Caffeine-induced calcium release from isolated sarcoplasmic reticulum of rabbit skeletal muscle

Caffeine prevents kidney stone formation by translocation of apical surface annexin A1 crystal-binding protein into cytoplasm: In vitro evidence

Recent large 3 cohorts have shown that caffeinated beverage consumption was associated with lower risk of kidney stone disease. However, its protective mechanisms remained unknown and had not been previously investigated. We thus evaluated protective effects of caffeine (1 μM–10 mM) on calcium oxalate monohydrate (COM) kidney stone formation, using crystallization, crystal growth, cell-crystal adhesion, Western blotting, and immunofluorescence assays. The results showed that caffeine reduced crystal number but, on the other hand, increased crystal size, resulting in unchanged crystal mass, consistent with crystal growth that was not affected by caffeine. However, caffeine significantly decreased crystal-binding capacity of MDCK renal tubular cells in a dose-dependent manner. Western blotting and immunofluorescence study of COM crystal-binding proteins revealed significantly decreased level of annexin A1 on apical surface and its translocation into cytoplasm of the caffeine-treated cells, but no significant changes in other COM crystal-binding proteins (annexin A2, α-enolase, HSP70, and HSP90) were observed. Moreover, caffeine decreased intracellular [Ca²⁺] but increased [Ca²⁺] secretory index. Taken together, our findings showed an in vitro evidence of the protective mechanism of caffeine against kidney stone formation via translocation of annexin A1 from apical surface into cytoplasm to reduce the crystal-binding capacity of renal tubular epithelial cells.

Mild stress of caffeine increased mtDNA content in skeletal muscle cells: the interplay between Ca2+ transients and nitric oxide

Caffeine increases mitochondrial biogenesis in myotubes by evoking Ca(2+) transients. Nitric oxide (NO) also induces mitochondrial biogenesis in skeletal muscle cells via upregulation of AMP-activated protein kinase (AMPK) activity and PGC-1α. However, the interplay and timing sequence between Ca(2+) transients and NO releases remain unclear. Herein, we tested the hypothesis that caffeine-evoked Ca(2+) transients triggered NO production to increase mtDNA in skeletal muscle cells. Ca(2+) transients were recorded with Fura-2 AM and confocal microscopy; mtDNA staining, mitochondrial membrane potential and NO level were determined using fluorescent probes PicoGreen, tetramethylrhodamine methyl ester (TMRM) and DAF-FM, respectively. In primary cultured myotubes, a subtle and moderate stress of caffeine increased mtDNA exclusively. Mitochondrial membrane potential and mtDNA were increased by 1 mM as well as 5 mM caffeine, whereas 10 mM caffeine did not change the fluorescence intensity of PicoGreen and TMRM. NO level in myocytes increased gradually following the first jump of Ca(2+) transients evoked by caffeine (5 mM) till the end of recording, when Fura-2 indicated that Ca(2+) transients recovered partly and even disappeared. Importantly, nitric oxide synthase (NOS) inhibitor (L-NAME) suppressed caffeine-induced mtDNA biogenesis, whereas NO donor (DETA-NO) increased mtDNA content. These data strongly suggest that caffeine-induced mtDNA biogenesis is dose-sensitive and dependent on a certain level of stress. Further, an increasing level of NO following Ca(2+) transients is required for caffeine-induced mtDNA biogenesis. Additionally, Ca(2+) transients, a usual and first response to caffeine, was either suppressed or attenuated by L-NAME, DETA-NO, AICAR and U0126, suggesting an inability to control [Ca(2+)](i) in these treated cells. There may be an important interplay between NO and Ca(2+) transients in intracellular signaling system involving NOS, AMPK and MEK.

Endoplasmic reticulum-mitochondria crosstalk in NIX-mediated murine cell death

Transcriptional upregulation of the proapoptotic BCL2 family protein NIX limits red blood cell formation and can cause heart failure by inducing cell death, but the requisite molecular events are poorly defined. Here, we show complementary mechanisms for NIX-mediated cell death involving direct and ER/sarcoplasmic reticulum-mediated (ER/SR-mediated) mitochondria disruption. Endogenous cardiac NIX and recombinant NIX localize both to the mitochondria and to the ER/SR. In genetic mouse models, cardiomyocyte ER/SR calcium stores are proportional to the level of expressed NIX. Whereas Nix ablation was protective in a mouse model of apoptotic cardiomyopathy, genetic correction of the decreased SR calcium content of Nix-null mice restored sensitivity to cell death and reestablished cardiomyopathy. Nix mutants specific to ER/SR or mitochondria activated caspases and were equally lethal, but only ER/SR-Nix caused loss of the mitochondrial membrane potential. These results establish a new function for NIX as an integrator of transcriptional and calcium-mediated signals for programmed cell death.

Effects of verapamil and gadolinium on caffeine-induced contractures and calcium fluxes in frog slow skeletal muscle fibers

In this work, we tested whether L-type Ca(2+ )channels are involved in the increase of caffeine-evoked tension in frog slow muscle fibers. Simultaneous net Ca(2+) fluxes and changes in muscle tension were measured in the presence of caffeine. Isometric tension was recorded by a mechanoelectrical transducer, and net fluxes of Ca(2+) were measured noninvasively using ion-selective vibrating microelectrodes. We show that the timing of changes in net fluxes and muscle tension coincided, suggesting interdependence of the two processes. The effects of Ca(2+)channel blockers (verapamil and gadolinium) were explored using 6 mM: caffeine; both significantly reduced the action of caffeine on tension and on calcium fluxes. Both caffeine-evoked Ca(2+) leak and muscle tension were reduced by 75% in the presence of 100 microM: GdCl(3), which also caused a 92% inhibition of net Ca(2+) fluxes in the steady-state condition. Application of 10 microM: verapamil to the bath led to 30% and 52% reductions in the Ca(2+)leak caused by the presence of caffeine for the peak and steady-state values of net Ca(2+) fluxes, respectively. Verapamil (10 microM): caused a 30% reduction in the maximum values of caffeine-evoked muscle tension. Gd(3+)was a more potent inhibitor than verapamil. In conclusion, L-type Ca(2+) channels appear to play the initial role of trigger in the rather complex mechanism of slow fiber contraction, the latter process being mediated by both positive Ca(2+)-induced Ca(2+ )release and negative (Ca(2+) removal from cytosol) feedback loops.

Regulation of Intracellular Calcium by Bupivacaine Isomers in Cardiac Myocytes from Wistar Rats

In this study we investigated the effects of a racemic mixture of bupivacaine (RS(+/-)bupivacaine) and its isomers (S(-)bupivacaine and R(+)bupivacaine) on the Ca2+ handling by ventricular myocytes from Wistar rats. Single ventricular myocytes were enzymatically isolated and loaded with the fluorescent Ca2+ indicator fura 2-am to estimate intracellular Ca2+ concentration during contraction and relaxation cycles. S(-)bupivacaine (10 muM) significantly increased peak amplitude and the rate of increase of Ca2+ transients in 155% +/- 54% (P < 0.05) and 194% +/- 94% (P < 0.01) of control. However, exposure to R(+)bupivacaine had no effect on either peak amplitude or rate of increase at any concentration tested. Saponin-skinned ventricular fibers were used to investigate the effect of bupivacaine on the intracellular Ca2+ regulation by sarcoplasmic reticulum (SR) and on the Ca2+ sensitivity of contractile system. S(-), R(+), and RS(+/-)bupivacaine induced Ca2+ release from SR (P < 0.01). In SR-disrupted skinned ventricular cells, bupivacaine and its isomers (5 mM) increased the sensitivity of contractile system to Ca(2+). S(-), RS(+/-), and R(+)bupivacaine significantly increased pCa50 from 5.8 +/- 0.1, 5.8 +/- 0.1, and 5.8 +/- 0.1, to 6.1 +/- 0.1 (P < 0.05), 6.0 +/- 0.1 (P < 0.05), and 6.1 +/- 0.1 (P < 0.05). Ca2+ release from SR through RyR2 activation could explain the increase of Ca2+ transients in cardiac cells. Increased intracellular Ca2+ in cardiac myocytes display a stereoselectivity to S(-)bupivacaine.

Effects of disulfiram on excitation-contraction coupling in rat soleus muscle

The aim of this study was to analyze whether disulfiram could affect excitation-contraction coupling in rat slow-twitch ( soleus) muscle.In small bundles of intact fibers, the amplitude and the time constant of relaxation of twitch and potassium contractures were dose-dependently and reversibly reduced by disulfiram at concentrations up to 27 microM. At larger concentrations (up to 67.5 microM) these effects were still present but less pronounced. In the presence of disulfiram (27 microM), the relationship between the amplitude of potassium contractures and membrane potential was shifted to more positive potentials whereas, the steady state inactivation curve was unchanged. These observations suggest that disulfiram has no effect on voltage sensors. In saponin-skinned fibers, the amount of Ca(2+) taken up, estimated by using the amplitude of 10 mM of caffeine contracture, was increased by disulfiram (27 microM). By contrast no significant modification was observed in the sensitivity of the ryanodine receptors to caffeine (contractures generated at 5 mM of caffeine) and in the myofibrillar Ca(2+) sensitivity (Triton X-100 skinned fibers). These results indicate that disulfiram induces a dose-dependent reversible effect on the contractile responses of soleus mammalian skeletal muscle by acting mainly on the sarcoplasmic reticulum Ca(2+)-ATPase activity.

The new compound, LASSBio 294, increases the contractility of intact and saponin-skinned cardiac muscle from Wistar rats

1. A new compound designated as LASSBio 294 (L-294), 3,4-methylenedioxybenzoyl-2-thienylhydrazone, was synthesized as an alternative therapeutic for cardiac dysfunction. 2. L-294 increased in a dose-dependent manner the spontaneous contractions of isolated hearts from Wistar rats with maximal effect (128.0+/-0.7% of control) observed at 25 microM. 3. The positive inotropic effect of L-294 was also observed in electrically stimulated cardiac tissues from Wistar rats. The maximal increment of twitches, at 200 microM, was 163.1+/-18.4% for atrial, 153.5+/-28.5% for papillary and 201.5+/-18.5% for ventricular muscles. 4. In saponin skinned ventricular cells: (a) L-294 present in the period of sarcoplasmic reticulum (SR) loading with Ca(2+) shifted the dose and caffeine-induced contracture curve; (b) L-294 (100 microM) increased 40% the Ca(2+) uptake into SR; (c) L-294 did not significantly alter the sensitivity of contractile proteins to Ca(2+) in SR-disrupted skinned ventricular cells. 5. Retrograde perfusion of the isolated heart from Wistar rats with L-294 (100 microM) did not cause any significant change in rhythm, heart rate (control, 220+/-14.7 b.p.m.; 246+/-24.6 b.p.m. for L-294), PR interval (control, 66.0+/-2.4 ms; 64.0+/-2.3 ms for L-294) or QRS duration (control, 28.8+/-3.4 ms; 32.0+/-2.0 ms for L-294). 6. These results suggest a novel mechanism for a positive cardioinotropic effect through an interaction with the Ca(2+) uptake/release process of the SR. The effect of L-294 could be explained by a pronounced increased accumulation of Ca(2+) into the SR.

Acute but not chronic caffeine impairs functional responses to ischaemia-reperfusion in rat isolated perfused heart

1. The effect of acute (50 micromol/L) and chronic (0.06% in drinking water for 14 days) caffeine on the response to ischaemia-reperfusion was studied in Wistar rat isolated perfused hearts. 2. Neither acute nor chronic caffeine modified normoxic heart rate or left ventricular pressures. However, acute caffeine decreased coronary flow by up to 20%, while chronic caffeine consumption increased coronary flow by approximately 15% and abolished the vasoconstrictor effect of acute caffeine (P<0.05). 3. After 15 min global ischaemia, chronic caffeine treatment did not alter the recovery of left ventricular diastolic pressure (LVDP), end-diastolic pressure (EDP) or heart rate during reperfusion, but did enhance coronary flow rate (P<0.05). Acute caffeine inhibited the recovery of LVDP and elevated postischaemic EDP in both caffeine-naive and chronic caffeine-treated groups. Acute caffeine also significantly inhibited coronary reflow in naive but not chronic caffeine-treated groups and produced a transient tachycardia during reperfusion in hearts from chronic caffeine-treated rats. 4. The incidence of arrhythmias was unaltered by chronic caffeine treatment, but was increased by acute caffeine in both naive and chronic caffeine hearts. 5. In conclusion, chronic caffeine intake alone has no detrimental effects on recovery from ischaemia; however, acute caffeine worsens postischaemic contractile function in hearts from naive and chronic caffeine-treated rats.

The Ca(2+)-ATPase of the sarcoplasmic reticulum in skeletal and cardiac muscle. An overview from the very beginning to more recent prospects

The discovery of the ATP-driven calcium pump in the sarcoplasmic reticulum membranes reaches back to the postwar (World War II) years and would not be possible without the generous support by the American scientific community. It was this community that in pre- and postwar years gave shelter to many European scientists, which in return stimulated scientific development in the United States. These pre- and postwar relations helped to establish the calcium pump as a physiologically relevant mechanism in all kinds of cells. The pump and its counterpart, the calcium release channel, proved to be controlled by various intrinsic mechanisms. Rising hydrogen concentrations as occurring in ischemic muscles switch off pump activity and counteract allosterically caffeine-induced calcium release (CICR). Rising phosphate or the presence of other calcium-precipitating anions, on the other hand, prevents pump inhibition by intraluminal calcium precipitation, which, simultaneously, can increase the quantity of releasable calcium. The inactivation of CICR by removing medium chloride must be considered as a hint of additional mechanisms by which calcium-dependent activity regulation can be modified.

Different effects of halothane, isoflurane and sevoflurane on sarcoplasmic reticulum of vascular smooth muscle in dog mesenteric artery

BACKGROUND

The direct effect of halothane on vascular smooth muscle is mediated in part via its effects on the sarcoplasmic reticulum (SR). Little information is available concerning the effects of other volatile anesthetics including isoflurane and sevoflurane, whose vascular effects differ from those of halothane. The aim of the present study was to compare the effects of halothane, isoflurane and sevoflurane on the SR by testing the contraction induced by caffeine in vascular smooth muscle.

METHODS

Rings without endothelium from isolated canine mesenteric artery were mounted in physiological saline solution (PSS) for isometric tension recording. After complete depletion of Ca2+ from the SR by adding 35 mM caffeine, the rings were exposed to normal Ca2+ containing PSS (Ca2+ loading), to Ca(2+)-free PSS for 10 min, and then to 15 mM caffeine to induce contraction. Anesthetics were administered during Ca2+ loading, the Ca(2+)-free phase and simultaneously with caffeine administration.

RESULTS

Halothane (0.5-2%) attenuated the caffeine-induced contraction of canine mesenteric artery when administered during Ca2+ loading in the SR (P < 0.001), whereas isoflurane and sevoflurane (1-4%) failed to affect the contraction. When given simultaneously with caffeine, halothane (1-2%) potentiated the caffeine-induced contraction (P < 0.05), but isoflurane and sevoflurane had no effect. When given before caffeine administration, halothane (0.5-2%), isoflurane (2-4%) and sevoflurane (4%) all potentiated the caffeine-induced contraction (P < 0.05).

CONCLUSION

It has been shown that halothane not only potentiates caffeine-induced Ca2+ release from the SR, but also induces contraction by releasing Ca2+ from the SR. We conclude that halothane decreases Ca2+ accumulation in the SR while exerting facilitative and additive effects on caffeine-induced Ca2+ release from the SR when applied before caffeine administration and simultaneously with caffeine, respectively, whereas isoflurane and sevoflurane lack both the ability to decrease Ca2+ accumulation and an additive effect on caffeine-induced Ca2+ release from the SR, but are able to facilitate Ca2+ release by caffeine.

An estimate of the participation of the sarcoplasmic reticulum in the intracellular Ca2+ regulation in adult and newborn ferret hearts

The aim of the present study was to estimate the participation of the sarcoplasmic reticulum in the Ca2+ regulation of the contraction of newborn ferret heart. Cyclopiazonic acid has been used to block the sarcoplasmic reticulum Ca2+ pump in adult and newborn (1 month and 5-6 day old) ferret ventricles of intact and saponin-treated preparations. Cyclopiazonic acid induced a decrease of the amplitude of the caffeine contractures generated in saponin skinned fibers. The sensitivity of the sarcoplasmic reticulum Ca2+ uptake to cyclopiazonic acid was similar in adult and newborn hearts. In intact preparations, cyclopiazonic acid (1-20 microM) induced a negative inotropic effect on the twitch with a prolongation in its kinetics. The maximal decrease in the amplitude of the twitch was larger in adult (92.4%) than in 1 month old (86.5%) and 5-6 day newborns (72.5%). Contrary to other species, where the sarcoplasmic reticulum Ca2+ pump is not functional in neonatal heart, it is proposed that ferret myocardium shows an early maturation of sarcoplasmic reticulum function.

Caffeine-evoked contractures in single slow (tonic) muscle fibres of the frog (Rana temporaria and R. esculenta)

Single slow (tonic) muscle fibres were dissected from cruralis muscles of Rana temporaria and R. esculenta. Increasing concentrations of caffeine were applied in Ringer solution, and contractures were measured isometrically. Sigmoid caffeine concentration-response curves were obtained, the threshold value being near 1.2 mmol/l, and maximum contractures being obtained with 10 to 20 mmol/l concentrations of caffeine. Contracture solutions were modified by varying the Ca2+ concentration or by replacing Ca2+ with 1.8 mmol/l Mg2+, Ni2+, Co2+ or with 0.1-5.0 mmol/l La3+. The effects of low pH (5.3), K+ (6,10 and 95 mmol/l), adenosine (10 mmol/l) and gallopamil (D600; 30 micromol/l) were examined too. The caffeine threshold was lowered by Mg2+, K+, 0 .1 mmol/l La3+ and D600, while all other substances including 0.5-5.0 mmol/l La3+ increased it. The amplitude of contractures evoked by high caffeine concentrations was unaffected. Caffeine (1-40 mmol/l) was also pressure injected into slow fibres. The composition of the solution was modified in a number of ways, but a contractile response was not observed or measured. Extracellular application of caffeine from the same pipettes evoked local contractures. Similar injection experiments in twitch fibres revealed the same results. These observations suggest that an extracellular binding site seems to be involved in the initiation of caffeine-evoked contractures in intact frog muscle fibres. Possible reasons for the ineffectiveness of intracellular caffeine are discussed.

Total and sarcoplasmic reticulum calcium contents of skinned fibres from rat skeletal muscle

1. The Ca2+ content of single mammalian skeletal muscle fibres was determined using a novel technique. Mechanically skinned fibres were equilibrated with varying amounts of the Ca2+ buffer BAPTA and were then lysed in a detergent-paraffin oil emulsion. The subsequent myofilament force response was used to estimate the additional amount of Ca2+ bound to BAPTA following lysis of intracellular membranes. 2. The total endogenous Ca2+ content (corrected for endogenous Ca2+ buffering) of fast-twitch (FT) and slow-twitch (ST) fibres at a myoplasmic pCa (-log[Ca2+]) of 7.15 was 1.32 +/- 0.02 and 1.35 +/- 0.08 mM per fibre volume, respectively. The sarcoplasmic reticulum (SR) component of these estimates was calculated as 1.01 and 1.14 mM, respectively, which normalized to SR volume corresponds to resting SR Ca2+ contents of 11 and 21 mM, respectively. 3. Equilibration of 'resting' fibres with low myoplasmic [Ca2+] (pCa 7.67-9.00) elicited a time-dependent decrease in Ca2+ content in both fibre types. Equilibration of resting fibres with higher myoplasmic [Ca2+] (pCa 5.96-6.32) had no effect on the Ca2+ content of ST fibres but increased the Ca2+ content of FT fibres. The maximum steady-state total Ca2+ content (3.85 mM) was achieved in FT fibres after 3 min equilibration at pCa 5.96. Equilibration at higher myoplasmic [Ca2+] was less effective, probably due to Ca2(+)-induced Ca2+ release from the SR. 4. Exposure of fibres to either caffeine (30 mM, pCa approximately 8, 2 min) or low myoplasmic [Mg2+] (0.05 mM, pCa approximately 9, 1 min) released approximately 85% of the resting SR Ca2+ content. The ability of caffeine to release SR Ca2+ was dependent on the myoplasmic Ca2+ buffering conditions. 5. The results demonstrate that the SR of ST fibres is saturated with Ca2+ at resting myoplasmic [Ca2+] while the SR of FT fibres is only about one-third saturated with Ca2+ under equivalent conditions. These differences suggest that the rate of SR Ca2+ uptake in FT fibres is predominantly controlled by myoplasmic [Ca2+] while that of ST fibres is more likely to be limited by the [Ca2+] within the SR lumen.

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.

Association of sorcin with the cardiac ryanodine receptor

Sorcin is a 22-kDa calcium-binding protein initially identified in multidrug-resistant cells; however, its patterns of expression and function in normal tissues are unknown. Here we demonstrate that sorcin is widely distributed in rodent tissues, including the heart, where it was localized by immunoelectron microscopy to the sarcoplasmic reticulum. A > 500-kDa protein band immunoprecipitated from cardiac myocytes by sorcin antiserum was indistinguishable in size on gels from the 565-kDa ryanodine receptor/calcium release channel recognized by ryanodine receptor-specific antibody. Association of sorcin with a ryanodine receptor complex was confirmed by complementary co-immunoprecipitations of sorcin with the receptor antibody. Forced expression of sorcin in ryanodine receptor-negative Chinese hamster lung fibroblasts resulted in accumulation of the predicted 22-kDa protein as well as the unexpected appearance of ryanodine receptor protein. In contrast to the parental host fibroblasts, sorcin transfectants displayed a rapid and transient rise in intracellular calcium in response to caffeine, suggesting organization of the accumulated ryanodine receptor protein into functional calcium release channels. These data demonstrate an interaction between sorcin and the ryanodine receptor and suggest a role for sorcin in modulation of calcium release channel activity, perhaps by stabilizing the channel protein.

Modulation of the ryanodine receptor sarcoplasmic reticular Ca2+ channel in skinned fibers of fast- and slow-twitch skeletal muscles from rabbits

This study was performed to compare skinned fibers from rabbit adductor magnus (AM) and soleus (SL) muscles with regard to the influence of caffeine, Ca2+ and Mg2+ on the depressive effects of ryanodine (RYA) on the caffeine-induced tension transients. Single skinned fibers were immersed in solutions to load Ca2+ into, and release Ca2+ from the SR (a load-release cycle). Three cycles were sequentially performed in each skinned fiber: (1) a control (no RYA), (2) a conditioning period in which activation was carried out in the presence of ryanodine plus various concentrations of the modulators, i.e. caffeine, Ca2+ or Mg2+, and (3) a test (no RYA) which monitored the release activity retained after the conditioning cycle. The depressive effect of RYA was found to be a function of [ryanodine], [caffeine], or [Ca2+], and an inverse function of [Mg2+], where [] denotes concentration. The half-maximal effects of RYA in AM (5 microM RYA) and SL (10 microM RYA), respectively, occurred at a pCa50 of 5.32 versus 5.43 without caffeine, or pCa50 of 7.24 versus 6.88 and pMg50 of 3.29 versus 3.61 with 25 mM caffeine, at a [caffeine] of 4.96 versus 7.29 mM, and at a [ryanodine] of 31.0 versus 101.6 microM. Thus, the RYA depression in skinned muscle fibers is modulated by caffeine, Ca2+, and Mg2+ in both muscle types, and AM is at least two- to fourfold more sensitive than SL.

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.

Ionic strength dependence of calcium, adenine nucleotide, magnesium, and caffeine actions on ryanodine receptors in rat brain

[3H]Ryanodine binding studies of ryanodine receptors in brain membrane preparations typically require the presence of high salt concentrations in assay incubations to yield optimal levels of binding. Here, radioligand binding measurements on rat cerebral cortical tissues were conducted under high (1.0 M KCl) and low (200 mM KCl) salt buffer conditions to determine the effects of ionic strength on receptor binding properties as well as on modulation of ligand binding by Ca2+, Mg2+, beta, gamma-methylene-adenosine 5'-triphosphate (AMP-PCP), and caffeine. In 1.0 M KCl buffer, labeled titration/equilibrium analyses yielded two classes of binding sites with apparent KD (nM) and Bmax (fmol/mg of protein) values of 2.4 and 34, respectively, for the high-affinity site and 19.9 and 157, respectively, for the low-affinity site. Unlabeled titration/equilibrium measurements gave a single high-affinity site with a KD value of 1.9 nM and a Bmax value of 95 fmol/mg of protein. The apparent KD value derived from association and dissociation studies was 20 pM. Equilibrium binding was activated by Ca2+ (KD/Ca2+ = 14 nM), inhibited by Mg2+ (IC50 = 5.0 mM), and unaffected by AMP-PCP or caffeine. In 200 mM KCl buffer conditions, labeled titration analyses gave only a single site with a KD value similar to and a Bmax value 1.8-fold greater than those obtained for the low-affinity site in 1.0 M KCl buffer. In unlabeled titration measurements, the KD value was fivefold lower, whereas the Bmax value was unaffected. The KD value derived from association and dissociation analysis was 2.4-fold greater in 200 mM KCl compared with 1.0 M KCl buffer conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Caffeine-induced calcium oscillations in heavy-sarcoplasmic-reticulum vesicles from rabbit skeletal muscle

Heavy-sarcoplasmic-reticulum vesicles from rabbit skeletal muscle show not only caffeine-induced calcium release in a medium allowing active calcium loading, but also oscillations in calcium concentration under appropriate conditions. The xanthine derivatives 7-isobutyl-1-methylxanthine and theophylline also induce oscillations under the same conditions. Calcium-releasing substances with other chemical structures such as adenosine nucleotides or calmodulin antagonists do not induce this effect. With the help of specific inhibitors such as ruthenium red, neomycin or magnesium it was demonstrated that the oscillation mechanism involves the ryanodine receptor/calcium channel. When ATP was substituted by GTP or ITP no oscillations occurred after caffeine application. The subsequent application of ATP, but not of adenosine 5'-[gamma-thio]triphosphate or adenosine 5'-[beta,gamma-methylene]triphosphate activated the oscillating mechanism, showing ATP to be an essential component of the oscillating system. We investigated the influence of the experimental conditions by altering the caffeine and ATP concentrations, calcium load, pH and ionic strength amongst other parameters. Potassium and anion channels are not involved in calcium oscillations of heavy sarcoplasmic reticulum, nor are the oscillations dependent on membrane potential.

Modulation of sarcoplasmic reticulum Ca(2+)-release channels by caffeine, Ca2+, and Mg2+ in skinned myocardial fibers of fetal and adult rats

Ryanodine causes depression of the caffeine-induced tension transient (ryanodine depression) in skinned muscle fibers, because it blocks the sarcoplasmic reticulum (SR) Ca(2+)-release channels [Su, J. Y. (1988) Pflügers Arch 411:132-136, 371-377; (1992) Pflügers Arch 421:1-6]. This study was performed to examine the sensitivity of SR Ca(2+)-release channels to ryanodine in fetal compared to adult myocardium and to investigate the influence of Ca2+, caffeine, and Mg2+ on ryanodine depression in skinned fibers. Ryanodine (0.3 nM-1 microM) caused a dose-dependent depression in skinned myocardial fibers of the rat, and the fetal fibers (IC50 approximately 74 nM) were 26-fold less sensitive than those of the adult (IC50 approximately 2.9 nM). The depression induced by 0.1 microM or 1 microM ryanodine was a function of [caffeine], or [Ca2+] (pCa < 6.0), which was potentiated by caffeine, and an inverse function of [Mg2+]. At pCa > 8.0 plus 25 mM caffeine, a 20% ryanodine depression was observed in both the fetal and adult fibers, indicating independence from Ca2+. Ryanodine depression in skinned fibers of the fetus was less affected than that seen in the adult by pCai, [caffeine]i, or 25 mM caffeine plus pCai or plus pMgi (IC50 approximately pCa 4.5 versus 5.1; caffeine 12.7 mM versus 2 mM; pCa 6.7 versus 7.3; and pMg 3.9 versus 3.3 respectively). The results show that the SR Ca(2+)-release channel in both fetal and adult myocardium is modulated by Ca2+, caffeine, and Mg2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of serine 2843 in ryanodine receptor-calcium release channel of skeletal muscle by cAMP-, cGMP- and CaM-dependent protein kinase

The aim of the present study was to determine the phosphorylation of the purified ryanodine receptor-calcium release channel (RyR) of rabbit skeletal muscle sarcoplasmic reticulum by the cAMP-dependent protein kinase (PK-A), cGMP-dependent protein kinase (PK-G) and Ca(2+)-, CaM-dependent protein kinase (PK-CaM) and the localization of phosphorylation sites. Phosphorylation was highest with PK-A (about 0.9 mol phosphate/mol receptor subunit), between one-half to two-thirds with PK-G and between one-third and more than two-thirds with PK-CaM. Phosphoamino acid analysis revealed solely labeled phosphoserine with PK-A and PK-G and phosphoserine and phosphothreonine with PK-CaM. Reverse-phase high-performance liquid chromatography (HPLC) of cyanogen bromide/trypsin digests of the phosphorylated RyR (purified by gel permeation HPLC) and two-dimensional peptide maps revealed one major phosphopeptide by PK-A and PK-G phosphorylation and several labeled peaks by PK-CaM phosphorylation. Automated Edman sequence analysis of the major phosphopeptide obtained from PK-A and PK-G phosphorylation and one phosphopeptide obtained from PK-CaM phosphorylation yielded the sequence KISQTAQTYDPR (residues 2841-2852) with serine 2843 as phosphorylation site (corresponding to the consensus sequence RKIS), demonstrating that all three protein kinases phosphorylate the same serine residue in the center of the receptor subunit, a region proposed to contain the modulator binding sites of the calcium release channel.

Caffeine and exercise performance. An update

Three principal cellular mechanisms have been proposed to explain the ergogenic potential of caffeine during exercise: (a) increased myofilament affinity for calcium and/or increased release of calcium from the sarcoplasmic reticulum in skeletal muscle; (b) cellular actions caused by accumulation of cyclic-3',5'-adenosine monophosphate (cAMP) in various tissues including skeletal muscle and adipocytes; and (c) cellular actions mediated by competitive inhibition of adenosine receptors in the central nervous system and somatic cells. The relative importance of each of the above mechanisms in explaining in vivo physiological effects of caffeine during exercise continues to be debated. However, growing evidence suggests that inhibition of adenosine receptors is one of the most important, if not the most important, mechanism to explain the physiological effects of caffeine at nontoxic plasma concentrations. Numerous animal studies using high caffeine doses have reported increased force development in isolated skeletal muscle in both in vitro and in situ preparations. In contrast, in vivo human studies have not consistently shown caffeine to enhance muscular performance during high intensity, short term exercise. Further, recent evidence supports previous work that shows caffeine does not improve performance during short term incremental exercise. Although controversy exists, the major part of published evidence evaluating performance supports the notion that caffeine is ergogenic during prolonged (> 30 min), moderate intensity (approximately 75 to 80% VO2max) exercise. The mechanism to explain these findings may be linked to a caffeine-mediated glycogen sparing effect secondary to an increased rate of lipolysis.

Calcium transients in isolated cardiac myocytes are altered by 1,1,1-trichloroethane

1,1,1-Trichloroethane is a widely used solvent that is annually linked to several cases of sudden death following accidental exposure or abuse. Sudden death is believed to be due to ventricular fibrillation or myocardial depression. The purpose of this study was to investigate the mechanism of myocardial depression by assessing the influence of 1,1,1-trichloroethane on intracellular Ca transients in single neonatal rat ventricular myocytes using spectrofluorometric analysis of fura-2-Ca binding. Cells were exposed to 1,1,1-trichloroethane in Hanks' balanced salt solution aliquoted as a 0.2% DMSO solution by a single pass suffusion in an environmentally controlled chamber. 1,1,1-Trichloroethane (0.25 mM-8 mM) reduced the height of electrically (1 Hz, 60 V, 10 ms) induced Ca transients concentration dependently and reversibly to a maximum of about 50% with no effect on diastolic Ca concentration. Video motion analysis revealed an inhibition of contractility in the same concentration range. 1,1,1-Trichloroethane inhibited cytosolic Ca increase in response to KCl-induced (90 mM) depolarizations and further decreased the limited Ca transients in ryanodine (1 microM) pretreated myocytes. Increased external Ca (5 mM) antagonized the effect of 0.5 mM 1,1,1-trichloroethane on the Ca transients. 1,1,1-Trichloroethane reduced the caffeine (10 mM) releasable Ca pool in myocytes. These results show that 1,1,1-trichloroethane inhibits Ca mobilization during excitation-contraction coupling in ventricular myocytes. An inhibitory action on the influx of extracellular Ca as well as on sarcoplasmic reticulum Ca release and sequestration is likely to be responsible for this action.

Adenosine(5')hexaphospho(5')adenosine stimulation of a Ca(2+)-induced Ca(2+)-release channel from skeletal muscle sarcoplasmic reticulum

Stimulation of a Ca(2+)-induced Ca(2+)-release channel from skeletal muscle sarcoplasmic reticulum by various adenosine(5')oligophospho(5')adenosines (ApnA, n = 2-6) by a rapid quenching technique using radioactive calcium was studied. Ap4A, Ap5A and Ap6A, as well as adenosine 5'-[beta, gamma-methylene]triphosphate (AdoPP [CH2]P), a non-hydrolyzable ATP analogue, stimulated the Ca(2+)-release channel, whereas Ap2A and Ap3A had no effect. At a concentration of 0.5 mM, the order of stimulation was AdoPP[CH2]P less than Ap4A less than Ap5A much less than Ap6A. As well as having the highest affinity (0.44 mM for half-maximal stimulation), Ap6A showed an extraordinarily high Hill coefficient of 3.3 (1.9 for AdoPP[CH2]P, 2.1 for Ap5A). The stimulating effect of Ap6A was reversible, yet its dissociation proceeded very slowly. Stimulation of Ca2+ release by Ap6A was counteracted by Mg2+ and ruthenium red. A 2',3'-dialdehyde derivative of Ap6A, which is a chemical probe for amino groups, stimulated irreversibly the Ca(2+)-release channel and modified some high-molecular-mass sarcoplasmic reticulum proteins, possibly including the channel protein. Our data suggest that Ap6A stimulates the Ca2+ channel by binding to the activation site of the channel subunit and simultaneously preventing the spontaneous decay of the Ca2+ channel by keeping together two of the four channel subunits by bridging them with its two adenosine groups.

Influence of caffeine, Ca2+, and Mg2+ on ryanodine depression of the tension transient in skinned myocardial fibers of the rabbit

Ryanodine, a blocker for Ca(2+)-release channels of the sarcoplasmic reticulum (SR Ca(2+)-release channels), induces depression of myocardial contraction in isolated intact muscle, which is consistent with depression of the caffeine-induced tension transient in skinned muscle fibers. In isolated SR, ryanodine binds to a specific receptor with high affinity, and this binding is enhanced by caffeine and increasing Ca2+ and decreased by increasing Mg2+. The aim of this study was to test the hypothesis that depression of myocardial contraction is mediated by changes in ryanodine-receptor binding properties. Accordingly, factors (caffeine, Ca2+, and Mg2+) affecting ryanodine-receptor binding properties in the isolated SR membrane were studied in skinned myocardial fibers from adult rabbits. The depression of the caffeine-induced tension transient by ryanodine (ryanodine depression) influenced by these three factors was measured. In a dose-dependent manner, increasing caffeine or Ca2+ concentrations enhanced the ryanodine depression. The concentrations for 50% ryanodine depression (IC50) approximated 7 mM for caffeine, and pCa 5.25 for Ca2+. When 1 microM ryanodine and 25 mM caffeine were combined, ryanodine depression was independent of Ca2+ at low Ca2+ concentrations (20%-30% at pCa greater than 8 and 7.5) and was a direct function of Ca2+ at higher concentrations (pCa 7.5-6.0 with IC50 approx. pCa 6.75). In contrast, increasing Mg2+ reduced the ryanodine depression with IC50 approximately equal to pMg 3.3. In conclusion, the caffeine- or Ca(2+)-enhanced, and Mg(2+)-reduced ryanodine depression observed in this study is consistent with known ryanodine-receptor binding properties.

Contractile properties and Ca2+ release activity of the sarcoplasmic reticulum in dilated cardiomyopathy

BACKGROUND

We performed a comparative study on Ca2+ release activity of the sarcoplasmic reticulum and calcium sensitivity of contractile apparatus of chemically skinned myocardial fibers obtained from four nonfailing human hearts and 13 excised hearts from patients with idiopathic dilated cardiomyopathy.

METHODS AND RESULTS

Ca2+ sensitivity of contractile apparatus was studied by following the isometric tension developed by chemically skinned myocardial fibers challenged with solutions of decreasing pCa. Ca2+ release from sarcoplasmic reticulum was monitored indirectly by measurement of the isometric tension developed by skinned fibers challenged with caffeine. We observed no significant difference of Ca2+ sensitivity and cooperativity between normal myocardium (pCa50 = 6.00 +/- 0.05; Hill coefficient, nHill = 2.07 +/- 0.10) and dilated cardiomyopathy (pCa50 = 6.03 +/- 0.07; nHill = 2.72 +/- 0.30) when the fibers were stretched to 130% of the resting length. We also found that both in normal myocardium and dilated cardiomyopathy, stretching to 150% of the resting length increased the Ca2+ sensitivity of the contractile system; pCa50 = 6.21 +/- 0.01 and 6.13 +/- 0.04 in normal and dilated cardiomyopathy, respectively, whereas in dilated cardiomyopathy there was a decrease of Hill coefficient with stretching that was not observed in the control group. The caffeine threshold in idiopathic dilated cardiomyopathy was markedly increased compared with the control group, 1.94 +/- 0.27 mmol/l and 0.29 +/- 0.04 mmol/l caffeine, respectively, whereas there were no significant differences in the extent and rate of caffeine-induced Ca2+ release.

CONCLUSIONS

These results indicate that in idiopathic dilated cardiomyopathy there is no alteration of contractile and regulatory proteins; on the contrary, the gating mechanism of the Ca2+ release channel of sarcoplasmic reticulum is abnormal, suggesting a possible involvement of the excitation-contraction coupling in the pathogenesis of this disease. It should also be taken into account, however, that the increased caffeine threshold in dilated cardiomyopathy would be a result of the enhanced resistance to the skinning procedure secondary to the modification of lipid species and/or content in sarcoplasmic reticulum membrane.

2,2,4-Trimethylpentane induces Ca2+ release from the sarcoplasmic reticulum terminal cisterns

Using quin2, the effects of aliphatic hydrocarbons on the system of Ca(2+)-induced Ca2+ release in isolated membranes of rabbit skeletal muscle terminal cisterns have been studied. The hydrocarbons were inserted into the membranes by means of hydrocarbon-containing liposomes. 2,2,4-Trimethylpentane (isooctane) caused a rapid release of 70-75% of Ca2+ taken up by the terminal cistern vesicles during the Ca(2+)-pump operation. This effect was inhibited by the caffeine-induced Ca2+ release blockers--Mg2+, ruthenium red and tetracaine. The same was observed with a decrease in the concentration of ATP that is known to activate the terminal cistern Ca2+ channels. The effect of 2,2,4-trimethylpentane on the longitudinal cistern fractions practically devoid of Ca(2+)-channels was insignificant. Heptane, hexane and octane caused a slow release of 5-10% of the accumulated Ca2+ from the terminal cistern vesicles; no such effect was induced by decane.

Contraction threshold and the "hump" component of charge movement in frog skeletal muscle

The delayed component of intramembranous charge movement (hump, I gamma) was studied around the contraction threshold in cut skeletal muscle fibers of the frog (Rana esculenta) in a single Vaseline-gap voltage clamp. Charges (Q) were computed as 50-ms integrals of the ON (QON) and OFF (QOFF) of the asymmetric currents after subtracting a baseline. The hump appeared in parallel with an excess of QON over QOFF by approximately 2.5 nC/mu F. Caffeine (0.75 mM) not only shifted the contraction threshold but moved both the hump and the difference between the ON and OFF charges to more negative membrane potentials. When using 10-mV voltage steps on top of different prepulse levels, the delayed component, if present, was more readily observable. The voltage dependences of the ON and OFF charges measured with these pulses were clearly different: QON had a maximum at or slightly above the contraction threshold, while QOFF increased monotonically in the voltage range examined. Caffeine (0.75 mM) shifted this voltage dependence of QON toward more negative membrane potentials, while that of QOFF was hardly influenced. These results show that the delayed component of intramembranous charge movement either is much slower during the OFF than during the ON, or returns to the OFF position during the pulse. Tetracaine (25 microM) had similar effects on the charge movement currents, shifting the voltage dependence on the ON charge in parallel with the contraction threshold, but to more positive membrane potentials, and leaving QOFF essentially unchanged. The direct difference between the charge movement measured in the presence of caffeine and in control solution was either biphasic or resembled the component isolated by tetracaine, suggesting a common site of caffeine and tetracaine action. The results can be understood if the released Ca plays a direct role in the generation of the hump, as proposed in the first paper of this series (Csernoch et al. 1991. J. Gen. Physiol. 97:845-884).

Current recording from sensory cilia of olfactory receptor cells in situ. I. The neuronal response to cyclic nucleotides

The olfactory mucosa of the frog was isolated, folded (the outer, ciliated side faced outward), and separately superfused with Ringers solution on each side. A small number of sensory cilia (one to three) were pulled into the orifice of a patch pipette and current was recorded from them. Fast bipolar current transients, indicating the generation of action potentials by the receptor cells, were transmitted to the pipette, mainly through the ciliary capacitance. Basal activity was near 1.5 spikes s-1. Exposure of apical membrane areas outside of the pipette to permeant analogues of cyclic nucleotides, to forskolin, and to phosphodiesterase inhibitors resulted in a dose-dependent acceleration of spike rate of all cells investigated. Values of 10-20 s-1 were reached. These findings lend further support to the notion that cyclic nucleotides act as second messengers, which cause graded membrane depolarization and thereby a graded increase in spike rate. The stationary spike rate induced by forskolin was very regular, while phosphodiesterase inhibitors caused (in the same cell) an irregular pattern of bursts of spikes. The response of spike rate was phasic-tonic in the case of strong stimulation, even when elicited by inhibitors of phosphodiesterase or by analogues of cyclic nucleotides that are not broken down by the enzyme. Thus, one of the mechanisms contributing to desensitization appears to operate at the level of the nucleotide-induced ciliary conductance. However, desensitization at this level was slow and only partial, in contrast to results obtained with isolated, voltage-clamped receptor cells.

Activation and inhibition of the calcium-release channel of isolated skeletal muscle heavy sarcoplasmic reticulum. Models of the calcium-release channel

Calcium-independent calcium efflux from heavy sarcoplasmic reticulum (HSR) of skeletal muscle was found to be biphasic, with half-times of 2-6 s and 200-400 s for the first and second phase, respectively. Calcium-, AMP- and caffeine-induced calcium efflux was triphasic, with half-times of 0.05-0.2 s, 1-5 s and 100-400 s for the first, second and third phases, respectively. This very fast first phase is certainly due to calcium efflux via the calcium-release channel of HSR vesicles. Both ruthenium red and neomycin inhibited the first phase of the calcium-independent calcium efflux and the first phase of the calcium-, AMP- or caffeine-induced calcium efflux completely, whilst the second phase was fully inhibited by ruthenium red only and partially inhibited by neomycin at high concentrations, indicating that the second phase of calcium release also occurs via the calcium-release channel. Various models for calcium efflux through the release channel have been tested by simulation. Activation and inhibition of the channel-mediated calcium efflux from HSR cannot be explained by two states of the calcium-release channel (open or closed), but requires the existence of at least three states. A channel with one open state and two closed states, resulting in a rapid inactivation, is the most simple model compatible with the experimental data. According to this model, activation is assumed to reduce inactivation of the channel, whilst inhibition assumes an acceleration of channel inactivation. This mechanism most likely applies to neomycin. An additional open-blocked state has to be assumed for inhibition by ruthenium red.

Blockage of a pump-related calcium-efflux pathway in light sarcoplasmic reticulum vesicles by Mops

Mops, used as a proton buffer, specifically enhances the accumulation of calcium or strontium by light sarcoplasmic reticulum vesicles driven by ATP or dinitrophenylphosphate as energy-yielding substrates when calcium-precipitating agents are absent. The enhancement of ion uptake by Mops is much greater for strontium than for calcium and is further increased when potassium is replaced by sodium as the dominant monovalent cation. Mops affects neither the activity of the calcium- or strontium-activated transport enzyme nor the active accumulation of calcium in the presence of oxalate, i.e. when the pump runs unidirectionally forward. Passive calcium and strontium efflux rates of approximately 40-50 nmol.mg-1.min-1 are considerably reduced when histidine/glycerophosphate or Tris/maleate are exchanged for Mops. The observed passive efflux rates and their modulation by Mops are too small, in relation to the rate of ion influx, to account for either the relatively small calcium and strontium load in the absence of precipitating agents or for its modulation by Mops. The results imply that the pump itself mediates ion efflux dependent on pump activity and the different degree of saturation of lumenal ion-binding sites by calcium and strontium, as well as their susceptibility to Mops.

pH modulates conducting and gating behaviour of single calcium release channels

Intracellular pH changes affect excitation-contraction coupling in skeletal and cardiac muscles. However the proton implication in modulating the sarcoplasmic reticulum Ca2+ release channel activity has never been visualized at single channel level. A large conducting Ca2+ release pathway has previously been characterized after incorporation of skeletal and cardiac sarcoplasmic reticulum vesicles into planar lipid bilayers. This channel has been activated by micromolar and millimolar concentrations of Ca2+ and ATP, respectively. The pH was independently varied on each side of the channels. Acidification of the cis-chamber (7.4 to 6.6) induced a modification of the gating behaviour, resulting in a decrease of the open probability. This effect was completely reversible. On the other hand, acidification of the trans-chamber (7.4 to 6.8) induced a reduction of the unitary conductance of the sarcoplasmic reticulum Ca2+ release channel.

Actions of caffeine on fast- and slow-twitch muscles of the rat

1. The effects of caffeine (0.2-20 mmol l-1) have been examined on calcium transients (measured with aequorin) and isometric force in intact bundles of fibres from soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscles of the rat. 2. At 25 degrees C, threshold caffeine concentration for an observable increase in resting [Ca2+]i was 0.2 and 1.0 mmol l-1 for soleus and EDL muscles respectively. Increases in resting force were first detectable at about 0.5 mmol l-1 caffeine for soleus muscles and 5.0 mmol l-1 caffeine for EDL muscles and occurred in the range 0.2-0.4 mumol l-1 [Ca2+]i for soleus and 0.7-0.9 mumol l-1 for EDL. 3. Caffeine potentiated the twitch responses of soleus and EDL in a dose-related manner. The soleus was more sensitive in this respect, with 50% potentiation occurring at 1 mmol l-1 caffeine compared with 3.5 mmol l-1 for the EDL. Concentrations of caffeine below 2 mmol l-1 potentiated Ca2+ transients associated with twitches in both soleus and EDL muscles with no apparent change in the decay rate constant. 4. High concentrations of caffeine (greater than 2 mmol l-1) further potentiated peak Ca2+ in the EDL but depressed it in the soleus. The rate of decay of the Ca2+ transient in high caffeine was significantly prolonged in the soleus but remained unaffected in the EDL. 5. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) had little effect on force or [Ca2+]i at concentrations known to significantly increase intracellular cyclic AMP levels. 6. The Ca2+ transient during fused tetani was characterized by an initial peak, a decline to a plateau level and sometimes a gradual rise towards the end of the stimulus train. Peak [Ca2+]i during normal tetani ranged between 1.1 and 2.4 mumol l-1 in the soleus and 1.9 and 4.0 mumol l-1 in the EDL. 7. Caffeine potentiated both force and [Ca2+]i during tetanus. Since the increase of the Ca2+ transient was significantly greater than potentiation of force, it is likely that saturation of myofilaments occurs. The primary effect of caffeine on the Ca2+ transient was an elevation of the plateau phase. 8. Caffeine concentrations below 5 mmol l-1 potentiate twitch and tetanic force in both fast- and slow-twitch mammalian skeletal muscles primarily by increasing both the basal and stimulus-evoked release of Ca2+ from the sarcoplasmic reticulum.

Rat vs. rabbit ventricle: Ca flux and intracellular Na assessed by ion-selective microelectrodes

Trans sarcolemmal Ca movements in rabbit and rat ventricular muscle were compared using extracellular double-barreled Ca-selective microelectrodes. In rabbit ventricle, steady-state twitches were associated with transient extracellular Ca (Cao) depletions, indicative of Ca uptake during the twitch. In contrast, steady-state twitches in rat ventricle were associated with net cellular Ca extrusion. Rest periods in rabbit ventricle lead to a net loss of cell Ca and resumption of stimulation induces a net uptake of Ca by the cells. Conversely, in rat ventricle rest periods lead to cellular Ca gain and resumption of stimulation induces a net Ca loss from the cells. Thus stimulation is associated with net Ca gain in rabbit ventricle and net Ca loss in rat ventricle. These observations provide an explanation for some of the functional differences between rat and rabbit ventricle (e.g., negative force-frequency staircase and rest potentiation in rat vs. positive staircase and rest decay in rabbit). Resting intracellular Na activity (alpha iNa) was 12.7 +/- 0.6 mM in rat and 7.2 +/- 0.5 mM in rabbit ventricle. This alpha iNa in rat ventricle is sufficiently high that Ca entry via Na-Ca exchange is thermodynamically favored at the resting membrane potential. This may explain why rest potentiation is observed in rat ventricle. In contrast, the lower alpha iNa in rabbit ventricle would favor Ca extrusion via Na-Ca exchange at rest (and consequent rest decay). In rat ventricle, the increase of intracellular [Ca] ([Ca]i) associated with contraction, coupled with the short action potential duration, strongly favor Ca extrusion via Na-Ca exchange and explain the observed Cao accumulation observed during twitches in rat. The high plateau of the rabbit ventricular action potential tends to prevent Ca extrusion via Na-Ca exchange during the contraction and explains the Cao depletions observed in rabbit. It is concluded that the higher alpha iNa and shorter action potential duration in rat vs. rabbit ventricle can explain many of the functional differences observed in these tissues.

Ruthenium red effect on mechanical and electrical properties of mammalian skeletal muscle

In this work we studied the effect of ruthenium red (RR) on the mechanical and electrical properties of rat diaphragm bundles in vitro (30 degrees C). Two concentrations of RR were used: 5 and 10 microM. We measured: 1) twitches, tetanus and caffeine contracture; 2) relation between mechanical tension and resting membrane potential (Vm); 3) contraction threshold by visualization of the contraction around the stimulated area of the fiber. The main finding are the following: a) RR potentiates the twitch tension. The tetanic tension is not affected and the time course of the caffeine contracture is shortened in the RR containing solutions; b) the relationship between mechanical tension and resting membrane potential (Vm) is shifted toward more negative values of Vm in RR; c) the mechanical threshold is lowered about 7 mV in the presence of RR; d) the rates of depolarization and repolarization of the action potential are decreased in the test solutions. We suggested that the shift in the mechanical threshold and the prolongation of the action potential are the main factors involved in the potentiating effect of RR. The mechanism by which RR shifts the mechanical threshold is not known.

Activation of the Ca2+ release channel of skeletal muscle sarcoplasmic reticulum by caffeine and related compounds

The caffeine-sensitive Ca2+ release pathway in skeletal muscle was identified and characterized by studying the release of 45Ca2+ from heavy sarcoplasmic reticulum (SR) vesicles and by incorporating the vesicles or the purified Ca2+ release channel protein complex into planar lipid bilayers. First-order rate constants for 45Ca2+ efflux of 1 s-1 were obtained in the presence of 1-10 microM free Ca2+ or 2 X 10(-9) M free Ca2+ plus 20 mM caffeine. Caffeine- and Ca2+-induced 45Ca2+ release were potentiated by ATP and Mg.ATP, and were both inhibited by Mg2+. Dimethylxanthines were similarly (3,9-dimethylxanthine) or more (1,7-, 1,3-, and 3,7-dimethylxanthine) effective than caffeine in increasing the 45Ca2+ efflux rate. 1,9-Dimethylxanthine and 1,3-dimethyluracil (which lacks the imidazole ring) did not appreciably stimulate 45Ca2+ efflux. Recordings of calcium ion currents through single channels showed that the Ca2+- and ATP-gated SR Ca2+ release channel is activated by addition of caffeine to the cis (cytoplasmic) and not the trans (lumenal) side of the channel in the bilayer. The single channel measurements further revealed that caffeine activated Ca2+ release by increasing the number and duration of open channel events without a change of unit conductance (107 pS in 50 mM Ca2+ trans). These results suggest that caffeine exerts its Ca2+ releasing effects in muscle by activating the high-conductance, ligand-gated Ca2+ release channel of sarcoplasmic reticulum.

Effect of acetylstrophanthidin on twitches, microscopic tension fluctuations and cooling contractures in rabbit ventricle

1. We have measured the effect of the aglycone acetylstrophanthidin (ACS) on twitches, cooling contractures and microscopic tension fluctuations in rabbit ventricular muscle. 2. Both developed twitches and cooling contractures are strengthened by applications of ACS in the range 1-4 microM. This positive inotropy averages 150-160% of control (zero ACS) in both twitches and cooling contractures. Cooling contracture magnitude is assumed to reflect the availability of sarcoplasmic reticulum (SR) Ca2+ for contraction (Bridge, 1986). We infer that ACS increases the availability of SR Ca2+ by enlarging SR Ca2+ stores and this may contribute to the positive inotropy. 3. However, twitches appear to increase at lower concentrations of ACS than those required to increase cooling contractures. This observation suggests that the initial ACS inotropy may be achieved without an increase in SR Ca2+. Furthermore, low doses of ACS produce positive inotropy in the presence of 10.0 mM-caffeine where cooling contractures are abolished. This also suggests that positive inotropy occurs in the absence of SR Ca2+ accumulation. 4. Rest decay of both cooling contractures and twitches is significantly slowed in 4 and 8 microM-ACS. We infer that ACS slows the rate of decline of SR Ca2+ available for contraction by slowing the rate at which Ca2+ is lost from the cell during rest. This suggests that ACS produces a net slowing of Ca2+ efflux during activity which in the absence of altered Ca2+ influx will result in net Ca2+ gain and presumably enlarged SR Ca2+ stores. 5. Increasing the concentration of ACS (6-10 microM) results in a decline in developed twitch tension, total tension and an increase in rest tension. Measurement of microscopic tension fluctuations indicates that as developed twitches decline, the root mean square (r.m.s.) of the tension fluctuations increases in a reciprocal manner. This supports the suggestion of others that the decline in developed twitch tension and the appearance of tension fluctuations are causally related. 6. Although ACS (6-10 microM) causes a decline in twitch tension, rapid cooling contractures remain elevated. We suggest that in the presence of Ca2+ oscillations the magnitude of cooling contractures reflects the sum of cytosolic Ca2+ and Ca2+ that is available for release. If microscopic tension fluctuations do represent Ca2+ moving between the SR and cytosol the sum of SR and cytosolic Ca2+ and hence cooling contracture might not decline.(ABSTRACT TRUNCATED AT 400 WORDS)

Caffeine contracture in guinea-pig ventricular muscle and the effect of extracellular sodium ions

1. The mechanisms underlying the virtual absence of caffeine contracture in guinea-pig heart in a Na+-rich external solution were reinvestigated in small (50-120 microns thick) bundles of intact and skinned papillary muscle fibres. 2. In Na+-free solution, the peak tension of 30 mM-caffeine contracture corresponded to the maximum tension of the skinned fibres, and was independent of changes in [Ca2+]o and [K+]o. In the presence of external Na+, the peak tension, which was at most several per cent of the maximum, was affected by [Ca2+]o, [Na+]o and [K+]o, and enhanced by Mn2+ and Ni2+. 3. In the absence of Ca2+, replacement of Na+ with K+ allowed caffeine to evoke a large contracture, showing that there was sufficient calcium stored in the cells under Na+-rich conditions. After treatment with 30 mM-caffeine in the Na+-rich, Ca2+-free solution, and upon replacement of all Na+ with Li+, caffeine was still able to produce a large contracture, which was dependent upon Ca2+ pre-loading of the cells before the first caffeine treatment and upon the subsequent duration in the Na+-free solution. 4. Replacement of Li+ with Na+ during the contracture led to rapid relaxation which was delayed by an increase in [Ca2+]o, depolarization by K+, and addition of La3+ and Mn2+. After Na+-induced complete relaxation in the absence of Ca2+, upon removal of the drugs and Na+, subsequent application of caffeine to the cells evoked a large contracture without Ca2+ reloading. 5. In the skinned fibres, 30 mM-caffeine increased the Ca2+ sensitivity of the contractile system and depressed the maximum tension. An increase in Na+ from 8.4 to 58.4 mM altered neither Ca2+ sensitivity nor the rate of tension development in the absence or presence of caffeine. 6. Increase in Na+ affected neither the rate nor the amount of Ca2+ uptake by the sarcoplasmic reticulum (SR) in the absence or presence of caffeine. Increasing Na+ slightly inhibited the caffeine-induced Ca2+ release from the SR, but more than 10 mM-caffeine produced SR Ca2+ depletion. 7. In the presence of a strong Ca2+ buffer, the steady level of Ca2+ uptake by the SR with 1 mM-caffeine was equal to the amount of Ca2+ remaining in the SR just after the application of caffeine, indicating that Ca2+ release was not inactivated.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenine nucleotides stimulate oxidation-induced calcium efflux from sarcoplasmic reticulum vesicles

Micromolar concentrations of copper (Cu2+) and cysteine induce rapid efflux of calcium from sarcoplasmic reticulum (SR) vesicles. This effect appears to be due to a Cu2+-catalyzed oxidation of the added cysteine to a critical sulfhydryl group on the release protein from sarcoplasmic reticulum (J. L. Trimm, G. Salama, and J. J. Abramson (1986) J. Biol. Chem. 261, 16092-16098). The data presented here indicate that adenine nucleotides synergistically stimulate copper/cysteine (oxidation)-induced calcium efflux from SR vesicles. The order of effectiveness in stimulating calcium efflux is ATP greater than AMP-PCP greater than cAMP greater than AMP greater than adenine approximately NAD approximately NADH. Non-adenine-containing nucleotides such as GTP, CTP, UTP, and ITP and the high energy phosphate compound, acetyl phosphate, were ineffective in stimulating oxidation-induced calcium efflux. The relative effectiveness of various adenine nucleotides in stimulating calcium-induced calcium efflux and oxidation-induced calcium efflux are identical, suggesting that a common mode of action is involved when calcium release is triggered by either method. The stimulatory effect of the adenine nucleotides on oxidation-induced efflux is independent of external magnesium concentration and independent of the magnesium gradient across the SR membrane.

Mechanisms of ryanodine-induced depression of caffeine-induced tension transients in skinned striated rabbit muscle fibers

Evidence suggests that ryanodine affects ligand-gated calcium channels in the sarcoplasmic reticulum (SR) resulting in depressed muscle contraction. In skinned fibers from striated muscle the effects of ryanodine were examined (1) on Ca2+ uptake and on Ca2+ release to differentiate whether the effects are on the pump or channel, and (2) during the tension transient, with ryanodine exposure at various times either simultaneous with or directly after exposure to caffeine. Of total calcium content in the SR, 25 mM caffeine released greater than 90% in papillary muscle (PM), approximately equal to 25% in soleus (SL), and approximately equal to 20% in adductor magnus (AM). Ryanodine (100 microM for 1-3 s for AM and SL; 1 microM for 7-10 s for PM), in the initial loading phase, did not significantly change, and in the initial release phase, markedly depressed the subsequent control caffeine-induced tension transients (C2) in all three muscle types. The depression increased with increasing time of exposure to ryanodine (10 microM) in the order of PM greater than AM greater than SL. Upon introduction of ryanodine after caffeine-induced tension transients, maximal depression was observed at half-maximum rise of the tension transient, followed by recovery of depression to completion in SL, and only partially in AM and PM at steady state of relaxation. The extent of recovery was in the order of SL greater than AM greater than PM. The data suggest that ryanodine affects Ca2+ releasing channel as a result of its binding to open channels.

Sites of action of dihydropyridine drugs in the mouse hemidiaphragm muscle

The effects of nifedipine and BAY K8644 on directly evoked isometric twitch and potassium (K+)- and caffeine-induced contractures were investigated in mouse hemidiaphragm preparations in which neuromuscular transmission had been irreversibly blocked. Both drugs caused initial potentiation of twitch which at high concentrations (greater than 3 x 10(-5) M) was followed by blockade. A simultaneous slow contracture was seen with nifedipine but not BAY K8644. Control K+ contractures were triphasic. The initial fast and slow phases of this contracture were potentiated by BAY K8644 at all times and concentrations. Both phases were potentiated by nifedipine at low concentrations but, during prolonged exposure to high concentrations, potentiation was replaced by an inhibition. The time course of activation and inactivation of the slow phase was also accelerated by all concentrations of nifedipine. The initial phase of caffeine-induced contracture was potentiated and resolved into two components. From these results at least three sites of action were postulated. Conventional binding to t-tubular Ca2+ channels was linked to effects on the slow phase of K+ contracture. An effect on Ca2+ release from the sarcoplasmic reticulum and an inhibition of Ca2+ transfer from uptake to release compartments in the sarcoplasmic reticulum are also postulated.

Ca2+ release from sarcoplasmic reticulum of skinned fast- and slow-twitch muscle fibers

We have performed a comparative study of Ca2+ release from the sarcoplasmic reticulum (SR) of chemically skinned fibers from rabbit fast- and slow-twitch skeletal muscle. Ca2+ fluxes have been indirectly monitored by following either tension development or the inhibition of net Ca2+-loading rate by a light-scattering method. Several drugs (Ca2+-release modulators) have been used to either trigger or block Ca2+ release. Our results indicate that caffeine, doxorubicin, and ryanodine activate Ca2+ release, whereas ruthenium red blocks Ca2+ release from both fast- and slow-twitch skinned fibers. Caffeine has greater affinity for slow SR, whereas doxorubicin, ruthenium red, and ryanodine have greater affinity for fast SR. Our results indicate that Ca2+-release mechanisms in fast and slow SR are homologous but not identical and that differences in twitch-contraction time might be also related to the inherent properties of the Ca2+-release mechanism.

Inhibition of calcium release from skeletal muscle sarcoplasmic reticulum by calmodulin

The effect of calmodulin on calcium release from heavy sarcoplasmic reticulum isolated from rabbit skeletal muscle was investigated with actively and passively calcium loaded sarcoplasmic reticulum vesicles and measured either spectrophotometrically with arsenazo III or by Millipore filtration technique. The transient calcium-, caffeine- and AMP-induced calcium release from actively loaded sarcoplasmic reticulum vesicles was reduced to 29%, 51% and 59% of the respective control value by 1 microM exogenous calmodulin. Stopped-flow measurements demonstrate that calmodulin reduces the apparent rate of caffeine-induced calcium release from actively loaded sarcoplasmic reticulum. The rate of calcium uptake measured in the presence of ruthenium red, which blocks the calcium release channel, was not affected by calmodulin or calmodulin-dependent phosphorylation of sarcoplasmic reticulum vesicles with ATP[S]. The rate of the calcium-, caffeine- and AMP-induced calcium release from passively loaded sarcoplasmic reticulum vesicles was reduced 1.4-2.0-fold by 1 microM exogenous calmodulin, i.e. the half-time of release was maximally increased by a factor of two, whilst calmodulin-dependent phosphorylation of a 57 kDa protein with ATP[S] had no effect. The data indicate that calmodulin itself regulates the calcium release channel of sarcoplasmic reticulum.

Drug-induced calcium release from heavy sarcoplasmic reticulum of skeletal muscle

Calcium release from isolated heavy sarcoplasmic reticulum of rabbit skeletal muscle by several calmodulin antagonistic drugs was measured spectrophotometrically with arsenazo III and compared with the properties of the caffeine-induced calcium release. Trifluoperazine and W7 (about 500 microM) released all actively accumulated calcium (half-maximum release at 129 microM and 98 microM, respectively) in the presence 0.5 mM MgCl2 and 1 mg/ml sarcoplasmic reticulum protein; calmidazolium (100 microM) and compound 48/80 (70 micrograms/ml) released maximally 30-40% calcium, whilst bepridil (100 microM) and felodipin (50 microM) with calmodulin antagonistic strength similar to trifluoperazine (determined by inhibition of the calcium, calmodulin-dependent protein kinase of cardiac sarcoplasmic reticulum) did not cause a detectable calcium release, indicating that this drug-induced calcium release is not due to the calmodulin antagonistic properties of the tested drugs. Calcium release of trifluoperazine, W7 and compound 48/80 and that of caffeine was inhibited by similar concentrations of magnesium (half-inhibition 1.4-4.2 mM compared with 0.97 mM for caffeine) and ruthenium red (half-inhibition for trifluoperazine, W7 and compound 48/80 was 0.22 microM, 0.08 microM and 0.63 micrograms/ml, respectively, compared with 0.13 microM for caffeine), suggesting that this drug-induced calcium release occurs via the calcium-gated calcium channel of sarcoplasmic reticulum stimulated by caffeine or channels with similar properties.

Effects of ryanodine on skinned myocardial fibers of the rabbit

Skinned fiber bundles from papillary muscle of rabbits were used to study the effects of ryanodine (1) on direct Ca2+ activation of the contractile proteins, and (2) on direct Ca2+ uptake and release from the sarcoplasmic reticulum (SR). Caffeine (25 mM) was used to release Ca2+ from the SR and to generate a tension transient. Each tension transient occurred after sequential immersion of the fiber bundles into five solutions: loading (uptake phase, [U]) and releasing (release phase, [R]). The height of free Ca2+-activated tension development of the contractile proteins, and the area of the tension transient generated by caffeine were assessed. (1) The direct free Ca2+-activated tension development of the contractile proteins was not significantly affected by ryanodine up to 0.1 mM, either at the submaximal or maximal free Ca2+ concentrations. (2) Ryanodine (1 nM-1 microM), in U, R, or in U and R, did not significantly change the immediate caffeine-induced tension transients. In the same preparation after ryanodine treatments, the second control caffeine-induced tension transients (C2, no ryanodine) were decreased in a dose-dependent manner (IC50 = 50 nM, 10 nM, 10 nM for R, U, and U and R, respectively). The depression caused by ryanodine on the SR was "activity"-dependent and not readily reversible. Total calcium content in the SR of C2 was not significantly changed by small quantities of ryanodine (less than 0.1 microM) and was decreased with greater amounts of ryanodine (greater than or equal to 0.1 microM). Thus, at low concentrations of ryanodine, the negative inotropic action is due to decrease Ca2+ release from the SR; at high concentration of ryanodine, it is due to decrease in calcium accumulation in the SR.

Effects of caffeine and cyclic adenosine 3',5'-monophosphate on adenosine triphosphate-dependent calcium uptake by lysed brain synaptosomes

The adenosine triphosphate-dependent calcium uptake by endoplasmic reticulum elements of lysed synaptosomes from rat brain cortex was studied. Caffeine exhibited a biphasic effect on this calcium uptake activity: concentrations of 1, 2, 5, 10 or 30 mM caffeine stimulated calcium uptake by 62, 111, 73, 88 and 60% respectively, whereas calcium uptake was inhibited by 55% at a 60-mM concentration of caffeine. Calcium release from endoplasmic reticulum elements of lysed brain synaptosomes was stimulated by 10 mM caffeine. Cyclic adenosine 3',5'-monophosphate stimulated calcium uptake in the lysed synaptosome preparation: exogenous concentrations of 0.05, 0.5, 5, 50, or 500 microM stimulated uptake by 67, 67, 95, 38 or 67% respectively. To explore the possibility that caffeine stimulated calcium uptake through inhibition of phosphodiesterase and consequent preservation of cyclic adenosine 3',5'-monophosphate, we have tested whether caffeine retained its ability to stimulate calcium uptake under conditions of maximal stimulation by cyclic adenosine 3',5'-monophosphate. The combined presence of 10 mM caffeine and 5 microM cyclic adenosine 3',5'-monophosphate resulted in an approximate doubling of the calcium uptake as compared to the uptake in the presence of the cyclic nucleotide alone, indicating that the stimulation due to caffeine does not occur via cyclic adenosine 3',5'-monophosphate.

Relation between action potential duration and mechanical activity on rat diaphragm fibers. Effects of 3,4-diaminopyridine and tetraethylammonium

The aim of this work was to study the electrical and mechanical properties of small bundles of rat diaphragm muscle treated with two blockers of the delayed potassium rectification channels: 3,4-diaminopyridine (3,4-DAP, 2.5 mM) and tetraethylammonium (TEA, 20 mM). Twitch tension was significantly potentiated by TEA and 3,4-DAP (39% and 59% respectively). Maximal tetanic tension was not affected by both drugs. The voltage dependence of the tension vs the resting membrane potential was shifted to lower values in TEA and 3,4-DAP. 3,4-DAP increased the caffeine contracture tension (2.5-10 mM) and lowered the caffeine contracture threshold. The duration of the action potential (measured at the level of -40 mV) was increased by TEA and 3,4-DAP solutions. This change was a consequence of the decrease in the rate of repolarization of the action potential. In addition, TEA reduced the amplitude and the rate of rise of the action potential. We suggested that the increment in the duration of the action potential and the shift of the mechanical threshold to more negative values of membrane potential might be the factors involved in the twitch potentiation induced by the TEA and 3,4-DAP solutions.

Effects of ryanodine on skinned skeletal muscle fibers of the rabbit

The mechanism(s) of ryanodine-induced contracture of skeletal muscle were studied in skinned fibers from soleus (SL) and adductor magnus (AM) (slow- and fast-twitch skeletal muscles) of rabbits. Pieces of SL or AM were homogenized (sarcolemma disrupted). Single fibers were dissected from the homogenate and mounted on photodiode force transducers. At concentrations 1-50 microM, ryanodine slightly but significantly increased the submaximal Ca2+-activated tension development of the contractile proteins in skinned fibers of AM but not of SL. Ryanodine in uptake phase or release phase increased caffeine-induced tension transients in the SR of both muscle types; however, no dose-response relation was found. Ryanodine greater than or equal to 1 microM decreased, however, the second control tension transients in a dose-dependent manner. The depression was nearly irreversible and "activity"-dependent. The concentrations of ryanodine that inhibited the second control tension transients by 50% were 10 microM and 5 microM for SL and AM, respectively, following ryanodine administration in the release phase, and 100 microM and 30 microM, respectively, for these preparations after the drug was present in the uptake phase. The quantity of calcium released from the SR by Triton X-100 and caffeine in the second control tension transient was unchanged by ryanodine at all concentrations tested when compared with that of the absence of ryanodine. The present findings suggest that the ability of ryanodine to increase immediate calcium release from the SR, and in AM but not SL, to increase the sensitivity of the contractile proteins to Ca2+ underlies the contracture caused by this agent in intact skeletal muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation and inhibition of the calcium gate of sarcoplasmic reticulum by high-affinity ryanodine binding

The occupancy of high-affinity ryanodine-binding sites of isolated heavy sarcoplasmic reticulum vesicles occurring in concentrated salt solutions affects ATP-dependent calcium accumulation and caffeine-induced calcium release. The initial suppression of calcium uptake is followed by a marked uptake activation resulting in a reduction of the final calcium level in the medium. Simultaneously, caffeine-induced calcium release is blocked. The dependence of inhibition of calcium uptake and caffeine-induced calcium release observed in assay media containing physiological concentrations of magnesium and ATP on the concentration of ryanodine corresponds to the drug's effectiveness in living muscles.

Effects of resting membrane potential and intactness of the T-tubules on caffeine contractures in rat skeletal muscle

We have studied the effects of changes in the resting membrane potential (Vm) and T-tubules on caffeine contracture (25 mM) elicited in rat soleus muscle in vitro at 34 degrees C. In high [K]o (30-140 mM, [K]o X [Cl]o constant) caffeine contractures were reduced by about 40-50% and had a faster time course than in normal Krebs ([K]o = 5 mM). Detubulation of the muscles by an osmotic treatment produces a reduction of about 30% in the caffeine contracture tension. Our results with high K solutions suggest a reduced sensitivity of the myofibrils to calcium released by caffeine. The effects of detubulation on caffeine contracture suggest that caffeine may have a direct effect on sarcolemma in addition to its well known action on the sarcoplasmic reticulum (SR). However, a depletion of the calcium content in the SR of depolarized muscle fibres as well as an anatomical damage produced by the osmotic treatment can not be ruled out as an explanation for the reduced caffeine contracture.