CARDIAC MUSCLE CONTRACTILITY 



183 



phosphate does not exchange readily with the bulk of 

 the intracellular phosphate (22, 124). [For further 

 details on these points see reference 113-] 



Action of Glxcosides on Muscle Proteins and Models 



In an attempt to define the locus of action of the 

 cardiac glycosides, many investigators have examined 

 the effect of ghcosides on a variety of muscle frac- 

 tions or models of the contractile mechanism. These 

 studies will be enumerated briefly. 



/) G-F transformation of actin. It was found in 1949 

 (153) and subsequently confirmed (58, 281, 334) that 

 the rate of polymerization of solutions of globular 

 actin to form the long chain called F actin was in- 

 creased in the presence of strophanthin or digitoxin. 

 However, Wollenberger (334) made an important 

 contribution to the problem by comparing the effects 

 of pairs of glycosides which were structurally similar 

 but one of which had no pharmacological activity 

 in whole animals. Both members of two such pairs in- 

 creased the rate of polymerization to the same extent, 

 showing that there was no correlation between in vivo 

 pharmacological activity and the effect on the iso- 

 lated protein. In the absence of such a correlation it is 

 difficult to conclude that the observed effect is re- 

 lated to the pharmacological action of the drug. Many 

 of the studies enumerated below cannot be judged be- 

 cause of the absence of control observations with a 

 pharmacologically inactive glycoside. 



2) Actomyosin viscosity. Addition of glycosides causes 

 a verv small decrease in the viscosity of actomyosin 

 solutions, but both pharmacologically active and in- 

 active compounds have the same effect (306, 309). 

 The drop in viscosity induced by ATP is not affected 

 by cardiac glycosides (68, 309). 



5) Muscle enzymes. ATPase activity is either in- 

 creased or decreased by ouabain, depending on the 

 purity of the preparation (68, 133). Digoxin appears 

 to have no effect on myosin ATPase activity, provided 

 the preparation is free of myokinase, but does enhance 

 the conversion of ADP to ATP which occurs in the 

 presence of myokinase (237). Strophanthin has little 

 or no effect on the activity of adenosine deaminase 

 prepared from mouse myocardium (217), but oua- 

 bain in very large doses inhibits deaminase acti\it\' of 

 guinea pig heart preparations (7). 



4) Potassium binding. The only in\estigator who has 

 made a correlation between in vivo pharmacological 

 activity and any effect on isolated contractile proteins 

 has been Waser (308), who has shown that moderate 

 concentrations of cardiac ijhcosides cause increased 



potassium binding by isolated actomyosin. He also 

 showed that these drugs decreased the thixotropy 

 (tendency to form a gel on standing) of actomyosin 

 (306, 307). In none of these studies did pharmacologi- 

 cally inactive glycosides have a corresponding action. 



5) Shortening of actomyosin threads and bands. Mallov & 

 Robb (204) first showed that the ATP-induced short- 

 ening of threads of skeletal muscle actomyosin was 

 increased by addition of a cardiac glycoside. This was 

 later confirmed for both skeletal (25) and heart (244) 

 muscle actomyosin. Kako & Bing (162a) showed that 

 the contractility ( % shortening vs. load) of actomyosin 

 bands prepared from hearts of patients with congestive 

 failure was significantly lower than that of normal 

 hearts. In the presence of digoxin plus io~"^m Ca the 

 contractility of the failure protein increased to the 

 normal range. 



6) Glycerol-extracted muscle. The divergent results ob- 

 tained with this preparation (69, 178, 288) probably 

 reflect the differences caused by small variations in the 

 glycerination procedure (see al.so section 11). 



In summary, glycosides have effects on various 

 properties of muscle cell proteins, but to date Waser is 

 the only investigator who claims a correlation between 

 a protein effect and in \'ivo pharmacological activity. 



Site of Glycoside Action 



It was stated at the beginning of this section that 

 glycosides are thought to act by altering the capacity 

 of the contractile protein to utilize the chemical 

 energy made available to it. This might occur by some 

 direct action of the drug on the contractile protein, as 

 suggested by some of the results noted in the previous 

 section, or it might be brought about indirectly by a 

 change in the cellular chemical environment induced 

 by the drug. There is a large body of literature in- 

 dicating that the glycosides can induce cellular chemi- 

 cal changes by interfering with normal membrane 

 transport of inorganic ions (see next section). It be- 

 comes pertinent to ask, then, whether the glvcosides 

 can enter the cell interior where interaction with the 

 contractile protein might occur, or whether they do 

 not penetrate beyond the cell membrane. It is clear 

 from a number of studies that glycosides bind to cer- 

 tain proteins, including actomyosin (70, 298, 305), and 

 their rapid disappearance from the blood suggests 

 that they are accumulated preferentially by a tissue 

 phase (284). Binding to serum albumin fractions has 

 been reported also (107, 254), but this seems to be due 

 to something associated with salt fractionation of the 

 albumin (either a small change in the albumin or 



