Basic study of the contractile force of the myocardium

Abstract

Since the experiments conducted in 1950 by WALTON and his co-workers in direct measurement of the isometric ventricular contractile force of the myocardium, several problems in this field have remained unsolved. Among these is the development of an efficient recording system for measuring these factors in living animals. This paper sets forth the results of the author's experiments aimed at developing such a system. The fundamental studies for this report were made on frogs and toads and were for the purpose of determining the influence of the pulse rate on the action potential and the contractile force of the myocardium. Method The toad was anesthetized with chloroform and placed in an upside-down position. The heart was exposed and its longitudinal contractile force was recorded by the suspension method. Needles were inserted at two points on the surface ofthe myocardium to record the action potential. The frog's heart was removed and a cross-section cut out. The cross-section was then cut in one place and stretched into an oblong. The strip wasthen place in a sepa-ration chamber filled with Ringer's solution. The tension was measured by a wire straingauge and the action potential was recorded by means of non-polarizable electrodes on both sides of the partition. The dog was anesthesized with thiopental-Na and the chest was cut open. The legsof a tiny metal bridge to which were attached semi-conductor strain-gauges, were fixed directly to the ventricular surface with a silk thread or Aron Alfa (α-ciano aclylate) paste. Using this method, the tension curve of the local myocardium was recorded. At the same time the left ventricular pressure curve, ECG and direnct-ECG were recorded. (Note: When artificial stimulation was used in these experiments, square wave pulse stimulation was employed.) Results & Summary 1) In the experiments conducted on frogs and toads, as the interval between the two stimuli approached a certain point, usually 1-1.2 sec., the second tension curve rose rapidly ; after that, as the interval increased, the tension curve tapered off gradually. In the action potential, the longer the interval between the two stimuli, the greater the second action potential. The point at which the action potential ceased to increase rapidly was found to be the same point at which the tension curve began to decrease from its highest point, i.e., 1-1.2 seconds between stimuli.