title: An internal viscous element limits unloaded velocity of sarcomere shortening in rat myocardium.
abstract: 1. Peak twitch force (F0) and sarcomere length (SL) were measured in trabeculae that had been dissected from the right ventricle of rat heart and that were superfused with a modified Krebs-Henseleit solution at 25 degrees C. Sarcomere length was measured by laser diffraction techniques. Force was measured with a silicone strain gauge. Unloaded velocity of sarcomere shortening (V0) was measured by the 'isovelocity release' technique. 2. At [Ca2+]o = 1.5 mM and SL below 1.9 microns, V0 increased in proportion to SL, while V0 was independent of SL above 1.9 microns. At [Ca2+]o = 0.5 mM, V0 was proportional to SL up to 2.2 microns. At [Ca2+]o = 0.2 mM, V0 was proportional to SL up to 2.3 microns which is the longest SL that we were able to study in our trabeculae. 3. A unique relationship was observed between V0 and F0, irrespective of whether F0 was altered by variation of [Ca2+]o or sarcomere length above slack length. 4. Passive viscosity (Fv) was measured during the pause between contractions in the presence of 1.5 mM [Ca2+bdo and in the range SL = 2.0-2.1 microns by applying 0.1 micron stretches at various velocities up to v = 30 microns s-1. The force response to stretch, corrected for the contribution of parallel elastic force, showed viscoelastic characteristics with an exponential increase to a maximum (Fv) during stretch and an exponential decline after the end of the stretch. Fv increased, by 0.3%F0 microns-1 s-1, in proportion to v < 5 microns s-1; the increase of Fv was smaller at higher v, suggesting non-Newtonian viscous properties. 5. The time constant of the increase of force during the stretch decreased (tau rise congruent to 7 ms to tau rise congruent to 4 ms) with increases in v (congruent to 4 microns s-1 to v congruent to 10 microns s-1; P = 0.02). The time constant of decay of force at the end of the stretch also decreased with increases in v (tau decay congruent to 8 ms at v congruent to 4 microns s-1 to tau decay congruent to 3 ms at v congruent to 30 microns s-1; P < 0.001). Calculated stiffness of the elastic term of the viscoelastic element was independent of v, i.e. 45-50 N mm-3.(ABSTRACT TRUNCATED AT 400 WORDS)
authors: Department of Medicine, University of Calgary, Alberta, Canada.
authors: P P
authors: PP
authors: de Tombe
authors: H E
authors: HE
authors: ter Keurs
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology; D004548:Elasticity; D004558:Electric Stimulation; D005260:Female; D017276:Friction; D006321:Heart / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D008954:Models, Biological; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012518:Sarcomeres / Q000502:physiology*; D016276:Ventricular Function; D014783:Viscosity
chemical_list: D002118:Calcium