COMPARISON OF UNITARY DISPLACEMENTS AND FORCES BETWEEN 2 CARDIAC MYOSIN ISOFORMS BY THE OPTICAL TRAP TECHNIQUE - MOLECULAR-BASIS FOR CARDIAC ADAPTATION

To provide information on the mechanism of cardiac adaptation at the m olecular level, we compared the unitary displacements and forces betwe en the 2 rat cardiac myosin isoforms, V-1 and V-3. A fluorescently lab eled actin filament, with a polystyrene bead attached, was caught by a n optical trap and brought close to a glass surface sparsely coated wi th either of the 2 isoforms, so that the actin-myosin interaction took place in the presence of a low concentration of ATP (0.5 mu mol/L). D iscrete displacement events were recorded with a low trap stiffness (0 .03 to 0.06 pN/nm). Frequency distribution of the amplitude of the dis placements consisted of 2 gaussian curves with peaks at 9 to 10 and 18 to 20 nm for both V-1 and V-3, suggesting that 9 to 10 nm is the unit ary displacement for both isoforms, The duration of the displacement e vents was longer for V-3 than for V-1. On the other hand, discrete for ce transients were recorded with a high trap stiffness (2.1 pN/nm), an d their amplitude showed a broad distribution with mean values between 1 and 2 pN for V-1 and V-3. The durations of the force transients wer e also longer for V-3 than for V-1. These results indicate that both t he unitary displacements and forces are similar in amplitude but diffe rent in duration between the 2 cardiac myosin isoforms, being consiste nt with the reports that the tension cost is higher in muscles consist ing mainly of V-1 than those consisting mainly of V-3.