Mode of action of diethyl ether on ATP-dependent Ca2+ transport by sarcoplasmic reticulum vesicles

Reversible thiol-dependent activation of ryanodine-sensitive Ca2+ release channel by etoposide (VP-16) phenoxyl radical

Many phenolic compounds can act as antioxidants by donating a proton to peroxyl radicals and quenching lipid peroxidation. Phenoxyl radicals produced this way or from metabolism by peroxidases, tyrosinase, or mixed-function oxidases, however, may react with sulfhydryl groups of proteins and other endogenous thiols. In this regard, phenolic compounds may have cytotoxic potential instead of antioxidant effects. We employed the anticancer drug, etoposide (VP-16), as a model phenolic compound to study the sensitivity of ryanodine-sensitive Ca2+ channel (RyR) to VP-16 phenoxyl radicals. The combination of VP-16 and tyrosinase, used to generate the etoposide phenoxyl radical, produced marked Ca2+ release from Ca2+-loaded RyR-rich vesicles prepared from terminal cisternae fraction of sarcoplasmic reticulum (SR). This effect was reversed by the SH-reagent, dithiothreitol (DTT), suggesting that cysteines within the RyR-protein complex were targets for modification by VP-16 phenoxyl radicals. VP-16/tyrosinase-induced release of Ca2+ was attenuated in vesicles prepared from longitudinal SR, which contain relatively little RyR. The effects of the VP-16 phenoxyl radical on Ca2+-ATPase in SR vesicles resembled those observed with caffeine or 4,4'-dithiodipyridine, both of which activate RyR Ca2+ release and lead to activation of Ca2+-ATPase via prolonged Ca2+ cycling. The addition of ruthenium red returned Ca2+-ATPase to its original level. Thus, under these conditions Ca2+-ATPase was not directly affected by VP-16 phenoxyl radical. The hypersensitive SH-groups on RyR are shown to be targets for oxidation of VP-16 phenoxyl radical, and suggest that other phenolic compounds could similarly disrupt Ca2+ homeostasis.

Nitric oxide activates skeletal and cardiac ryanodine receptors

The endothelial-derived relaxing factor, nitric oxide (NO.) has been shown to depress force in smooth and cardiac muscles through the activation of guanylyl cyclase and an increase in cGMP. In fast skeletal muscle, NO (i.e. NO-related compounds) elicits a modest decrease in developed force, but in contracting muscles NO increases force by a mechanism independent of cGMP. We now demonstrate an alternative mechanism whereby NO triggers Ca2+ release from skeletal and cardiac sarcoplasmic reticulum (SR). NO delivered in the form of NO gas, NONOates (a class of sulfur-free compounds capable of releasing NO), or S-nitrosothiols (R-SNO) oxidized or transnitrosylated regulatory thiols on the release channel (or ryanodine receptor, RyR), resulting in channel opening and Ca2+ release from skeletal and cardiac SR. The process was reversed by sulfhydryl reducing agents which promoted channel closure and Ca2+ reuptake by ATP-driven Ca2+ pumps. NO did not directly alter Ca(2+)-ATPase activity but increased the open probability of RyRs reconstituted in planar bilayers and inhibited [3H]-ryanodine binding to RyRs. The formation of peroxynitrite or thiyl radicals did not account for the reversible R-SNO-dependent activation of RyRs. Ca2+ release induced by nitric oxide free radicals (NO.) was potentiated by cysteine providing compelling evidence that NO. in the presence of O2 formed nitrosylated cysteine followed by the transnitrosation of regulatory thiols on the RyR to activate the channel. These findings demonstrate direct interactions of NO derivatives with RyRs and a new fundamental mechanism to regulate force in striated muscle.

Alteration of the L-type calcium current in guinea-pig single ventricular myocytes by heptaminol hydrochloride

1. The effects of heptaminol on calcium current amplitude and characteristics were studied in single ventricular myocytes of guinea-pig by use of the whole cell configuration of the patch clamp technique. 2. A concentration-dependent decrease in ICa amplitude was observed. At heptaminol concentration as low as 10(-6) M, this effect was observed in only two cells (n = 6). At 10(-5) M the reduction of ICa was of 30 +/- 15% (n = 11). 3. The current recovery from inactivation at -40 mV holding potential (HP) seemed less sensitive to perfusion with heptaminol (greater than 10(-6) M). However, at -80 mV HP the overshoot of the recovery curve was decreased by heptaminol. 4. Both at -40 mV and -80 mV HP, heptaminol (10(-5) M) significantly increased the steady state inactivation of ICa. 5. As previously proposed by others to explain the effects of membrane active substances, the effects of heptaminol may result from alterations in cell membrane properties and possibly from an increase in intracellular free calcium ion concentration.

Sulphydryl reagents trigger Ca2+ release from the sarcoplasmic reticulum of skinned rabbit psoas fibres

1. By analogy with studies on sarcoplasmic reticulum (SR) vesicles, Ca2+ release induced by heavy metals and mercaptans (e.g. cysteine) was investigated in rabbit skinned psoas fibres through measurements of isometric tension. 2. Heavy metals (at 2-5 microM) elicited phasic contractions by triggering Ca2+ release from the SR and had the following order of potency: Hg2+ > Cu2+ > Cd2+ > Ag+ > Ni2+. Higher concentrations produced tonic contractions due to maintained high Ca2+ permeability of SR membranes. 3. Contractions induced by heavy metals required a functional and Ca(2+)-loaded SR, were dependent on [Ca2+]free, blocked by Ruthenium Red (RR), inhibited by free Mg2+ and reduced glutathione (GSH) but not by oxidized glutathione (GSSG). Such contractions were not elicited through direct interaction(s) of heavy metals with the myofilaments. 4. In the presence of catalytic concentrations of Hg2+ or Cu2+ (2-5 microM), additions of cysteine (25-100 microM) elicited rapid twitches, producing 70% of maximal force with a time to half-peak of 2 s. Contractions induced by cysteine plus a catalyst required a functional SR network, were dependent on free [Mg2+] and were blocked by RR or GSH but not by GSSG. 5. In the presence of Hg2+ (2-5 microM), low concentrations of cysteine (10 microM) elicited tonic contractures, but subsequent or higher additions of cysteine (50-100 microM) caused further SR Ca2+ release and tension, followed by rapid and full relaxation. 6. High cysteine (200-250 microM, without Cu2+ or Hg2+) blocked contractions elicited by Cl- induced depolarization of sealed T-tubules. High cysteine probably acted as a sulphydryl reducing agent which promoted rapid relaxation of the fibres through the closure of Ca(2+)-release channels and ATP-dependent re-uptake of Ca2+ by the SR. 7. In some batches of skinned fibres (approximately 10%), cysteine (5-50 microM) alone (without Hg2+ or Cu2+ catalyst) produced rapid twitches. This implied that the catalyst(s) necessary to promote the sulphydryl oxidation reaction with exogenously added cysteine may be present in intact fibres but is usually lost by the skinning procedure. 8. The data demonstrate that skeletal fibres contain a highly reactive and accessible sulphydryl site on an SR protein which can be reversibly oxidized and reduced to respectively, open and close SR Ca(2+)-release channels. A model of sulphydryl-gated excitation-contraction coupling is proposed where the voltage sensor on the T-tubule membrane directly oxidizes sulphydryl sites on SR Ca(2+)-release channels.

The effects of n-alkanols on the lipid/protein interface of Ca(2+)-ATPase of sarcoplasmic reticulum vesicles

The effects of ethanol, n-butanol, n-hexanol and n-octanol on lipid-protein interactions in sarcoplasmic reticulum vesicles (SRV) are investigated using the C-14 nitroxide spin-labeled phosphatidylcholine. n-Alkanols, which activate the Ca(2+)-dependent ATPase of sarcoplasmic reticulum but decrease net Ca2+ uptake by the vesicles, are shown to affect the lipids interacting with the protein surface. Spectral analysis revealed that increasing concentrations of the alcohols progressively displace and mobilize lipids from the lipid/protein interface. For butanol, hexanol and octanol maximally activated SRV, 23 to 30% of the protein-interacting lipids are displaced. Thus, the displacement of more than 30% of the annular lipids by these alkanols cause inhibition of the enzyme. The motional properties of the labels that remain restricted by the protein surface are unaffected by the alcohols. The degree of mobilization attained by the labels displaced from the interface is much greater than that observed in alcohol-treated dispersions of extracted lipids. We propose that the alcohol molecules interfere with the protein-lipid interactions creating fluid clusters around the proteins. These fluidized regions would affect the enzyme conformation, perturbing its function. Fluidized annular lipids apparently increase the number of ion-conducting defects around the enzyme, increasing Ca2+ efflux, and thereby reducing net uptake.

Effects of tetracaine and procaine on skinned muscle fibres depend on free calcium

The local anaesthetics, tetracaine and procaine have previously been found to block, induce or potentiate Ca2+ release from the sarcoplasmic reticulum (SR) of skeletal muscle depending on the preparation, experimental conditions and design. We now show that low concentrations of tetracaine and procaine block SR Ca2+ release whereas high concentrations induce release from the SR of amphibian and mammalian skinned fibres. Both actions depend on pCa, such that a shift in pCa can alter their effect from blocking to releasing Ca2+. In skinned fibres with Ca2+-loaded SR, tetracaine (1 mM) produced a tonic contraction with a time to half-peak of 15-20 s and a magnitude reaching 80% of maximum force. Ca2+ release by tetracaine or procaine occurred at pCa less than or equal to 6.5 and was not blocked by Ruthenium Red (RR) (25 mM). This action of tetracaine was attributed to SR Ca2+ release rather than to a displacement of bound Ca2+ because fibres lacking a functional SR due to pre-treatment with quercetin (100 mM), A 23187 (100 micrograms ml-1) or Triton X-100 (1%) did not contract after additions of tetracaine. Lower concentrations of tetracaine (0.5 mM) and procaine (less than or equal to 10 mM) blocked contractions due to caffeine (at pCa greater than or equal to 6.73), sulphydryl oxidizing agents, or Ca2+-induced Ca2+ release (CICR). The inhibition of CICR as a function of pCa was difficult to measure quantitatively since lowering pCa to elicit CICR twitches was sufficient to initiate tetracaine-induced tonic contractions. Experiments with isolated SR vesicles showed that 1 mM tetracaine inhibited CICR, over a wide range of pCa but 3-5 mM tetracaine induced rapid Ca2+ release. The opposite effects of tetracaine and procaine depend mostly on their concentration in SR vesicles and/or pCa in skinned fibres. Blockade of release seems to occur via the CICR pathway, and induction of release through an increase in SR membrane permeability.