The Sarcoplasmic Reticulum: Storage and Release of Calcium

Abstract

It is now well known that the contractile system of muscle fibres becomes activated when the intracellular calcium ion concentration increases to about 1 to 10 µM and that it is turned off when it is lowered to below about 0.1 µM. Here, we shall describe the structures and mechanisms that raise and lower the intracellular free calcium ion concentration quickly during the contraction-relaxation cycle of various types of muscle. In very thin muscle fibres, such as those of the frog heart (Fabiato 1983), vertebrate smooth muscle (Deth and van Breemen 1974), and in the myotome of Branchiostoma lanceolatum (Hagiwara et al. 1971; but cf. Melzer 1982a, b), the activating calcium ions seem to originate, at least partly, from the extracellular fluid and enter the cell through calcium channels of the cell membrane during activation. Bianchi and Shanes (1959) detected a small calcium influx even in stimulated skeletal muscle, but Sandow (1965) calculated that the amount of calcium entering the stimulated muscle cell is too small and its diffusion into the interior of the fibre too slow (cf. also Hill 1948) to account for the rapid onset of contraction. In these fast and wide-fibred muscles the activator calcium is released from and recuperated by the sarcoplasmic reticulum (Hasselbach 1964) during contraction and relaxation (Sects. 2.2 and 2.3).