DUGALD E. S. BROWN 101 



sion of tlie cleavage furrow in the yea urcliin egg. Here the action is on 

 the cortical gel of the egg which is solated by pressure to a degree depending 

 on the temperature. If, in both muscle and the cortical gel, the decrease 

 in tension results from the conversion of a fibrous form of protein to the 

 globular form with a decrease in volume of about 120 cc/mole, then the 

 entropy change in the muscle would be about three times that for the 

 cortical gel. The difference might turn out to depend on the fact that 

 contraction in muscle involves a protein complex with three active sites 

 while the gel has but one. 



In regard to the pressure effect at constant temperature, two situations 

 in the treppe heart are critical. First, at 5°-8°C as shown by the tem- 

 perature dependence, the 'active state' is apparently sufficiently pro- 

 longed to permit maximum tension to develop and here pressure inhibits 

 tension. Second, and in contrast, at 20°C the tension is reduced and 

 compression increases the tension to a level equaling that of the maximum 

 at 9°C and atmospheric pressure. 



In regard to the inhibition at 5°C the plot of logi,, [y/(l — y)] against 

 pressure is linear and the decrease in tension proceeds with a decrease 

 in volume of 120 cc/mole (fig. 45-2). This is attributed to the con- 

 version of fibrous to globular protein in equilibrium (B). 



At 20°C, pressure is remarkably effective in increasing the tension. 

 In experiments designed to study this pressure relation, hearts which gave 

 only 20% of the maximum tension at 20°C were prepared. These hearts 

 permitted measurements over a considerable range of tensions. The ten- 

 sion data, again plotted as logio [y/(l - y)] against pressure, are 

 linear and show that the change in tension, which under these conditions 

 is in the direction of an increase rather than a decrease, proceeds in 

 accordance with a volume change of 710 cc/mole (fig. 4C-1). At first 

 glance this appears to be an unreasonable value, but it deserves con- 

 sideration in the light of recent results on the production of contractures 

 by withdrawal of chloride (28) and the addition of potassium (41), 

 respectively. In both types of contractures induced by ionic changes, the 

 relation between tension and ion concentration is described by logio 

 [y/(l — y)] = n log [ion] where for CI" and K+, n is between 5 and 6. 

 Here, the value of n is twice as large as the value of n with respect to pH 

 or Ca++ in the glycerated fiber. In muscle, however, both chloride with- 

 drawal and potassium excess cause depolarization of the cell membrane. 

 The view is currently held that the contracture is a consequence of the de- 

 polarization of the cell. If, in these ion-induced contractures, depolarization 

 produces an activator Ca , each unit of which induces the formation of 

 two AAIar^ units, a value of n = 6 would result. 



In the case of the cardiac twitch, it is therefore suggested that the 



