Experiments on the Diastolic and Resting Lengths of Cardiac Strips

Abstract

Changes in the length of the restingmuscleand in the diastolic length of the contracting muscle were studied in ventricular strips, papillary muscles, and atrial appendages of mammalian hearts by means of an isotonic lever and optical or photoelectric recording. Spontaneous changes of resting and diastolic length were observed during the first 20-30 min. In exceptional cases, periodic undulation were noted. Increase in the temp. of the bath was frequently followed by increase in resting length (relaxation). Decrease in the temp. of the bath was constantly followed by a decrease in length (contraction). High calcium concn. was followed by diastolic shortening of the strip, then arrest of contractions (contracture). On the other hand, high K concn. was followed by arrest of contractions without demonstrable relaxation. Ouabain, in concns. of 1:15-1:250 million, consistently caused decrease of resting and diastolic lengths (contraction). This shortening reached a maximum of 13%. Epinephrine and nor-[image]-epinephrine in concns. 1:500,000-1:100 million, also caused a decrease of resting and diastolic lengths. Possible differences in the mechanism of action of epinephrine and ouabain are discussed. Acetylcholine had no effect on diastolic and resting lengths. Cocaine and procaine, in concns. ranging from 1:50,000 to 1:1 million, caused an increase of resting and diastolic lengths (relaxation). Stimulation of the muscular strip with a rapid succession of electrical impulses led to decrease of diastolic length. The phenomenon was more constant and more evident if the strip had been cooled or had been treated with high Ca concns. or ouabain before stimulation.