The Dependence of Muscle Contraction and Relaxation on the Intracellular Concentration of Free Calcium Ions

Abstract

Calcium released from the sarcoplasmic reticulum into the sarcoplasm at the onset of activation triggers contraction in which intracellular free calcium is the major determinant of contractile activity. What is the evidence? In 1966, Jöbsis and O’Connor made an important discovery. Toad muscle fibres loaded with the calcium indicator murexide turned faintly red upon electrical stimulation just before the fibre contracted, indicating the formation of a calcium-murexide complex and a rise in Ca²⁺ concentration. Since then calcium-measuring techniques have been greatly improved and applied to many types of muscle, as reviewed by Ashley and Campbell (1979), Blinks et al. (1982), and Thomas (1982). Now the methodology for the determination of the intracellular Ca²⁺ concentration has become a biological discipline in its own right as new methods are being developed by which the Ca²⁺ concentration can be determined more precisely and conveniently. In the following sections we shall see that the quantitative relationship between intracellular Ca²⁺ concentration and contraction is similar in different kinds of invertebrate muscle (Sect. 3.1) and vertebrate muscle (Sect. 3.2), whereas the temporal relationships may vary widely.