Reciprocal regulation between taurine and glutamate response via Ca<Superscript>2+</Superscript>- dependent pathways in retinal third-order neurons

Although taurine and glutamate are the most abundant amino acids conducting neural signals in the central nervous system, the communication between these two neurotransmitters is largely unknown. This study explores the interaction of taurine and glutamate in the retinal third-order neurons. Using specific antibodies, both taurine and taurine transporters were localized in photoreceptors and Off-bipolar cells, glutamatergic neurons in retinas. It is possible that Off-bipolar cells release juxtaposed glutamate and taurine to activate the third-order neurons in retina. The interaction of taurine and glutamate was studied in acutely dissociated third-order neurons in whole-cell patch-clamp recording and Ca²⁺ imaging. We find that taurine effectively reduces glutamate-induced Ca²⁺ influx via ionotropic glutamate receptors and voltage-dependent Ca²⁺ channels in the neurons, and the effect of taurine was selectively inhibited by strychnine and picrotoxin, but not GABA receptor antagonists, although GABA receptors are present in the neurons. A CaMKII inhibitor partially reversed the effect of taurine, suggesting that a Ca²⁺/calmodulin-dependent pathway is involved in taurine regulation. On the other hand, a rapid influx of Ca²⁺ through ionotropic glutamate receptors could inhibit the amplitude and kinetics of taurine-elicited currents in the third-order neurons, which could be controlled with intracellular application of BAPTA a fast Ca²⁺ chelator. This study indicates that taurine is a potential neuromodulator in glutamate transmission. The reciprocal inhibition between taurine and glutamate in the postsynaptic neurons contributes to computation of visual signals in the retinal neurons.     

Taurine-activated currents in isolated neurons of the rat cerebellum

Taurine-activated currents were investigated in rat cerebellar neurons using techniques of voltage clamping at the membrane and intracellular perfusion. Activation of both chloride and calcium conductance at the membrane were produced by applying taurine to the membrane surface. The dose-response curve for taurine-activated current is in the 1×10^–4–1×10^–1 M concentration region. The dissociation constant of the taurine-receptor complex equals 2×10^–3 M. Activation of taurine-induced currents is a cooperative process: Hill's coefficient –2. It was found that bicuculline and strychnine exert a blocking action on taurine-activated currents, while pentobarbital and oxazepam potentiate taurine action.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 6, November–December, 1990, pp. 780–786.     

13C nuclear magnetic resonance study of the complexation of calcium by taurine

¹³C Nuclear magnetic resonance chemical shifts, ¹J_C-C scalar coupling constants, spin-lattice relaxation times, and nuclear Overhauser effects were determined for taurine-[1, 2 ¹³C] and a taurine-[1 ¹³C] and taurine-[2 ¹³C] mixture in the presence and absence of calcium. Ionization constants for taurine amino and sulfonic acid groups and chemical shifts of N-methylene and S-methylene carbons of the taurine cation, zwitterion, and anion were obtained from simultaneous least squares analysis of ¹³C titration curves of both taurine carbons. Comparison of taurine titration shifts to values for related compounds reveals some unusual electronic properties of the taurine molecule. Stability constants of 1:1 calcium complexes with taurine zwitterions and anions, as well as their ¹³C chemical shifts, were obtained by least squares analysis of titration curves measured in the presence of calcium. The stability constants of calcium-taurine complexes were significantly lower than previous values and led to estimates that only approximately one percent of intracellular calcium of mammalian myocardial cells would exist in a taurine complex. The implications of these results with respect to the effect of taurine on calcium ion flux are discussed.     

Epileptiform postsynaptic currents in primary culture of rat cortical neurons: Calcium mechanisms

In this study we demonstrate that the primary culture of rat cortical neurons is a convenient model for investigations of epileptogenesis mechanisms and specifically, of the postsynaptic epileptiform currents (EC) reflecting periodical asynchronous glutamate release. In particular, we have revealed that in primary culture of cortical neurons EC can appear spontaneously or can be triggered by the withdrawal of magnesium block of NMDA receptor channels or by shutting down GABAergic inhibition. EC were found to depend on intracellular calcium oscillations. The secondary calcium release from intracellular stores was needed for EC synchronization. EC were suppressed by the influences causing either neuronal calcium overload or decrease of intracellular calcium concentration. Calcium entry into neurons in the case of NMDA receptor hyperactivation or in the case of calcium ionophore ionomycin treatment eliminated EC. The suppression of EC also occurred after a decrease of intracellular calcium concentration induced by BAPTA loaded into the neurons or by stimulation of calcium removal from cells via Na⁺/Ca²⁺ exchanger by 1 nM ouabain. Partial dependence of EC on action potential generation was found. Thus, EC in neurons are activated by intracellular periodic calcium waves within a limited concentration window.     

Taurine increases mitochondrial buffering of calcium: role in neuroprotection

Summary. We have determined the role of mitochondria in the sequestration of calcium after stimulation of cerebellar granule cells with glutamate. In addition we have evaluated the neuroprotective role of taurine in excitotoxic cell death. Mitochondrial inhibitors were used to determine the calcium buffering capacity of mitochondria, as well as how taurine regulates the ability of mitochondria to buffer intracellular calcium during glutamate depolarization and excitotoxicity. We report here that pre-treatment of cerebellar granule cells with taurine (1 mM, 24 h) significantly counteracted glutamate excitotoxicity. The neuroprotective role of taurine was mediated through regulation of cytoplasmic free calcium ([Ca²⁺] _i ), and intra-mitochondrial calcium homeostasis, as determined by fluo-3 and ⁴⁵Ca²⁺-uptake. Furthermore, the overall mitochondrial function was increased in the presence of taurine, as assessed by rhodamine accumulation into mitochondria and total cellular ATP levels. We specifically tested the hypothesis that taurine reduces glutamate excitotoxicity through both the enhancement of mitochondrial function and the regulation of intracellular (cytoplasmic and intra-mitochondrial) calcium homeostasis. The role of taurine in modulating mitochondrial calcium homeostasis could be of particular importance under pathological conditions that are characterized by excessive calcium overloads. Taurine may serve as an endogenous neuroprotective molecule against brain insults. Authors’ address: Abdeslem El Idrissi, Biology Department and Center for Developmental Neuroscience, College of Staten Island/CUNY, 6S-134 Staten Island, NY 10314, U.S.A.     

Modulation of Calcium Channels by Taurine Acting Via a Metabotropic-like Glycine Receptor

Taurine is one of the most abundant free amino acids in the central nervous system, where it displays several functions. However, its molecular targets remain unknown. It is well known that taurine can activate GABA-A and strychnine-sensitive glycine receptors, which increases a chloride conductance. In this study, we describe that acute application of taurine induces a dose-dependent inhibition of voltage-dependent calcium channels in chromaffin cells from bovine adrenal medullae. This taurine effect was not explained by the activation of either GABA-A, GABA-B or strychnine-sensitive glycine receptors. Interestingly, glycine mimicked the modulatory action exerted by taurine on calcium channels, although the acute application of glycine did not elicit any ionic current in these cells. Additionally, the modulation of calcium channels exerted by both taurine and glycine was prevented by the intracellular dialysis of GDP-β-S. Thus, the modulation of voltage-dependent calcium channels by taurine seems to be mediated by a metabotropic-like glycinergic receptor coupled to G-protein activation in a membrane delimited pathway.     

Calcium Signaling in <Emphasis Type="Italic">Carassius</Emphasis> Cerebellar Neurons: Role of the Mitochondria

We studied the involvement of the mitochondria playing the role of a calcium store in the control of calcium exchange in cerebellar neurons of a fish species tolerant to hypoxia, crucian (Carassius gibelio). In our experiments we used an ionophore, CCCP, that blocked accumulation of calcium by the above organelles. The intracellular concentration of free Ca²⁺ ([Ca²⁺] _і ) was measured using a calcium-sensitive dye, Fura-2AM, and the microfluorescent technique. We found that cerebellar neurons of Carassius gibelio possess a well-expressed system clearing the cytoplasm from excessive Ca²⁺, and the mitochondria are actively involved in this process. Under conditions of suppression of the process of accumulation of calcium by the mitochondria under the action of CCCP, the amplitude of calcium transients increased by about 50%. In addition, the decay phase of depolarization-induced intracellular calcium transients was slowed down considerably. Therefore, our experiments are indicative of the significant role of the mitochondria in the control of calcium dynamics in cerebellar neurons of Carassius gibelio in the course of functional activity of these cells.     

Interaction between the actions of taurine and angiotensin II

Summary. The amino acid, taurine, is an important nutrient found in very high concentration in excitable tissue. Cellular depletion of taurine has been linked to developmental defects, retinal damage, immundeficiency, impaired cellular growth and the development of a cardiomyopathy. These findings have encouraged the use of taurine in infant formula, nutritional supplements and energy promoting drinks. Nonetheless, the use of taurine as a drug to treat specific diseases has been limited. One disease that responds favorably to taurine therapy is congestive heart failure. In this review, we discuss three mechanisms that might underlie the beneficial effect of taurine in heart failure. First, taurine promotes natriuresis and diuresis, presumably through its osmoregulatory activity in the kidney, its modulation of atrial natriuretic factor secretion and its putative regulation of vasopressin release. However, it remains to be determined whether taurine treatment promotes salt and water excretion in humans with heart failure. Second, taurine mediates a modest positive inotropic effect by regulating [Na⁺]_i and Na⁺/Ca²⁺ exchanger flux. Although this effect of taurine has not been examined in human tissue, it is significant that it bypasses the major calcium transport defects found in the failing human heart. Third, taurine attenuates the actions of angiotensin II on Ca²⁺ transport, protein synthesis and angiotensin II signaling. Through this mechanism taurine would be expected to minimize many of the adverse actions of angiotensin II, including the induction of cardiac hypertrophy, volume overload and myocardial remodeling. Since the ACE inhibitors are the mainstay in the treatment of congestive heart failure, this action of taurine is probably very important. Received November 10, 1998, Accepted May 19, 1999     

Analysis of CaM-kinase signaling in cells

A change in intracellular free calcium is a common signaling mechanism that modulates a wide array of physiological processes in most cells. Responses to increased intracellular Ca²⁺ are often mediated by the ubiquitous protein calmodulin (CaM) that upon binding Ca²⁺ can interact with and alter the functionality of numerous proteins including a family of protein kinases referred to as CaM-kinases (CaMKs). Of particular interest are multifunctional CaMKs, such as CaMKI, CaMKII, CaMKIV and CaMKK, that can phosphorylate multiple downstream targets. This review will outline several protocols we have used to identify which members and/or isoforms of this CaMK family mediate specific cellular responses with a focus on studies in neurons. Many previous studies have relied on a single approach such as pharmacological inhibitors or transfected dominant-negative kinase constructs. Since each of these protocols has its limitations, that will be discussed, we emphasize the necessity to use multiple, independent approaches in mapping out cellular signaling pathways.     

Effect of ischemia,calcium depletion and repletion,acidosis and hypoxia on cellular taurine content

Summary.  Occlusion of the left main coronary artery led to a time-dependent release of taurine from the heart. Upon reperfusion, there was a second phase of taurine release, which exceeded the amount of taurine that exited the heart during the 45 min ischemic insult. To obtain information on the mechanism underlying the release of taurine, three variables were examined, acidosis, hypoxia and calcium overload. It was found that large amounts of taurine also leave the cell during the calcium paradox, a condition induced by perfusing the heart with calcium containing buffer following a period of calcium free perfusion. However, little taurine effluxes the hearts exposed to buffer whose pH was lowered to 6.6. Isolated neonatal cardiomyocytes subjected to chemical hypoxia also lost large amounts of taurine. However, the amount of taurine leaving the cells appeared to be correlated with the intracellular sodium concentration, [Na⁺]_i. The data suggest that taurine efflux is regulated by [Na⁺]_i and cellular osmolality, but not by cellular pH. Received November 15, 2001 Accepted January 15, 2002 Published online October 3, 2002 Acknowledgements This study was supported with a grant from the Taisho Pharmaceutical Company. Authors' address: Dr. Stephen W. Schaffer, Department of Pharmacology, University of South Alabama, School of Medicine, Mobile, Alabama, U.S.A., E-mail: sschaffe@jaguarl.usouthal.edu     

Differences of Ca2+ handling properties in identified central olfactory neurons of the antennal lobe

Information processing in neurons depends on highly localized Ca²⁺ signals. The spatial and temporal dynamics of these signals are determined by a variety of cellular parameters including the calcium influx, calcium buffering and calcium extrusion. Our long-term goal is to better understand how intracellular Ca²⁺ dynamics are controlled and contribute to information processing in defined interneurons of the insect olfactory system. The latter has served as an excellent model to study general mechanisms of olfaction. Using patch-clamp recordings and fast optical imaging in combination with the ‘added buffer approach’, we analyzed the Ca²⁺ handling properties of different identified neuron types in Periplaneta americana's olfactory system. Our focus was on two types of local interneurons (LNs) with significant differences in intrinsic electrophysiological properties: (1) spiking LNs that generate ‘normal’ Na⁺ driven action potentials and (2) non-spiking LNs that do not express voltage-activated Na⁺ channels. We found that the distinct electrophysiological properties from different types of central olfactory interneurons are strongly correlated with their cell specific calcium handling properties: non-spiking LNs, in which Ca²⁺ is the only cation that enters the cell to contribute to membrane depolarization, had the highest endogenous Ca²⁺ binding ratio and Ca²⁺ extrusion rate.     

Spontaneous calcium transients regulate neuronal plasticity in developing neurons

Calcium ions play critical roles in neuronal differentiation. We have recorded transient, repeated elevations of calcium in embryonic Xenopus spinal neurons over periods of 1 h in vitro and in vivo, confocally imaging fluo 3-loaded cells at 5 s intervals. Calcium spikes and calcium waves are found both in neurons in culture and in the intact spinal cord. Spikes rise rapidly to approximately 400% of baseline fluorescence and have a double exponential decay, whereas waves rise slowly to approximately 200% of baseline fluorescence and decay slowly as well. Imaging of fura 2-loaded neurons indicates that intracellular calcium increases from 50 to 500 nM during spikes. Both spikes and waves are abolished by removal of extracellular calcium. Developmentally, the incidence and frequency of spikes decrease, whereas the incidence and frequency of waves are constant. Spikes are generated by spontaneous calcium-dependent action potentials and also utilize intracellular calcium stores. Waves are produced by a mechanism that does not involve classic voltage-dependent calcium channels. Spikes are required for expression of the transmitter GABA and for potassium channel modulation. Waves in growth cones are likely to regulate neurite extension. The results demonstrate the roles of a novel signaling system in regulating neuronal plasticity, that operates on a time scale 10⁴ times slower than that of action potentials. © 1995 John Wiley & Sons, Inc.     

Effects of apelin-13 in mice model of experimental pain and peripheral nociceptive signaling in rat sensory neurons

Objective: Apelin-13 is an endogenous peptide with potential analgesic action, although the sites of its analgesic effects remain uncertain and the results are even controversial with regard to its pain modulating action. This study evaluated the possible pain-modulating action of peripherally administered apelin-13 using heat-induced, withdrawal latency to the thermal stimuli, acute pain model in mice. Involvement of peripheral mechanisms was tested, by using the intracellular calcium concentrations as a key signal for nociceptive transmission, in cultured rat dorsal root ganglion (DRG) neurons. Methods: DRG neurons were cultured on glass coverslips following enzymatic digestion and mechanical agitation, and loaded with the calcium-sensitive dye Fura-2 acetoxymethyl ester (1?µM). Intracellular calcium responses in individual DRG neurons were quantified by ratiometric calcium imaging technique. Results: Peripheral injection of a single dose of apelin-13 (100?mg/kg and 300?mg/kg) significantly decreases the latency to painful stimuli in a dose and time-dependent manner (p?<?0.01, p?<?0.05, respectively, n?=?8 each). Apelin-13 (0.1?µM and 1?µM) did not produce a significant effect on cytoplasmic Ca²⁺ ([Ca²⁺]_i) responses, evoked by membrane depolarization, in cultured rat DRG neurons. Conclusion: Together these results indicate that apelin-13 can cause increased pain sensitivity after peripheral administration, but this effect does not involve calcium mediated signaling in primary sensory neurons.     

Calcium signalling in cortical and thalamic neurons of rats with streptozotocin-induced diabetes

Intracellular Ca²⁺ transients were measured with the use of a Ca²⁺-sensitive fluorescent indicator, fura-2, in neocortical and thalamic neurons in brain slices from control rats and rats with uncompensated streptozotocin-induced diabetes. The transients were evoked by high-potassium (50 mM)-induced membrane depolarization. The amplitude of depolarization-induced Ca²⁺ transients demonstrated a tendency to increase under diabetic conditions, beeing more expressed in cortical neurons compared with thalamic ones. The transients in cortical neurons from diabetic animals became also more susceptible to the blocking action of nifedipine (100μM) and less sensitive to Ni²⁺ (50μM), indicating that diabetic changes affect mostly Ca²⁺ transients triggered by high-voltage activated (L-type) calcium channels. The duration of a statistically significant increase was observed in the residual elevation of intracellular Ca²⁺ changes. However, a statistically significant increase was observed in the residual elevation of intracellular Ca²⁺ measured 60 sec after termination of membrane depolarization in both cortical and thalamic neurons, indicating alterations in the mechanisms that restore the resting level of Ca²⁺ in the cytosol. It is concluded that uncomensated insulin-dependent diabetes, which according to earlier data substantially alters calcium signalling in primary sensory neurons, also affects such signalling in the neurons of higher brain structures including the thalamus and cortex.     

Mutual influence of plasma membrane structures and intracellular calcium stores on the formation of calcium signals in primary nociceptive neurons

Participation of different calcium-regulating mechanisms in the formation of intracellular calcium signals in rat primary sensory neurons was studied using two-wavelength fluorescent microscopy. Mitochondria were shown to be the most powerful intracellular calcium-regulating structures in the investigated neurons. These organelles were involved in the modulation of calcium signals induced either by Ca²⁺ entry from the extracellular medium or by Ca²⁺ release from endoplasmic reticulum (ER). Analysis of the mitochondrial calcium exchange showed that the efficiency of mitochondria depended on whether calcium entered the cytosol from ER or from the extracellular solution. Depletion of ER by activation of ryanodine-sensitive, inositol-3-phosphate-sensitive receptors of ER or by activation of the leak channels via the block of ATPases in ER activated the store-operated calcium entry from the extracellular medium to cytosol. The kinetics of the rising phase of these Ca²⁺ transients depended on the way of ER depletion. This allows suggesting the existence of different activation mechanisms for the studied signals. The block of the mitochondrial calcium uniporter resulted in a rapid recovery of the intracellular calcium concentration after the Ca²⁺ transient induced by store-operated calcium influx. We conclude that mitochondrial calcium uptake can prevent calcium-dependent inactivation of store-operated calcium channels.     

Rapid action of estradiol in primate GnRH neurons: the role of estrogen receptor alpha and estrogen receptor beta

Estrogens play a pivotal role in the control of female reproductive function. Recent studies using primate GnRH neurons derived from embryonic nasal placode indicate that 17β-estradiol (E₂) causes a rapid stimulatory action. E₂ (1 nM) stimulates firing activity and intracellular calcium ([Ca²⁺]_i) oscillations of primate GnRH neurons within a few min. E₂ also stimulates GnRH release within 10 min. However, the classical estrogen receptors, ERα and ERβ, do not appear to play a role in E₂-induced [Ca²⁺]_i oscillations or GnRH release, as the estrogen receptor antagonist, ICI 182,780, failed to block these responses. Rather, this rapid E₂ action is, at least in part, mediated by a G-protein coupled receptor GPR30. In the present study we further investigate the role of ERα and ERβ in the rapid action of E₂ by knocking down cellular ERα and ERβ by transfection of GnRH neurons with specific siRNA for rhesus monkey ERα and ERβ. Results indicate that cellular knockdown of ERα and ERβ failed to block the E₂-induced changes in [Ca²⁺]_i oscillations. It is concluded that neither ERα nor ERβ is required for the rapid action of E₂ in primate GnRH neurons.     

Physiological and chemical analysis of neurotransmitter candidates at a fast excitatory synapse in the jellyfish Cyanea capillata (Cnidaria, Scyphozoa)

Motor nerve net (MNN) neurons in the jellyfish Cyanea capillata communicate with one another by way of fast, bidirectional excitatory chemical synapses. As is the case with almost all identified chemical synapses in cnidarians, the identity of the neurotransmitter at these synapses is unclear. MNN neurons are large enough for stable intracellular recordings. This, together with the fact that they can be exposed, providing unlimited access to them and to their synapses, prompted a study of the action of a variety of neurotransmitter candidates, including those typically associated with fast synapses in higher animals. Only the amino acids taurine and β-alanine produced physiological responses consistent with those of the normal EPSP in these cells. Moreover, chemical analysis revealed that both taurine and β-alanine are present in the neurons and released by depolarization. These various findings strongly suggest that either or both of these amino acids, or a closely related compound is the neurotransmitter at the fast chemical synapses between MNN neurons.     

大鼠全脑缺血后海马CA1区锥体神经元L型钙通道电流的改变(英)

已有研究表明在脑缺血期间及再灌流后早期,海马CA1锥体神经元细胞内钙浓度明显升高,这一钙超载被认为是缺血性脑损伤的重要机制之一.电压依赖性钙通道是介导正常CA1神经元钙内流的主要途径.实验观察了脑缺血再灌流后早期海马CA1锥体神经元电压依赖性L型钙通道的变化.以改良的四血管闭塞法制作大鼠15 min前脑缺血模型,在急性分离的海马CA1神经元上,采用膜片钳细胞贴附式记录L型电压依赖性钙通道电流.脑缺血后CA1神经元L型钙通道的总体平均电流明显增大,这是由于通道的开放概率增加所致.进一步分析单通道动力学显示,脑缺血后通道的开放时间变长,通道的开放频率增大.研究结果提示L型钙通道功能活动增强可能参与了缺血后海马CA1锥体神经元的细胞内钙浓度升高.     

Comparative effects of sodium pyrithione evoked intracellular calcium elevation in rodent and primate ventral horn motor neurons

Oral administration of sodium pyrithione (NaP) causes hindlimb weakness in rodents, but not in primates. Previous work using Aplysia neurons has demonstrated that NaP produces a persistent influx of Ca²⁺ ions across the plasma membrane. To determine whether this also occurs in mammalian neurons and whether this could underlie the inter-species difference between rodents and primates, we have tested the effects of NaP on intracellular Ca²⁺ levels ([Ca²⁺]_i) in rat and monkey motor neurons in vitro. Motor neurons present in spinal cord slices from rhesus monkey embryos (E37 and 56) and from rat E16 were dissected and cultured on glass coverslips. Following 2 weeks (rhesus) or 2-3 days (rat) in culture, neurons were loaded with fura-PE3/AM, and examined for [Ca²⁺]_i changes in response to NaP. Rhesus motor neurons were identified by immunostaining for Islet-1 (MN specific antigen) and neuron specific enolase (NSE). Motor neurons from both species exhibited dose-dependent NaP-evoked increases in [Ca²⁺]_i However, the dose-response curve for the Rhesus motor neurons was significantly shifted to the right of the rat dose-response curve, whereas the overall amplitude of the Ca²⁺ rise was similar in both species. As shown previously for the Aplysia neurons, the action of NaP is attenuated by SKF 96365, an inhibitor of store-operated calcium entry. In contrast the action of NaP is unaffected by nifedipine and tetrodotoxin, blockers of voltage-dependent Ca²⁺ and Na⁺ channels, respectively, or by ouabain, an inhibitor of the plasma membrane Na⁺/K⁺ ATPase. Our results indicate that the NaP-induced increase in [Ca²⁺]_i is conserved across species and suggest that the toxicological sensitivity of rodent over primate to pyrithione could be due to the enhanced sensitivity of rodent motor neurons to NaP-evoked intracellular Ca²⁺ elevation.     

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

Summary 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 (10M) (Pasantes-Morales and Ordóñez, 1982; Lombardini, 1991). In this report, ATP-dependent Ca²⁺ uptake in retinal membrane preparations was found to be inhibited by 5M 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.