Ruthenium red inhibits the mitochondrial Ca2+ uptake in intact bovine spermatozoa and increases the cytosolic Ca2+ concentration

Mitochondria in Injury, Inflammation and Disease of Articular Skeletal Joints

Inflammation is an important biological response to tissue damage caused by injury, with a crucial role in initiating and controlling the healing process. However, dysregulation of the process can also be a major contributor to tissue damage. Related to this, although mitochondria are typically thought of in terms of energy production, it has recently become clear that these important organelles also orchestrate the inflammatory response via multiple mechanisms. Dysregulated inflammation is a well-recognised problem in skeletal joint diseases, such as rheumatoid arthritis. Interestingly osteoarthritis (OA), despite traditionally being known as a ‘non-inflammatory arthritis’, now appears to involve an element of chronic inflammation. OA is considered an umbrella term for a family of diseases stemming from a range of aetiologies (age, obesity etc.), but all with a common presentation. One particular OA sub-set called Post-Traumatic OA (PTOA) results from acute mechanical injury to the joint. Whether the initial mechanical tissue damage, or the subsequent inflammatory response drives disease, is currently unclear. In the former case; mechanobiological properties of cells/tissues in the joint are a crucial consideration. Many such cell-types have been shown to be exquisitely sensitive to their mechanical environment, which can alter their mitochondrial and cellular function. For example, in bone and cartilage cells fluid-flow induced shear stresses can modulate cytoskeletal dynamics and gene expression profiles. More recently, immune cells were shown to be highly sensitive to hydrostatic pressure. In each of these cases mitochondria were central to these responses. In terms of acute inflammation, mitochondria may have a pivotal role in linking joint tissue injury with chronic disease. These processes could involve the immune cells recruited to the joint, native/resident joint cells that have been damaged, or both. Taken together, these observations suggest that mitochondria are likely to play an important role in linking acute joint tissue injury, inflammation, and long-term chronic joint degeneration - and that the process involves mechanobiological factors. In this review, we will explore the links between mechanobiology, mitochondrial function, inflammation/tissue-damage in joint injury and disease. We will also explore some emerging mitochondrial therapeutics and their potential for application in PTOA.

Electrical recordings of the mitochondrial calcium uniporter in Xenopus oocytes

The mitochondrial calcium uniporter is a Ca²⁺ channel that has been hard to characterize electrophysiologically. Tsai and Tsai establish a method that permits efficient electrophysiological recordings of the human uniporter in Xenopus oocytes and demonstrate characteristic uniporter behaviour.

The mitochondrial calcium uniporter is a multisubunit Ca²⁺ channel that mediates mitochondrial Ca²⁺ uptake, a cellular process crucial for the regulation of oxidative phosphorylation, intracellular Ca²⁺ signaling, and apoptosis. In the last few years, genes encoding uniporter proteins have been identified, but a lack of efficient tools for electrophysiological recordings has hindered quantitative analysis required to determine functional mechanisms of this channel complex. Here, we redirected Ca²⁺-conducting subunits (MCU and EMRE) of the human uniporter to the plasma membrane of Xenopus oocytes. Two-electrode voltage clamp reveals inwardly rectifying Ca²⁺ currents blocked by a potent inhibitor, Ru360 (half maximal inhibitory concentration, ~4 nM), with a divalent cation conductivity of Ca²⁺ > Sr²⁺ > Ba²⁺, Mn²⁺, and Mg²⁺. Patch clamp recordings further reveal macroscopic and single-channel Ca²⁺ currents sensitive to Ru360. These electrical phenomena were abolished by mutations that perturb MCU-EMRE interactions or disrupt a Ca²⁺-binding site in the pore. Altogether, this work establishes a robust method that enables deep mechanistic scrutiny of the uniporter using classical strategies in ion channel electrophysiology.

Distinct mechanosensitive Ca2+ influx mechanisms in human primary synovial fibroblasts

Synovial cells are exposed to continually changing dynamic forces and are implicated in the maintenance of joint homeostasis. However, the mechanisms of synovial cell responses to mechanical stress are unclear. In this study, we investigated the difference between the mechanosensitive channels of human primary synovial fibroblasts (SFBs) and human primary dermal fibroblasts (DFBs) in response to mechanical stretch by uniaxial cyclic stretching and mechanical cell membrane deformation in vitro. Cyclic stretching induced orientation of SFBs and DFBs perpendicularly to the stretching direction. Furthermore, uniaxial stretching increased intracellular Ca(2+) levels in both cell types. The perpendicular orientation of DFBs was blocked by gadolinium (III) chloride (Gd(3+), a mechanosensitive Ca(2+) channel blocker) or ruthenium red (RR, a nonselective Ca(2+) channel blocker). However, Gd(3+) did not block the stretch-induced perpendicular orientation in SFBs, while RR inhibited this orientation. Similarly, Ca(2+) influx in DFBs induced by uniaxial stretching and membrane deformation was inhibited by Gd(3+), RR, and GsMTx-4 (another mechanosensitive Ca(2+) channel blocker), while only RR inhibited Ca(2+) influx in SFBs. Our results suggest that SFBs respond to mechanical stretch through mechanosensitive channels that are distinct from those of DFBs.

Voltage and calcium transients in basal dendrites of the rat prefrontal cortex

Higher cortical functions (perception, cognition, learning and memory) are in large part based on the integration of electrical and calcium signals that takes place in thin dendritic branches of neocortical pyramidal cells (synaptic integration). The mechanisms underlying the synaptic integration in thin basal dendrites are largely unexplored. We use a recently developed technique, multisite voltage-calcium imaging, to compare voltage and calcium transients from multiple locations along individual dendritic branches. Our results reveal characteristic electrical transients (plateau potentials) that trigger and shape dendritic calcium dynamics and calcium distribution during suprathreshold glutamatergic synaptic input. We regularly observed three classes of voltage-calcium interactions occurring simultaneously in three different zones of the same dendritic branch: (1) proximal to the input site, (2) at the input site, and (3) distal to the input site. One hundred micrometers away from the synaptic input site, both proximally and distally, dendritic calcium transients are in tight temporal correlation with the dendritic plateau potential. However, on the same dendrite, at the location of excitatory input, calcium transients outlast local dendritic plateau potentials by severalfold. These Ca2+ plateaus (duration 0.5-2 s) are spatially restricted to the synaptic input site, where they cause a brief down-regulation of dendritic excitability. Ca2+ plateaus are not mediated by Ca2+ release from intracellular stores, but rather by an NMDA-dependent small-amplitude depolarization, which persists after the collapse of the dendritic plateau potential. These unique features of dendritic voltage and calcium distributions may provide distinct zones for simultaneous long-term (bidirectional) modulation of synaptic contacts along the same basal branch.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Transient receptor potential-like channels mediate metabotropic glutamate receptor EPSCs in rat dopamine neurones

Transient receptor potential (TRP) channels form cationic channels activated by diverse factors including mechanical stimuli, changes in osmolarity, pH and temperature, as well as the exogenous irritant, capsaicin. Metabotropic glutamate receptors have also recently been linked to TRP channel activation in neurones of the substantia nigra, hippocampus and cerebellum, suggesting a novel role for such channels in synaptic communication via endogenous neurotransmitters. We tested this for dopamine neurones in rat brain slices by characterizing the current-voltage relationship and pharmacology of EPSCs mediated by group I metabotropic glutamate receptor subtype 1 (mGluR1). Slow inward currents (273 +/- 35 pA peak amplitude, 381 +/- 25 ms latency, holding potential (V(h)) =-73 mV) representing evoked mGluR1 EPSCs were isolated in the presence of antagonists of AMPA, NMDA, GABA(A), GABA(B), muscarinic and glycine receptors. CPCCOEt (100 microM), an mGluR1 antagonist, blocked the residual EPSC in all recordings. mGluR1-activated EPSCs reversed polarity near -10 mV, consistent with the involvement of a cationic channel. Extracellular application of the non-selective TRP channel blockers SKF 96365, flufenamic acid and ruthenium red caused reversible inhibition of mGluR1-activated EPSCs. These characteristics parallel those of mGluR1 activation with an agonist and indicate the involvement of a TRP-like channel in mGluR1-mediated EPSCs.

Involvement of transient receptor potential-like channels in responses to mGluR-I activation in midbrain dopamine neurons

We investigated the involvement of store-operated channels (SOCs) and transient receptor potential (TRP) channels in the response to activation of the group I metabotropic glutamate receptor subtype 1 (mGluR1) with the agonist (S)-3,5-dihydroxyphenylglycine (DHPG, puff application) in dopamine neurons in rat brain slices. The mGluR1-induced conductance reversed polarity close to 0 mV and at more positive potentials when extracellular potassium concentrations were increased, indicating the involvement of a cationic channel. DHPG currents but not intracellular calcium responses were reduced by low extracellular sodium concentrations but were not affected by sodium channel blockers, tetrodotoxin and saxitoxin or by inhibition of the h-current with cesium. Abolition of calcium responses with intracellular BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; 10 mm) did not affect current responses, indicating they were not calcium activated. Extracellular application of non-selective SOCs and TRP channel blockers 2-aminoethoxydiphenylborane (2-APB), SKF96365, ruthenium red and flufenamic acid (but not gadolinium) reduced DHPG current and calcium responses. Intracellular application of ruthenium red and 2-APB did not affect DHPG currents, indicating that IP3 and ryanodine receptors did not mediate their actions. Single-cell PCR revealed the presence of TRPC1 and 5 mRNA in most dopamine neurons and subtypes 3, 4 and 6 in some. Store depletion evoked calcium entry indicative of SOCs, providing the first functional observation of such channels in native central neurons. Store depletion with either cyclopiazonic acid or ryanodine abolished calcium but not current responses to DHPG. The electrophysiological and pharmacological properties of the mGluR1-induced inward current are consistent with the involvement of TRP channels whereas calcium responses are dependent on the function of SOCs in voltage clamp recordings.

A novel O2-sensing mechanism in rat glossopharyngeal neurones mediated by a halothane-inhibitable background K+ conductance

Modulation of K+ channels by hypoxia is a common O2-sensing mechanism in specialised cells. More recently, acid-sensitive TASK-like background K+ channels, which play a key role in setting the resting membrane potential, have been implicated in O2-sensing in certain cell types. Here, we report a novel O2 sensitivity mediated by a weakly pH-sensitive background K+ conductance in nitric oxide synthase (NOS)-positive neurones of the glossopharyngeal nerve (GPN). This conductance was insensitive to 30 mM TEA, 5 mM 4-aminopyridine (4-AP) and 200 microM Cd2+, but was reversibly inhibited by hypoxia (O2 tension (PO2) = 15 mmHg), 2-5 mM halothane, 10 mM barium and 1 mM quinidine. Notably, the presence of halothane occluded the inhibitory effect of hypoxia. Under current clamp, these agents depolarised GPN neurones. In contrast, arachidonic acid (5-10 microM) caused membrane hyperpolarisation and potentiation of the background K+ current. This pharmacological profile suggests the O2-sensitive conductance in GPN neurones is mediated by a class of background K+ channels different from the TASK family; it appears more closely related to the THIK (tandem pore domain halothane-inhibited K+) subfamily, or may represent a new member of the background K+ family. Since GPN neurones are thought to provide NO-mediated efferent inhibition of the carotid body (CB), these channels may contribute to the regulation of breathing during hypoxia via negative feedback control of CB function, as well as to the inhibitory effect of volatile anaesthetics (e.g. halothane) on respiration.

Evidence against a functional ATP-dependent calcium extrusion mechanism in bovine epididymal sperm

Bovine epididymal sperm resuspended in ionic buffers take up relatively large amounts of calcium. This uptake, which is almost entirely mitochondrial, apparently bypasses the sperm cytosol. The direct mitochondrial loading is an unusual aspect of sperm calcium uptake, which suggests that the plasma membrane region surrounding the mitochondria should be highly permeable to calcium, whereas the membrane domains surrounding the head and tail regions of sperm should be impermeable. This study was undertaken to determine the role of a plasma membrane calcium ATPase in sperm calcium homeostasis. Kinetics of calcium (45Ca2+) uptake into intact and permeabilized caudal epididymal sperm confirmed that mitochondrial calcium uptake occurs with virtually no resistance from the surrounding plasma membrane. Cytoplasmic calcium accumulation by sperm depleted of intracellular ATP, measured in the presence of mitochondrial calcium uptake inhibitors, showed no increase upon energy depletion as would be expected if an ATP-dependent calcium extrusion mechanism were present. Furthermore, lowering the incubation temperature to further reduce the activity of the calcium ATPase in these energy-depleted sperm was also without effect on calcium accumulation. The calcium ATPase inhibitor vanadate, even at high concentrations, failed to increase intracellular 45Ca2+ accumulation. However, vanadate was effective in inhibiting motility showing that the compound was accumulated into sperm to inhibit flagellar dyenin ATPase. Therefore, the lack of effect of vanadate on 45Ca2+ accumulation was not due to its inability to enter sperm. Other calcium ATPase inhibitors such as quercetin, thapsigargin, and cyclopiazonic acid, which readily demonstrate ATP-dependent calcium extrusion in other somatic cells, were also without effect on sperm calcium accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

The function of calcium in plant graviperception

The fundamental question of gravitational biology is how do plants perceive a gravity. Recent experimental results have demonstrated that Ca second-messenger system has an essential role in induction of graviresponsiveness. Our data, that stimuli of various nature cause a rise of hyaloplasm Ca level revealed by means of pyroantimonate method, as well as complete inhibition of the gravitropism in roots of pea seedlings, provide indirect but consistent evidence of this role of Ca ions. A possible explanation for these results is that they may be due to an unbalanced and undirectional influx of Ca ions in statocytes from cell walls or from intracellular Ca stores, while in the presence of the Earths 1 g vector, this process occurs directionally, along this vector. It is possible that a target for the gravity stimulus is the flux mechanism of Ca to statocytes, including participation of the phosphatidylinositol system and calmodulin. The data that have become available from space flight experiments will be reviewed and an attempt will be made to compare these results with ground-based observations.

Regulation of calcium transport in bovine spermatozoa

Calcium uptake into bovine epididymal spermatozoa is enhanced by introducing phosphate in the suspending medium (Babcock et al. (1975) J. Biol. Chem. 250, 6488-6495). This effect of phosphate is found even at a low extracellular Ca2+ concentrations (i.e., 5 microM) suggesting that phosphate is involved in calcium transport via the plasma membrane. Bicarbonate (2 mM) cannot substitute for phosphate, and a relatively high bicarbonate concentration (20 mM) causes partial inhibition of calcium uptake in absence of Pi. In the presence of 1-2 mM phosphate, 20 mM bicarbonate enhances Ca2+ uptake. The data indicate that the plasma membrane of bovine spermatozoa contains two carriers for Ca2+ transport: a phosphate-independent Ca2+ carrier that is stimulated by bicarbonate and a phosphate-dependent Ca2+ carrier that is inhibited by bicarbonate. Higher phosphate concentrations (i.e., 10 mM) inhibit Ca2+ uptake into intact cells (compared to 1.0 mM phosphate) and this inhibition can be relieved partially by 20 mM bicarbonate. This effect of bicarbonate is inhibited by mersalyl. Calcium uptake into the cells is enhanced by adding exogenous substrates to the medium. There is no correlation between ATP levels in the cells and Ca2+ transport into the cell. ATP levels are high even without added exogenous substrate and this ATP level is almost completely reduced by oligomycin, suggesting that ATP can be synthesized in the mitochondria in the absence of exogenous substrate. Calcium transport into the sperm mitochondria (washed filipin-treated cells) is absolutely dependent upon the presence of phosphate and mitochondrial substrate. Bicarbonate cannot support Ca2+ transport into sperm mitochondria. There is good correlation between Ca2+ uptake into intact epididymal sperm and into sperm mitochondria with the various substrates used. This indicates that the rate of calcium transport into the cells is determined by the rate of mitochondrial Ca2+ uptake and respiration with the various substrates.

Calcium transport in bovine sperm mitochondria: effect of substrates and phosphate

Calcium uptake into filipin-treated bovine spermatozoa is completely inhibited by the uncoupler CCCP or by ruthenium red. Both Pi and mitochondrial substrates are required to obtain the maximal rate of calcium uptake into the sperm mitochondria. Bicarbonate and other anions such as lactate, acetate or beta-hydroxybutyrate do not support a high rate of calcium uptake. There are significant differences among various mitochondrial substrates in supporting calcium uptake. The best substrates are durohydroquinone, alpha-glycerophosphate and lactate. Pyruvate is a relatively poor substrate, and its rate can be greatly enhanced by malate or succinate but not by oxalacetate or lactate. This stimulation is blocked by the dicarboxylate translocase inhibitor, butylmalonate and can be mimiced by the non-metabolized substrate D-malate. The Ka for pyruvate was found to be 17 microM and 67 microM in the presence and absence of L-malate, respectively. The Ka for L-malate is 0.12 mM. It is suggested that in addition to the known pyruvate/lactate translocase there is a second translocase for pyruvate which is malate/succinate-dependent and does not transport lactate. In the presence of succinate, glutamate stimulates calcium uptake 3-fold, and this effect is not inhibited by rotenone. In the presence of glutamate plus malate or oxalacetate there is only an additive effect. It is suggested that glutamate stimulates succinate transport and/or oxidation in bovine sperm mitochondria. The alpha-hydroxybutyrate is almost as good as lactate in supporting calcium uptake. Since the alpha-keto product is not further metabolized in the citric acid cycle, it is suggested that lactate can supply the mitochondrial needs for NADH from its oxidation to pyruvate by the sperm lactate dehydrogenase x. Thus, when there is sufficient lactate in the sperm mitochondria, pyruvate need not be further metabolized in the citric acid cycle in order to supply more NADH.

Calcium uptake in human spermatozoa: characterization and mechanisms

Basal 45Ca2+ influx was analyzed in human seminal spermatozoa using a method that allows these highly reactive cells to be easily and safely handled. The uptake was a time-dependent process, with its maximum at 400 s. The kinetics of 45Ca2+ transport was saturating as a function of extracellular Ca2+ concentration with a Km of 429 microM and a Vmax of 1.6 nmol 45Ca2+/mg protein/2.5 min. Depolarizing conditions and the calcium channel blocker verapamil did not affect the uptake; based on this, the presence of operating calcium channels in seminal spermatozoa is excluded. The independence of 45Ca2+ uptake on external concentration of both Na+ and Ca2+ suggests that Na+/Ca2+ exchange does not occur in these cells. The anticalmodulin drug trifluoperazine, the mitochondrial inhibitor antimycin A, and the SH reagents N-ethylmaleimide and mersalyl all inhibited the ion transport. A calmodulin-regulated, energy-requiring, proteinaceous Ca2+ transporter seems to be the main operating mechanism of calcium uptake in human seminal gametes.

Transport mechanism for calcium and phosphate in ram spermatozoa

Calcium uptake into ejaculated ram spermatozoa is highly enhanced by the addition of extracellular phosphate. Under identical conditions, extracellular calcium stimulates the uptake of phosphate by the cells. Both calcium and phosphate uptake are comparably inhibited by the sulfhydryl reagent mersalyl. The I50 was found to be 6.36 and 10.14 nmol mersalyl per mg protein for phosphate and calcium uptake, respectively. Calcium uptake is inhibited by mersalyl whether phosphate is present or not. Extracellular fructose causes a 5-fold increase in calcium uptake. When fructose and phosphate are present in the cell's medium, there is an additive effect, which indicates that two independent systems are involved in calcium transport into the cell. Ruthenium red, which blocks Ca2+ transport into the mitochondria, causes 70% and 95% inhibition of calcium uptake in the absence or in the presence of fructose, respectively. Ruthenium red does not affect phosphate uptake unless calcium was present in the incubation medium. The stimulatory effect of fructose upon calcium uptake can be mimicked by L-lactate and can be inhibited by the glycolytic inhibitor 2-deoxyglucose. Fructose and L-lactate stimulate mitochondrial respiration in a comparable way. Oligomycin, which inhibits mitochondrial ATP synthesis, does not inhibit Ca2+ uptake. This indicates that ATP is not involved in the mechanism by which mitochondrial respiration stimulates Ca2+ uptake. The calcium channel blocker, verapamil, inhibits Ca2+ uptake in the presence or absence of extracellular phosphate. The phosphate-dependent calcium transport mechanism is more sensitive to verapamil than is the phosphate-independent transporter. In summary, the data indicate that the plasma membrane of mammalian spermatozoa contains a calcium/phosphate symporter, a phosphate-independent calcium carrier and a calcium-independent phosphate carrier.

Effects of calcium chelators, divalent cations and sulfhydryl reagents on calcium uptake and motility of bovine spermatozoa

Addition of 1 mM Ca/EGTA complex (1:1 ratio) to an incubation medium containing 1.5 mM Ca2+ produced a notable increase in the Ca2+ cycling in ejaculated bovine spermatozoa. Similar results were also obtained with the Ca/EDTA and Ca/EDTA complexes or with the heavy metal chelator DTPA (50 microM). Ba2+, Ni2+ or Co2+ added at 0.1 mM concentration abolished the stimulatory effect of the Ca/EGTA complex on Ca2+ cycling, whereas it did not affect the calcium movement in the absence of the calcium chelator complex. It is concluded that small amounts of these cations should be bound to the plasma membrane of bovine spermatozoa and inhibit the cellular calcium influx. 0.1 mM Cd2+ and NEM or 1 mM diamide produced a calcium efflux from the spermatozoa together with an inhibition of cellular motility and an increase in glutamate-oxaloacetate transaminase release. Conversely the impermeant sulfhydryl reagent mersalyl caused a net calcium efflux but did not alter the cellular motility nor the transaminase release. It is suggested that the permeant thiol reagents could decrease the spermatozoal mobility by impairing the mitochondrial ATP-synthesis.

Evidence for a delta pH-driven Ca2+ uptake in EGTA-treated bovine spermatozoa

Calcium efflux from ejaculated bovine spermatozoa occurred upon incubation in Ca2+/EGTA buffers with Ca2+ ion concentrations ranging from 0.1 microM to 1 nM. Both total cellular calcium and cytosol free Ca2+ concentrations, the latter measured with Quin 2, were inversely correlated with the Ca2+ activity of the medium. An influx of radioactive 45Ca2+ parallel to a net efflux of calcium took place in spermatozoa incubated in 45Ca2+/EGTA buffers with 45Ca2+ activity of 0.01 microM or 0.1 microM. The uptake of the radioactive isotope was higher in spermatozoa incubated at pH 7.8 than that found at pH 6.8, increased in the presence of acetate or amiloride but decreased when ammonium chloride or monensin was added to the incubation mixture. Addition of acetate produced a decrease of the cytoplasmic pH, determined with the indicator carboxyfluorescein, whereas addition of NH4Cl or monensin caused a pH increase. Addition of either nigericin or monensin to spermatozoa suspended in a choline medium containing low concentrations of Na+, K+ and Ca2+ produced a cytosolic acidification, the subsequent addition of Ca2+ caused a cytosolic alkalinization parallel to an increase of the cytosolic free Ca2+. Addition of CaCl2 to EGTA-pretreated spermatozoa resuspended in a poorly buffered medium induced an evident decrease of extracellular pH suggesting a cellular proton extrusion. Both monensin and nigericin caused an increase of the calcium transport in spermatozoa suspended in a choline medium containing a physiological concentration of 1.5 mM CaCl2. Taken together the present results indicate that, under the experimental conditions used, a delta pH-driven Ca2+ uptake occurs in ejaculated bovine spermatozoa and suggest that Ca2+ is taken up in exchange with H+.

Inhibitory effect of ionized free intracellular calcium enhanced by ruthenium red and m-chloro-carbonylcyanide phenyl hydrazon on the evoked release of acetylcholine

In order to understand the relationship between the free ionized calcium concentration in the axon terminals and the transmitter release we have investigated the effect of ruthenium red (RuR) and m-chloro-carbonylcyanide phenyl hydrazon (CCCP), mitochondrial uncoupler agents on the liberation of acetylcholine from myenteric plexus of guinea-pig ileum. Both compounds are able to enhance intracellular free Ca2+. In the presence of RuR and CCCP the spontaneous release of radioactivity from isolated myenteric-plexus preparation, previously loaded with (methyl-3H)-choline chloride was significantly enhanced while that evoked by electrical stimulation was decreased. Atropine did not affect the electrically evoked release in the presence of CCCP, indicating that in the effect of CCCP the presynaptic muscarinic receptors mediated negative feedback modulation does not play any role. Our findings are consistent with the hypothesis that the enhanced level of intracellular free Ca2+ enhances the spontaneous while depresses the electrically evoked release of transmitter.