Excitation-contraction coupling (EC) describes the events that lead from electrical stimulation of muscle to the initiation of muscle contraction. The time course of Ca2+ release from the SR during muscle contraction is of great interest. Ashley and Ridgway (1968) were the first to study this relationship. They monitored changes in Ca2+ concentration during muscle contraction by injecting aequorin, a Ca2+-binding bioluminescent protein, into muscle fibers. Upon Ca2+-binding aequorin emits light that can be measured; following the emission of light, aequorin is inactivated and the bound Ca2+ is released.
An aequorin-injected muscle fiber was electrically stimulated and first the action potential was registered. This was followed by light emission reflecting changes in intracellular Ca2+ concentration. When the Ca2+-mediated light output reached its peak, tension developed, by the time maximal tension was produced the Ca2+-mediated light output died away (Fig. EC1).
Fig. EC1. Time relationship of action potential, Ca transient and tension. (From Ashley and Ridgway, 1968).
Currently, fluorescent Ca2+ indicators, such as fura-2 and quin-2, are used to measure changes in intracellular Ca2+ concentrations in the nanomolar to micromolar range. These indicators are excited at slightly longer wavelength when they are free of Ca2+ than when in their Ca2+- bound form. By measuring the ratio of fluorescent intensity at two excitation wavelengths the free Ca2+ concentration can be calculated.
The release of Ca2+ from SR when studied in frog muscle by confocal microscopic imaging of the fluorescent indicator, fluo-3, reveals that the release occurs largely in the form of discrete events, termed Ca2+ sparks. It is not clear whether the sparks are the results of opening of individual channels or of concerted opening of channel clusters (Rios et al., 1999).
Sequence of events: Upon stimulation of the muscle, an action potential propagates over the sarcolemma, travels through the T-tubules and elicits Ca2+ release from the SR into the sarcoplasm. Ca2+ binds to TN and the inhibition of actin-myosin combination that prevails at rest is lifted, and contraction ensues. Ca2+ is the link between excitation and contraction. Fig. EC2 shows that in the resting muscle (A) the membrane is negative on the inside and positive on the outside (left part of the Figure). In the shortened muscle (B), one notes the reversal of polarity upon stimulation and Ca2+ release from the terminal cisternae of SR toward the filaments (right part of the Figure).
Fig. EC2. Sequence of events during excitation-contraction coupling. (From Keeton, 1972).
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