Abstract-To provide information on the mechanism of cardiac adaptation at the molecular level, we compared the unitary displacements and forces between the 2 rat cardiac myosin isoforms, V 1 and V 3 . A fluorescently labeled actin filament, with a polystyrene bead attached, was caught by an optical trap and brought close to a glass surface sparsely coated with either of the 2 isoforms, so that the actin-myosin interaction took place in the presence of a low concentration of ATP (0.5 mol/L). Discrete displacement events were recorded with a low trap stiffness (0.03 to 0.06 pN/nm). Frequency distribution of the amplitude of the displacements 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 unitary displacement for both isoforms. The duration of the displacement events was longer for V 3 than for V 1 . On the other hand, discrete force transients were recorded with a high trap stiffness (2.1 pN/nm), and 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 were also longer for V 3 than for V 1 . These results indicate that both the unitary displacements and forces are similar in amplitude but different in duration between the 2 cardiac myosin isoforms, being consistent with the reports that the tension cost is higher in muscles consisting mainly of V 1 than those consisting mainly of V 3 . (Circ Res. 1998;82:1029-1034.)Key Words: cardiac myosin Ⅲ unitary displacement Ⅲ unitary force Ⅲ laser optical trap M ammalian ventricular muscle myosin is divided into 3 different isoforms, V 1 , V 2 , and V 3 . 1 Actin-activated MgATPase activity is highest for V 1 and lowest for V 3 . Contractile properties of these isoforms have been studied in muscle preparations, in which myosin isoform composition was modified either hormonally 2 or by imposing overload. 3,4 All these studies indicated that the maximum unloaded shortening velocity (V max ) of muscle preparations correlated well with their V 1 isoform content, suggesting the fast crossbridge cycling rate in this isoform. In these experiments with muscle preparations, however, it was difficult to preclude the possible influence of concomitant changes of other cellular components on V max . In addition, it was difficult to estimate the number of crossbridges generating contractile force.In vitro motility assay systems are effective in studying the kinetics of the ATP-dependent interaction between purified actin and myosin molecules. In accordance with the results for V max in muscle preparations, the unloaded velocity of actin-myosin sliding in vitro has been shown to be much faster for V 1 than for V 3 isoforms. 5,6 Using the centrifuge microscope, with which constant centrifugal forces are applied as loads on in vitro actin-myosin sliding, we showed that the shape of force-velocity curves was markedly different between V 1 and V 3 isoforms, reflecting their different interaction kinetics with actin.7 Recent developmen...

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To facilitate cardiac muscle research, we developed a novel method by which the force and length of a single ventricular myocyte can be recorded with a pair of carbon graphite fibers attached firmly to both ends. One fiber was stiff, whereas the other fiber was compliant to allow the recording of force and shortening during twitch contractions. The image of the compliant carbon fiber was projected onto a pair of photodiodes, and their output was fed to a piezoelectric transducer after variable amplifications to alter the effective compliance of the carbon fiber. Thus contraction of the myocyte was induced under virtually isometric conditions as well as under auxotonic conditions. We obtained a bell-shaped relation between the compliance under an auxotonic load and the work output of the myocyte, which was directly related to myocyte performance in the heart. Because it is easy to attach myocytes to the experimental apparatus, the present method would allow us to study cardiac muscle mechanics at the cellular and molecular levels.

Abstract-Reduced myocardial vasodilatation (MVD) in hypercholesterolemics without overt coronary stenosis has been reported.However, the status of MVD in hypertriglyceridemics has not yet been clarified. The aim of this study was to investigate whether MVD is impaired in patients with hypertriglyceridemia without overt coronary stenosis. Twenty-three hypertriglyceridemics (10 normocholesterolemic hypertriglyceridemics [HTGs] and 13 mixed combined hyperlipidemics [MCHLs]) and 13 age-matched controls were studied. All patients were proven to have more than one normal coronary artery, as diagnosed by coronary angiography, and those segments that were perfused by anatomically normal coronary arteries were used in the study. Myocardial blood flow (MBF) during dipyridamole (DP) loading and baseline MBF were measured by using positron emission tomography and [13 N]ammonia, after which MVD was calculated. Baseline MBF (mL ⅐ min Ϫ1 ⅐ 100 g Ϫ1) was comparable among HTG (76.0Ϯ26.1), MCHL (77.0Ϯ26.1), and controls (80.3Ϯ38.5). However, MBF during DP loading was significantly lower in MCHL (159Ϯ52.5) than in control subjects (292Ϯ166, PϽ.01), while it was comparable in HTG (202Ϯ104) and controls. MVD was significantly reduced in both HTG (2.70Ϯ1.09, PϽ.05) and MCHL (2.07Ϯ.70, PϽ.01) compared with controls (3.73Ϯ1.14). MVD in MCHLs tended to be reduced compared with that in HTGs, but the difference was statistically insignificant (Pϭ.08). There was a significant relationship between MVD and both plasma triglycerides (rϭϪ.47, PϽ.01) and plasma total cholesterol (rϭϪ.55, PϽ.01). When controls and HTGs were combined, the relationship between MVD and plasma total triglycerides became more prominent (rϭϪ.55, PϽ.05), and the significant relationship between cholesterol level and MVD disappeared. Multivariate regression analysis has revealed that the triglyceride level (Fϭ5.2, PϽ.05) was independently related to MVD (rϭ.69, PϽ.01). In conclusion, MVD was reduced in hypertriglyceridemics in anatomically normal coronary arteries. Hypertriglyceridemia is an independent factor for this abnormality. (Arterioscler Thromb Vasc Biol. 1998;18:294-299.) Key Words: hyperlipidemia Ⅲ hypertriglyceridemia Ⅲ atherosclerosis Ⅲ myocardial vasodilatation Ⅲ PET R educed MVD has been thought to be a factor in coronary stenosis.1 It has been shown that MVD can be altered without evidence of ischemia in hypercholesterolemics 2-4 and that there is a significant inverse relationship between MVD and plasma TG levels in hyperlipidemics without evidence of ischemia. 3,5 In fact, alterations in MVD without overt coronary stenosis have been indicated in a variety of conditions; for example, hypertrophic cardiomyopathy, 6 dilated cardiomyopathy, 7 and diabetes with and without associated hypertension, 8 -10 and in normal segments in patients with myocardial infarction. 11 We have reported a reduced MVD without overt coronary stenosis in patients with hypercholesterolemia. 12However, most recent studies, including our own, involved patients with pure hyperchole...