200 1. lODOACETATE AND lODOACETAMIDE 



reliable inhibitor for this purpose. Some of the characteristics of this rigor 

 in skeletal muscle will be described in this section, and in subsequent sec- 

 tions rigor in other types of muscle will be treated, following which the 

 various theories of iodoacetate rigor will be summarized. 



(A) Mechanical characteristics of rigor. The muscle treated with iodoace- 

 tate changes from a soft-sticky to a dry-hard state, just as in post-mortem 

 rigor (Bendall, 1960), and there is a loss of extensibility, plasticity, and 

 excitability. Although the shortening of an unloaded muscle in rigor is the 

 same as in maximal isotonic tetanus (around 60%), the tension developed 

 under loaded or isometric conditions is less than in a tetanus (Sandow and 

 Schneyer, 1955). Spontaneous iodoacetate rigor in unstimulated muscle 

 generates 16-17% of maximal tetanus tension, while activity rigor in stim- 

 ulated muscle generates 35-40% of maximal tetanus tension; the latter 

 is approximately equivalent to the twitch tension. The tension developed 

 in iodoacetate rigor may be a good deal more than in post-mortem rigor 

 (Lippay and Patzl, 1935). Rigor tension is dependent on a number of fac- 

 tors — especially the nutritional, metabolic, and contractile activities of 

 the muscle previous to rigor — but is generally not dependent on the iodo- 

 acetate concentration, i.e., when rigor begins to develop it proceeds to com- 

 pletion in an all-or-none manner. However, the kinetics of rigor vary with 

 the iodoacetate concentration. The initiation of rigor follows a lag period 

 which depends on the rate of penetration of the inhibitor into the muscle, 

 the rate of reaction within the muscle cells, the level of high-energy phos- 

 phate bonds present, and the rate at which ATP is being used or hydrolyzed. 

 The higher the iodoacetate concentration, the shorter this lag period up 

 to a point. Thus the lag period in frog sartorius is around 10 min at 0.2-1 

 mM iodoacetate (and presumably this would not be decreased at higher 

 concentrations), 45 min at 0.1 mM, and 180 min at 0.02 mM (Sandow and 

 Schneyer, 1955). Once rigor begins it develops at about the same rate except 

 for the lowest concentration of iodoacetate, where it is definitely slower. 

 The maximal shortening reached is the same in each case. 



It has been known since the earliest studies that the lag period for rigor 

 depends on the activity of the muscle, and rigor has been classified into 

 different types on this basis. Lippay and Patzl (1935) stated that there 

 are three kinds of rigor: (1) that of intact muscle with blood supply and 

 innervation, with a lag period around 1 hr, (2) that of muscle stimulated 

 to contract through its nerve, with a lag period of several minutes, and (3) 

 that of denervated unstimulated muscle, with a lag period of several hours. 

 Sandow and Schneyer (1955) similarly classified rigor into spontaneous rigor 

 and activity rigor, depending on whether the muscle is resting or contracting 

 following exposure to iodoacetate. Indeed, a muscle that is fatigued before 

 exposure to iodoacetate will pass into rigor much more rapidly than a nor- 

 mal muscle. Although the time relations may be quite different under these 



