Brief Communication Modulation of Crossbridge Kinetics by Myosin Isoenzymes in Skinned Human Heart Fibers

Skinned fibers from the normal human heart with the f3-myosin heavy chain (ventricular fibers) revealed both a higher force generation per cross section and a higher Ca2' sensitivity than skinned fibers with the a-myosin heavy chain (atrial fibers). The relation between isometric ATPase activity and isometric tension of atrial fibers was higher than that of ventricular fibers. Since the ATPase-tension relation equals the rate constant for the transition from force-generating into non-force-generating crossbridge states (gapp), myosin heavy chain isoenzymes seem to have different crossbridge turnover kinetics. Modulation of gapp by myosin heavy chain isoenzymes could explain the different contractile behavior of atrial and ventricular fibers. gapp was independent of Ca2+. (Circulation Research 1991;68:614-618)