Pharmacology of calcium release from sarcoplasmic reticulum

Caffeine Use in Sports, Pharmacokinetics in Man, and Cellular Mechanisms of Action

Caffeine is the most widely consumed psychoactive 'drug' in the world and probably one of the most commonly used stimulants in sports. This is not surprising, since it is one of the few ergogenic aids with documented efficiency and minimal side effects. Caffeine is rapidly and completely absorbed by the gastrointestinal tract and is readily distributed throughout all tissues of the body. Peak plasma concentrations after normal consumption are usually around 50 microM, and half-lives for elimination range between 2.5-10 h. The parent compound is extensively metabolized in the liver microsomes to more than 25 derivatives, while considerably less than 5% of the ingested dose is excreted unchanged in the urine. There is, however, considerable inter-individual variability in the handling of caffeine by the body, due to both environmental and genetic factors. Evidence from in vitro studies provides a wealth of different cellular actions that could potentially contribute to the observed effects of caffeine in humans in vivo. These include potentiation of muscle contractility via induction of sarcoplasmic reticulum calcium release, inhibition of phosphodiesterase isoenzymes and concomitant cyclic monophosphate accumulation, inhibition of glycogen phosphorylase enzymes in liver and muscle, non-selective adenosine receptor antagonism, stimulation of the cellular membrane sodium/potassium pump, impairment of phosphoinositide metabolism, as well as other, less thoroughly characterized actions. Not all, however, seem to account for the observed effects in vivo, although a variable degree of contribution cannot be readily discounted on the basis of experimental data. The most physiologically relevant mechanism of action is probably the blockade of adenosine receptors, but evidence suggests that, at least under certain conditions, other biochemical mechanisms may also be operational.

Pharmacology and preclinical pharmacokinetics of peppermint oil

The principal pharmacodynamic effect of peppermint oil relevant to the gastrointestinal tract is a dose-related antispasmodic effect on the smooth musculature due to the interference of menthol with the movement of calcium across the cell membrane. The choleretic and antifoaming effects of peppermint oil may play an additional role in medicinal use. Peppermint oil is relatively rapidly absorbed after oral administration and eliminated mainly via the bile. The major biliary metabolite is menthol glucuronide, which undergoes enterohepatic circulation. The urinary metabolites result from hydroxylation at the C-7 methyl group at C-8 and C-9 of the isopropyl moiety, forming a series of mono- and dihydroxymenthols and carboxylic acids, some of which are excreted in part as glucuronic acid conjugates. Studies with tritiated I-menthol in rats indicated about equal excretion in feces and urine. The main metabolite indentified was menthol-glucuronide. Additional metabolites are mono- or di-hydroxylated menthol derivatives.

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.

Evidence on the operation of ATP-induced capacitative calcium entry in breast cancer cells and its blockade by 17beta-estradiol

Little is known about the regulation of cytosolic calcium Ca(2+) levels ([Ca(2+)](i)) in breast cancer cells. We investigated the existence of capacitative calcium entry (CCE) in the tumorigenic cell line MCF-7 and its responsiveness to ATP. MCF-7 cells express purinergic receptors as well as estrogen receptors (ER). Depletion of calcium stores with thapsigargin (TG, 500 nM) or ATP (10 microM) in the absence of extracellular Ca(2+), resulted in a rapid and transient elevation in [Ca(2+)](i). After recovery of basal levels, Ca(2+) readmission (1.5 mM) to the medium increased Ca(2+) influx (twofold over basal), reflecting pre-activation of a CCE pathway. Cells pretreated with TG were unable to respond to ATP, thus indicating that the same Ca(2+) store is involved in their response. Moreover, IP(3)-dependent ATP-induced calcium mobilization and CCE were completely blocked using compound U-73122, an inhibitor of phospholipase C. Compound 2-APB (75 microM) and Gd(3+) (10 microM), antagonists of the CCE pathway, completely prevented ATP-stimulated capacitative Ca(2+) entry. CCE in MCF-7 cells was highly permeable to Mn(2+) and to the Ca(2+) surrogate Sr(2+). Mn(2+) entry sensitivity to Gd(3+) matched that of the Ca(2+) entry pathway. 17Beta-estradiol blocked ATP-induced CCE, but was without effect on TG-induced CCE. Besides, the estrogen blockade of the ATP-induced CCE was completely abolished by preincubation of the cells with an ER monoclonal antibody. ER alpha immunoreactivity could also be detected in a purified plasma membrane fraction of MCF-7 cells. These results represent the first evidence on the operation of a ATP-responsive CCE pathway in MCF-7 cells and also indicate that 17beta-estradiol interferes with this mechanism by acting at the cell surface level.

Glucosylceramide and glucosylsphingosine modulate calcium mobilization from brain microsomes via different mechanisms

We recently demonstrated that elevation of intracellular glucosylceramide (GlcCer) levels results in increased functional Ca2+ stores in cultured neurons, and suggested that this may be due to modulation of ryanodine receptors (RyaRs) by GlcCer (Korkotian, E., Schwarz, A., Pelled, D., Schwarzmann, G., Segal, M. and Futerman, A. H. (1999) J. Biol. Chem. 274, 21673-21678). We now systematically examine the effects of exogenously added GlcCer, other glycosphingolipids (GSLs) and their lyso-derivatives on Ca2+ release from rat brain microsomes. GlcCer had no direct effect on Ca2+ release, but rather augmented agonist-stimulated Ca2+ release via RyaRs, through a mechanism that may involve the redox sensor of the RyaR, but had no effect on Ca2+ release via inositol 1,4,5-trisphosphate receptors. Other GSLs and sphingolipids, including galactosylceramide, lactosylceramide, ceramide, sphingomyelin, sphingosine 1-phosphate, sphinganine 1-phosphate, and sphingosylphosphorylcholine had no effect on Ca2+ mobilization from rat brain microsomes, but both galactosylsphingosine (psychosine) and glucosylsphingosine stimulated Ca2+ release, although only galactosylsphingosine mediated Ca2+ release via the RyaR. Finally, we demonstrated that GlcCer levels were approximately 10-fold higher in microsomes prepared from the temporal lobe of a type 2 Gaucher disease patient compared with a control, and Ca2+ release via the RyaR was significantly elevated, which may be of relevance for explaining the pathophysiology of neuronopathic forms of Gaucher disease.

Capacitative calcium influx in human epithelial breast cancer and non-tumorigenic cells occurs through Ca2+ entry pathways with different permeabilities to divalent cations

The operation of capacitative Ca(2+) entry (CCE) in human breast cancer (SKBR3) and non-tumorigenic (HBL100) cell lines was investigated as an alternative Ca(2+) entry route in these cells. Ca(2+) readdition after thapsigargin-induced store depletion showed activation of CCE in both cell lines. SKBR3 cells exhibited retarded store depletion and CCE decay kinetics compared to the non-tumorigenic HBL100 cells, suggesting alterations in Ca(2+) homeostasis. CCE was also highly permeable to Mn(2+) and to a lesser extent to Sr(2+), but not to Ba(2+). In HBL100 cells, CCE is contributed (30%) by a Ca(2+)/Mn(2+) permeable route insensitive to low (1 microM) Gd(3+) and a Ca(2+)/Sr(2+)/Mn(2+) permeable non-selective pathway (70%) sensitive to 1 microM Gd(3+). In SKBR3 cells, the relative contribution to CCE of both routes was opposite to that in non-tumorigenic cells.

Sustained norepinephrine contraction in the rat portal vein is lost when Ca(2+) is replaced with Sr(2+)

Agonist-induced activation of smooth muscle involves a rise in intracellular Ca(2+) concentration and sensitization of myosin light chain phosphorylation to Ca(2+). Sr(2+) can enter through Ca(2+) channels, be sequestered and released from sarcoplasmic reticulum, and replace Ca(2+) in activation of myosin light chain phosphorylation. Sr(2+) cannot replace Ca(2+) in facilitation of agonist-activated Ca(2+)-dependent nonselective cation channels. It is not known whether Sr(2+) can replace Ca(2+) in small G protein-mediated sensitization of phosphorylation. To explore mechanisms involved in alpha-receptor-activated contractions in smooth muscle, effects of replacing Ca(2+) with Sr(2+) were examined in rat portal vein. Norepinephrine (NE) at >3.0 x 10(-7) M in the presence of Ca(2+) resulted in a strong sustained contraction, whereas this sustained component was absent in the presence of Sr(2+); only the amplitude of phasic contractions increased. Pretreatment with low (approximately 0.05 mM) free Ca(2+) followed by 2.5 mM Sr(2+) resulted in a sustained component of the NE response. In beta-escin-permeabilized preparations, phenylephrine in the presence of GTP or guanosine 5'-O-(3-thiotriphosphate) alone induced sensitization to Sr(2+). In conclusion, a Ca(2+)-regulated membrane/channel process is required for the sustained component of NE responses in rat portal vein. Sensitization alone is not responsible for the sustained phase of the NE contraction.

Identification of the cardiac ryanodine receptor channel in membrane blebs of sarcoplasmic reticulum

Blebs of the sarcoplasmic reticulum (SR) membrane of heart muscle cells were generated after saponin perforation of the plasma membrane followed by complete hypercontraction of the cell. Although characteristic proteins of the plasma membrane, namely the beta1-adrenoreceptor and Galphai, were stained by monoclonal antibodies in the hypercontracted cells, these proteins could not be detected in the adjacent blebs. Monoclonal antibodies to the cardiac ryanodine receptor (RyR2), calsequestrin and SERCA2 bound at different amounts to surface components of the blebs and to components of the hypercontracted cells. From the immunofluorescence signals we conclude that the blebs are mainly constituted of corbular and junctional SR membrane, and only to a lesser extent of network SR membrane. Deconvolution microscopy revealed that the membrane location of RyR2, calsequestrin and SERCA2 in the bleb is comparable to native SR membrane. At the bleb membrane giga-ohm seals could be obtained and patches could be excised in a way that single-channel currents could be measured, although these are not completely identified.

Methyl p-hydroxybenzoate (E-218) a preservative for drugs and food is an activator of the ryanodine receptor Ca(2+) release channel

1. Haloperidol is a drug used in the management of several psychotic disorders and its use has been linked to Neuroleptic Malignant Syndrome. In the present study we have investigated the effect of a commercial preparation of haloperidol, Serenase, on skeletal muscle sarcoplasmic reticulum. 2. Addition of Serenase to isolated terminal cisternae caused a rapid release of calcium. We tested whether the active Ca(2+)-releasing substance was haloperidol or another compound present in the preparation. 3. Our results show that methyl p-hydroxybenzoate, one of the preservatives and a commonly used anti-microbial agent (E-218) is an activator of Ca(2+) release (E.C. 50=2.0 mM), mediated by a ruthenium red-sensitive Ca(2+) release channel present in skeletal muscle terminal cisternae.

Effects of halothane on the membrane potential in skeletal muscle of the frog

Halothane has many effects on the resting membrane potential (V(m)) of excitable cells and exerts numerous effects on skeletal muscle one of which is the enhancement of Ca(2+) release by the sarcoplasmic reticulum (SR) resulting in a sustained contracture. The aim of this study was to analyse the effects of clinical doses of halothane on V(m), recorded using intracellular microelectrodes on cleaned and non stimulated sartorius muscle which was freshly isolated from the leg of the frog Rana esculenta. We assessed the mechanism of effects of superfused halothane on V(m) by the administration of selective antagonists of membrane bound Na(+), K(+) and Cl(-) channels and by inhibition of SR Ca(2+) release. Halothane (3%) induced an early and transient depolarization (4.5 mV within 7 min) and a delayed and sustained hyperpolarization (about 11 mV within 15 min) of V(m). The halothane-induced transient depolarization was sensitive to ryanodine (10 microM) and to 4-acetamido-4'-isothiocyanatostilbene 2,2' disulphonic acid (SITS, 1 mM). The hyperpolarization of V(m) induced by halothane (0.1 - 3%) was dose-dependent and reversible. It was insensitive to SITS (1 mM), tetrodotoxin (0.6 microM), and tetraethylammonium (10 mM) but was blocked and/or prevented by ryanodine (10 microM), charybdotoxin (CTX, 1 microM), and glibenclamide (10 nM). Our observations revealed that the effects of halothane on V(m) may be related to the increase in intracellular Ca(2+) concentration produced by the ryanodine-sensitive Ca(2+) release from the SR induced by the anaesthetic. The depolarization may be attributed to the activation of Ca(2+)-dependent Cl(-) (blocked by SITS) channels and the hyperpolarization to the activation of large conductance Ca(2+)-dependent K(+) channels, blocked by CTX, and to the opening of ATP-sensitive K(+) channels, inhibited by glibenclamide.

Induction of skeletal muscle contracture and calcium release from isolated sarcoplasmic reticulum vesicles by sanguinarine

The benzophenanthrine alkaloid, sanguinarine, was studied for its effects on isolated mouse phrenic-nerve diaphragm preparations. Sanguinarine induced direct, dose-dependent effects on muscle contractility. Sanguinarine-induced contracture was partially inhibited when the extracellular Ca(2+) was removed or when the diaphragm was pretreated with nifedipine. Depletion of sarcoplasmic reticulum (SR) internal calcium stores completely blocked the contracture. Sanguinarine induced Ca(2+) release from the actively loaded SR vesicles was blocked by ruthenium red and dithiothreitol (DTT), consistent with the ryanodine receptor (RyR) as the site of sanguinarine action. Sanguinarine altered [(3)H]-ryanodine binding to the RyR of isolated SR vesicles, potentiating [(3)H]-ryanodine binding at lower concentrations and inhibiting binding at higher concentrations. All of these effects were reversed by DTT, suggesting that sanguinarine-induced Ca(2+) release from SR occurs through oxidation of critical SH groups of the RyR SR calcium release channel.

The role of cyclic nucleotides and calcium in the relaxation produced by amrinone in rat aorta

(1) The vasorelaxation produced by the phosphodiesterase 3 (PDE3) inhibitor, amrinone was investigated in isolated rat aorta denuded of endothelium. In the presence of extracellular Ca(2+), amrinone, milrinone and 3-isobutyl-1-methylxanthine (IBMX), relaxed endothelium-denuded rat aortic rings constricted with phenylephrine. While the actions of milrinone and IBMX were inhibited by the protein kinase G (PKG) inhibitor, Rp-8-Bromo guanosine-3',5' monophosphothioate (Rp-8-Br-cGMPS; 0.5 mM), that of amrinone was only slightly affected; whereas the protein kinase A (PKA) inhibitor, Rp-adenosine-3',5' cyclic monophosphothioate (Rp-cAMPS; 0.5 mM) had no effect on any agent. (2) Amrinone (100 microM) inhibited (45)Ca(2+) influx through receptor- or store-operated Ca(2+) channels following stimulation with phenylephrine (1 microM) or thapsigargin (1 microM). In contrast, amrinone had no effect on KCl (120 mM)-stimulated Ca(2+) influx. (3) In the absence of extracellular Ca(2+), amrinone (30 microM) inhibited the constriction produced by phenylephrine, 5-hydroxytryptamine (5HT) and U46619, and this effect was not affected by Rp-cAMPS or Rp-8-Br-cGMPS. (4) The intracellular mechanism of action of amrinone may involve the phospholipase C (PLC)-inositol 1,4,5 trisphosphate (IP(3))-intracellular Ca(2+) signal transduction pathway. However, amrinone (100 microM) had no effect on either basal- or noradrenaline (100 microM)-stimulated PLC activity. Similarly, IP(3) stimulated a concentration-dependent release of Ca(2+) from rat brain microsomes that was not affected by amrinone (30 and 100 microM). (5) In conclusion, the vasorelaxant action of amrinone does not involve adenosine 3',5' cyclic monophosphate (cAMP) or involve guanosine 3',5' cyclic monophosphate (cGMP) but may include an inhibition of Ca(2+) influx through receptor- or store-operated Ca(2+) channels, although it does not directly affect intracellular Ca(2+) release.

Involvement of the Glu724-Pro760 region of the dihydropyridine receptor II-III loop in skeletal muscle-type excitation-contraction coupling

Our previous study (El-Hayek, R., Antoniu, B., Wang, J. P., Hamilton, S. L., and Ikemoto, N. (1995) J. Biol. Chem. 270, 22116-22118) suggested the hypothesis that skeletal muscle-type excitation-contraction coupling is regulated by two domains (activating and blocking) of the II-III loop of the dihydropyridine receptor alpha1 subunit. We investigated this hypothesis by examining conformational changes in the ryanodine receptor induced by synthetic peptides and by transverse tubular system (T-tubule) depolarization. Peptide A, corresponding to the Thr671-Leu690 region, rapidly changed the ryanodine receptor conformation from a blocked state (low fluorescence of the conformational probe, methyl coumarin acetamide, attached specifically to the ryanodine receptor) to an activated state (high methyl coumarin acetamide fluorescence) as T-tubule depolarization did. Peptide C, corresponding to the Glu724-Pro760 region, blocked both conformational changes induced by peptide A and T-tubule depolarization. Its ability to block peptide A-induced and depolarization-induced activation was considerably impaired by replacing the portion of peptide C corresponding to the Phe725-Pro742 region of the loop with cardiac muscle-type sequence. These results are consistent with the model that depolarization-induced activation of excitation-contraction coupling and blocking/repriming are mediated by the peptide A region and the peptide C region (containing the critical Phe725-Pro742 sequence) of the II-III loop, respectively.

2-Hydroxycarbazole induces Ca2+ release from sarcoplasmic reticulum by activating the ryanodine receptor

2-Hydroxycarbazole was shown to induce Ca2+ release from skeletal muscle and cardiac muscle sarcoplasmic reticulum at concentrations between 100-500 microM. This release was blocked by both 1 mM tetracaine and 30 microM ruthenium red which inhibit the ryanodine receptor or by pre-treatment with 10 mM caffeine which depletes the ryanodine receptor-containing Ca2+ stores. This, in addition to the fact that 2-hydroxycarbazole has little effect on Ca2+ ATPase activity, indicates that it activates Ca2+ release through the ryanodine receptor. The apparent EC50 value for release from both skeletal muscle and cardiac muscle sarcoplasmic reticulum was approximately 200 microM and maximal release occurred at 400-500 microM, making it approximately 20 times more potent than caffeine. The dose-dependency in the extent of Ca2+ release induced by 2-hydroxycarbazole was also apparently highly cooperative for both preparations. That 2-hydroxycarbazole was able to mobilize Ca2+ from non-muscle cell microsomes and in intact TM4 cells (which contain ryanodine receptors), makes this compound a more potent and commercially available alternative to caffeine in studying the role of this intracellular Ca2+ channel in a variety of systems.

Biochemical mechanisms of calcium mobilisation induced by mastoparan and chimeric hormone-mastoparan constructs

Ca2+ efflux, Ca(2+)-ATPase, and membrane permeability measurements were used to investigate the biochemical mechanisms of Ca2+ release induced by mastoparan (MP) and the chimeric hormone-MP constructs incorporating galanin (galparan) or vasopressin antagonist (M375 and M391) moieties. Comparative studies utilised preparations of porcine cerebellar microsomes and rabbit skeletal muscle sarcoplasmic reticulum (SR). MP and chimeric peptides galparan, M375 and M391 induce Ca2+ release over a range of concentrations from 0.3-10 microM. Comparison of MP and three chimeric, N-terminal extended, constructs indicates that N-terminal extension modifies the biological properties of MP, producing changes in efficacy which are enzyme-isoform-specific. Biochemical studies indicate that the chimeric analogues and MP inhibit Ca(2+)-ATPases and directly activate the ryanodine receptor (RyR) to release Ca2+ from both heavy SR (HSR) and microsomes. The same peptides have no effect on the InsP3 receptor (InsP3R). Other actions that include modest changes in membrane permeability may also contribute to the Ca(2+)-mobilising action of MP and chimeric constructs.

Emodin-induced muscle contraction of mouse diaphragm and the involvement of Ca2+ influx and Ca2+ release from sarcoplasmic reticulum

1. The effects on skeletal muscle of emodin, an anthraquinone, were studied in the mouse isolated diaphragm and sarcoplasmic reticulum (SR) membrane vesicles. 2. Emodin dose-dependently caused muscle contracture, simultaneously depressing twitch amplitude. Neither tubocurarine nor tetrodotoxin blocked the contraction suggesting that it was caused myogenically. 3. The contraction induced by emodin persisted in a Ca2+ free medium with a slight reduction in the maximal force of contraction. The contraction induced by emodin in the Ca2+ free medium was completely blocked when the internal Ca2+ pool of the muscle was depleted by ryanodine. These data suggest that the contraction caused by emodin is due to the release of Ca2+ from the intracellular ryanodine-sensitive pool. 4. In contrast to the effect seen in the Ca2+ free medium, emodin induced a small but consisted contraction in the ryanodine-treated muscle in Krebs medium. The contraction was blocked in the presence of dithiothreitol and was partially blocked by nifedipine, suggesting that oxidation of a sulphhydryl group on the external site of dihydropyridine receptor is involved. 5. Emodin dose-dependently increased Ca2+ release from actively loaded SR vesicles and this effect was blocked by ruthenium red, a specific Ca2+ release channel blocker, and the thiol reducing agent, DTT, suggesting that emodin induced Ca2+ release through oxidation of the critical SH of the ryanodine receptor. 6. [3H]-ryanodine binding was dose-dependently potentiated by emodin in a biphasic manner. The degree of potentiation of ryanodine binding by emodin increased dose-dependently at concentrations up to 10 microM but decreased at higher concentrations of 10-100 microM. 7. These data suggest that muscle contraction induced by emodin is due to Ca2+ release from the SR of skeletal muscle, as a result of oxidation of the ryanodine receptor and influx of extracellular Ca2+ through voltage-dependent Ca2+ channels of the plasma membrane.

Imaging caffeine-induced Ca2+ transients in individual fast-twitch and slow-twitch rat skeletal muscle fibers

Fast-twitch and slow-twitch rat skeletal muscles produce dissimilar contractures with caffeine. We used digital imaging microscopy to monitor Ca2+ (with fluo 3-acetoxymethyl ester) and sarcomere motion in intact, unrestrained rat muscle fibers to study this difference. Changes in Ca2+ in individual fibers were markedly different from average responses of a population. All fibers showed discrete, nonpropagated, local Ca2+ transients occurring randomly in spots about one sarcomere apart. Caffeine increased local Ca2+ transients and sarcomere motion initially at 4 mM in soleus and 8 mM in extensor digitorum longus (EDL; approximately 23 degrees C). Ca2+ release subsequently adapted or inactivated; this was surmounted by higher doses. Motion also adapted but was not surmounted. Prolonged exposure to caffeine evidently suppressed myofilament interaction in both types of fiber. In EDL fibers, 16 mM caffeine moderately increased local Ca2+ transients. In soleus fibers, 16 mM caffeine greatly increased Ca2+ release and produced propagated waves of Ca2+ (approximately 1.5-2.5 microns/s). Ca2+ waves in slow-twitch fibers reflect the caffeine-sensitive mechanism of Ca2(+)-induced Ca2+ release. Fast-twitch fibers possibly lack this mechanism, which could account for their lower sensitivity to caffeine.

Comparative studies on the induction of muscle contracture in mouse diaphragm and Ca2+ release from sarcoplasmic reticulum vesicles by organotin compounds

Effects of organotins, including triethyltin and tributyltin, on skeletal muscle were studied with diaphragm and isolated sarcoplasmic reticulum membrane vesicles. Triethyltin induced muscle contracture in mouse diaphragm while tributyltin had comparatively less potency and efficacy in inducing the muscle contracture. The contracture induced by tributyltin was inhibited when the diaphragm was pretreated with low Ca2+ medium or caffeine while the contracture induced by triethyltin persisted in the Ca2+-free medium but was inhibited by pretreatment of caffeine. Pretreatment of dithiothreitol blocked the contracture induced by tributyltin but not that by triethyltin. Triethyltin dose-dependently induced Ca2+ release from sarcoplasmic reticulum vesicles and inhibited the Ca2+-ATPase activity. These results suggested that triethyltin induced contracture in mouse diaphragm was mainly by induction of Ca2+ release and inhibition of Ca2+ uptake of the internal Ca2+ storage site the sarcoplasmic reticulum, while the tributyltin induced contracture might be due to enhancement of extracellular Ca2+ influx which further induce the release of internal Ca2+ through the Ca2+-induced Ca2+ release mechanism.

Histidine-containing dipeptides as endogenous regulators of the activity of sarcoplasmic reticulum Ca-release channels

It is shown that histidine-containing dipeptide carnosine (beta-alanyl-L-histidine), which is present in skeletal muscles in millimolar concentrations, decreases the rate of Ca2+ accumulation by the heavy fraction of sarcoplasmic reticulum from rabbit skeletal muscles. This effect results from the ability of carnosine to induce a rapid Ca2+ release from the heavy sarcoplasmic reticulum vesicles via activation of the ruthenium red-sensitive Ca-channels. The effect of carnosine is dose-dependent that indicates the presence of saturable site(s) for carnosine in the molecules of Ca-channels. The C0.5 value carnosine (the concentration that induces the half-maximal Ca2+ release) is 8.7 mM. The 1 N-methylated derivative of carnosine, i.e., anserine, also induces a rapid Ca2+ release with the half-maximal effect at 2.7 mM. Conversely, neither histidine nor beta-alanine (both separately and in the mixture) cause Ca2+ release. In addition, carnosine increases the sensitivity of Ca-channels to their well-known activators (caffeine, AMP, and Ca2+) and decreases inhibitory effect of low concentrations of Mg2+. It is concluded that carnosine as a component of skeletal muscles can be an endogenous regulator of the sarcoplasmic reticulum Ca-channel activity.

Pharmacology of Ca2+ release from red beet microsomes suggests the presence of ryanodine receptor homologs in higher plants

Cyclic ADP-ribose (cADPR) is known to release Ca2+ from plant vacuoles, implying that this NAD+ metabolite may possess a second messenger role in plants. The degree to which the plant cADPR-gated Ca2+ release mechanism resembles cADPR action in animals has been evaluated. cADPR-elicited Ca2+ release from red beet microsomes was inhibited by 1 mM procaine but insensitive to heparin. Furthermore, pre-release of Ca2+ from red beet vesicles by either 5 mM caffeine or micromolar levels of ryanodine precluded further Ca2+ mobilisation by cADPR. Thus, this study argues strongly for conservation between the plant and animal cADPR-elicited Ca2+ release mechanisms.

Gold ion inhibits silver ion induced contracture and activates ryanodine receptors in skeletal muscle

Effects of Au3+ on Ag(+)-induced contractures and Ca2+ release channel activity in the sarcoplasmic reticulum were studied in frog skeletal muscles. Single fibres spontaneously produced phasic and tonic contractures upon addition of 5-20 microM Ag+ or more than 50 microM Au3+. Simultaneous application of 5 microM Ag+ and 20 microM Au3+ inhibited contractures induced by Ag+. Au3+ applied immediately after development of Ag(+)-induced contractures shortened the duration of the phasic contracture and markedly decreased the subsequent tonic contracture. Pretreatment of fibres with Au3+ inhibited the Ag(+)-induced phasic contracture. Ca2+ release channels incorporated into planar lipid bilayers were activated in response to Au3+ at 20 to 200 microM. A close relationship was observed between Ca2+ release channel open probability and amplitude of the Au(3+)-induced tonic contracture. Channel activity was inhibited by 5 microM ruthenium red. We conclude that extracellular Au3+ at low concentrations modifies the interaction of Ag+ with voltage sensors in the transverse tubules to inhibit the Ag(+)-induced contracture and, if it enters the cell, Au3+ may directly activate the sarcoplasmic reticulum Ca2+ release channel to partially contribute to the tonic contracture.

Ca(2+)- and pH-dependent halothane stimulation of Ca2+ release in sarcoplasmic reticulum from frog muscle

The effect of halothane on calcium release kinetics was studied in triad-enriched sarcoplasmic reticulum vesicles from frog skeletal muscle. Release from vesicles passively equilibrated with 3 mM 45CaCl2 was measured in the millisecond time range by use of a fast-filtration system. Halothane (400 microM) increased release rate constants at pH 7.1 and 7.4 as a function of extravesicular pCa. In contrast, halothane at pH 6.8 produced the same stimulation of release from pCa 7.0 to 3.0; no release took place in these conditions in the absence of halothane. Halothane shifted the calcium activation curve at pH 7.1, but not at pH 7.4, to the left and increased channel open probability at pH 7.1 in the cis pCa range of 7.0 to 5.0. These results indicate that cytosolic pCa and pH modulate the stimulatory effects of halothane on calcium release. Furthermore, halothane stimulated release in frog skeletal muscle at low pH and resting calcium concentration, indicating that in frog muscle halothane can override the closing of the release channels produced by these conditions, as it does in malignant hyperthermia-susceptible porcine muscle.

Induction of calcium release from sarcoplasmic reticulum of skeletal muscle by xanthone and norathyriol

1. Effects of xanthone and its derivative, 1,3,6,7-tetrahydroxyxanthone (norathyriol), on Ca2+ release and ryanodine binding were studied in isolated sarcoplasmic reticulum (SR) vesicles from rabbit skeletal muscle. 2. Both xanthone and norathyriol dose-dependently induced Ca2+ release from the actively loaded SR vesicles which was blocked by ruthenium red, a specific Ca2+ release inhibitor, and Mg2+. 3. Xanthone and norathyriol also dose-dependently increased apparent [3H]-ryanodine binding. Norathyriol, but not xanthone, produced a synergistic effect on binding activation when added concurrently with caffeine. 4. In the presence of Mg2+, which inhibits ryanodine binding, both caffeine and norathyriol, but not xanthone, could restore the binding to the level observed in the absence of Mg2+. 5. Xanthone activated the Ca(2+)-ATPase activity of isolated SR vesicles dose-dependently reaching 70% activation at 300 microM. 6. When tested in mouse diaphragm, norathyriol potentiated the muscle contraction followed by twitch depression and contracture in either a Ca(2+) -free bathing solution or one containing 2.5 mM Ca2+. These norathyriol-induced effects on muscle were inhibited by pretreatment with ruthenium red or ryanodine. 7. These data suggest that xanthone and norathyriol can induce Ca2+ release from the SR of skeletal muscle through a direct interaction with the Ca2+ release channel, also known as the ryanodine receptor.

Sarcoplasmic reticulum Ca2+ pump in pig coronary artery smooth muscle is regulated by a novel pathway

Coronary artery smooth muscle expresses an alternative splice (SERCA2b) of the sarcoplasmic reticulum (SR) Ca2+ pump gene SERCA2, which is also expressed in cardiac muscle (SERCA2a), but how the activity of this transporter is regulated in the coronary artery is not known. SERCA2a in the cardiac muscle can be regulated via phospholamban or, as recently reported, by a direct phosphorylation of this protein by calmodulin kinase (Xu, A., C. Hawkins, and N. Narayanan. J.Biol. Chem. 268:8394-8397, 1993). Because both SERCA2a and SERCA2b contain this calmodulin kinase phosphorylation site, we examined the effect of endogenous calmodulin kinase phosphorylation of the SR Ca2+ pump in the coronary artery. SR-enriched membranes were isolated from coronary artery smooth muscle and washed in ethylene glycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid to remove bound calmodulin. When these membranes were incubated with MgATP2- in the presence of Ca2+/calmodulin, a 115-kDa protein was phosphorylated. This phosphorylated 115-kDa protein was identified as SERCA2b in Western blots and by immunoprecipitation using a SERCA2-selective antibody. Preincubating the membranes in MgATP2- in the presence of Ca2+/calmodulin stimulated the subsequent Ca2+ uptake in the presence of oxalate plus MgATP2- and azide. The stimulation of Ca2+ uptake was inhibited by including the SR Ca2+ pump inhibitors thapsigargin and cyclopiazonic acid in the Ca2+ uptake medium or by including the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide or the calmodulin kinase II peptide fragment 290-309 in the phosphorylation solution. Thus an endogenous calmodulin-dependent kinase phosphorylated SERCA2b and activated it. Phospholamban could not be detected in these membranes in Western blots. Therefore, the regulation of the SR Ca2+ pump activity in coronary artery smooth muscle may involve a direct phosphorylation of the pump protein by an endogenous calmodulin-dependent kinase.

Differentiation of BC3H1 smooth muscle cells changes the bivalent cation selectivity of the capacitative Ca2+ entry pathway

Differentiation of BC3H1 cells leads to expression of a variety of proteins characteristic of smooth muscle and to changes in the behaviour of intracellular Ca2+ stores. Treatment of both differentiated and undifferentiated cells with thapsigargin (2 microM) emptied their intracellular Ca2+ stores, and in the presence of extracellular Ca2+ caused an increase in cytosolic [Ca2+] that rapidly reversed after its removal. The amplitudes of these capacitative Ca2+ entry signals were 101 +/- 8 nM (n = 42) in differentiated cells and 188 +/- 16 nM (n = 35) in undifferentiated cells. Mn2+ entry in thapsigargin-treated cells, measured by recording the quenching of cytosolic fura 2 fluorescence, was 374 +/- 26% (n = 34) and 154 +/- 7% (n = 41) of control rates in differentiated and undifferentiated cells, respectively. Empty stores caused Ba2+ entry to increase to 282 +/- 20% (n = 8) of its basal rate in differentiated cells and to 187 +/- 20% (n = 8) in undifferentiated cells. Rates of Ca2+ extrusion, measured after rapid removal of extracellular Ca2+ from cells in which capacitative Ca2+ entry had been activated, were similar in differentiated (t1/2 = 23 +/- 2 s, n = 7) and undifferentiated (23 +/- 1 s, n = 6) cells. The different relationships between capacitative Ca2+ and Mn2+ signals are not, therefore, a consequence of more active Ca2+ extrusion mechanisms in differentiated cells, nor are they a consequence of different fura 2 loadings in the two cell types. We conclude that during differentiation of BC3Hl cells, the cation selectivity of the capacitative pathway changes, becoming relatively more permeable to Mn2+ and Ba2+. The change may result either from expression of a different capacitative pathway or from modification of the permeation properties of a single pathway.

Attenuation of endothelin-1-induced calcium response by tyrosine kinase inhibitors in vascular smooth muscle cells

Although tyrosine kinases play an important role in cell growth and have been implicated in regulation of smooth muscle contraction, their role in agonist-induced myoplasmic Ca2+ responses is unclear. We examined effects of the tyrosine kinase inhibitors genistein and methyl 2,5-dihydroxycinnamate (MDHC) on the endothelin-1 (ET-1)-induced Ca2+ response and determined underlying mechanisms for the effects. Freshly isolated smooth muscle cells from porcine coronary arteries were loaded with fura 2 ester, and myoplasmic free Ca2+ (Ca2+ (m)) concentration was estimated with fura 2 microfluorometry. Both genistein and MDHC inhibited the initial transient Cam2+ response to ET by 54 and 81%, respectively (P < 0.05), in the presence of extracellular Ca2+. Genistein also significantly delayed the Cam2+ response, with the latent period from ET-1 application to the beginning of the Cam2+ response being increased from 1.08 +/- 0.17 to 2.65 +/- 0.52 min (P < 0.05). In the absence of extracellular Ca2+, genistein inhibited the ET-1-induced Cam2+ response by 93% (P < 0.05). The Cam2+ responses to caffeine (5 mM) or inositol trisphosphate (IP3) applied intracellularly via a patch-clamp pipette were not affected by genistein. Both genistein and MDHC also abolished the sustained Cam2+ response to ET-1. However, the Cam2+ response to depolarization by 80 mM K+ was not inhibited by MDHC and only inhibited 22% by genistein (P < 0.05). These results indicate that 1) activation of tyrosine kinases is an important regulatory mechanism for the ET-1-induced Cam2+ response in vascular smooth muscle and 2) tyrosine kinases mediate ET-1-induced Ca2+ release with no direct effect on IP3-mediated Ca2+ release. Thus ET-1-mediated signaling upstream of IP3 interaction with the Ca2+ stores is regulated by tyrosine kinases.

The permeability transition pore as a mitochondrial calcium release channel: a critical appraisal

Mitochondria from a variety of sources possess an inner membrane channel, the permeability transition pore. The pore is a voltage-dependent channel, activated by matrix Ca2+ and inhibited by matrix H+, which can be blocked by cyclosporin A, presumably after binding to mitochondrial cyclophilin. The physiological function of the permeability transition pore remains unknown. Here we evaluate its potential role as a fast Ca2+ release channel involved in mitochondrial and cellular Ca2+ homeostasis. We (i) discuss the theoretical and experimental reasons why mitochondria need a fast, inducible Ca2+ release channel; (ii) analyze the striking analogies between the mitochondrial permeability transition pore and the sarcoplasmic reticulum ryanodine receptor-Ca2+ release channel; (iii) argue that the permeability transition pore can act as a selective release channel for Ca2+ despite its apparent lack of selectivity for the transported species in vitro; and (iv) discuss the importance of mitochondria in cellular Ca2+ homeostasis, and how disruption of this function could impinge upon cell viability, particularly under conditions of oxidative stress.

Regulation of the human histamine H1 receptor stably expressed in Chinese hamster ovary cells

1. The human H1 receptor gene expressed in Chinese hamster ovary cells (CHOhumH1) encodes a classical histamine H1 receptor with a pharmacology similar to that of the H1 receptor found in guinea-pig cerebellum and the endogenously expressed human H1 receptor in 1321N1 astrocytoma cells as determined by [3H]-mepyramine binding studies. 2. In CHOhumH1 cells, histamine induced a concentration-dependent rise in inositol phosphates (EC50 2.23 +/- 0.97 microM) and a rapid increase of [Ca2+]i, followed by a sustained increase of [Ca2+]i upon addition of 100 microM histamine. 3. Short-term exposure of CHOhumH1 cells to histamine (100 microM) resulted in a decrease of subsequent histamine-induced Ca2+ responses. The histamine-induced desensitization appeared to be heterologous as the ATP-induced Ca2+ response was also found to be affected. 4. The process of heterologous histamine-induced desensitization of the Ca2+ response in CHOhumH1 cells can be ascribed to an alteration at the level of the intracellular Ca2+ pool, as the Ca2+ response of caffeine (10 mM), which releases Ca2+ from intracellular Ca2+ stores was also attenuated upon short-term histamine exposure. 5. In CHOhumH1 cells the PKC activator, PMA, was found to inhibit the histamine (100 microM)-induced Ca2+ response concentration-dependently (IC50 0.2 +/- 0.03 microM) as well as the ATP (100 microM)-induced Ca2+ response. However, this inhibition was only partial and less effective than histamine-pretreatment. Moreover, in CHOhumH1 cells PKC downregulation induced by long-term exposure to PMA (1 microM) did not affect the histamine-induced desensitization nor did pretreatment with the specific PKC inhibitor Ro-31-8220 (10 microM), indicating that in CHOhumH1 cells PKC is probably not involved in the heterologous desensitization. 6. Long-term treatment of CHOhumH1 cells with histamine or other H1 agonists resulted in a time- and concentration-dependent decrease in the number of H1 receptor binding sites (maximal reduction: 47 +/- 5%). 7. Long-term exposure of CHOhumH1 cells to ATP or PMA did not affect H1 receptor density. 8. Both histamine (100 microM)- and ATP (100 microM)-induced Ca2+ responses were affected upon long-term exposure of cells to histamine (100 microM), which might be explained by an alteration at a level distant from the receptor. 9. These results show that in CHOhumH1 cells the human histamine H1 receptor is susceptible to short-term and long-term receptor regulation in which PKC does not seem to play a role. The CHOhumH1 cells therefore provide an excellent model system for studying the mechanism(s) of PKC-independent H1 receptor regulation.

4-Chloro-m-cresol, a potent and specific activator of the skeletal muscle ryanodine receptor

The aim of the present study was to determine the effects of 4-chloro-m-cresol (4-CmC), a preservative often added to drugs intravenously administered, on the skeletal muscle sarcoplasmic reticulum (SR) Ca2+ release channel/ryanodine receptor. In heavy SR vesicles obtained from rabbit back muscles, 4-CmC stimulated (Ca2+)-activated [3H]ryanodine binding with an EC50 of about 100 microM. In the same concentration range, 4-CmC directly activated the isolated Ca2+ release channel reconstituted into planar lipid bilayers. The sensitivity to 4-CmC was found to be higher when applied to the luminal side of the channel suggesting binding site(s) different from those of nucleotides and caffeine. In skeletal muscle fibre bundles obtained from biopsies of patients susceptible to malignant hyperthermia, a skeletal muscle disease caused by point mutations in the ryanodine receptor, 4-CmC evoked caffeine-like contractures. Contrary to caffeine which induces contractures in millimolar concentrations, the threshold concentration for 4-CmC was 25 microM compared to 75 microM for non-mutated control fibres. Since these data strongly indicate that 4-CmC specifically activates SR Ca2+ release also in intact cell systems, this substance might become a powerful tool to investigate ryanodine receptor-mediated Ca2+ release in muscle and non-muscle tissue.

Caffeine inhibits Ca2+ uptake by subplasmalemmal calcium stores ('alveolar sacs') isolated from Paramecium cells

Caffeine inhibits 45Ca2+ sequestration by subplasmalemmal calcium stores ('alveolar sacs') of low thapsigargicin sensitivity which we have isolated from the ciliated protozoan, Paramecium tetraurelia. Inhibition depends on caffeine concentration, with an IC50 of 31.8 mM. According to kinetic evaluation this is compatible with non-competitive inhibition of Ca2+ uptake, rather than with superimposed 45Ca2+ release during sequestration. It remains to be analysed whether this mechanism might be of possible relevance also for Ca2+-mediated activation in vivo in this or in any other secretory system. Such an effect could also operate indirectly, e.g., by Ca2+-release induction via sequestration inhibition. This is the first description of caffeine-mediated inhibition of Ca2+ uptake by calcium stores from a secretory system. Our data are compatible with some observations with sarcoplasmic reticulum from striated muscle fibers.

Ligand-gated channel of the sarcoplasmic reticulum Ca2+ transport ATPase

In resting muscle, cytoplasmic Ca2+ concentration is maintained at a low level by active Ca2+ transport mediated by the Ca2+ ATPase from sarcoplasmic reticulum. The region of the protein that contains the catalytic site faces the cytoplasmic side of the membrane, while the transmembrane helices form a channel-like structure that allows Ca2+ translocation across the membrane. When the coupling between the catalytic and transport domains is lost, the ATPase mediates Ca2+ efflux as a Ca2+ channel. The Ca2+ efflux through the ATPase channel is activated by different hydrophobic drugs and is arrested by ligands and substrates of the ATPase at physiological pH. At acid pH, the inhibitory effect of cations is no longer observed. It is concluded that the Ca2+ efflux through the ATPase may be sufficiently fast to support physiological Ca2+ oscillations in skeletal muscle, that occur mainly in conditions of intracellular acidosis.

Pharmacological modulators of the inositol 1,4,5-trisphosphate receptor

Elevation of cytosolic calcium concentrations, induced by many neurotransmitters, plays a crucial role in neuronal function. Some neurotransmitters produce the second messenger InsP3 which activates an intracellular calcium channel (InsP3 receptor) usually located in the endoplasmic reticulum. This article undertakes a comprehensive survey of most pharmacological modulators of the InsP3 receptor so far reported. This review discusses in detail competitive antagonists, non-competitive antagonists and thiol reactive reagents, highlighting their modes of action and in some cases indicating drawbacks in their use.

Voltage control of calcium transients elicited by caffeine and tetracaine in cultured rat muscle cells

Cultured hind limb skeletal muscle cells from newborn rats were used to study the effect of caffeine and tetracaine upon intracellular Ca2+ release under voltage or current clamp conditions. Free [Ca2+]i was measured using the fluorescent calcium-sensitive dye Fluo-3. A field containing one or several myotubes was observed with a video camera and image analysis of fluorescence changes was performed. Addition of 100-500 microM tetracaine to the external saline elicited strong fluorescence responses in non-clamped cells, but significantly lower responses in cells clamped at -90 mV. At the same time, tetracaine inhibited voltage induced calcium release. Voltage and tetracaine modulation over the action of caffeine (500 microM) was also observed. Pretreatment of cells with 10 microM nifedipine abolished the caffeine induced fluorescence response in non-clamped cells. These findings suggest that, in cultured muscle cells, calcium release through the caffeine and tetracaine sensitive pathways is controlled by both membrane potential and the dihydropyridine receptor.

A conformational change in the junctional foot protein is involved in the regulation of Ca2+ release from sarcoplasmic reticulum. Studies on polylysine-induced Ca2+ release

We investigated both conformational changes in the junctional foot protein (JFP) and Ca2+ release from sarcoplasmic reticulum (SR) in parallel after stimulation of triadic vesicles by the JFP-specific ligand, polylysine. To monitor protein conformational change, the JFP was labeled in a site-directed fashion with the fluorescent conformational probe methylcoumarin acetate (MCA) (Kang, J. J., Tarcsafalvi, A., Carlos, A. D., Fujimoto, E., Shahrokh, Z., Thevenin, B. J.-M., Shohet, S. B., and Ikemoto, N. (1992) Biochemistry 31, 3288-3293). The induction of SR Ca2+ release by polylysine produced a rapid increase in the fluorescence intensity of the JFP-bound MCA. The polylysine concentration dependence of the fluorescence change was essentially the same as that of Ca2+ release, suggesting that the two events are tightly coupled. However, the rate constant of MCA fluorescence change was much larger than that of Ca2+ release; i.e. the conformational change preceded Ca2+ release. Prevention of protein conformational change by lysine (0.2 M) inhibited Ca2+ release from SR. Inhibition of Ca2+ release by Mg2+ (5 mM), however, had little effect on the conformational change. These results suggest that binding of polylysine to the JFP produces conformational changes in the protein, which in turn activates the Ca2+ channel, leading to Ca2+ release from the SR.

Vasopressin stimulation of Ca2+ mobilization, two bivalent cation entry pathways and Ca2+ efflux in A7r5 rat smooth muscle cells

1. Arg8-vasopressin (AVP)-regulated Ca2+ transport were investigated in fura-2-loaded A7r5 cells using both single cell and population measurements. 2. AVP evokes an initial concentration-dependent rise in cytosolic free Ca2+ concentration ([Ca2+ ]i) to a peak which is independent of extracellular Ca2+, and a sustained Ca2+ signal that results from a balance between stimulation of Ca2+ entry and efflux. 3. Depletion of intracellular Ca2+ stores with thapsigargin, ionomycin, or prior treatment with AVP in Ca2(+)-free medium activates 'capacitative' entry of Ca2+, Ba2+ or Mn2+. Capacitative Mn2+ entry is inhibited by refilling stores with Ca2+; neither Sr2+ nor Ba2+ substitute for Ca2+ to give this effect. 4. In cells with empty stores, AVP stimulates further bivalent cation entry, and the effect persists when extracellular Na+ is replaced by N-methyl-D-glucamine or under depolarizing condition (extracellular KCl concentration ([KCl]o), 135 mM). This effect of AVP is not therefore merely a consequence of AVP causing membrane hyperpolarization or stimulation of Na(+)-Ca2+ exchange, but results from opening of a bivalent cation influx pathway. 5. Several lines of evidence indicate that AVP-stimulated bivalent cation entry is not a consequence of more complete emptying of the intracellular stores and consequent further activation of the capacitative pathway. AVP stimulates Ba2+ entry when the intracellular Ca2+ stores have been both emptied by ionomycin and prevented from refilling by thapsigargin. Mn2+ permeates the capacitative pathway, but AVP does not further increase Mn2+ entry, confirming that AVP does not further activate the capacitative pathway and that the two pathways differ in their permeability to Mn2+. When the extracellular [Sr2+] is low, empty stores do not stimulate detectable Sr2+ entry, but addition of AVP causes substantial Sr2+ entry. 6. A decrease in [Ca2+]i occurs when 50 nM AVP is added during a sustained elevation of [Ca2+]i evoked by thapsigargin. Since AVP does not inhibit the capacitative pathway, this result suggests that AVP stimulates Ca2+ extrusion. 7. We conclude that stimulation of Ca2+ mobilization, two modes of bivalent cation entry, and Ca2+ efflux all contribute to the complex concentration-dependent effects of AVP in A7r5 smooth muscle cells.

Activation and labelling of the purified skeletal muscle ryanodine receptor by an oxidized ATP analogue

We have tested the periodate-oxidized ATP analogue 2',3'-dialdehyde adenosine triphosphate (oATP) as a ligand for the skeletal muscle ryanodine receptor/Ca(2+)-release channel. Ca2+ efflux from passively loaded heavy sarcoplasmic reticulum vesicles of skeletal muscle is biphasic. oATP stimulates the initial phase of Ca2+ release in a concentration-dependent manner (EC50 160 microM), and the efflux proceeds with a half-time in the range 100-200 ms. This oATP-modulated initial rapid Ca2+ release was specifically inhibited by millimolar concentrations of Mg2+ and micromolar concentrations of Ruthenium Red, indicating that the effect of oATP was mediated via the ryanodine receptor. The purified Ca(2+)-release channel was incorporated into planar lipid bilayers, and single-channel recordings were carried out to verify a direct interaction of oATP with the ryanodine receptor. Addition of oATP to the cytoplasmic side activated the channel with an EC50 of 76 microM, which is roughly 30-fold higher than the apparent affinity of ATP. The oATP-induced increase in the open probability of the ryanodine receptor displays a steep concentration-response curve with a Hill coefficient of approximately 2, which suggests a co-operativity of the ATP binding sites in the tetrameric protein. oATP binds to the ryanodine receptor in a quasi-irreversible manner via Schiff base formation between the aldehyde groups of oATP and amino groups in the nucleotide binding pocket. This allows for the covalent specific incorporation of [alpha-32P]oATP by borhydride reduction. A typical adenine nucleotide binding site cannot be identified in the primary sequence of the ryanodine receptor. Our results demonstrate that oATP can be used to probe the structure and function of the nucleotide binding pocket of the ryanodine receptor and presumably of other ATP-regulated ion channels.

Possible involvement of Ca(2+)-induced Ca2+ release mechanism in Ag(+)-induced contracture in frog skeletal muscle

To determine if an Ag(+)-induced contracture is associated with the Ca(2+)-induced Ca2+ release mechanism in the sarcoplasmic reticulum, effects of Ca(2+)-induced Ca2+ release modulators on the Ag(+)-induced contracture were studied with single fibers of frog toe skeletal muscle. The fiber treated with 1 mM caffeine contracted significantly much more than controls without caffeine at Ag+ concentrations below 1 microM. Procaine shifted the Ag+ concentration-tension curve to the right, dose-dependently. When 10 mM procaine was applied to contracting fibers not treated with caffeine, the duration of 5 microM Ag(+)-induced contracture was shortened with a little decrease in tension amplitude, that was different from the effect of procaine on caffeine contracture. In caffeine solution, 0.5 microM Ag+ caused a long-lasting contracture with sometimes two peaks. 2 mM procaine led to disappearance of such two peaks, resulting in shortening of the contracture. K+ contracture was potentiated by 1 mM caffeine only at lower concentrations of K+, and inhibited by 10 mM procaine. These results suggest that the Ag(+)-induced contracture is composed of two components: Ca(2+)-induced Ca2+ release-dependent and -independent. 5 microM Ag(+)-induced contracture slowly relaxed with a wavy tension pattern to the resting level when 0.05 mM dithiothreitol was applied around peak of the tension. This relaxation was accelerated by procaine application. These findings may be explained by attributing a portion of Ag(+)-induced contracture to the effect of Ca2+ released through the Ca(2+)-induced Ca2+ release mechanism in the sarcoplasmic reticulum.

Alteration of intracellular Ca2+ transients in COS-7 cells transfected with the cDNA encoding skeletal-muscle ryanodine receptor carrying a mutation associated with malignant hyperthermia

Malignant hyperthermia (MH), an inherited neuromuscular disease triggered by halogenated inhalational anaesthetics and skeletal-muscle relaxants, appears to be due to an alteration of intracellular Ca2+ homoeostasis. MH occurs in 1 out of 20,000 anaesthetized adults and is characterized by hypermetabolism, skeletal-muscle rigidity and elevation in body temperature, which is frequently fatal [MacLennan and Phillips (1992) Science 256, 789-794]. The defect responsible for the disease may lie within the mechanism controlling the release of Ca2+ from sarcoplasmic reticulum via the ryanodine-receptor (RYR) Ca2+ channel; in fact a point mutation in the RYR has been associated with MH in some human families, as well as in the MH-susceptible pig. To date, however, no direct evidence has been obtained demonstrating that the point mutation is both necessary and sufficient to cause functional alterations in RYR-mediated Ca2+ release. In the present report we show that the presence of the Arg-to-Cys point mutation in the recombinant RYR expressed in COS-7 transfected cells causes abnormal cytosolic Ca2+ transients in response to 4-chloro-m-cresol, an agent capable of eliciting in vitro contracture of MH-susceptible muscles.

Effects of bipyridylium compounds on calcium release from triadic vesicles isolated from rabbit skeletal muscle

1. The effects of 1,1'-diheptyl-4,4'-bipyridinium dibromide (DHBP), a viologen for electrochromic memory display agent, on calcium release and ryanodine binding were studied with triad-rich sarcoplasmic reticulum (SR) vesicles isolated from rabbit skeletal muscle. 2. DHBP inhibited the calcium release induced by 2 mM caffeine and 2 micrograms ml-1 polylysine with an IC50 value of 5 micrograms ml-1 and 4 micrograms ml-1 respectively. 3. DHBP inhibited [3H]-ryanodine binding in a dose-dependent manner with an IC50 of 2.5 micrograms ml-1 and 90-100% inhibition at 20-30 micrograms ml-1. 4. Calcium uptake by SR was inhibited in the presence of caffeine and this inhibition was antagonized by concomitant addition of DHBP. 5. The effect of DHBP on muscle twitches was studied on the mouse diaphragm. Muscle twitches elicited by direct electrical muscle stimulation and contractions induced by either 10 mM caffeine or 1 microM ryanodine were blocked by pretreatment with DHBP. 6. Data from this study provided evidence that DHBP blocked the calcium release from SR by direct interaction with the calcium release channel, also known as the ryanodine receptor. A possible use of this agent as a specific inhibitor for calcium release and as a muscle relaxant was suggested.

Induction of Ca2+ oscillations by selective, U73122-mediated, depletion of inositol-trisphosphate-sensitive Ca2+ stores in rabbit pancreatic acinar cells

The effect of the putative inhibitor of phospholipase C activity, U73122, on the Ca2+ sequestering and releasing properties of internal Ca2+ stores was studied in both permeabilized and intact rabbit pancreatic acinar cells. U73122 dose dependently inhibited ATP-dependent Ca2+ uptake in the inositol (1,4,5)-trisphosphate-[Ins(1,4,5)P3]-sensitive, but not the Ins(1,4,5)P3-insensitive, Ca2+ store in acinar cells permeabilized by saponin treatment. In a suspension of intact acinar cells, loaded with the fluorescent Ca2+ indicator, Fura-2, U73122 alone evoked a transient increase in average free cytosolic Ca2+ concentration ([Ca2+]i,av), which was largely independent of external Ca2+. Addition of U73122 to cell suspensions prestimulated with either cholecystokinin octapeptide or JMV-180 revealed an inverse relationship in size between the U73122- and the agonist-evoked [Ca2+]i,av transient. Moreover, thapsigargin-induced inhibition of intracellular Ca(2+)-ATPase activity resulted in a [Ca2+]i,av transient, the size of which was not different following maximal prestimulation with either U73122 or agonist. These observations suggest that U73122 selectively affects the Ins(1,4,5)P3- casu quo agonist-sensitive internal Ca2+ store, whereas thapsigargin affects both the Ins(1,4,5)P3-sensitive and -insensitive Ca2+ store. Digital-imaging microscopy of Fura-2-loaded acinar cells demonstrated that U73122, in contrast to thapsigargin, evoked sustained oscillatory changes in [Ca2+]i. The U73122-evoked oscillations were abolished in the absence of external Ca2+. The ability of U73122 to generate external Ca(2+)-dependent Ca2+ oscillations suggests that depletion of the agonist-sensitive store leads to an increase in Ca2+ permeability of the plasma membrane and that the Ins(1,4,5)P3-insensitive Ca2+ pool is necessary for the Ca2+ oscillations.

Surface charge potentiates conduction through the cardiac ryanodine receptor channel

Single channel currents through cardiac sarcoplasmic reticulum (SR) Ca2+ release channels were measured in very low levels of current carrier (e.g., 1 mM Ba2+). The hypothesis that surface charge contributes to these anomalously large single channel currents was tested by changing ionic strength and surface charge density. Channel identity and sidedness was pharmacologically determined. At low ionic strength (20 mM Cs+), Cs+ conduction in the lumen-->myoplasm (L-->M) direction was significantly greater than in the reverse direction (301.7 +/- 92.5 vs 59.8 +/- 38 pS, P < 0.001; mean +/- SD, t test). The Cs+ concentration at which conduction reached half saturation was asymmetric (32 vs 222 mM) and voltage independent. At high ionic strength (400 mM Cs+), conduction in both direction saturated at 550 +/- 32 pS. Further, neutralization of carboxyl groups on the lumenal side of the channel significantly reduced conduction (333.0 +/- 22.5 vs 216.2 +/- 24.4 pS, P < 0.002). These results indicate that negative surface charge exists near the lumenal mouth of the channel but outside the electric field of the membrane. In vivo, this surface charge may potentiate conduction by increasing the local Ca2+ concentration and thus act as a preselection filter for this poorly selective channel.

Ryanodine and theophylline-induced depletion of energy stores in amphibian muscle

The effects of high and low levels of ryanodine on theophylline-induced energy depletion were studied in isolated frog sartorius muscle. Whereas low concentrations of ryanodine (1-10 microM) did not change high energy phosphate contents (PE) after 60 min, high levels (100 microM) reduced resting energy contents by 60% after 60 min. Subcontracture levels of theophylline (2 mM), in the presence of high ryanodine, produced an 80% PE depletion, suggesting possible additive or synergistic effects of these two agents. In contrast to theophylline-induced depletion, neither the ryanodine-induced depletion nor the theophylline-plus-ryanodine-induced depletion of PE seemed sensitive to inhibition by 1 mM procaine. This suggests that there may be differences in the mechanisms whereby methylxanthines and ryanodine deplete energy stores and evoke contractures in amphibian skeletal muscle.

Characterization study of the ryanodine receptor and of calsequestrin isoforms of mammalian skeletal muscles in relation to fibre types

We have investigated high-affinity ryanodine-binding sites in membrane preparations from representative fast-twitch and slow-twitch muscles of the rabbit and rat, as well as from human mixed muscle. Our results, obtained in high-ionic strength binding buffer, demonstrate extensive similarities in binding affinity for [3H]ryanodine (Kd: about 10 nM) and a two-fold to four-fold difference in membrane density of the ryanodine receptor between fast-twitch and slow-twitch muscle of the rat and rabbit, respectively. The [3H]ryanodine-pCa relationship for the Ca(2+)-activation curve of ryanodine binding was found to be similar for all mammalian muscles, as tested at 20 nM ryanodine. With 10 mM caffeine or 50 microM doxorubicin the pCa for half-maximal activation of [3H]ryanodine binding invariably shifted from an average pCa value of 6.5 to pCa 7.1-7.3. IC50 values for the inhibition of [3H]ryanodine binding by Ruthenium Red, a Ca(2+)-release channel blocker, did not differ significantly (range 0.3-1.0 microM). The Ca(2+)-dependence curve (range 1 nM-10 mM free Ca2+) that we have observed at 5 nM ryanodine, for [3H]ryanodine binding to terminal cisternae from rabbit fast-twitch, as well as slow-twitch muscle, is bell-shaped and differs from that obtained with cardiac terminal cisternae from the same species. Cardiac ryanodine receptor is also clearly distinguishable for electrophoretic mobility, Cleveland's peptide maps, and, most strikingly, for total lack of cross-reactivity with polyclonal antibody to fast skeletal RyR. By the same properties, the ryanodine receptor of fast- and slow-twitch muscle appear to be the same or a similar protein. On investigating the composition of calsequestrin in rat and human skeletal muscles, both in membrane-bound form and after purification by phenyl-Sepharose chromatography, we have been able to show that, independent of the animal species, the cardiac isoform, as characterized by the identical amino-terminal amino-acid sequence, pattern of immunoreactivity, and lack of Ca(2+)-dependent shift in mobility on SDS-PAGE, is exclusively expressed in slow-twitch fibres, together with the main fast-skeletal calsequestrin isoform. While our experimental findings strongly argue for the presence of only one population of skeletal-specific Ca(2+)-release channels in junctional terminal cisternae of mammalian fast-twitch and slow-twitch muscle, they at the same time suggest the existence of differences in calsequestrin modulation of Ca(2+)-release, depending on its isoform composition.

Enhancement of Ca2+ release channel activity by phosphorylation of the skeletal muscle ryanodine receptor

The Ca2+ release channel of rabbit skeletal muscle sarcoplasmic reticulum (SR) can be phosphorylated by membrane associated protein kinase(s) utilizing endogenously synthesized or exogenously added ATP. The channel protein has been enriched in non-phosphorylated and phosphorylated form from heavy SR following solubilization with CHAPS (3-[(3-cholamidopropyl)dimethylammonio-1-propane-sulfonate) and ultracentrifugation on a linear sucrose/CHAPS gradient. Reconstitution of the isolated channels into planar bilayers shows that phosphorylation enhances the open probability by increasing the sensitivity towards micromolar Ca2+ and ATP. The phosphorylation induced enhancement of the channel activity can be reversed by purified protein phosphatase 2A.