the Nernst equation, the relationship between Erev and the [Cl-]i/[Cl-]o ratio fitted well with a slope of 51 mV per decade change in the concentration ratio, consistent with a Cl(-)-selective conductance. 5. The permeability ratios of this swelling-activated conductance to chloride, methanesulphonate (MSA) and aspartate (Asp) were calculated as PCl:PMSA:PASP = 1:0.36:0.02, with the ion selectivity sequence of Cl- > MSA- >> Asp-, which suggests the swelling-activated conductance is slightly permeable to other anions. 6. Application of a Cl- channel blocker, diphenylamine-2-carboxylate (DPC, 200 microM), substantially suppressed the swelling-activated current without shifting the Erev of this current. The effect of DPC was independent of membrane potential. 7. This evidence demonstrates that hyposmotic swelling of cultured chick heart cells activates a channel-mediated Cl- conductance which may be associated with the integrated response of volume-regulatory mechanisms.

Na+/K+ pump inhibition induces cell shrinkage in cultured chick cardiac myocytes

Myocardial cell swelling occurs in ischemia and in reperfusion injury before the onset of irreversible injury. Swelling has been attributed to failure of the Na+/K+ pump and the accumulation of intracellular Na+. To evaluate the role of the pump-leak model of cell volume maintenance, short term changes in cell volume in response to Na+/K+ pump inhibition were studied in aggregates of cultured embryonic chick cardiac myocytes using optical and biochemical methods. Exposure to 100 microM ouabain over 20 min induced cell shrinkage of approximately 10%. Cell water was also decreased by Na+/K+ pump inhibition; incubation for 1 hr either in the presence of 100 microM ouabain or in K(+)-free solution reduced cell water by 18.4% and 28.4% respectively. When exposed to ouabain in the absence of extracellular Ca2+, the aggregates swelled by approximately 15%, indicating that extracellular Ca2+ was required for the ouabain-induced shrinkage to occur. Ouabain still caused shrinkage, however, in the presence of the Ca2+ channel blockers verapamil (10 microM) and nifedipine (10 microM), suggesting that Na+/Ca2+ exchange, rather than Ca2+ channels, is the route for Ca2+ influx during Na+/K+ pump inhibition. Efflux of amino acids (taurine, aspartate, glutamate, glycine and alanine) from confluent monolayers of chick heart cells exposed to ouabain for 20 min was nearly double that observed in control solution. These results suggest that, during Na+/K+ pump inhibition, chick heart cells can limit accumulation of intracellular sodium by means of Na+/Ca2+ exchange, and that a rise in intracellular [Ca2+], also mediated by Na+/Ca2+ exchange, promotes the loss of amino acids and ions to cause cell shrinkage. Therefore, swelling during ischemic injury may not result from Na+/K+ pump failure alone, but may reflect the exhaustion of alternative volume regulatory transport mechanisms.