The link between skeletal and smooth muscle : Evolution of E-C coupling factor

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In this study, I have investigated the link factor which converts the smooth muscle to the skeletal muscle using marine animals.Marine animals have a large varieties of combinations of organelles, which control the contraction of the muscle cells, i. e., sarcoplasmic reticulum (SR), transvers tubular structure (T system), Ca channel, ryanodine receptor and DHP receptor.Heart muscles of Ascidia Ciona, which has SR but not T system, requires extracellular Ca for its contraction and usually behaves like smooth muscle.But by adding caffeine or replacing external Cl to SCN, heart muscle contracted by electrical stimulations in the absence of external Ca like skeletal muscle.On the other hand, on pelagic tunicate Doliolum, which lacks T and SR organelles, muscle contracted only in the presence of external Ca, even if caffeine was present.It is suggested that the presence of SR is essential for skeletal muscle type contraction and the E-C coupling factor exist on SR even if it is not active.The coupling factor suggested to be existed in smooth muscle SR but the coupling might be very loose.By developing the coupling, smooth muscle maight be converted to skeletal muscle.