Effect of Tension on Induction of Automaticity by Epinephrine in Papillary Muscle.

• It has often been reported that increased arterial blood pressure leads to the production of cardiac arrhythmias. 1-0 Similarly, increased pacemaker activity occurs when various cardiac tissues are stretched 7> 8 and recently Keatinge 9 also showed that quiescent isolated rabbit atrium or ventricle will start to beat when the pressure inside is raised. The present experiments were designed to determine the effects of increase in tension upon induction of pacemaker activity (auto-maticity) in cat papillary muscle. An isolated preparation of this muscle is especially suitable for such a study since it is normally quiescent but can be excited by epinephrine. The threshold dose of epinephrine required to induce repetitive contractions remains the same for several hours I0 and changes in threshold due to increased tension on the muscle can readily be determined. Methods Papillary muscles of 1.0 to 1.5 mm in diameter were taken from the right ventricles of 34 cats (0.4 to 0.7 kg) killed by a blow on the head. These muscles were suspended vertically in a 10-ml bath containing Krebs-Henheleit solution bubbled with 95% oxygen and 5% carbon dioxide and kept at 37°C. Tension was recorded iso-metrically with a Baldwin strain gauge and a Grass polygraph. The upper end of the muscle was attached by a thread to the strain gauge. The lower end was attached to a glass rod which could be raised or lowered in relation to the strain gauge by a Palmer rack-work "x" block. Slow changes in tension on the muscle were made by altering the muscle length in this way. In experiments where tension was to be applied rapidly, a 4 or 9 g weight was suspended from the strain gauge. The tension on the muscle was then adjusted by moving the rack so that the total tension registered on the polygraph record was 5 or 10 g. The tension on the muscle was therefore 1 g. The tension could be changed rapidly to 5 or 10 g by removing the weight with a lever. The time course of the tension change when the weight was removed was followed in several experiments by running the polygraph paper at 50 mm/sec. The tension change was always completed within 0.2 sec. Tension changes due to removal and immediate replacement of the weight were completed within 0.8 sec. A tension of one gram was kept on each muscle before tests were begun. Only …