The Sarcoplasmic Calcium Pump
Abstract
The sarcoplasmic reticulum belongs to the most highly specialized membrane structures. The unique function the sarcoplasmic membranes (SP-membranes) are involved in is the regulation of muscle activity. Considerable quantities of calcium (0.1–0.2 µM/ml fibre water) (Weber and Herz, 1963) are assumed to be released from the reticular membranes during a few (1–2) milliseconds when muscular activity sets in. For relaxation these very quantities have to be removed. This is accomplished rapidly by the SP-membranes. Furthermore in order to keep the contractile protein in the state of rest, the membranes must maintain the level of ionized calcium in the myoplasma below 10-7 M (Port Zehl, Caldwell, and Rüegg, 1969). Every or nearly every myofibril is surrounded by these membranes. Therefore, the distance of diffusion of calcium is small so that the calcium movement to and from the myofibrils can take place during the short time interval in which the contraction relaxation cyclus does occur. It has been estimated from electron micrographs that the frog sartorius muscle contains 5 – 10 mg of membrane protein/g of muscle. The surface covered by the system approximates 10000 cm2/g of muscle (Peachey, 1965). From various muscles the SP-membranes have been isolated as membrane fragments with good yields. Astonishingly, the isolated membrane fragments form vesicles which are tightly closed so that the transport of calcium can result in its accumulation inside the vesicles.
Keywords
Calcium Uptake Calcium Oxalate Calcium Transport Calcium Oxalate Crystal Oxalate ConcentrationPreview
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