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To study the mechanical properties of various crossbridge states in the anterior byssus retractor muscle (ABRM) of Mytilus, the tension response of the glycerinated ABRM to a step increase of the length was examined in rigor solutions with saturating Ca2+ (Rca) and without added Ca2+ (R), rigor ones with ADP (AD) and with both ADP and saturating Ca2+ (ADca), and a low ATP, activating one. The application of ADca to a rigor ABRM caused a slow tension development of less than 0.083 kg/cm2 (n = 6) probably because of contaminant ATP in the presence of 0.25 mM p,p-di(adenosine-5')pentaphosphate (Ap5A) and 10 U/ml hexokinase with 2 mM glucose. The instantaneous stiffness in ADca was slightly smaller than that in the low ATP solution and greater than those in R, Rca, and AD, giving evidence that the stretch response reflected the mechanical property of the crossbridges. The rate of the tension decay during the initial 5 ms after the length change was completed was slowest in the ADca among the solutions examined, while during the initial 30-90 ms it was faster in the low ATP solution than R, Rca, AD, and ADca with little difference of the rate in the latter four solutions. The difference in the time course of the tension decay in between the low ATP solution and ADca may be taken to indicate that the high stiffness in ADca was not due to the formation of the tension-generating crossbridges but to the crossbridges with both bound Ca and ADP (AMCaADP) made directly from the rigor crossbridge (AM). Consequently, it was thought that AMCaADP was stiffer than AM and the crossbridges with either bound Ca (AMCa) or ADP (AM.ADP), the latter three kinds of the crossbridges being formed directly from AM, not as a result of ATP hydrolysis.

Effects of Calcium and ADP on Tension Responses to Step Length Increases in Glycerinated Mytilus Smooth Muscle.