Loss Of Taurine Research Articles

Taurine efflux and intracellular pH during astrocyte volume regulation.

Cytotoxic cerebral edema is characterized by enlarged astroglial cells. In tissue culture, osmotically swollen astrocytes return toward normal volume over a period of 15-30 min in a process termed regulatory volume decrease (RVD). RVD is due, in part, to net efflux of taurine and other amino acids. Our objective in these studies was to examine changes in astrocyte intracellular pH (pHi) which may be related to taurine loss during RVD. We hypothesized net efflux of anionic taurine abandons a proton inside the cell, thus lowering pHi. Primary cultures of cerebral astrocytes were prepared from neonatal rats pups and grown on glass coverslips. Confluent cells were loaded at 37 degrees C with the fluorescent pH indicator BCECF. Fluorescence intensity ratios for excitation wavelengths of 440 nm and 494 nm (530 nm emission) were computed every 2 sec. Intensity ratios were calibrated to pHi at the end of each experiment using 140 mM KCl plus 8.6 microM nigericin at pH 7.4. pHi was measured in isoosmotic Hepes-buffered saline (290 mOsm) and then in hypoosmotic Hepes-buffered saline (200 mOsm) in the presence of 0.5 mM amiloride. Some solutions also contained 150 microM niflumic acid (NA). Cellular taurine content was determined in parallel studies using HPLC. Changes in pHi were compared between groups using Student's t-test with Bonferroni correction. Significance was assumed if p < 0.05. In isoosmotic saline, mean +/- SEM pHi was 7.58 +/- 0.04 and decreased to 7.35 +/- 0.09 after adding amiloride. Hypoosmotic exposure caused a further drop in pHi of 0.29 +/- 0.03 within 15 min. Recovery of pHi in isoosmotic saline was amiloride-sensitive. Subsequent hypoosmotic exposure after recovery in isoosmotic saline produced a change in pHi which was 81 +/- 9% of the change measured during the initial hypoosmotic exposure. Taurine content decreased from 147 +/- 6 nmol/(mg protein) to 116 +/- 7 nmol(mg protein) during the 15 min hypoosmotic exposure in 0.5 mM amiloride. NA significantly reduced the hypoosmotically induced change in pHi to 0.17 +/- 0.02 while completely blocking taurine loss. Assuming an intracellular buffering power of 13 mM, the NA-sensitive intracellular acidification of cells during hypoosmotic exposure in the presence of 0.5 mM amiloride corresponds to 1.6 mequiv/l additional intracellular H+. This increase in intracellular H+ content is equivalent to approximately 32% of the NA-sensitive loss of taurine. The association of changes in pHi with taurine efflux is supported by these data; however, efflux of other weak acids may contribute to intracellular acidification during astrocyte RVD and a significant portion of taurine must leave the cell with a proton.

Regulation of the taurine content in Ehrlich ascites tumour cells.

Taurine plays quantitatively an important role as an organic osmolyte in anisosmotic cell volume regulation in Ehrlich ascites tumour cells3. It is the saturable, pH-sensitive, Na+- and Cl−-dependent β-system that accounts for taurine accumulation9,10,15, whereas the swelling-activated taurine channel accounts for net loss of cellular taurine following hypotonic exposure15,16.

Ursodeoxycholic acid in cystic fibrosis-related liver disease: a systematic review.

Treatment of progressive lung disease in cystic fibrosis (CF) has improved greatly over the last 30 years, leading to improvement in survival well into adult lifel. This has led to clinicians paying increasing attention to both treating and delaying progression of disease in other affected organs. A recent epidemiological study2 reported that the clinical prevalence of cystic fibrosis-related liver disease is relatively uncommon in young children but rises to a peak in adolescence. The pathogenesis of liver involvement in CF remains unclear, partly because there are still insufficient long-term data on large numbers of patients. The end point seen at post-mortem is focal biliary cirrhosis which is the typical histological lesion found in those CF patients with liver disease and is characterized by portal ducts containing an inspissated granular eosinophilic material, bile duct proliferation, chronic inflammatory infiltration and some fibrosis3. This may progress to multilobular cirrhosis which has a reported incidence of up to 4% in one clinic4. Periportal changes were also seen with excessive mucus in the biliary tract3. These observations led to the suggestion that the development of focal biliary cirrhosis may be related to mucus obstruction in the intrahepatic ducts3 but this remains uncertain. Recently, the cystic fibrosis transmembrane regulator (CFTR) was found to be located at or near the apical membrane of the intrahepatic bile duct epithelium5. This observation has been followed by a hypothesis that a primary defect in biliary chloride transport may cause abnormal bile flow and thus contribute to the pathophysiology. In CF the faecal losses of bile acids and taurine (a bile acid conjugate) are high, resulting in a greater proportion of glycine-conjugated bile acids which are potentially more hepatotoxic than taurine-conjugated bile acids and may contribute to the bile duct injury6. It is likely that these mechanisms may lead eventually to intrahepatic bile duct irregularities seen at endoscopic retrograde cholangiography7 and to the observed post-mortem findings of CF liver disease. No association between genotype and development of liver disease has been found so far. A recent study analysing the risk factors for the development of liver disease associated with CF showed no difference in allele frequencies between those with and those without liver involvement8. Early detection and assessment of progression of CFliver disease can be difficult. By the time clinical liver disease is evident, usually portal hypertension and cirrhosis have already developed. At this stage, the only potentially curative treatment may be liver transplantation9. Biochemical markers of liver function may not be useful in the assessment of liver involvement because any hepatocyte dysfunction is due to the secondary effect of the primary intrahepatic ductal lesion and the level of abnormality does not always correlate with clinical evidence of liver disease6. Abnormalities of these tests may also be due to an effect other than CF liver disease such as an adverse effect of a drug treatment or bacterial infectionl. Ultrasound appearances of liver size and irregular texture, as well as Doppler studies of altered direction and velocity of portal vein flow can be used to assess the presence and progression of liver disease or portal hypertension1 1. A recent study has suggested that measuring biliary excretion of disopropylphenyl-carboxymethyl iminodiacetic acid (DISIDA) by radioisotope scanning with hepatic scintigraphy may be useful in assessing early liver involvement12. Recent developments in therapy have been directed towards attempting to improve biliary secretion and bile acid composition. Ursodeoxycholic acid (UDCA) is a naturally occurring hydrophilic bile acid which constitutes less than 4% of the total bile acid pool. In 1990, Erlinger et al. 13 showed that UDCA improves bile acid flow by inducing an increased bicarbonate-rich bile flow. Hydrophilic UDCA is less hepatotoxic than primary bile acids10. Initially UDCA was used to dissolve gallstones but more recently has been used as a possible treatment for other adult chronic cholestatic liver diseases such as primary biliary cirrhosis14 and primary sclerosing cholangitis1 . Following these trials, UDCA was found to improve liver function tests in CF liver disease16 and has since been used in the treatment of and hopefully in the prevention of progressive liver disease in CF in recent years. 6 'Respiratory Unit, Alder Hey Children's Hospital, Eaton Road, Liverpool L12 2AP; 2Department of Mathematical Sciences, University of Liverpool L69 3BX, England

Increased potassium, chloride, and taurine conductances in astrocytes during hypoosmotic swelling.

Membrane conductances during hypoosmotic swelling were characterized in rat astrocytes in primary tissue culture. Using whole cell patch clamp techniques, mean +/- SEM cell conductance in isoosmotic phosphate-buffered saline (PBS) was 55.6 +/- 5.8 pS/pF. Cell conductance (mean +/- SEM) increased from this initial value to 187 +/- 46%, 561 +/- 188%, and 1216 +/- 376% within 9 min of exposure to 220 mOsm, 190 mOsm, and 145 mOsm PBS, respectively. With each of these hypoosmotic exposures, no change occurred in membrane capacitance. When CsCl replaced KCl in the microelectrode solution, a similar conductance increase was obtained at each osmolality. However, when gluconate salts were used in place of chloride salts in the electrode solution, no significant conductance increase was observed with 190 mOsm PBS. With a KCl microelectrode solution, all conductance increase which occurred in 190 mOsm PBS was inhibited by 200 microM niflumic acid, but not by 5 mM BaCl(2). Both niflumic acid and BaCl(2) inhibited 60-80% of the conductance increase of cells in 145 mOsm PBS. Using a microelectrode solution containing taurine as the major anion, membrane conductance increased 5-fold when cells were placed in 250 mOsm medium. This conductance increase was completely inhibited by 200 microM niflumic acid. Thus, independent chloride and potassium conductances are activated by hypoosmotic swelling of cultured astrocytes while plasma membrane area is unaltered. The chloride conductance pathway is activated at a significantly lower degree of hypoosmotic exposure than that which activates the potassium pathway and may be permeable to anionic taurine. These conductance pathways may mediate diffusive loss of potassium, chloride, and taurine from these cells during volume regulation following hypoosmotic swelling.

Neuroactive amino acids in hepatic encephalopathy.

There is abundant evidence to suggest that alterations of excitatory and inhibitory amino acids play a significant role in the pathogenesis of hepatic encephalopathy (HE) in both acute and chronic liver diseases. Brain glutamate concentrations are reduced in patients who died in hepatic coma as well as in experimental HE, astrocytic reuptake of glutamate is compromised in liver failure and postsynaptic glutamate receptors (both NMDA and non-NMDA subclasses) are concomitantly reduced in density. Recent studies in experimental acute liver failure suggest reduced capacity of the astrocytic glutamate transporter in this condition. Together, this data suggests that neuron-astrocytic trafficking of glutamate is impared in HE. Other significant alterations of neuroactive amino acids in HE include a loss of taurine from brain cells to extracellular space, a phenomenon which could relate both to HE and to brain edema in acute liver failure. Increased concentrations of benzodiazepine-like compounds have been reported in human and experimental HE. Clinical trials with the benzodiazepine antagonist flumazenil reveal a beneficial effect in some patients with HE; the mechanism responsible for this effect, however, remains to be determined.

Choroid plexus taurine transport

The putative osmoregulatory agent, taurine, is lost from the brain during hypo-osmotic stress or ischemia, but the regulatory mechanisms involved in this loss have not been fully elucidated. In this study, we have examined taurine transport by the isolated rat choroid plexus, one element of the brain-blood interface, and examined how it may be regulated as part of brain volume regulation. Choroid plexus taurine uptake was Na- and Cl-dependent with a V max and K m of 6.5 0.3 pmol/mg/min and 232 33 μM. The latter is substantially greater than the normal CSF taurine concentration and this may be important in removing taurine released into the CSF during parenchymal cell swelling. Taurine uptake also appears calmodulin dependent as it was reduced by 84 and 91% in the presence of 25 μM trifluoperazine and 100 μM W-7, two calmodulin inhibitors. Taurine efflux from choroid plexus was stimulated by trifluoperazine, taurine, and hypo-osmotic stress. The latter two effects were reduced by niflumic acid, suggesting that taurine and hypo-osmotic stress act on the same pathway. The stimulation of efflux by hypo-osmotic stress decreased with time, whereas the effect of external taurine was sustained. If this efflux pathway is involved in the movement of taurine from choroid plexus to blood, these results suggest that changes in extracellular taurine may be more important than the direct effect of hypo-osmolality in the long-term loss of taurine from the brain.

Dietary Antibiotics Decrease Taurine Loss in Cats Fed a Canned Heat-Processed Diet

In a crossover design, cats were fed a canned heat-processed diet (18 g dry matter/kg initial body wt) either with (+) or without (-) antibiotics [a mixture of penicillin G, procaine (25 mg/18 g diet) and tetracycline (50 mg/18 g diet)]. The (-/+) group received no antibiotics during the first 5-wk period and received antibiotics during the second 5-wk period; the (+/-) group received the reverse. Plasma, whole blood, urinary and fecal concentrations of taurine, fecal bile acid excretion and cholyltaurine hydrolase activities were measured. Consumption of antibiotics for 5 wk resulted in a lower rate of depletion of plasma taurine. Taurine concentrations decreased more over the first 5 wk in cats in the (-/+) group than in cats in the (+/-) group [from 116 ± 26 to 26 ± 6 µmol/L (-/+) and from 109 ± 6 to 77 ± 7 µmol/L (+/-) for plasma, and from 546 ± 8 to 292 ± 29 µmol/L (-/+) and from 560 ± 11 to 431 ± 20 µmol/L (+/-) for whole blood]. Urinary total taurine excretions during the 5th week were 54 µmol/d for the (-/+) group and 135 µmol/d for the (+/-) group (pooled SEM, ±13). Fecal total taurine excretions during the 5th week were 184 and 53 µmol/d for the (-/+) and (+/-) groups, respectively, (pooled SEM ± 9). Most of the fecal taurine was unconjugated (free). Fecal bile acid excretions during the 5th week were 235 ± 18 and 106 ± 11 µmol/d for the (-/+) and (+/-) groups, respectively. Dietary antibiotics suppressed fecal cholyltaurine hydrolase activity of cats. Fecal cholyltaurine hydrolase activities during the 5th week were 279 ± 54 and 42 ± 10 nmol cholic acid released · min-1 · g dry feces-1 in the (-/+) and (+/-) groups, respectively. After the crossover, mean values for the groups were reversed, showing that the observed changes were due to the antibiotic treatment. These results support the hypothesis that the dietary taurine requirement of cats is largely determined by the extent of microbial degradation of taurine in the gastrointestinal tract.

Taurine and water channels are co-localized in renal tubule cells and other tissues. Immunocytochemical studies in rats.

One of the best established roles of taurine is that of an osmoregulator7. Many cell types have been shown to sustain a loss of taurine upon exposure to hypoosmotic stress11 while an increased uptake of this amino acid can be induced by hyperosmotic conditions12. The latter effect has been demonstrated in renal cells in culture, thus supporting the notion that taurine acts as an organic osmolyte in the kidney along with polyhydric alcohols and methylamines5.

Effects of taurine on calcium in platelets and their aggregation.

Taurine concentrations are 400-fold greater in platelets than in plasma1, 5. The role of taurine in platelets has not been elucidated. For cardiologists, platelet taurine content is likely to have diagnostic value, since a decrease in taurine concentration in these cells indirectly reflects changes in taurine level in body tissues in general and in heart in particular11. Paasonen et al. 12 have reported that the taurine level in patients with hypertension does not differ from control values but increases in patients with myocardial infarction. In platelets, taurine transport decreases both in hypertension and in cardiovascular failure9. Marked losses of taurine from animals or humans affects the functional state of not only such organs as heart13 and retina8 but also of platelets. Taurine is known to model Ca2+ responses to PAF3. Taurine can inhibit PAF-induced aggregation of guinea pig platelets and aggregation of human platelets induced by ADP, serotonin and epinephrine2. Hayes et al. 9 reported the interrelation of taurine status and platelet aggregation in cat. They discovered that marked loss of taurine from platelets make them more sensitive to aggregation. These authors assume that this effect can be mediated via changes in GSH and altered thromboxane liberation. We suppose that this effect of taurine is not its only platelet action. Taurine may possess a homeostatic action that is revealed in certain pathological conditions. For this reason we have studied taurine effects in cells obtained from donors with different rhythm disturbances.

Blood-brain barrier taurine transport during osmotic stress and in focal cerebral ischemia.

Little is known about blood to brain taurine transport despite substantial evidence suggesting a role of taurine in brain volume regulation during osmotic stress or conditions inducing cell swelling, such as ischemia. We have made measurements of the taurine influx rate constant (K1) with [3H]taurine in three conditions: raised plasma taurine concentrations induced by infusion with 50 mM taurine (10 microliters/100 g/min); osmotic stress induced by i.p. injections of 1.5 M NaCl (2 ml/100 g) or distilled water (10 ml/100 g); and 4 h of middle cerebral artery occlusion (MCAo). In rats with MCAo, additional determinations were made of tissue water and taurine contents, and blood-brain barrier passive permeability with [3H]alpha-aminoisobutyric acid. Taurine infusion increased plasma taurine from 110 +/- 63 microM (SD) to 407 +/- 63 (p < 0.001) and decreased taurine K1 at the blood-brain barrier by 70% (p < 0.001), signifying saturable uptake that maintained unidirectional influx constant. Similarly, although hypo- and hyperosmolality increased and decreased plasma taurine concentration, respectively, a reciprocal relationship between K1 and plasma taurine in these experiments ensured that unidirectional fluxes of taurine into brain were unchanged by osmotic stress. During MCAo, the taurine K1 was reduced 80% in the ipsilateral ischemic tissue compared with the contralateral nonischemic tissue (p < 0.001). This decline may be due to a release of taurine into the brain circulation, because there was a concomitant loss of tissue taurine of 7.4 +/- 2.4 mmol/g dry weight (p < 0.05). Alternately, if taurine uptake is sodium dependent, the decline might reflect a disruption of the endothelial sodium gradient.

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.

Failure of Oral Taurine Supplementation to Influence Skin-Flap Survival in Rats

Taurine was given orally to rats to determine its influence on survival of subdermal plexus skin flaps. Flaps were raised on the dorsum of 40 rats divided into groups given 0, 10, 30, or 100 mg/kg taurine daily starting 2 days before and continuing 14 days after surgery. No significant difference was found between groups for percentage of distal flap necrosis, although mean area of necrosis was less for the 30 mg/kg group. Oral taurine did not result in significant elevation of plasma taurine concentrations at day 16 of administration, although the 30 mg/kg group maintained a higher mean value. Analysis of skin taurine concentrations in flaps failed to detect significant differences between groups at each of three zones (proximal-normal, middle-transition, and distal-sloughed). All groups, except the 10 mg/kg group, had significantly lower taurine concentrations in the distal zone than in the normal and transition zones. In each group there was a trend toward lower taurine concentration from proximal to distal, suggesting that loss of tissue taurine may occur with tissue necrosis. Subjectively, no differences in skin histopathology were noted, but less severe skin lesions were more common in the 10 mg/kg group. Daily oral taurine supplementation rates of 10, 30, or 100 mg/kg appear not to significantly affect survival of subdermal plexus skin flaps in rats.

Diet-Induced Cholecystokinin Release in Cats

Cats have a dietary requirement for taurine as a re sult of a limited ability to synthesize taurine and ob ligate taurine conjugation to bile acids. Recent research indicates that increased taurocholic acid turnover may contribute to the taurine depletion that is observed in cats fed certain heat-processed canned diets or purified soy protein-containing diets (Hickman et al. 1992). The mechanism by which these diets increase bile se cretion and taurine loss is not known. Possibilities in clude increased loss of taurine-conjugated bile salts through binding to undigestible dietary components, increased bacterial degradation as a result of altered gut microflora, a direct increase in bile secretion and enterohepatic recycling due to diet-mediated eleva tions in cholecystokinin (CCK)7 release, or a combi

Effects of extracellular bicarbonate ions and pH on volume-regulatory taurine efflux from rat cerebral cortical slices in vitro: evidence for separate neutral and anionic transport mechanisms

Net efflux of [ 3H]taurine from cells in pre-loaded slices of rat cerebral cortex has previously been shown to occur via an anionic pathway, believed to consist of exchange of anionic taurine for extracellular Cl −, and operating under both isomotic and hypoosmotic conditions, and a calmodulin-dependent mechanism, activated by hypoosmotic stress: the latter may comprise conductive channels (Law, R.O. (1994) Biochim. Biophys. Acta 1221, 21–28). Experiments have now been performed to examine two inter-related problems; firstly, how anion/anion exchange (assuming 1:1 stochiometry) could contribute to the regulation of brain cell volume, and secondly, whether the hypoosmotically-activated component of efflux represents a second anionic transport process or loss of neutral taurine. The former process has been shown to be strongly dependent upon extracellular pH and bicarbonate concentration, being accelerated by low pH (7.0) and high (60 mmol/1) bicarbonate, and retarded by alkalinization (pH 7.8) and low (2.5 mmol/l) bicarbonate. Taurine efflux is inhibited by acetazolamide, with accompanying cell swelling in both isoosmotic and hypoosmotic media. It is hypothesized that inwardly directed bicarbonate transport, in exchange for intracellular Cl −, operates in parallel with efflux of anionic taurine in exchange for extracellular Cl −, and it is the subsequent conversion of bicarbonate to CO 2 and water, under the influence of carbonic anhydrase, that effects a volume-regulatory decrease in internal osmotic potential. The dependence of taurine efflux upon pH and bicarbonate persists in the presence of trifluoperazine (an inhibitor of calmodulin activation) but is abolished in hypoosmotic media by the anion transport inhibitor niflumic acid. Cell depolarization in high K + has no effect on taurine efflux, which is envisaged as involving parallel electroneutral anion exchange processes (taurine/Cl − and Cl −/bicarbonate) augmented, in hypoosmotic media, by diffusive efflux of neutral taurine.

Short-Term Cell Volume Regulation in Mytilus Californianus Gill

Long-term acclimation of Mytilus californianus to 60% artificial sea water (585 mosmol l-1; ASW) led to a 30-40% decrease in the taurine (53.5-36.9 mumol g-1 wet mass) and betaine (44.8-26.2 mumol g-1 wet mass) content of gill tissue, compared with that of control animals held in 100% ASW (980 mosmol l-1). The K+ content of gills did not change following long-term acclimation to reduced salinity. In contrast, losses of all three solutes during a brief (60 min) exposure to 60% ASW were less than or equal to 15%. Nevertheless, the swelling of gill cells that occurred after acute exposure to 60% ASW was followed by a return towards the control volume. Direct optical measurement of single gill filaments confirmed that, during an acute exposure to reduced salinity, ciliated lateral cells increased in cell height (volume) and then underwent a regulatory volume decrease (RVD) with a half-time of approximately 10 min. This short-term RVD was completely inhibited by exposure to 1 mmol l-1 quinidine, a K+ channel blocker, but only when the drug was applied to the basolateral aspect of the gill epithelium. Application of 1 mumol l-1 valinomycin relieved the inhibition by quinidine of the gill RVD. However, addition of valinomycin did not accelerate the rate of RVD observed in the absence of quinidine. These results indicate that long-term acclimation of Mytilus californianus gill in dilute sea water involves primarily losses of taurine and betaine, whereas short-term regulation of cell volume may involve an electrically conductive loss of intracellular K+ and a counter ion.

A loss of taurine and other amino acids from ventricles of patients undergoing bypass surgery.

To study the changes in amino acid content of left ventricles of patients during cardiac surgery that involves cardiopulmonary bypass and cold cardioplegia. Biopsy specimens (up to 10 mg wet weight) from the left ventricle of 30 patients undergoing coronary artery bypass graft and valve replacement surgery on cardiopulmonary bypass (protected by cold cardioplegia with St Thomas' solution) were taken immediately before the infusion of the cardioplegic solution and just before the removal of the cross clamp, and were analysed for their amino acid content. Of the most abundant cellular amino acids in the left ventricle taurine, glutamine, glutamate, and aspartate, but not alanine, showed a significant fall during the period of cross clamping. A rise in intracellular sodium (Na) is known to occur during cold cardioplegic arrest so that an activation of an amino acid/Na efflux, similar to that seen in animal experiments, seems a likely mechanism. The anomalous behaviour of alanine suggests some recovery of metabolism. The loss of alpha amino acids (by contrast with the loss of taurine) will depress protein synthesis and reduce energy reserves after cardiac surgery. Attempts to preserve the concentrations of intracellular alpha amino acids must be balanced against the need to regulate intracellular Na concentration and hence intracellular pH and calcium ions. The presence of alpha amino acids in the cardioplegic solution (or in a resuscitation solution) should maintain the intracellular concentrations and favour activation of the taurine/Na symport to oppose the rise in intracellular Na concentration. Because the reservoir of tissue taurine is limited, the potential benefits of increasing the concentration of taurine in the heart by diet before surgery and addition of alpha amino acids to the cardioplegic solution merits further assessment.

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

Mechanisms of volume regulation in hyposomotically treated cultured chick heart cell preparations were studied using optical, biochemical, and nuclear magnetic resonance methods. This approach afforded the resolution of time-dependent responses that might ordinarily be obscured by the complex morphology of intact cardiac muscle preparations. In hyposmotic solutions, cells swelled to a peak volume within 3 min and slowly regulated toward original volume (regulatory volume decrease, RVD). Upon return of the cells to isosmotic solution following hyposmotic treatment, the cells shrank to a steady-state volume that was substantially less than the initial volume in control solution. A vigorous RVD could also be elicited by hyposmotic swelling under Cl(-)-free conditions. Measurement of both inorganic cation loss via atomic absorption spectroscopy and organic solute loss via 1H-nuclear magnetic resonance and high-pressure liquid chromatographic techniques revealed that the RVD observed following exposure to hyposomotic solutions was mediated in part by a substantial loss of taurine, glutamate, aspartate, and glycine as well as loss of inorganic ions (Na+,K+). The hyposmotically activated transport of amino acids was also associated with the production of glutamate and aspartate. The volume regulatory release and production of amino acids have significant implications for the metabolic and functional integrity of cardiac cells.

Interdependence of intracellular taurine and sodium in guinea pig heart.

The aim was to investigate the effects of raising intracellular taurine on the intracellular sodium activity (aNa1) in isolated guinea pig ventricular myocytes, and the effect of procedures that raise intracellular sodium on taurine concentration in the perfused guinea pig ventricular tissue. Taurine was introduced into the sarcoplasm of isolated ventricular myocytes, either during cell isolation or by diffusion from a penetrating micropipette, and the effect on aNai was measured using an ion sensitive microelectrode. Guinea pig hearts, mounted on a Langendorff apparatus, were perfused with a variety of physiological media and the level of taurine in the ventricles determined using high pressure liquid chromatography. An increase in intracellular taurine caused by its presence during cell isolation or by diffusion from a micropipette significantly reduced the aNai of isolated myocytes at rest, during perfusion with Ca depleted solutions, or on inhibition of the Na pump. In the guinea pig ventricles, taurine at 13.0(SEM 0.6) mmol.kg-1 wet weight comprised up to 45% of the free amino acids; since plasma taurine was 64(13) mumol.litre-1, this means that in vivo a large outwardly directed gradient for taurine exists (equivalent to a free energy of 13.7 KJ.mol-1). Upon perfusion with Ca,Mg free Tyrode solution (which raises intracellular sodium markedly), a time dependent loss of taurine occurred. Both the rate of loss and the total amount lost were increased when the Na pump was also inhibited. This loss of tissue taurine was not due to release from dead or lysed cells, as it was antagonised by procedures known to reduce the rise of aNai during Ca depletion, was inhibited by beta alanine (an inhibitor of taurine transport), and the fall in tissue taurine was not correlated with the appearance of lactate dehydrogenase in the effluent. The data from isolated myocytes and perfused guinea pig hearts were consistent with the presence of a Na/taurine symport which is activated to cause efflux of Na and taurine when either rise above their physiological level.

Taurine Balance is Different in Cats Fed Purified and Commercial Diets

Ileal fluxes, urinary losses and taurine balance were determined in six taurine-replete and four taurine-depleted cats. Digesta samples collected at the terminal ileum were used to assess ileal flux of taurine. Four diets were tested: a commercial diet in two forms (heat-processed and frozen) and two purified diets containing either 1225 or 0 mg taurine/kg diet. Five-day balance trials were performed on d 3–7 with measurement of food intake and taurine in urine and ileal digesta. Substantially greater quantities of total taurine (free + bound) were found in ileal digesta from cats fed the heat-processed rather than the frozen preserved diet (205 vs. 101% of the average daily taurine intake, respectively), with calculated taurine balances of -609 vs. -212 μmol/d, respectively. The quantity of taurine in ileal digesta from taurine-replete cats fed the 1225 or 0 mg taurine/kg purified diets was not significantly different, indicating that taurine found at the terminal ileum is mostly of endogenous origin. Taurine-depleted cats had significantly lower amounts of taurine in ileal digesta, with a taurine balance of -77 μmol/d. These results demonstrate that a heat-processed diet causes substantially greater losses of taurine from the intestine than does a frozen diet. This phenomenon may explain the inability of some heat-processed diets to maintain normal plasma taurine concentrations in cats.

The metabolic basis for the taurine requirement of cats.

Taurine is an essential dietary nutrient for cats because the rate of synthesis under most conditions is less than the rate of loss from the body. A deficiency of taurine results in pathological states affecting a number of organ systems, (Morris et al., 1990). Definition of the minimal dietary requirements of cats for taurine has been particularly challenging. Unlike many other nutrients, the loss of taurine from the body is dependent on the composition of the diet, and the method by which the diet is processed. The minimal concentrations of taurine in the diet to satisfy each physiological function of taurine at this time are unknown and likely to be somewhat different for each function. For example, two clinical conditions attributed to taurine deficiency in cats, feline central retinal degeneration (FCRD) and dilated cardiomyopathy do not always occur together in the same cat, each can occur without presence of the other (Pion et al., 1987). The purpose of this paper is to briefly review two important factors that determine the requirement for this nutrient, i.e. endogenous synthesis of taurine and loss of taurine from the body.