Abstract
Deprivation of external calcium causes sudden potentiation of the twitch response of single muscle fibers. The potentiation was 64 +/- 8%. Potentiation is simultaneous with membrane depolarization occurring after Ca(++) removal. This depolarization amounted to 9 +/- 2 mv. Ca(++) removal also alters the action potential. 3 min after calcium withdrawal, action potential amplitude fell by 36 +/- 3 mv; maximum rates of rise and fall of the spike decreased by 55 +/- 5 and 63 +/- 5% respectively. Changes in shape of the A. P. differ from those seen with other potentiators of the twitch response, such as Zn(++). After short exposure to calcium-free media, potassium-induced contractures show potentiation of peak tension. The S-shaped curve relating potassium contracture tension to log [K](o) shifts to the left after such treatment. Calcium deprivation also increased the rate of relaxation of the contractures. This effect depends on the duration of calcium deprivation, and is probably related to the effect of calcium lack on the membrane. The change in relaxation occurred immediately after calcium deprivation, and was reversed by sudden readmission of calcium. Relaxation of twitch and tetanus responses also were affected by Ca lack, but not as rapidly as potassium contractures. The results suggest that external calcium is not directly involved in the process responsible for tension development, supporting the view that this process is mediated by translocation of intracellular calcium. The relaxation process, however, appears to be rapidly affected by deprivation of external calcium.
Highlights
Deprivation of external calcium causes sudden potentiation of the twitch response of single muscle fibers
T h e disappearance of the contractile response has been attributed to the depolarization of the fiber membrane which occurs in calcium-free media [18, 20], and to a change in the excitationcontraction coupling (ECC) system [19, 23]
It m a y be seen that the twitch is potentiated immediately after the solution change, and that the fiber is able to maintain the contractile response for a long period before the twitch tension falls below the value obtained in the solution with normal calcium content
Summary
Deprivation of external calcium causes sudden potentiation of the twitch response of single muscle fibers. Lfittgau [21] has shown that removal of external calcium causes a fast depolarization of about 10 my, followed by a slower one which progresses at a rate of 1 my per min. In this case, the loss of the contractile response should occur after a relatively long delay if it is caused by depolarization of the membrane. The experiments described in this paper were carried out to extend this information further by studying the short-term effects of calcium deprivation on the electrical and mechanical responses of muscle fibers
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