Absence Of Taurine Research Articles

Taurine prevents beta-glycerophosphate-induced calcification in cultured rat vascular smooth muscle cells.

Vascular calcification is an ectopic calcification that commonly occurs in atherosclerosis. Because taurine was previously shown to protect against cardiovascular diseases, the effect of taurine on vascular calcification was evaluated in calcified vascular smooth muscle cells (VSMCs) of rat in vitro in the present study. Osteoblastic differentiation, calcification, and proliferation in VSMCs were detected in the presence and absence of taurine. Alkaline phosphatase (ALP), cellular calcium content, and (45)Ca accumulation were measured as the indicators of osteoblastic differentiation and calcification. Incubation of VSMCs with Beta-glycerophosphate for 10 days induced an osteoblast-like morphological change. The activity of ALP was enhanced. Calcium content and (45)Ca uptake were increased in these cells. Calcification of these VSMCs was demonstrated with Beta-glycerophosphate treatment. In association with these alterations, cell proliferation, detected by cell counting, [(3)H]thymidine ([(3)H]TdR), and [(3)H]leucine ([(3)H]Leu) incorporation, was also increased in these calcified VSMCs. Taurine at 20 mmol/l decreased calcium content, (45)Ca(2+) uptake, and ALP activity both after early and late treatment, in which a reduction of the cell count, [(3)H"]TdR, and [(3)H]Leu incorporation of calcified VSMCs was also noted. Compared with the calcified group, morphological changes in the VSMCs of the early-treated group were deferred. These results demonstrated that calcification of VSMCs could be alleviated by taurine. Taurine treatment appeared to be more beneficial when the treatment was started earlier.

Taurine attenuates calcium-dependent, Fas-mediated neutrophil apoptosis.

The pathway involved in Fas-mediated neutrophil apoptosis remains to be fully elucidated. We examined whether this pathway involved either oxygen-dependent or calcium-dependent mechanisms. We also investigated whether taurine, a powerful antioxidant and regulator of intracellular calcium fluxes, could inhibit Fas-mediated neutrophil apoptosis. Neutrophils were stimulated with Fas monoclonal antibody in the presence or absence of taurine. Fas receptor ligation resulted in significant neutrophil apoptosis at 18 h. Engagement of the Fas receptor rapidly resulted in a significant decrease in intracellular calcium. Apoptosis was inhibited and intracellular calcium levels were maintained in the presence of calcium ionophore A23187 or taurine. Fas ligation did not result in an increase in intracellular reactive oxygen species. We have demonstrated that Fas-mediated neutrophil apoptosis occurs after a decrease in intracellular calcium and is reactive oxygen intermediate independent. Furthermore, the amino acid taurine attenuates this pathway of neutrophil apoptosis by calcium regulation. This newly identified role of taurine in the inhibition of Fas-mediated neutrophil apoptosis may have significant implications for future manipulation of host pro-inflammatory cell function.

Stopped-flow kinetic analysis of Escherichia coli taurine/alpha-ketoglutarate dioxygenase: interactions with alpha-ketoglutarate, taurine, and oxygen.

Taurine/alpha-ketoglutarate dioxygenase (TauD), a member of the broad class of non-heme Fe(II) oxygenases, converts taurine (2-aminoethanesulfonate) to sulfite and aminoacetaldehyde while decomposing alpha-ketoglutarate (alphaKG) to form succinate and CO(2). Under anaerobic conditions, the addition of alphaKG to Fe(II)TauD results in the formation of a broad absorption centered at 530 nm. On the basis of studies of other members of the alphaKG-dependent dioxygenase superfamily, we attribute this spectrum to metal chelation by the substrate C-1 carboxylate and C-2 carbonyl groups. Subsequent addition of taurine perturbs the spectrum to yield a 28% greater intensity, an absorption maximum at 520 nm, and distinct shoulders at 480 and 570 nm. This spectral change is specific to taurine and does not occur when 2-aminoethylphosphonate or N-phenyltaurine is added. Titration studies demonstrate that each TauD subunit binds a single molecule of Fe(II), alphaKG, and taurine. In addition, these studies indicate that the affinity of TauD for alphaKG is enhanced by the presence of taurine. alpha-Ketoadipate, the other alpha-keto acid previously shown to support TauD activity, and alpha-ketocaproate lead to the formation of weak 520 nm-like spectra with Fe(II)TauD in the presence of taurine; however, corresponding spectra at 530 nm are not observed in the absence of taurine. Pyruvate and alpha-ketoisovalerate fail to elicit absorption bands in this region of the spectrum, even in the presence of taurine. Stopped-flow UV-visible spectroscopy reveals that the 530 and 520 nm spectra associated with alphaKG-Fe(II)TauD and taurine-alphaKG-Fe(II)TauD are formed at catalytically competent rates ( approximately 40 s(-)(1)). The rate of chromophore formation was independent of substrate or enzyme concentration, suggesting that alphaKG binds to Fe(II)TauD prior to the formation of a chromophoric species. Significantly, the taurine-alphaKG-Fe(II)TauD state, but not the alphaKG-Fe(II)TauD species, reacts rapidly with oxygen (42 +/- 9 s(-)(1)). Using the data described herein, we develop a preliminary kinetic model for TauD catalysis.

Two types of K(+) channels at the basolateral membrane of proximal tubule: inhibitory effect of taurine.

The cell-attached configuration of the patch-clamp technique was used to investigate the effects of taurine on the basolateral potassium channels of rabbit proximal convoluted tubule. In the absence of taurine, the previously reported ATP-blockable channel, K(ATP), was observed in 51% of patches. It is characterized by an inwardly rectifying current-voltage curve with an inward slope conductance of 49 +/- 5 pS (n = 15) and an outward slope conductance of 13 +/- 6 pS (n = 15). The K(ATP) channel open probability (P(o)) is low, 0.15 +/- 0.06 (n = 15) at a -V(p) = -100 mV (V(p) is the pipette potential), and increases slightly with depolarization. The gating kinetics are characterized by one open time constant (tau(o) = 5.0 +/- 1.9 ms, n = 6) and two closed time constants (tau(C1) = 5. 2 +/- 1.5 ms, tau(C2) = 140 +/- 40 ms; n = 6). In 34% of patches, a second type of potassium channel, sK, with distinct properties was recorded. Its current-voltage curve is characterized by a sigmoidal shape, with an inward slope conductance of 12 +/- 2 pS (n = 4). Its P(o) is voltage independent and averages 0.67 +/- 0.03 (n = 4) at -V(p) = -80 mV. Both its open time and closed time distributions are described by a single time constant (tau(o) = 96 +/- 19 ms, tau(C) = 10.5 +/- 3.6 ms; n = 4). Extracellular perfusion of 40 mM taurine fails to affect sK channels, whereas K(ATP) channel P(o) decreases by 75% (from 0.17 +/- 0.06 to 0.04 +/- 0.02, n = 7, P < 0.05). In conclusion, the absolute basolateral potassium conductance of rabbit proximal tubules is the resulting combination of, at least, two types of potassium channels of roughly equal importance: a high-conductance low-open probability K(ATP) channel and a low-conductance high-open probability sK channel. The previously described decrease in the basolateral absolute potassium conductance by taurine is, however, mediated by a single type of K channel: the ATP-blockable K channel.

Inhibition of rat vascular smooth muscle cell proliferation by taurine and taurine analogues

The growth of rat aorta vascular smooth muscle cells (VSMCs) was measured in the presence and absence of taurine. Concentrations of taurine as low as 0.3 mM in the culture medium inhibited the proliferation of the cells, as monitored by measuring cell count, and also inhibited the rate of DNA synthesis, as examined by measuring [ 3H]thymidine incorporation into DNA. However, even at the highest concentration of taurine (30 mM), the doubling time of the VSMCs was only increased by 38%. Protein content of the VSMCs was decreased by 30 mM taurine. [ 3H]Leucine incorporation into newly synthesized protein was not affected by the highest concentration of taurine tested (30 mM), indicating that taurine did not inhibit protein synthesis but rather decreased total protein content by inhibiting cellular proliferation. The effects of other amino acids such as alanine, glycine, and serine and of various taurine analogues such as β-alanine, guanidinoethanesulfonic acid (GES), and isethionic acid also were tested at a concentration of 20 mM for their effects on the growth of the VSMCs. Alanine, glycine, and serine had only a minimal effect or no effect on cell count, quantity of protein, and incorporation of [ 3H]thymidine into DNA. GES, β-alanine, and isethionic acid had a significant effect on cell count, protein content, and incorporation of [ 3H]thymidine into DNA. β-Alanine was the only analogue tested that significantly depressed [ 3H]leucine incorporation into newly synthesized protein. It is concluded that taurine, GES, and isethionic acid inhibited proliferation of VSMCs but did not alter normal protein synthesis or survivability of VSMCs. In contrast, other amino acids, alanine, glycine and serine, had minimal effects on VSMC proliferation and protein synthesis, whereas β-alanine appeared to be toxic, inhibiting both VSMC synthesis and de novo protein synthesis.

Taurine might be acting as a trophic factor in the retina by modulating phosphorylation of cellular proteins

Protein phosphorylation is involved in the regeneration of the nervous system. Taurine modulates the phosphorylation of specific proteins in the retina, and also increases outgrowth from ganglion cells. In order to test the possible role of protein phosphorylation on the outgrowth from the retina and on the trophic effect of taurine, in vitro studies were performed in the presence of phorbol and nonphorbol protein kinase C activators, the protein kinase C inhibitor tamoxifen, and phosphatase inhibitors. After crush of the optic nerve, explants of the goldfish retina were cultured and the outgrowth was evaluated by measuring the length and the density of neurites. The activation of protein kinase C decreased the outgrowth from the explants and impaired the stimulatory effect of taurine. Phosphatase inhibitors produced a similar effect on basal and taurine-modulated outgrowth. In certain concentrations, some of these drugs did not affect the emission of neurites in the absence of taurine, but decreased the effect of the amino acid. Tamoxifen also reduced the outgrowth, probably acting at other cellular levels or indicating that the regulation of outgrowth by phosphorylation is a complex and dual process. The response to the drugs, evaluated by length or density of fibers, was not the same, since rate of outgrowth was more affected than density, which suggests that both parameters are modulated at differential stages or sensitivities to the tested agents.

Effects of urea on taurine efflux and cell volume in incubated rat cerebral cortical minislices

The effects of urea on the rate of efflux of preloaded taurine and volume regulation have been examined in incubated minislices from rat superficial cerebral cortex. As external urea was increased in the range 0-100 mmol/L, there was a concentration-dependent slowing of cellular taurine efflux. Cell volumes progressively increased over the range 0-50 mmol/L urea, but decreased slightly in 100 mmol/L. Urea had no effect on cell volume in the absence of taurine. Retardation of efflux, and cell swelling in the presence of 50 mmol/L urea were entirely abolished by trimethylamine (100 mumol/L). TMA had no effect on either variable in the absence of urea. It is suggested that impaired loss of taurine and accompanying cell swelling may be factors contributing to the neurological disturbances accompanying uremia.

Cloning and Characterization of the Promoter Region of the Rat Taurine Transporter Gene: Transcriptional Regulation by Taurine and Estradiol 1809

Expression of the renal taurine transporter gene is regulated by intracellular taurine content. In certain cell lines, transcription is regulated by taurine availability. We attempted to define the nature of the signal for this adaptive response. A clone containing a segment of the 5′- upstream region of rat taurine transporter genomic DNA was obtained by screening a P1 library using PCR probes derived from rat brain taurine transporter cDNA (rB16a). The DNA (≈1.6 kb pairs) was isolated and characterized. The 5′-flanking region of the gene contains typical eukaryotic promoter elements, including a TATA box, four consensus binding sites for SP1, and two estrogen receptor half-site sequences (AGGTCA). The DNA fragment was also analyzed by transient transfection. When linke to a firefly luciferase reporter gene, it enhanced transcription 100-fold in two renal cell lines, and was influenced by the presence or absence of taurine. Promoter activity was also enhanced by treatment with β-17 estradiol in human breast cancer cells, and in rat cardiomyocytes, but not in renal cells.

Effect of taurine on chelerythrine inhibition of calcium uptake and ATpase activity in the rat retina

Taurine potentiates calcium uptake in whole retinal homogenates as well as in rod outer segments and mitochondrial fractions. The aim of this study was to correlate taurine potentiation of calcium uptake with its effects on other cellular processes through the use of chelerythrine (CHT), a modulator of protein kinase C (PKC), ATPase activity, and, as recently shown, of retinal protein phosphorylation. CHT inhibited calcium uptake only when ATP was present, and inhibition increased significantly in conditions of ATP excess. Taurine potentiated ATP-dependent calcium uptake but decreased the potency of ATP to induce uptake activity. CHT inhibition of calcium uptake exhibited similar potencies in the presence and absence of taurine, and this inhibition seemed to be independent of PKC inhibition. Because of the ATP-dependence of the observed effect, total ATPase activity was studied using similar treatments. In the absence of taurine, CHT inhibited ATPase activity with the same potency ( ic 50 ∼ 59.3 μM) as with calcium uptake inhibition ( ic 50 ∼ 87.9 μM), presenting a possible mechanism of action of CHT. In the presence of taurine, no such correlation was observed, suggesting an ATPase'independent mechanism of action. In fact, taurine did not potentiate ATPase activity, but rather it decreased the potency of CHT inhibition of ATPase, effects incongruent with the effects of taurine on calcium uptake and on CHT inhibition of calcium uptake. Enzyme kinetic experiments provided more supporting data. Taurine was found to cause an increase in the affinity of the ATP substrate for the ATPase enzyme, contradicting the aforementioned effect of taurine to decrease the potency of ATP to induce calcium uptake. Thus, taurine seems to increase calcium uptake through a hitherto unreported mechanism distinct from its modulation of ATPase activity.

Conversion of taurine into N-chlorotaurine (taurine chloramine) and sulphoacetaldehyde in response to oxidative stress.

N-Chlorotaurine (taurine chloramine), formed by treating taurine with hypochlorous acid, was shown to decompose to sulphoacetaldehyde with a first-order rate constant of 9.9+/-0.5 x 10(-4).h-1 at 37 degrees C in 0.1 M phosphate buffer, pH 7.4. Rat liver homogenates accelerated this decay in a process that was proportional to tissue-protein concentration and saturable, with maximum velocity (Vmax) and Km values of 0.28+/-0.01 nmol/min per mg of protein and 37+/-9 microM respectively. This activity was found to be lost on heat denaturation, but retained after dialysis. There was no detectable formation of sulphoacetaldehyde when taurine itself was incubated with the tissue homogenates under the same conditions. Activation of human neutrophils (1.67 x 10(6) cells/ml) with latex beads resulted in a respiratory burst of oxygen-radical production, the products of which were partially sequestered by 12.5 mM taurine. Under these conditions sulphoacetaldehyde was generated at a constant rate of 637+/-18 pmol/h per ml for over 7 h. A non-activated neutrophil suspension contained constant levels of 1.42+/-0.02 nmol/ml sulphoacetaldehyde, as did activated cells incubated in the absence of taurine, a basal level which may indicate a steady turnover of taurine in these cells. Such formation of chlorotaurine and its decay to the aldehyde may be the first steps in the metabolism of taurine to isethionate (2-hydroxyethanesulphonate) that has been demonstrated by various authors to occur in vivo.

Pharmacological characterization of the effects of taurine on calcium uptake in the rat retina.

Taurine is known to increase ATP-dependent calcium ion (Ca2+) uptake in retinal membrane preparations and in isolated rod outer segments (ROS) under low calcium conditions (10 microM) (Pasantes-Morales and Ordóñez, 1982; Lombardini, 1991). In this report, ATP-dependent Ca2+ uptake in retinal membrane preparations was found to be inhibited by 5 microM cadmium (Cd2+), suggesting the involvement of cation channel activation. The activation of cGMP-gated cation channels, which are found in the ROS, is a crucial step in the phototransduction process. An inhibitor of cGMP-gated channels, LY83583, was found to inhibit taurine-stimulated ATP-dependent Ca2+ uptake but had no effect on ATP-dependent Ca2+ uptake in the absence of taurine, indicating that taurine may be increasing ATP-dependent Ca2+ uptake through a mechanism of action involving the opening of cGMP-gated channels. The activation of cGMP-gated channels with dibutyryl-cGMP and with phosphodiesterase inhibition using zaprinast caused an increase in ATP-dependent Ca2+ uptake in isolated ROS, but not in taurine-stimulated ATP-dependent Ca2+ uptake. LY83583 had the same effects in isolated ROS as in retinal membrane preparations. Another inhibitor of cGMP-gated channels, Rp-8-Br-PET-cGMPS, produced the same pattern of inhibition in isolated ROS as LY83583. Thus, there appears to be a causal link between taurine and the activation of the cGMP-gated channels in the ROS under conditions of low calcium concentration, a connection that suggests an important role for taurine in the visual signalling function of the retina.

Unique pharmacological interactions of taurine and chelerythrine in the retina.

The effects of taurine and chelerythrine (CHT) on ATP-dependent calcium uptake and the phosphorylation of the approximately 20 kDa phosphoprotein in the retina were compared. In the absence of the CHT, taurine stimulated ATP-dependent calcium uptake and attenuated the phosphorylation of the approximately 20 kDa phosphoprotein. On the other hand, CHT produced the opposite results in the absence of taurine. When the two agents were used simultaneously, it was found that CHT non-competitively inhibited the action of taurine to stimulate calcium uptake, while taurine non-competitively inhibited the action of CHT to stimulate the phosphorylation of the approximately 20 kDa phosphoprotein. The data present an unusual pharmacological mechanism for controlling the signal transduction pathway involving the two distinct cellular processes being studied. Given the unique data, a control system is proposed in which the function of the approximately 20 kDa phosphoprotein is linked to the stimulation of ATP-dependent calcium uptake.

Interaction between taurine and angiotensin II: Modulation of calcium transport and myocardial contractile function

Angiotensin II modulates several aspects of cardiac function, including myocardial contractility, heart rate and myocyte growth. Most of these actions are intimately associated with alterations in calcium transport. Since taurine also modulates calcium transport, we examined possible interactions between taurine and angiotensin II at the level of the major cellular extruder of calcium, the Na−-Ca2+ exchanger. Over a concentration range of 0.5–25 mM, Turne served as an effective inhibitor of angiotensin II-mediated stimulation of the exchanger. An Arrhenius plot of Na+-Ca2+ exchange activity revealed that angiotensin II (2 nM) increased transporter activity by reducing the activation energy of the transport process. Taurine (25 mM) inhibited the angiotensin II effect by partially preventing the reduction in activation energy. However, neither agent significantly altered the transition temperature, ruling out a change in membrane fluidity or an alteration in the rate limiting step of the transporter as a cause of the observed effects. Since the Na+-Ca2+ exchanger plays an important role in the handling of [Ca2+]i by the myocardium, the effect of taurine on angiotensin II's modulation of contractile function was also examined. Hearts perfused with buffer containing angiotensin 11 experienced a slight positive isotropic effect in the absence of taurine but this was converted to a negative inotropic effect in the presence of taurine. The data suggest that Turine inhibits some, but not all of the actions of angiotensin II. The possibility that a phosphorylation event is the site of the angiotensin II-taurine interaction is discussed.

Hypotonicity increases basolateral taurine permeability in rabbit proximal convoluted tubule

The permeabilities of the basolateral membrane of rabbit proximal convoluted tubule (PCT) to taurine (PTau) and glucose (PGlc) were estimated under control and hypotonic conditions using the initial rate of increase in cellular volume (CV) induced on isotonic replacement of 40 mM mannitol by one or the other of these substrates. Under control conditions, addition of taurine led to an increase in CV at an initial rate of 7.1 +/- 1.7%/min, leading to a cell swelling of 30.2 +/- 4.8% after 5 min (n = 6). Addition of glucose led to an increase in CV at an initial rate of 30.0 +/- 3.8%/min, leading to a cell swelling of 25.7 +/- 3.1% after 5 min (n = 7). After a period of recovery of 5 min in the absence of taurine or glucose, a 40 mosmol/kg hypotonic shock induced a cell swelling of 14.2 +/- 1.3 and 16.1 +/- 5.2%, respectively, followed by an almost complete volume regulatory decrease after 5 min. At that time, addition of taurine under continuous hypotonicity induced an increase in CV at an initial rate 2.57 +/- 0.17 times larger than that observed under the isotonic condition (P < 0.005), while addition of glucose induced an initial increase in CV identical to that observed under the isotonic condition. The increases in CV observed on addition of taurine were completely abolished in the absence of sodium under both isotonic and hypotonic conditions. The permeability to K+ was also estimated, in the absence of sodium, using the initial rate of increase in CV induced on isotonic replacement of 40 mM N-methyl-D-glucamine by K+.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoprotective effects of taurine in isolated rat hepatocytes

This study examined the protection of isolated rat hepatocytes against carbon tetrachloride, hydrazine and 1,4-naphthoquinone (1,4-NQ) toxicity. Hepatocytes were incubated with various concentrations of toxicant in the presence and absence of taurine (0–15 m m). The presence of taurine significantly decreased the cytotoxicity of each compound as measured by trypan blue uptake and lactate dehydrogenase leakage. The protection was related to the concentration of taurine, with a significant effect at 10 m m for all three compounds. When ATP was measured, however, taurine failed to protect against the depletion caused by hydrazine, whereas depletion due to 1,4-NQ was significantly ameliorated. The results suggest that taurine may protect cells from cytotoxicity as reflected by membrane damage but biochemical events underlying the toxicity, such as ATP depletion, may not be affected. Taurine may be a useful tool for the investigation of mechanisms of cytotoxicity.

Alterations in the immune system in cats fed diets with excess cystine

As part of an ongoing biochemical study in nutrition we examined blood profiles, serum chemistry, lymphocyte transformation and lymphoid pathology in cats fed a diet containing 5% cystine with and without taurine. Automated blood counts of whole blood samples showed a decrease in red blood cell counts accompanied by a significant decrease in hemoglobin and hematocrit in cats fed 5% cystine in the absence of taurine compared to cats fed 0.05% taurine (control). A significant increase was noted in serum cholesterol in cats fed cystine and cystine/taurine compared to cats fed control diets. There were no significant differences in lymphocyte transformation using leukocytes isolated from the spleen and blood with the mitogens, phytohemagglutinin and pokeweed. However, lymphocyte transformation of both spleen and blood without mitogen from the excess cystine group were significantly higher than leukocytes from the 0.05% taurine group (control). Pathological examination of regional lymph nodes, livers, and spleens showed histological abnormalities in cats fed the excess cystine diet. These results indicate that there are alterations in the immune system of cats fed a diet containing 5% cystine with and without dietary taurine.

Taurine depletion in very low birth weight infants receiving prolonged total parenteral nutrition: Role of renal immaturity

In a prospective, controlled study, plasma and urinary taurine concentrations were determined weekly, between postnatal weeks 3 and 18, in (1) seven sick infants (gestational age less than 28 weeks, birth weight less than or equal to 1000 gm) who received a taurine-free total parenteral nutrition solution for 32 to 49 days (group P) and who subsequently were formula fed and (2) eight sick infants matched by gestational age and birth weight, who received formula or human milk from day 3 to 4 of life (group E). Ten healthy full-term infants ranging in age from 1 to 18 weeks and fed with formula provided normal values (group C). Significantly lower mean plasma taurine values (range 1.59 to 3.43 mumol/dl) were found between postnatal weeks 3 and 7 in group P compared with group E (range 5.54 to 6.97 mumol/dl) and with group C (5.6 +/- 0.34 mumol/dl). After initiation of feeding, plasma taurine concentrations in group P increased to normal. Markedly elevated values of mean fractional excretion of taurine, 38% to 56%, were found between weeks 3 and 5 in group P and E compared with group C (15.5 +/- 3.2%). In contrast, during the same period, low urinary taurine values (4.9% to 6.7%) were found in two larger, older infants receiving total parenteral nutrition whose plasma taurine values were in the normal range. After week 5, urinary taurine values were in the control range in all groups. We conclude that the absence of taurine in total parenteral nutrition solutions administered to very low birth weight infants and the limited ability of the immature kidney to adapt to low taurine intake by "up-regulation" of tubular taurine reabsorption may result in depleted taurine body pools during the first weeks of life. This inability to conserve taurine by the immature nephron could potentially have a deleterious effect on the developing brain and retina in these infants, and indicates a possible need for taurine supplementation.

Opposing interactions of ionophores (valinomycin and monensin) on calcium ion uptake in rat retinal preparations.

Valinomycin is a potent inhibitor of taurine-stimulated ATP-dependent calcium ion uptake in rat retinal membrane preparations but had no effect on ATP-dependent calcium ion uptake in the absence of taurine and no effect on ATP-independent calcium ion uptake. The presence of potassium ions in the buffer systems were required for valinomycin to be inhibitory. On the contrary, monensin stimulated calcium ion uptake in the ATP-dependent system but had no effect on ATP-independent calcium ion uptake. The crude retinal homogenate was also fractionated into various subcellular components. The fraction which contained photoreceptor cell synaptosomes (P1) had a higher specific activity for taurine-stimulated ATP-dependent calcium ion uptake than the crude homogenate or either the fractions which contained synaptosomes derived from the plexiform layer (P2) or rod outer segments (ROS). No differences in calcium ion uptake were observed in the various subcellular fractions compared to the homogenate when assayed for ATP-dependent calcium ion uptake. Valinomycin inhibited both ATP-dependent and taurine-stimulated ATP-dependent calcium ion uptake in the P1, P2, and ROS fractions while monensin stimulated the ATP-dependent calcium ion uptake in the subcellular fractions.

Effect of taurine on a muscle intracellular membrane

Sarcoplasmic reticulum from rat skeletal muscle had rates of calcium oxalate uptake dependent upon the speed with which the muscle was removed and homogenized. Rapidly isolated sarcoplasmic reticulum (muscle removed and homogenized within 20 min) showed a 25% increase in rate of calcium oxlate uptake in the presence of 15 mM taurine, and the total sequestering capacity was also increased. The rate of calcium oxalate uptake by more slowly isolated sarcoplasmic reticulum (1 h between sacrifice of first animal and homogenization of the muscle) could be increased by 30% by performing the isolation with 10–15 mM taurine present in all media, compared with a paired isolation in the absence of taurine. Exposure of sarcoplasmic reticulum to taurine throughout isolation led to an increased yield of microsomes and sarcoplasmic reticulum. These effects of taurine on sarcoplasmic reticulum were not duplicated by phosphate, isethionate, KCl, cysteine or histidine, but aminopropane-sulfonic acid showed similar effects. Taurine slowed the rate of loss of calcium transport and ATPase activities of sarcoplasmic reticulum caused by phospholipase C. It is suggested that taurine may function as a membrane stabilizer.