The rate constant for dissociation of ADP from actomyosin subfragment 1 (Si) has been measured in this laboratory and elsewhere for a variety of vertebrate muscle types. We have made the following observations: (i) In solution, the dissociation of ADP from actomyosin-Sl limits the rate of dissociation of actomyosin-Sl-ADP by ATP and, presumably, also limits the rate of crossbridge detachment in contracting muscle. (it) For muscle types in which the rate of ADP dissociation from actomyosin-Sl is slow enough to measure using stopped-flow methods, the rate constants are nearly the same as the theoretical value for the minimum allowable rate constant for dissociation of an attached crossbridge. Therefore, ADP dissociation is sufficiently slow to be the molecular step that limits the maximum shortening velocity of these muscles. (i) Variation with muscle type of the rate constant for ADP dissociation may be a general phylogenetic mechanism for regulating shortening velocity.that one or more of the rate constants of the ATP hydrolysis mechanism limits the muscle contraction rate. For a molecular step to limit the shortening velocity of muscle, it must have the following attributes:(i) The rate constant for such a step must be consistent with both the maximum working length of an individual crossbridge and the rate of axial displacement of the thick and thin filaments observed in contracting muscle. The minimum allowable rate constant, kmin, for the conversion of an attached crossbridge state in muscle (corresponding to the AM states in the top line of Eq. 1) to other attached or detached states (corresponding to the M states in the bottom line of Eq. 1) can be estimated from the maximum rate of contraction (the unloaded shortening velocity) and the distance over which a crossbridge can remain attached, using Eq. 2, Muscle contraction is thought to occur as the result of a cyclic association and dissociation of crossbridges formed between myosin molecules in the thick filaments and F-actin molecules in the thin filament, involving concomitant hydrolysis of ATP. This cycle can lead to relative sliding of the actin and myosin filaments and result in the production of work. A rationale for studying the kinetic mechanism of ATP hydrolysis by actomyosin-Sl in solution is that it may directly relate to the observed physiological properties of muscle. A condensed version of the kinetic mechanism, consistent with recent observations (1, 2), is shown in Eq. 1. The second-order reactions of nucleotide and phosphate binding have been shown to be two-step reactions (3-5), but they are condensed here to single steps:where M represents myosin subfragment-1 (Si) and AM represents actomyosin S1. Different types of vertebrate muscles have shortening velocities that vary by almost two orders of magnitude. Barany showed that the steady-state rate of hydrolysis of MgATP by actomyosin is correlated with muscle shortening velocity (6).However, it has not been demonstrated that the rate of ATP hydrolysis directly limits shortening ve...

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Single intramuscular injections of aflatoxin B1 into the tail muscle of Penaeus stylirostris produced 24-and 96-h median lethal doses of 100.5 (78.3 to 129.0) and 49.5 (29.8 to 82.3) mg/kg, respectively. A toxicity curve showed no threshold at the levels tested. The mortality response in a feeding study with P. vannamei was not dose dependent, but tissue and organ damage were similar to that seen in injected animals.