solution. At higher pH values the fibers were 

 relatively plastic. The authors think that parallel 

 with the actoniyosin system which produces initial 

 tension of the adductor there is a second, or 

 paramyosin system, capable of maintaining the 

 tension developed by the first one by crystallization 

 of the paramyosin component caused by pH 

 shift within the muscle. The theory was tested 

 by Hayashi, Rosenbluth, and Lamont, (1959) on 

 the nmscle extracts of Mercenaria (Venus) mer- 

 cenaria and Spisula solidissima. The results of 

 these experiments tend to support the hypothesis 

 that crystallization of paramyosin elTectively 

 freezes the adductor nmscle at any state of 

 contraction. 



In two papers dealing with the fine structure of 

 the small fibers of the oyster {C. angulata) and 

 other bivalves, Hanson and Lowy (1959, 1961) 

 have proposed two possible explanations of the 

 mechanism by which the closing muscles of mol- 

 lusks maintain tension "very economically," 

 i.e., without using much energy. According to 

 their view, based on examination of electron 

 micrographs of muscle, the thick filaments of the 

 fibril (see p. 157 and fig. 144) are discontinuous 

 and do not contract; they slide as the nmscle 

 shortens the relative portions of the thick and thin 

 filaments. The tension is maintained by cross 

 links between the two types of filaments. Accord- 

 ing to their view the alternative hypothesis, which 

 supposes that tension is maintained by change 

 in the physical state of the protein within a 

 paramyosin system, is diflRcult to reconcile with 

 their observations. The sliding or so-called 

 interdigitatory model of the contractile structm-e 

 is based primarily on the studies of striated muscle 

 (Huxley, 1960), and the extension of the theory to 

 nonstriated muscles of bivalves is very attractive. 

 It is impossible, however, to state at present which 

 of the two theories interprets correctly the catch 

 mechanism. Further experimental studies are 

 needed to solve the puzzle which for a century 

 has baffled the biologist. 



In spite of the substantial advance of bio- 

 chemical investigations, the problem of the lock- 

 ing mechanism recjuires further study. So far 

 no evidence has been presented to show that the 

 shift in the pH needed for the crystallization of 

 paramyosin actually takes place in the whole 

 living muscle of a bivalve. It seems that the 

 solution to the locking paradox should consider 

 the problem in its entirety, by taking into account 



all the biochemical and biophysical processes 

 which accompany the prolonged tonus of the 

 adductor muscle. 



Chemical changes during muscular activity 



Chemical changes occurring during the contrac- 

 tion and relaxation of the nmscle are extraordinarily 

 complex. The reader interested in this problem 

 should consult the textbooks of general physiology 

 (Scheer, 1948), biochemistry (Needham, 1932; 

 Baldwin, 1957), and particularly the comprehen- 

 sive reviews of more recent works given by Szent- 

 Gyorgyi (1951) and Weber (1958). Most of the 

 work on the chemistry of muscular contraction 

 has been performed on vertebrate muscles. In 

 general the results were found to apply to the 

 muscles of scallops {Pecten, Astropecten, Chlamys), 

 sea mussels (Mytilus), edible oysters {Ostrea, 

 Crassostrea), and Anodonta. 



A complex chain of events is involved in mus- 

 cular contractions. I will consider only the high 

 points. Glycogen appears to be the principal, 

 if not the only source of energy in this process. 

 Its content in the adductor muscles of bivalves 

 varies from less than 1 to about 3 percent. The 

 immediate source of energy for muscular con- 

 traction is not derived, however, from the break- 

 down of glycogen. Considerable quantities of 

 phosphate are released by the organic compounds 

 called phosphagens. These substances contain 

 (Weber, 1958, p. 5) an energy-rich phosphate 

 bond and, therefore, are the "stores of immediately 

 available energy." Creatine phosphate, identified 

 as a phosphagen of vertebrate muscle, does not 

 (iccur in mollusks; its place is taken by arginine 

 phosphate. Phosphagen decreases during con- 

 traction and is formed again during rest. After 

 prolonged contractions the tissues of the fatigued 

 muscle become acidic due to the accumulation of 

 lactic acid. Glaister and Kerly (1936) found that 

 iodoacetate, which inhibits the formation of lactic 

 acid in Mytilus nmscle does not materially inter- 

 fere with muscular contraction. 



The key substance involved in the energy trans- 

 formatiDU in the muscles is, however, adenosine- 

 triphosphate (ATP); the presence of ATP is a 

 prerequisite to contraction. According to Szent- 

 Gyorgyi's theory ATP has a great affinity to 

 myosin and is strongly linked to it. Excitation 

 of the muscle implies the formation of actomyosin 

 (from actin -\- nayosin), a process which does not 

 take place in the absence of ATP (Szent-Gyorgyi, 



THE ADDUCTOR MUSCLE 



167 



