ATP-induced Contraction of Sarcomeres*

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Abstract
Single sarcomeres and myofibrillar fragments were isolated from chicken pectoral muscle and the properties of their contraction induced by ATP were measured mainly in 0.04 M KC1 at pH 7.0 and 25[degree]C, in the presence of different concentrations of ATP, MgCl2, CaCl2. ethylenedlamine tetraacetate (EDTA and l,2-bis-(2-dicarboxy methyl amino ethoxy) ethane (EGTA). Changes in the band patterns of myofibrillar fragments during contraction induced by ATP were as follows: in the 1st stage, a dense band appeared at the outer edge of the original H-zone. This band formation was accompanied by a gradual disappearance of the H-zone. In the 2nd stage, the H-zone disappeared completely, the I-band shortened considerably and a dense band developed in the mid region of the sarcomere. Contraction band was formed at the Z-line and bands of lower density appeared at the outer edges of the original H-zone. At the next stage the latter bands fused, and then disappeared. The minimum ATP concentration required for contraction of myofibrillar fragments decreased and approached a constant value on increasing the concentration of MgCl2. Contraction was unaffected by addition of CaCl2 or by treatment with EGTA. Contraction was inhibited by EDTA. The rate of ATP-hydrolysis depended on the concentration of MgATP2-. The average rate (Vs) of the initial sarcomere shortening stage was measured under a microscope from the time-course of the increase in the number of contracted myofibrillar fragments. Vs was related to the rate of ATP-hydrolysis. The average amount of hydrolysis of ATP necessary to cause contraction of myofibrillar fragments to the stage of disappearance of I-bands and formation of faint contraction bands was found to be 5.6[long dash]7.2 [mu]moles Pi/g of protein.

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