Treatment of hypertension with oral taurine: experimental and clinical studies.

Oral taurine treatment has been studied extensively as a hypotensive agent. Several rat models of hypertension have been used to prove that dietary taurine supplementation can alleviate high blood pressure, among other cardiovascular problems. Experimental models mentioned in this review are the spontaneously hypertensive rat, the DOCA-salt rat, the Dahl-S rat, the renovascular hypertensive rat, the hyperinsulinemic rat and the ethanol-treated rat. The beneficial effects of taurine were also demonstrated in studies involving human subjects suffering essential hypertension. Taurine supplementation of 6 g/day for as little as 7 days resulted in measurable decreases in blood pressure in these patients. In both rat and human studies, the effects of taurine appeared to be dependent on the modulation of an overactive sympathetic system. However, taurine has positive effects on other types of cardiovascular problems and thus may act through more than one mechanism.

Cytokine-induced iNOS expression in C6 glial cells: transcriptional inhibition by ethanol.

The effect of cytokines, lipopolysaccharide, and ethanol on inducible nitric-oxide synthase (iNOS) expression was studied in C6 glial cells. Maximal induced activity, measured by the accumulation of nitrite in culture medium, occurred following treatment with lipopolysaccharide and interferon-gamma. Each cytokine alone was ineffective, whereas an optimal combination of interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma was near maximal, indicating synergistic interactions. Other combinations caused submaximal activity. Ethanol is known to suppress iNOS expression in C6 cells induced by a phorbol ester plus lipopolysaccharide. The current work shows ethanol also suppresses cytokine-induced iNOS expression and reduces interleukin-1beta and tumor necrosis factor-alpha potency without affecting interferon-gamma potency. Ethanol-mediated reductions in cytokine-induced iNOS mRNA and immunoreactive protein levels suggested an effect on gene transcription. Therefore, C6 cells stably expressing 1846 and 526 base fragments of the rat iNOS gene promoter fused to a luciferase reporter gene were prepared and characterized and used to study the effect of ethanol on iNOS promoter activity. Promoter activity in stable transfected C6 cells was inhibited by ethanol exposure with a similar concentration dependence as observed for inhibition of nitrite production, indicating that iNOS inhibition by ethanol is transcriptional. Furthermore, ethanol inhibition of the 526 base fragment activity, which lacks interferon-gamma enhancement of lipopolysaccharide-induced luciferase activity, confirmed that interferon-gamma-responsive elements do not participate in acute ethanol-induced inhibition of rat iNOS gene transcription.

The role of taurine in the pathogenesis of the cardiomyopathy of insulin-dependent diabetes mellitus.

The cellular and molecular physiology and pathology of insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM) are mostly studied and understood through the use of animal models. Fundamental differences between the IDDM and NIDDM animal models may help to explain the etiology behind diabetic cardiomyopathy, one of the most severe complications of IDDM. Experimental rat models of IDDM exhibit a characteristic increase in tissue levels of taurine in the heart, a change that is not seen in NIDDM rats. This article deals with the causes and possible consequences of this observation which may contribute to the development of diabetic cardiomyopathy. Modulation of pyruvate dehydrogenase (lipoamide) (PDH; EC 1.2.4.1) activity was found to be a possible mode for taurine involvement. PDH is a mitochondrial protein and is the rate-limiting step in the generation of acetyl CoA from glycolysis. In IDDM, PDH activity is decreased through a mechanism that includes the stimulation of the de novo synthesis of a kinase activator protein (KAP) which phosphorylates PDH and inactivates the enzyme. This lesion does not occur in NIDDM rat hearts. Taurine is known to inhibit the phosphorylation of PDH in vitro, and in taurine-depleted rats PDH phosphorylation is known to increase. Thus, the increased levels of taurine in the diabetic heart may be inhibiting this phosphorylation which in turn may be stimulating the synthesis of KAP through a negative feedback process. The main argument for this theory would be the lack of change in both the taurine levels and the activity of PDH in the NIDDM rat model.

Stabilization of calcium uptake in rat rod outer segments by taurine and ATP.

Calcium ion (Ca2+) uptake was measured in rod outer segments (ROS) isolated from rat retina in the presence of varying concentrations of CaCl2 in the incubation buffer (1.0-2.5 mM). It is known that taurine increases Ca2+ uptake in rat ROS in the presence of ATP and at low concentrations of CaCl2 (Lombardini, 1985a); taurine produces no significant effects when CaCl2 concentrations are increased to 1.0 and 2.5mM. With the removal of both taurine and ATP, Ca2+ uptake in rat ROS increased significantly in the presence of 2.5mM CaCl2. Taurine treatment in the absence of ATP was effective in decreasing Ca2+ uptake at the higher levels of CaCl2 (2.0 and 2.5 mM). Similar effects were observed with ATP treatment. The data suggest that taurine and ATP, alone or in combination, limit the capacity of the rat ROS to take up Ca2+ to the extent that a stable uptake level is achieved under conditions of increasing extracellular Ca2+, indicating a protective role for both agents against calcium toxicity.

Stimulatory effect of taurine on calcium ion uptake in rod outer segments of the rat retina is independent of taurine uptake.

Taurine stimulates ATP-dependent Ca(2+) uptake in the rat rod outer segments (ROS). This stimulation has been linked to the function of the cyclic nucleotide-gated cation channel, implying an important physiologic role for taurine in visual signal transduction. Calmodulin (CaM) has been reported to affect taurine transport in the choroid plexus and also to inhibit the cyclic nucleotide-gated channel; thus, the effects of the competitive CaM inhibitors trifluoperazine (TFP) and N-(8-aminooctyl)-5-iodonaphthalene-1-sulfonamide (J-8) were studied on Ca(2+) and taurine uptake in the rat ROS. Pretreatment of the ROS preparation with TFP and J-8 for 5 min before measurement of Ca(2+)-uptake activity produced inhibition of the effects of taurine on ATP-dependent Ca(2+) uptake. Both TFP and J-8 also were effective in inhibiting high-affinity taurine uptake. In both uptake systems, inhibition by TFP was noncompetitive. These data initially suggested that the stimulatory effects of taurine on ATP-dependent Ca(2+) uptake are dependent on taurine uptake. However, competitive inhibition of taurine uptake by guanidinoethane sulfonate did not produce any effect on the stimulatory effects of taurine. Previous studies have proposed that taurine binds directly to the plasma membrane, and our study demonstrated that TFP inhibits taurine binding to the ROS. In addition, our study demonstrated that taurine uptake is unaffected by varying the concentration of Ca(2+) and that the effects of TFP are independent of Ca(2+), suggesting that TFP acts through a CaM-independent mechanism.

Effects of short chain alkanols on the inducible nitric oxide synthase in a glial cell line.

1. Ethanol inhibits inducible nitric oxide synthase (iNOS) expression in C6 glioma cells by an unknown mechanism. Because relatively high concentrations are needed for inhibition in drug-naive cells (IC50 approximately = to 150 mM), suppression due to cytotoxicity is one possible mechanism that has not been ruled out. Therefore, the present study examined the effects of ethanol and other alkanols on C6 glioma cell viability and iNOS activity to better understand the mechanism for inhibition. 2. iNOS expression was induced in cell culture with lipopolysaccharide and phorbol ester treatment. Nitrite accumulation in culture medium, the in vitro conversion of [3H]-L-arginine to [3H]-L-citrulline, and immunoblotting were used to quantify iNOS induction and activity. Trypan blue exclusion, extracellular release of lactate dehydrogenase, and quantity of total cell protein were used as measures of viability. 3. Short chain alkanols, methanol through 1-heptanol, concentration-dependently inhibited nitrite accumulation. Longer chain alkanols, 1-octanol and 1-decanol, did not except at cytotoxic concentrations. Experiments indicated short chain alkanol inhibition was not due to direct actions on iNOS catalytic activity, but that it transpires during iNOS induction. Immunoblots showed reduced iNOS protein levels. 4. Correlation analysis ruled out iNOS inhibition as being due to decreased cell number, total cell protein, or cell viability. In contrast, there was significant correlation with physical measures of lipophilicity. 5. In conclusion, inhibition of iNOS expression by ethanol and other short chain alkanols is not due to cytotoxicity. Instead, the strong correlation with lipophilicity suggests the inhibition derives from an interaction with unknown hydrophobic cellular sites.

Taurine uptake activity in the rat retina: protein kinase C-independent inhibition by chelerythrine.

Taurine, a regulatory amino acid of various biochemical processes in the retina, requires an efficient uptake system to maintain the high physiological concentration of taurine in the retina. Taurine uptake was characterized in both whole retinal preparations and in isolated rod outer segments (ROS) in terms of uptake kinetics and possible protein kinase C (PKC)-dependent regulation. Two uptake systems, a high- and a low-affinity system, were found in whole retinal preparations while only the high-affinity system was found in the isolated ROS. All the uptake systems characterized were inhibited by guanidinoethane sulfonate (GES), a well-known competitive inhibitor of taurine uptake. Stimulation and inhibition of PKC activity with phorbol myristate acetate and with staurosporine, respectively, produced no significant effect on taurine uptake. On the other hand, chelerythrine (CHT), a documented potent PKC inhibitor, was found to cause significant inhibition of the two taurine uptake systems, presumably through a PKC-independent mechanism. The data demonstrate that CHT may be a useful tool in studying taurine uptake in the retina and specifically in the ROS.

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.

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 (IC50 approximately 59.3 microM) as with calcium uptake inhibition (IC50 approximately 87.9 microM), 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.

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.

Suppression by ethanol of inducible nitric oxide synthase expression in C6 glioma cells.

Exposure to lipopolysaccharide (LPS) combined with phorbol-12-myristate-13-acetate (PMA) stimulates de novo synthesis of inducible nitric oxide synthase (NOS-2) in C6 glioma cells. Ethanol dose-dependently inhibits C6 cell NOS-2 activity, as measured by nitrite accumulation in culture medium, when present during LPS plus PMA treatment. The present study reports on mechanisms related to this inhibition. Ethanol added directly to cytosolic extracts did not inhibit NOS-2 catalytic activity, nor did ethanol decrease nitrite accumulation when added to cultures 24 hr after LPS plus PMA treatment. In contrast, NOS-2 enzymatic activity was significantly decreased in cytosolic extracts from cultures simultaneously exposed to ethanol and LPS plus PMA for 24 hr. Immunoblot analysis showed a coincident decrease in NOS-2 protein immunoreactivity. RNA analysis revealed that NOS-2 mRNA was decreased at both 12 and 24 hr during LPS plus PMA induction in the presence of ethanol. Subsequent experiments confirmed that 12-hr exposure to ethanol was sufficient to inhibit LPS/PMA-induced NOS-2 activity. Ethanol exposure also inhibited NOS-2 activity induced by LPS plus interferon-gamma, by LPS plus tumor necrosis factor-alpha and by tumor necrosis factor-alpha alone. These data point to an inhibitory ethanol effect at a site downstream from cytokine receptor activation and second messenger signal transduction mechanisms leading to suppression of NOS-2 gene expression in C6 cells.