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

Taurine modulates calcium influx through L-type voltage-gated calcium channels in isolated cochlear outer hair cells in guinea pigs

Taurine has been proposed to play a role in calcium modulation. To explore the effect of taurine on intracellular calcium homeostasis of isolated cochlear outer hair cells and on the gentamycin-induced inhibition of calcium influx evoked by high K(+) depolarization, we employed fluo-3 imaging of intracellular calcium ([Ca(2+)](i)) via confocal laser scanning microscopy to measure real-time changes of [Ca(2+)](i). We found that the sole application of taurine (5, 10, 20 mM) induced a transient [Ca(2+)](i) increase in a concentration-dependent manner, which was inhibited either by the application of an L-type calcium-channel blocker nifedipine or a calcium-free medium. Pre-incubation with 1mM gentamicin induced inhibition of [C(a)(2+)](i) elevation evoked by high K(+). Short-term (10 min) exposure with a high level of taurine (20 mM) prevented this inhibition. These results indicated that taurine at a high concentration was able to promote calcium influx through L-type calcium channels in isolated outer hair cells and antagonize gentamycin-induced inhibition of calcium elevation evoked by high K(+) by its calcium homeostatic effect.

Taurine: evidence of physiological function in the retina

Taurine is a free amino acid found in high millimolar concentrations in mammalian tissue and is particularly abundant in the retina. Mammals synthesize taurine endogenously with varying abilities, with some species more dependent on dietary sources of taurine than others. Human children appear to be more dependent on dietary taurine than adults. Specifically, it has been established that visual dysfunction in both human and animal subjects results from taurine deficiency. Moreover, the deficiency is reversed with simple nutritional supplementation with taurine. The data suggest that taurine is an important neurochemical factor in the visual system. However, the exact function or functions of taurine in the retina are still unresolved despite continuing scientific study. Nevertheless, the importance of taurine in the retina is implied in the following experimental findings: (1) Taurine exhibits significant effects on biochemical systems in vitro. (2) The distribution of taurine is tightly regulated in the different retinal cell types through the development of the retina. (3) Taurine depletion results in significant retinal lesions. (4) Taurine release and uptake has been found to employ distinct regulatory mechanisms in the retina.

Taurine antagonizes calcium overload induced by glutamate or chemical hypoxia in cultured rat hippocampal neurons

Taurine has been proposed to have antiexcitotoxic and antihypoxic activity. To explore the effect of taurine on neuronal calcium overload evoked by glutamate or hypoxia, we employed fluo-3 imaging of intracellular calcium ([Ca2+]i) in confocal laser scanning microscope to measure real-time changes of [Ca2+]i arose from glutamate/2,4-dinitrophenol (DNP, mimic hypoxia) and taurine in cultured rat hippocampal neurons. We found that 3 mM taurine could inhibit [Ca2+]i elevation ascribed to 0.5 mM glutamate or 0.2 mM DNP. Low (0.5 mM) or high (12 mM) level of taurine displayed no significantly depressant effect. However, sole application of taurine could increase [Ca2+]i transiently. The results indicate that taurine in moderate concentration may exert antiexcitotoxic and antihypoxic effect partially via its antagonism to [Ca2+]i overload.

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.

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.