216 1. MALONATE 



felt that such an action might have bearing on the mechanisms whereby 

 the cardioactive glycosides stimulate the failing heart. Only the major 

 results will be summarized here. The positive inotropic action occurs most 

 strongly when glucose is present, less in substrate-free medium, and not at 

 all with pyruvate or a-hydroxybutyrate as substrate (Covin and Berman, 

 1956). These results suggested that malonate might stimulate the Embden- 

 Meyerhof glycolytic pathway, resulting in an accelerated conversion of glu- 

 cose and glycogen to pyruvate. If this were so, pyruvate should produce a 

 comparable positive inotropic effect and it does in both substrate-depleted 

 and glucose-supplemented strips. Furthermore, iodoacetate at 0.2 mM 

 blocks the stimulation by malonate, whereas it does not affect the response 

 to pyruvate. The chelation of Ca++ was shown to contribute to the depression 

 produced by malonate at high concentrations (20-50 mM), and it probably 

 reduces the amount of stimulation seen at the lower concentrations since 

 lowering the Ca++ to the degree calculated to occur in 10 mM malonate 

 depresses the contractile activity 23%. The effects of malonate on the oxida- 

 tion of C^*-labeled substrates by ventricle strips were then studied in cham- 

 bers in which the respiration and contractile activity could be determined 

 simultaneously (Rice and Berman, 1961). Malonate at 5.6 mM under con- 

 ditions in which a positive inotropic effect is observed has very little effect 

 on the utilization of glucose- 1-C'^, glucose-6-C^*, and pyruvate-2-C^*, slight 

 inhibition of glucose oxidation being noted although this is possibly not 

 significant. These results indicate that the stimulatory action is not related 

 to (1) acceleration of glucose metabolism, (2) inhibition of the cycle, or (3) 

 stimulation of the pentose-phosphate pathway. It was found that C^^Og is 

 produced from malonate-2-C^* in ventricle strips, and possibly part of the 

 positive inotropic action in substrate-depleted strips is related to the oxida- 

 tion of malonate via formation of acetyl-CoA and its incorporation into 

 the cycle. However, the explanation for the greater effect of malonate in 

 the presence of glucose and the inhibition of its action by iodoacetate is 

 not immediately evident. It may be noted that other metabolic inhibitors, 

 such as fluoride, arsenate, fluoroacetate, and dehydroacetate, can exert po- 

 sitive inotropic actions under the appropriate conditions, so this paradoxical 

 effect of malonate is not unique. 



Wenzel and Siegel (1962) determined the dose-response curves for the 

 positive inotropic effects of malonate and ouabain on the rat ventricle strip, 

 and then constructed an isobologram, plotting the malonate concentration 

 against the ouabain concentration for a chosen contractile stimulation. 

 Since the isobol sags, i.e., is concave upwards, they claimed it is clear that 

 potentiation occurs and that this indicates the sites of action of malonate 

 and ouabain are different. There is some doubt that a moderately sagging 

 isobol can be interpreted as potentiation, inasmuch as pure summation 

 often elicits such a curve (see Figs. 1-10-7, 8). 



