Determinants of Loaded Shortening Velocity in Single Cardiac Myocytes Permeabilized with

Force-velocity relations were obtained from single cardiac myocytes isolated by enzymatic digestion of rat myocardium and permeabilized with the pore-forming staphylococcal toxin c-hemolysin. Single cardiac myocytes were attached to a force transducer and piezoelectric translator and viewed with an inverted microscope to allow periodic monitoring of sarcomere length during experiments. Permeabilized cells were activated by immersion in a bath of known [Ca 2+j. We report that the Ca 2+ sensitivity of cells obtained by enzymatic digestion and permeabilized using a-hemolysin is similar to that reported previously for mechanically disrupted ventricular myocardium; however, the tension-pCa relation is less steep in the new preparation. During isotonic measurements, force was clamped to various loads using a rapid-response servo system. All recordings of shortening under load were distinctly curvilinear, and analysis of data involved fitting each shortening recording with a single exponential curve and calculating the value of the slope at the initial time of the load clamp. In addition, the presence of significant resting force at initial sarcomere lengths in these cells required that the possibility of alteration of velocity due to the presence of resting force be addressed. The maximum shortening velocity in fully Ca2+-activated single ventricular myocytes studied by this method was 2.83 muscle lengths per second on average. The basis for curvilinear shortening is postulated to be multifactorial in cardiac muscle, involving