Muscle excitation/contraction coupling

What is the role of intracellular Ca2+ waves due to Ca2+ release from the SR In many cell types stimulation results in Ca2+ waves rather than a maintained increase of [Ca2+] . Summation of such waves in many cells can result in a maintained contraction. Indeed, recent work suggests that these waves are implicated in the genesis of vascular tone (Peng et al 2001). It is important for us to consider how widespread in smooth muscle excitation-contraction coupling are such waves. 

Taggart MJ 2001 Smooth muscle excitation-contraction coupling a role for caveolae and caveolins News Physiol Sci 16 61-65... 

Cavl.l L-type CClS skeletal muscle excitation-contraction coupling, gene transcription dihydropyridines (DHPs) lethal... 

Although it is convenient for physiologists to categorize smooth muscle excitation-contraction coupling mechanisms, it is important to understand that electromechanical and pharmacomechanical coupling mechanisms are complexly intertwined in intact... 

No. Voltage-gated calcium channels are found only in smooth and cardiac muscle. Excitation-contraction coupling in skeletal muscle depends on the release of intracellular calcium from the sarcoplasmic reticulum. 

Excitation-contraction coupling (EC coupling) is the mechanism underlying transformation of the electrical event (action potential) in the sarcolemma into the mechanical event (muscle contraction) which happens all over the muscle. In other words, it is the mechanism governing the way in which the action potential induces the increase in the cytoplasmic Ca2+ which enables the activation of myofibrils. 

Four different localizations of fatigue can be identified (a) decreased central command (b) decreased activation of the muscle membrane and the T-tubular system (c) decreased Ca release from the SR and (d) decreased response to the Ca release by the contractile proteins. The first two are partly extra-muscular while c and d are intramuscular responses to the excitation of the muscle membrane and often defined as excitation-contraction coupling. 

From this brief summary of excitation-contraction coupling it is obvious that Ca is an important link between the activated membrane and the contractile proteins, and thus a regulator of tension development. Westerblad et al. (1991) defined three factors which explain the force decrease in fatigued muscle reduced Ca " release from the SR, reduced Ca sensitivity of the myofilaments, and reduced maximum Ca -activated tension. 

It has been shown that inositol triphosphate (IP3) is involved in the excitation-contraction coupling in smooth muscle (Vergara et al., 1985), but presently no clear evidence has been reported for a similar involvement in skeletal muscle. If IP3 functions as a messenger for Ca release, it would bridge the gap between muscle metabolic changes and Ca release, as ATP is a prerequisite for IP3 regeneration. 

Schneider, M.F. Chandler, W.K. (1973). Voltage dependent charge movement in skeletal muscle A possible step in excitation-contraction coupling. Nature 242,244-246. 

Vergara, J., Tsien, R.Y., Delay, M. (1985). Inositol 1,4, 5-trisphosphate Possible chemical link in excitation-contraction coupling in muscle. Proc. Natl. Acad. Sci. USA 82,6352-6356. 

A different but very interesting scenario involving L-type Ca channels is seen in skeletal muscle, where the major component of these Ca channels plays two roles. Skeletal muscle does not require extracellular Ca for excitation-contraction coupling, rather it utilizes Ca stored in the sarcoplasmic reticulum. The role of the L-type channel proteins as true Ca channels in skeletal muscle appears to be of secondary importance, but may be to provide Ca to the cells over longer periods of time. The main role of the L-type channel protein(s)... 

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Somlyo AP 1985 Excitation-contraction coupling and the ultrastructure of smooth muscle. Circ Res 57 497-507... 

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Cheng H, Lederer WJ, Cannell MB 1993 Calcium sparks elementary events underlying excitation—contraction coupling in heart muscle. Science 262 740—744 Collier ML, Thomas AP, Berlin JR 1999 Relationship between L-type Ca2+ current and unitary sarcoplasmic reticulum Ca2+ release events in rat ventricular myocytes. J Physiol (Lond)... 

Tanabe T, Beam KG, Adams BA, Niidome T, Numa S 1990 Regions of the skeletal muscle dihydropyridine receptor critical for excitation-contraction coupling. Nature 346 567-569... 

Calcium release events in excitation-contraction coupling in smooth muscle... 

Bolton TB, Aaronson PI, MacKenzie I 1988 Voltage-dependent calcium channel in intestinal and vascular smooth muscle cells. Ann NY Acad Sci 522 32 -2 Bolton TB, Prestwich SA, Zholos AV, Gordienko DV 1999 Excitation-contraction coupling in gastrointestinal and other smooth muscles. Annu Rev Physiol 61 85—115 Bramich NJ, Hirst GDS 1999 Sympathetic neuroeffector transmission in the rat anococcygeus muscle. J Physiol 516 101—115... 

Cheng H, Lederer WJ, Cannell MB 1993 Calcium sparks elementary events underlying excitation-contraction coupling in heart muscle. Science 262 740—744... 

Burdyga TV, Wray S 1999b The effect of cyclopiazonic acid on excitation-contraction coupling in guinea-pig ureteric smooth muscle role of the sarcoplasmic reticulum. J Physiol 517 855-865... 

Devine CE, Somlyo AV, Somlyo AP 1971 Sarcoplasmic reticulum and excitation-contraction coupling in mammalian smooth muscles. J Cell Biol 52 690-718 Ebashi S 1991 Excitation—contraction coupling and the mechanism of muscle contraction. Annu Rev Physiol 53 1—16... 

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