Effects of stretch on mechanical and electrical properties of cardiac muscle

A comparative study was made of effects of stretch on cat papillary muscle and auricle strips, hamster ventricular wall, ground squirrel ventricular strip, chicken auricle and ventricle, terrapin auricle and ventricle strips, and carp ventricle. In all species mild stretch increased contractile tension, excessive stretch decreased it, and velocity of conduction was constant with mild stretch but decreased with excessive stretch. In homeotherms under mild stretch, contraction time was unchanged and increased contractile tension was accompanied by increase in velocity of contraction. Under excessive stretch contraction time increased despite decreasing contractile tension. In poikilotherms contraction time and contractile tension were increased by mild stretch. Contraction time decreased with excessive stretch. Stretch increased relaxation time in all species. The evidence suggests that decrease in contractile tension with stretch is related, at least in the initial phases, to a change in excitability and conduction. In all species mild stretch was without effect on transmembrane action potentials, whereas at excessive stretch resting potentials and overshoot declined and rise time of the action potential was greatly increased. At excessive stretch surface-recorded potentials exhibited polyphasic deflections. All observed responses to stretch were reversible. Immediately after stretch the fibers were slightly elongated but developed a higher maximal contractile tension, and contraction time also was slightly increased.