Caffeine Contracture in Frog Cardiac Muscle after Exposure

Transcription

1 Short Communication Japanese Journal of Physiology, 30, , 1980 Caffeine Contracture in Frog Cardiac Muscle after Exposure to High Concentrations of Calcium Moto MATSUMURA and Kazuhiko NARITA Department of Physiology, Kawasaki Medical School, Kurashiki, Japan Summary Caffeine induced contracture in frog auricular muscle at room temperature when the muscle was exposed to a Ca-rich solution. The initial transient tension lasted sec, and it was inhibited by procaine. It was dependent on caffeine concentration and the time of exposure to a Ca-rich solution prior to caffeine attack, but was little dependent on Ca concentration during caffeine contracture. CHAPMAN and LEOTY (1976) showed that caffeine induced the transient contracture in mammalian cardiac muscle. On the other hand, it has been known that the frog cardiac muscle usually responds to caffeine with the potentiated twitch (KIMOTO, 1972; CHAPMAN and MILLER, 1974), and that caffeine contracture takes place when it is previously treated with potassium-rich or sodium-free solution (SuzuKI, 1962; KIMOTO et al., 1974; CHAPMAN and MILLER, 1974) or when the surrounding medium is rapidly cooled (SAKAI and KURIHARA, 1974). It is interesting for the study of excitation-contraction coupling in cardiac muscle to note other situations in which caffeine contracture can be induced. KIMOTO et al. (1974) briefly reported the presence of caffeine contracture in the solution of 4-fold Ca concentration. The present work was carried out to investigate the properties of caffeine contracture observed in Ca-rich solution. The trabecullar muscle fibers were dissected from the inner surface of the atrium of the bullfrog, Rana catesbeiana. The thin muscle preparation was placed in a small 0.3 ml chamber (0.3 ~0.3 ~3.0 cm), and the bathing solutions were quickly exchanged within 2 sec. One end of the muscle was fixed and the other end was connected to a glass rod attached to the anode of the RCA 5734 transducer. The muscle length was close to that in situ. The tension was displayed on a pen recorder (Yokogawa Electric Works, Ltd., Type 3046). All the experiments were carried out at room temperature, ranging from 21 to 25 Ž. The composition of the standard Ringer solution was as follows (mm); NaCl 117, KCl 2.0, CaCl2 1.8, glucose 10 and HEPES buffer 1, giving ph of 7.2. The Ca-rich Ringer solution was prepared by increasing CaCl2 concentration, other electrolytes concentrations Received for publication August 1,

2 138 M. MATSUMURA and K. NARITA Fig. 1. A-D show caffeine contracture in frog cardiac muscle, caffeine concentration is 10 mm in A, 20 mm in B, 30 mm in C and 10 mm again in D. The change in external Ca concentration before and during caffeine contracture is illustrated below the records. Arrows indicate the exposure time to Ca-rich solution and it is 12 min throughout A to F. At the time marked, muscle was washed with the standard Ringer solution. 25 Ž. E and F are from another muscle preparation, showing the time course of the initial transient tension on faster speed as well as the inhibitory action of procaine. Procaine was added 12 min prior to caffeine. 24 Ž. being unchanged. Caffeine anhydrate and procaine-hcl was added to the standard or Ca-rich Ringer solution. The experiments were started at least 2 hr after dissection so that the muscle was in the steady hypodynamic state. Figure 1, A-D shows the contracture of frog auricular muscle induced by caffeine of different concentrations. The experimental procedure was as follows: at first the muscle was immersed in the standard Ringer solution, which was rapidly exchanged with 16.2 mm-ca-rich Ringer solution, and then after 12 min this solution was exchanged again with 16.2 mm- Ca-rich Ringer solution containing caffeine. The peak tension in contracture depended on the caffeine concentration and the maximum was obtained at a concentration of 30 mm or more. The time course of the contracture could be divided into the initial transient and late sustained phases. The initial tension attained a peak within 5 sec (Fig. 1 E and Fig. 2 F) and decreased rapidly, while the late tension was maintained for 30 min as long as the muscle was exposed to caffeine. Since the composition other than CaCl2 was not changed, the Ca-rich solutions were hypertonic, which might make the muscle tend to be susceptible to caffeine (CAPUTO, 1966). The effects of tonicity of 16.2 mm Ca-Ringer solution on the caffeine contracture, however, could be excluded because the caffeine contracture was not detected in NaCl hypertonic solution of excess 22.0 mm NaCl or only very small tension was observed in sucrose hypertonic solution of excess 32.0 mm sucrose. Procaine inhibited only the initial tension in caffeine contracture. As shown in Fig. 1 E and F, if the muscle was previously treated by 10 mm procaine before exposure to 30 mm caffeine the initial tension in contracture was completely Japanese Journal of Physiology

3 CAFFEINE CONTRACTURE IN CARDIAC MUSCLE 139 abolished. Similar but weak inhibitory action of procaine was also observed when it was applied simultaneously with caffeine. On the other hand, the late tension was not decreased but seemed to be slightly increased. The initial tension was augmented if the external Ca concentration was increased from 5.4 mm to 10.8, 16.2 and 21.6 mm. In other series of experiments, the muscle was first exposed to 16.2 mm Ca solution and then to 16.2 mm Ca- 30 mm caffeine or 1.8 mm Ca-30 mm caffeine solution. The initial tension in two kinds of solutions showed little difference. Therefore, it is not the Ca concentration during caffeine application but the one prior to caffeine application that affects the size or duration of the initial tension. In that sence, caffeine contracture is independent of calcium concentration. The late tension was closely related to Ca concentration, and attained maximum at 16.2 or 21.6 mm. The minimum Ca concentration for the development of 30 mm caffeine contracture was 5.4 or 7.2 mm, consistent with the results of KIMOTO et al. (1974). The increase in Ca concentration above 21.6 mm caused a small contracture and application of caffeine elicited further tension development. The initial tension was also dependent on the time of exposure of muscle to Ca-rich solution but the late tension was not (Fig. 2 A-D). It was sometimes Fig. 2. Effect of exposure time to Ca-rich solution on the initial tension. External Ca concentration is 16.2 mm throughout A to D, and the exposure time is 3 min in A, 6 min in B, 9 min in C, 3 min again in D. E shows the relationship between the area under the initial tension curve and the exposure time. Results from 5 experiments are shown by different marks. The ordinate is scaled in an arbitrary unit so that the area at 6 min exposure is a control (large circle). The line is drawn by eye. The measured area is illustrated by the dotted area in F Ž. Vol.30, No.1, 1980

4 140 M. MATSUMURA and K. NARITA observed that the initial tension in the first caffeine attack was shorter than that in later one (compare A to D). The area under the tension curve within 20 or 30 sec was measured and adopted as an index of capacity for initial contracture. The relationship between them is shown in Fig. 2 E. The results indicate that the capacity of muscle to respond to caffeine with the transient tension is stored during the period of immersion in Ca-rich solution and that the storage process progresses with a half time of around 3 min at Ž. Seasonal variation was observed. Autumn frogs were more sensitive to caffeine than winter or spring frogs. The present study showed that caffeine induced contracture in frog cardiac muscle after exposure to Ca-rich solution. Like the cooling contracture of SAKAI and KURIHARA (1974) and the contracture presented by other authors (FOZZARD, 1977), the contracture was composed of initial transient and late sustained tensions. The initial tension in caffeine contracture observed here was similar to that in ferret heart muscle reported by CHAPMAN and LEOTY (1976), in the manner that it was transient and was little dependent on Ca concentration during caffeine application. Moreover, the initial tension was abolished by procaine, which is known to inhibit Ca-induced Ca release mechanism (THORENS and ENDO, 1975). Then, it is suggested that the initial tension is related to Ca release from the internal stored sites, and that the sites accumulate a sufficient amount of Ca during exposure to Ca-rich solution (cf. CHAPMAN, 1979, pp.27-30). The late sustained tension seems to be caused by the inflow of Ca from the extracellular sites, because it lasted as long as the muscle was exposed to Ca-rich solution containing caffeine and was dependent on external Ca concentration. JUNDT et al. (1975) showed that the Ca turnover rate across the membrane was increased by caffeine in the presence of Na. We have, however, no direct evidence concerning the origin of late tension. REFERENCES CAPUTO, C. (1966) Caffeine- and potassium-induced contractures of frog striated muscle fibers in hypertonic solution. J. Gen. Physiol., 50: CHAPMAN, R. A. (1979) Excitation-contraction coupling in cardiac muscle. Frog. Biophys. Mol. Biol., 35: CHAPMAN, R. A. and LEOTY, C. (1976) The time-dependent and dose-dependent effects of caffeine on the contraction of the ferret heart. J. Physiol. (Lond.), 256: CHAPMAN, R. A. and MILLER, D. J. (1974) The effect of caffeine on the contraction of the frog heart. J. Physiol. (Lond.), 242: FOZZARD, H. A. (1977) Heart: Excitation-contraction coupling. Annu. Rev. Physiol., 39: JUNDT, H., PORZIG, H., REUTER, H., and STUCKI, J. W. (1975) The effect of substances releasing intracellular calcium ions on sodium dependent calcium efflux from guinea-pig auricles. J. Physiol. (Lond.), 246: KIMOTO, Y. (1972) Effects of caffeine on the membrane potentials and contractility of the guinea pig atrium. Jpn. J. Physiol., 22: KIMOTO, Y., SAITO, M., and GoTO, M. (1974) Effects of caffeine on the membrane potentials, Japanese Journal of Physiology

5 CAFFEINE CONTRACTURE IN CARDIAC MUSCLE 141 membrane currents and contractility of the bull frog atrium. Jpn. J. Physiol., 24: SAKAI, T. and KURIHARA, S. (1974) The rapid cooling contracture of toad cardiac muscles. Jpn. J. Physiol., 24: SUZUKI, K. (1962) Studies on the mechanism of the excitation-contraction coupling in cardiac muscle, with special reference to the caffeine-contracture. Jpn. J. Physiol., 12: THORENS, S. and ENDO, M. (1975) Calcium-induced Ca release and "depolarization-induced" Ca release: Their physiological significance. Proc. Jpn. Acad., 51: Vol.30, No.1, 1980