Sulfhydryls on frog skeletal muscle membrane participate in contraction

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Abstract
To examine the molecular mechanism underlying contractile activation, we studied effects of a sulfhydryl reagent, N-(7-dimethylamino-4-methylcoumarinyl)maleimide (DACM), on twitch, Ag(+)-induced contraction, and K+ and caffeine contractures in single toe muscle fibers of frog. DACM suppressed twitch and Ag(+)-induced contraction, dose dependently, but not caffeine contracture. K+ contracture also was decreased appreciably by exposure to 40 microM DACM for 10 min. DACM elicited no shift of the mechanical threshold or inhibition of resting potential but slightly inhibited action potential. Increase of the fluorescence intensity produced by binding of 10 microM DACM to sulfhydryl groups was depressed by brief pretreatment with 100 microM Ag+. When exposed to 1 mM dithiothreitol (DTT) within 5 s of the rising phase of 5 microM Ag(+)-induced contraction, the fiber rapidly decreased the tension to the resting level. In this case, reapplication of 5 microM Ag+ after washing out DTT elicited a new contraction similar to the first Ag(+)-induced contraction. The second contraction amplitude depended on the time between the onset of the first Ag(+)-induced contraction and DTT application. If DTT was applied after more than 16 s, tension no longer developed on the second exposure to Ag+ or K+. The experiments provide evidence that crucial sulfhydryl groups participate in muscle activation. The possible role of the sulfhydryl group on the transverse tubular membrane in tension development is discussed.