The effects of the naturally occurring amino acid taurine (2-aminoethanesul-phonic acid) on isometric force development were investigated using skinned muscle fibre preparations. In atrial and ventricular pig heart muscles, as well as in fibres of slow abdominal extensor muscle of crayfish, an increase of submaximal isometric force was observed in Ca2+-activated skinned fibre preparations at physiological concentrations of taurine. The maximal isometric force remained unaffected in all preparations. It is assumed that taurine increases the Ca2+ sensitivity of the force-generating myofilaments in mammalian hearts and tacean slow skeletal muscle fibres.
Taurine occurs widely in animal tissues, often at concentrations exceeding those of most other free amino acids (Jacobsen and Smith, 1968). The highest concentrations have been detected in excitable tissues such as nerve and muscle (Huxtable, 1980), and a variety of physiological effects have been reported. The most established function is its contribution to osmoregulation in the muscle tissues of many marine animals, especially crustaceans (Shaw, 1958; Allen and Garrett, 1971; Dalla Via, 1989).
Taurine is also present in considerable amounts in heart tissues, constituting up to 50 % of the free amino acid pool in mammals (Awapara et al. 1950). Here, too, taurine has been proposed to maintain cardiac osmolarity (Thurston et al. 1981).
In addition, taurine modulates numerous Ca2+-dependent processes in heart and other tissues (Chovan et al. 1980; Kramer et al. 1981; Huxtable, 1989; Dolara et al. 1973). Taurine influences cardiac contractility, inducing antiarrhythmic activity (Read and Welty, 1963) and exerting positive (in low-Ca2+ media) as well as negative (in high-Ca2+ media) inotropic effects on intact mammalian hearts (Dietrich and Diacono, 1971; Schaffer et al. 1978; Franconi et al. 1982). Furthermore, the positive inotropic effects of cardiac glycosides are potentiated by taurine (Guidotti et al. 1971).
Direct effects of taurine on force generation of the isolated contractile machinery have not yet been reported. We measured the isometric force production of Ca2+-activated skinned muscle fibre preparations (see Stephenson, 1981) of pig heart and slow skeletal muscles of crayfish in the presence and absence of physiological concentrations of taurine. Some of our results have been published in abstract form (Galler and Hutzler, 1988).
Materials and methods
Biological materials and skinning procedures
Pig hearts were obtained from the local slaughterhouse. Crayfish (Pacifastacus leniusculus, Dana) originated from the University of Konstanz. Tissue concentrations of taurine were determined using a slightly modified HPLC technique described earlier (Haller and Lackner, 1987). For mechanical experiments, fibre bundles of pig heart muscle (trabeculae of the right atrium and musculi papillares in the right ventricle) were chemically skinned in a solution containing 50% glycerol, 1% Triton X-100, 2mmol 1−1 dithioerythritol (DTE), 10mmoll−1 NaN3, 5 mmol I−1 ATP, 5 mmol I−1 MgCl2, 5 mmol 1−1 EGTAand 20 mmol 1−1 imidazole, pH 7.0 adjusted with KOH. Since preliminary mechanical experiments suggested that there are intracellular Ca2+-sequestering vesicles, we extended the skinning procedure to 24 h at a temperature of about 4°C. Such treated preparations did not show any Ca2+-translocating activity, but the velocity of force development was generally reduced. Maximum isometric tension was also diminished (about 1.5 Ncm−2). We have chosen these preparations to exclude any possible effects of Ca2+ translocation due to the activity of sarcoplasmic reticulum.
Skinned skeletal muscle fibres of crayfish were prepared by drying abdominal muscles (using silica gel, at − 20°C) that had been frozen quickly in liquid nitrogen (see Stienen et al. 1983). For mechanical measurements, single fibres of superficial abdominal extensor muscles (Parnas and Atwood, 1966) were used.
For heart muscle preparations, the relaxation solution contained Mopso [3-(N-morpholino)-2-hydroxypropanesulphonic acid] (20 mmol 1−1), K2H2EGTA (10 mmol 1−1), Na2H2ATP (10 mmol 1−1), sodium phosphocreatine (10 mmol I−1), magnesium propionate (13.4mmoll−1) and taurine (0 or 5mmoll−1). The maximal activating solution had the same composition, except that K2H2EGTA was substituted by CaH2EGTA (10 mmol 1−1) and the magnesium propionate concentration was 12.9 mmol I−1. The ionic strength of the solutions was 0.11 mol 1−1. For the crayfish slow muscle preparation, relaxation and maximal activating solutions (ionic strength, 0.22 mol 1−1; solutions modified after Moisescu and Thieleczek, 1979) both contained Mopso (60 mmol 1−1), Na2H2ATP (8mmoll−1), sodium phosphocreatine (10mmoll−1), caffeine (15mmoll−1) and taurine (0 or 5 mmol 1−1). In addition, the relaxation solution contained K2H2EGTA (50 mmol 1−1) and magnesium propionate (8.3 mmol I−1) and the maximal activating solution contained CaH2EGTA (50 mmol I−1) and magnesium propionate (7.4 mmol I−1). In all cases, pH was adjusted to 6.70 at a temperature of 22±0.5°C.
In preliminary experiments, different ion compositions of the bath solutions were tested to obtain optimal conditions for reproducible force values of the skinned muscle fibre preparations. A relatively low pH of 6.70 appeared most appropriate. In addition, the adjustment of the required free Ca2+ concentrations in the EGTA-buffered solutions is facilitated at this relatively low pH. For crayfish fast skeletal muscle fibre preparations no condition could be detected in which force transitions were reproducible enough to study effects on myofibrillar Ca2+ sensitivity. Thus, taurine effects could not be investigated in crayfish fast muscle fibres.
To obtain solutions with different Ca2+ concentrations, relaxing and maximal activating solutions were mixed in different ratios. Creatine kinase (50i.u. ml−1) was added to all solutions immediately before the mechanical experiments. Precise concentrations of free calcium and magnesium ions in all bath solutions were achieved with an iterative computer program based on the equilibrium constants listed by Martell and Smith (1977). The free magnesium ion concentration in the bath solutions was 1 mmol I−1 in the case of crayfish skeletal muscle fibres and 3 mmol 1−1 in the case of the pig heart preparations. Free Ca2+ concentrations of the activating solutions with and without taurine were measured with a Ca2+-selective electrode (Schefer et al. 1986) calibrated with the solutions described by Tsien and Rink (1980) (ionic strength, 0.13 mol I−1). Differences, due to different ionic strengths, of the activity coefficients of Ca2+ in the calibration and experimental solutions were corrected by applying the Debye Hückel formalism as described by Meier et al. (1980).
Isometric force measurements
The skinned fibres were mounted horizontally between a fixed glass needle and a force transducer (AE801, SensoNor, Horton, Norway) with nitrocellulose dissolved in acetone. The preparations were 0.5–2 mm long with diameters of 70–200 μm. After mounting, the fibres were immediately incubated in the relaxing solution. To improve the pattern of cross-striation, length changes of about 1 % of total fibre length were induced for about 5 min by moving the glass needle sinusoidally with a vibrator (Ling V101). Sarcomere length of the muscle fibres.was measured with a laser beam (He-Ne laser from Spectra physics, model 102; 4 mW) according to the method described by Zite-Ferenczi and Rüdel (1978).
Sarcomere length of the heart preparations was set at 2.1 μm; that of crayfish slow skeletal muscle fibres was not changed after incubating the stiff dried fibres mounted at the apparatus for force measurements. It ranged from 7 to 10μm.
The skinned fibre preparations were activated by incubation in solutions with different Ca2+ concentrations. An automatic cuvette transporting system provided a rapid change of bath solutions. Force signals were measured with a bridge amplifier connected to a chart recorder (Gould, Brush 220).
Taurine concentrations in muscle tissues
Concentrations of taurine in different tissues and animals are listed in Table 1. The amount of taurine in atrial muscles of pig is smaller than in ventricular muscles (P<0.05). In crayfish, slow abdominal extensor muscles contain up to seven times more taurine than do fast abdominal extensor muscles. The ratio varies considerably in different animals.
Maximal isometric force
Maximal activating solutions with or without taurine were applied with or without intermittent relaxation to test the effects of taurine on maximum isometric force development. Up to concentrations of 20 mmol 1−1, no significant influence of taurine on maximum isometric force could be detected.
Submaximal isometric forces
Successive activations of crayfish slow skeletal muscle fibres by addition of Ca2+ were interrupted by total relaxation, whereas in pig heart preparations, Ca2+ concentration was raised gradually without intermittent relaxations. Activating solutions with and without taurine were applied alternately (Fig. 1).
In several experiments, the preparations were preincubated in a taurine-containing relaxation solution for different times (0–30 min) prior to the submaximal activations in the presence of taurine. In other experiments, crayfish slow muscle preparations were incubated continuously in taurine-containing media, but at the end of the experiments submaximal activations were applied in taurine-free media. Here, preincubation in taurine-free relaxation solution lasted 1 min. In all these cases of different incubation procedures, similar taurine effects were observed.
In most of the experiments on crayfish slow muscle, preparations were kept in taurine-free media for about 15 min during the first incubations in relaxation and activation solutions. Pig heart preparations were kept in taurine-free media for more than 24 h during the skinning procedure and the first mechanical experiments. When changing from taurine-free to taurine-containing media or vice versa, preincubations lasting from about 20s (crayfish preparations) to 2 min (pig heart preparations) in the corresponding relaxation solutions were applied.
Submaximal forces were enhanced by taurine in both cardiac and crayfish slow muscles. At about half-maximal activation the observed increase was 15.2±9.7 % (S.D.) of maximal force in atrial preparations (N=8), 17.9±13.0% (S.D.) in ventricular preparations (N=10), and 13.2±6.5% (S.D.) in crayfish slow skeletal muscle fibres (N=12) in the presence of 5 mmol I−1 taurine. Relative force plotted against pCa of the activating solutions and fitted with a Hill equation (Altringham and Johnston, 1982) revealed a shift of the curve towards lower Ca2+ values in the presence of taurine (Fig. 2). The increase of pCa50 (the negative logarithm of Ca2+ concentration for half-maximal activation) is significant (paired t-test, P<0.001) and similar for different preparations: pig atrial fibres, 0.13±0.09pCaunits (±S.D., N=7); pig ventricular fibres, 0.15±0.11 pCa units (N=10); crayfish slow muscle fibres, 0.10±0.04pCa units (N=12). Taurine did not significantly change the slope of the pCa-force curve in any preparation.
Our measurements of taurine concentrations in different muscle tissues correspond with the findings in skeletal muscles of chicken (Airaksinen and Partanen, 1985) and fish (Haller and Lackner, 1987), where higher concentrations were found in the slow muscles than in the fast muscles. The higher taurine levels in pig ventricular muscles compared to those in atrial muscles also fit the above pattern, since ventricular muscle is considered to be slow and atrial muscle to be fast (Morano et al. 1988).
Physiological concentrations of taurine in pig heart and crayfish slow skeletal muscle fibres shift the pCa-force curve to lower Ca2+ concentrations without affecting the maximal force. These findings cannot be explained by a change of Ca2+ activity within the muscle fibre preparations as a result of Ca2+ movement through the sarcoplasmic reticulum. Pig heart preparations were treated with the non-ionic detergent Triton X-100, which removes membranes of sarcoplasmic reticulum (Meisheri and Rüegg, 1983). In the case of crayfish slow skeletal muscle fibres, all solutions for mechanical experiments contained 15 mmol I−1 caffeine, which prevents accumulation of Ca2+ in the sarcoplasmic reticulum (Nagasaki and Kasai, 1983). In addition, a constant Ca2+ level was maintained using high concentrations of the Ca2+ buffer EGTA(50mmol 1−1). The possibility, therefore, that taurine acts via a change of Ca2+ concentration mediated through the sarcoplasmic reticulum can be excluded. We assume instead that taurine increases the Ca2+ sensitivity of the force-generating structures in pig heart and crayfish slow skeletal muscles.
As to the mechanism of action of taurine, its dipolar character may be responsible for the effects mentioned above, since taurine weakens charge dependent protein-protein interactions. The troponin-I-actin binding which inhibits the force-generating myosin-actin interaction (El-Saleh et al. 1986) could theoretically be weakened by taurine. This would lead to the observed calcium-sensitizing effect (Rüegg, 1987).
The positive inotropic effect of taurine on living mammalian hearts (e.g. Dietrich and Diacono, 1971; Dolara et al. 1973) is usually thought to be caused by an increase in intracellular Ca2+ concentration mediated by taurine. Franconi et al. (1982) measured the force and tissue Ca2+ concentrations of superfused guinea pig ventricular strips at different external CaCl2 concentrations in the presence and absence of taurine. They found only a coarse positive correlation between the taurine-mediated change in contractility and the taurine-induced change in the Ca2+ concentration of the heart tissue at different external CaCl2 concentrations. The maxima of the two parameters appeared at different external CaCl2 concentrations. The maximal taurine-mediated increase of internal Ca2+ concentration occurred at an external CaCl2 concentration of 1.8 mmol I−1, whereas the maximal taurine-induced positive inotropic effect appeared at an external CaCl2 concentration of 0.9 mmol I−1. Thus, in the presence of taurine, the increase of force is not always strictly correlated with an increase of internal Ca2+ concentration. Therefore, the positive inotropic effect of taurine may not be an exclusive effect of increased intracellular Ca2+ in intact heart muscle fibres; our study suggests that it may be partially due to enhanced Ca2+ sensitivity of force generating myofilaments.
The potentiation of the positive inotropic effect of cardiac glycosides on intact mammalian hearts by taurine (Guidotti et al. 1971) may also be explained by the Ca2+-sensitizing action. For these reasons, further attention should be paid to taurine as a potential moderate cardiotonic drug.
Since changes in taurine concentration appear in mammalian hearts and in crustacean skeletal muscles in different physiological states, the Ca2+-sensitizing effect of taurine could have physiologically important implications. Congestive heart failure in humans (Huxtable and Bressler, 1974) and dogs (Peterson et al. 1973) is accompanied by an increased taurine concentration in the heart tissues affected. Probably taurine is increased because of its Ca2+-sensitizing effects on myofibrils.
In some crustacean species (e.g. Carcinus, Eriocheir and Palaemon; see Shaw, 1958; Allen and Garrett, 1971; Dalla Via, 1989) changes in sarcoplasmic taurine concentration were found to be dependent on extracellular osmolarity. Implications for muscle contraction are still unknown.
Apart from the naturally occurring sarcoplasmic imidazoles carnosine and N-acetyl histidine (Harrison et al. 1986), no endogenous Ca2+ sensitizers have yet been found. Non-endogenous Ca2+-sensitizing drugs (submazole, Herzig et al. 1981; caffeine, Wendt and Stephenson, 1983) are imidazole derivatives. With taurine, a Ca2+ sensitizer with different chemical properties has been found. It is possible that other sarcoplasmic amino acids and peptides (e.g. glutamine and glutathione) may also have modulatory effects on the Ca2+ sensitivity of myofibrils.
This work is dedicated to Professor Dr W. Wieser (Institut für Zoologie, Universitat Innsbruck) on the occasion of his 65th birthday. The authors wish to thank Professor J. C. Rüegg (Heidelberg) and his group for a gentle introduction into the skinned fibre technique and their encouragement. Our thanks are also due to Professor W. Rathmayer (Konstanz) for generous and helpful support, Professor W. Wieser (Innsbruck) for reading the manuscript and D. Günzel and S. Lanka (Konstanz) for valuable assistance and support. J. Vogt, W. Kühnel and S. Hahn (Konstanz) are acknowledged for constructing our mechanical apparatus and Drs D. Ammann and T. Bührer (Zürich) for providing some Ca2+-selective membranes. This work was supported by the DFG (SFB156) and by the FWF (P-7162/BIO).