Myofilament-generated tension oscillations during partial calcium activation and activation dependence of the sarcomere length-tension relation of skinned cardiac cells.

Abstract

During partial Ca2+ activation, skinned cardiac cells with sarcoplasmic reticulum destroyed by detergent developed spontaneous tension oscillations consisting of cycles (0.1-1 Hz) of rapid decrease of tension corresponding to the yield of some sarcomeres and slow redevelopment of tension corresponding to the reshortening of these sarcomeres. Myofilament-generated tension oscillations were never observed during the full activation induced by saturating with free Ca2+ or by rigor tension induced by decreasing MgATP in the absence of free Ca2+ or when the mean sarcomere length (SL) of the preparation was > 3.10 .mu.m during partial Ca2+ activation. A stiff parallel elastic element borne by a structure that could be digested by elastase hindered the study of the SL-active tension diagram in 8-13 .mu.m wide skinned cells from the rat ventricle, but this study was possible in 2-7 .mu.m wide myofibril bundles from the frog or dog ventricle. During rigor the tension decreased linearly when SL was increased from 2.35-3.80 .mu.m. During full Ca2+ activation the tension decreased by < 20% when SL was increased from 2.35 to .apprx. 3.10 .mu.m. During partial Ca2+ activation the tension increased when SL was increased from 2.35-3.00 .mu.m. From this observation of an apparent increase in the sensitivity of the myofilaments to Ca2+ induced by increasing SL during partial Ca2+ activation, a proposed model describes the tension oscillations and permits derivation of the maximal velocity of shortening (Vmax). Vmax was increased by increasing free Ca2+ or decreasing free Mg2+ but not by increasing SL.