Effect of extracellular calcium on contractile activation in guinea‐pig ventricular muscle.

Abstract

SUMMARY1. The problem of whether or not the alterations in twitch tension of mammalian cardiac muscle induced by changes in extracellular Ca concentration ([Ca].) are due to the Na-Ca exchange process across the surface membrane and/or the changes in the amount of Ca in the sarcoplasmic reticulum has been re-examined by using thin bundles (70-120 ,sm diameter) dissected from guinea-pig papillary muscle.2. The observed time course ofthe change in the twitch tension due to a step change in [Ca]o was compared with that computed on a basis of the diffusion process of Ca ions in a circular cylinder and of the steady-state relation between [Ca]. and twitch tension.3. After a sudden decrease in [Ca]. from 2 mm to various lower concentrations, the isometric twitch tension of the thin bundles first fell rapidly and monotonically and then showed a much smaller and slower secondary fall. The correspondence of the observed time course of the rapid phase with the predicted time course and the observed half-time of the rapid phase ranging from 1-0 to 2-5 s indicate that the rapidity of the twitch response may be dominated by simple diffusion of Ca ions through the extracellular space. If so, the effective diffusion constant of Ca ions inside the bundles was 1-4+002 x 10-6 cm2/S (mean+s.E., n = 9). 4. The magnitude and direction of the step change in [Ca]0 or the change at different stimulus frequencies gave rise to dissimilar time courses of the contractile change; the difference in the rapid time courses due to these factors could be explained by the simple diffusion models, but not in the much slower phase.5. The half-time for the Ca effect was the same as that for the rapid effect of Na ions in the external solutions.6. The time course of twitch decline due to [Ca]0 decrease in the Na-free (Li) solution was identical to that predicted from the time course measured in the Na-rich solution and the steady-state relation between [Ca]0 and tension in the Na-free solution.7. The half-time of Ca leak from the sarcoplasmic reticulum in the skinned cardiac muscle was 40-60 s in the presence of 10 mM-EGTA, much shorter than that of the

Free‐calcium and force transients during depolarization and pharmacomechanical coupling in guinea‐pig smooth muscle.

“…Therefore, the removal of extracellular Ca2+ could limit the ability of the sarcoplasmic reticulum to maintain its Ca2+ content, and cause a greater or lesser loss of stored Ca2+, depending on the leak-pump ratio of the sarcoplasmic reticulum membrane. Muscle-specific differences in Ca2+ retention by the sarcoplasmic reticulum have been demonstrated: the sarcoplasmic reticulum in skinned cardiac muscle is less able to retain its Ca2+ content than the sarcoplasmic reticulum of frog skeletal muscle (Kitazawa, 1984). Therefore, our results contribute to the suspicion that the loss of contractility due to the removal of extracellular Ca2+ is not unambiguous evidence of extracellular Ca2+ being a significant source of activator Ca2+ in normal Ca2+-containing solution (for review, Johansson & Somlyo, 1980;Somlyo, 1985).…”

The effects of metabolic inhibition on intracellular calcium and pH in isolated rat ventricular cells.

“…Figure 3 shows the effects on fluorescence of adding saponin at a concentration of 50 ,Ig ml-'. This has been shown in isolated rat cardiac muscle to make the cell membrane Ca2+ AND pH DUIRINVG METABOLIC I2NHIBITION 397 leaky to large molecular weight substances without affecting the sarcoplasmic reticular membrane (Kitazawa, 1984). The addition of saponin released the Fura-2 as evidenced by the decrease of fluorescence at 360 nm (the isosbestic wavelength).…”

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

“…Second, solutions were exchanged very rapidly in two ways. One was a rapid perfusion method, which was the same as that described previously (Kitazawa, 1984) except for the use of a different experimental trough with a small volume of 0-25 ml and a force transducer (AE801, AME, Horten, Norway). The other was the so-called bubble chamber method originally designed by Horiuti: the ends of the fibre bundles were tied with a silk monofilament to the fine tips of two tungsten needles, one of which was connected to a force transducer.…”

“…On the basis of the estimated concentration profile derived from the diffusion equation (see Fig. 1 in Kitazawa, 1984), when transient caffeine contracture produced in individual cells is very short-lived, e.g. only about 0-5 s, it was estimated that hardly any significant development of tension would be observed in the bundle even if each cell were to produce a large tension, because of slow diffusion and disturbance of tension measurement upon rapid perfusion for 0 5 s. If the lifetime is 4 s or more, as shown by Chapman & Leoty (1976), then it would be possible to measure a substantial contracture in the bundle sufficiently with the perfusion apparatus used in this study.…”

“…First, small bundles (50-120 ,um in diameter, about 1 mm in length) dissected from papillary muscles of guineapig right ventricles were used. The method used for dissection has been previously described in detail (Kitazawa, 1984). Second, solutions were exchanged very rapidly in two ways.…”