Lactic acid in mammalian cardiac muscle. —Part I. The stimulation maximum

Abstract

The earlier work of Fletcher and Hopkins (1), and the more recent researches of A. V. Hill (2), Meyerhof (3) and their collaborators on the role of lactic acid in muscular contraction, have shown that oxygen is not required for the production of lactic acid, but only for its removal during the recovery process. This fact, coupled with the ability of a skeletal muscle to respond to stimulation until its lactic acid content is many times the resting value, enables the skeletal muscle to draw on its future supply of oxygen and to go into “oxygen debt.” The question naturally arises, are these facts also true of the heart muscle? Can the heart respond to stimulation when it has accumulated as large a concentration of lactic acid as is found in the skeletal muscle? In other words, How does the stimulation maximum of lactic acid of the heart compare with that of the skeletal muscle? A priori certain differences might be expected in view of the differences in the activity and nutrition of cardiac and skeletal muscles. In the first place the heart is not called upon to increase its energy output to anything like the extent of the skeletal muscle; nor does the heart have any long periods of comparative rest. It is always active and is contracting roughly one-third of the time. In addition the heart is provided with a very efficient circulation and has the first call on the oxygenated blood. For these reasons there seems to be no apparent need for the heart to have the ability of accumulating lactic acid to the same extent as the skeletal muscle; indeed, in view of the necessity for its constant activity, it would seem dangerous for it not to possess a mechanism by which the removal of lactic acid keeps pace with its production.