An indirect proof of stretch-induced Ca++ release from the sarcoplasmic reticulum in glycerinated skeletal and heart muscle preparations (author's transl)

Abstract

The existence of still functioning vesicles of the sarcoplasmic reticulum (SR) in glycerinated rabbit psoas muscle fibers and cat myocardium is demonstrated by the use of activation (force development and shortening) of the contractile system as an indicator for "sarcoplasmic", free Ca++ concentration and by electron microscopic studies. Furthermore, the conditions for Ca++ release and uptake by the SR vesicles were analyzed. (1) As indicated by electron microscopic studies, glycerinated skeletal and heart muscle preparations contain SR vesicles (diameter 0.1--0.2 mu). Detergent treatment as used by Julian (1971) causes neither morphological change in the contractile system nor in the SR vesicles. (2) The delay in tension development of glycerinated preparations after an increase in free Ca++ concentration of the bathing medium is demonstrated to be caused at least partly by Ca++ uptake in these SR vesicles. (3) The SR vesicles of glycerinated preparations are able to accumulate Ca++ just at concentrations which are subthreshold for tension development. The accumulated Ca++ can be detected by a transient force development resulting from abolishment of the Ca++ storage capacity of the SR vesicles caused by high concentrations of detergents. (4) Ca++-induced Ca++ release is also demonstrated in glycerinated preparations. After Ca++ loading this release mechanism occurs at Ca++ concentrations in the bathing medium starting at about 10(-7,48) Mol/l both in skeletal and heart muscle preparations. (5) Quick stretch of Ca++-loaded, relaxed preparations caused local contractions of the myofibrils in both single fibers and fiber bundles of glycerinated rabbit psoas muscle. Skeletal fiber bundles and cat myocardium show a transient tension development parallel to these local contractions. These effects are demonstrated to be due to a stretch-induced Ca++ release from the SR vesicles. (6) Length-controlled quick stretches or quick releases of partly activated preparations cause sinusoidal tension oscillations superimposed on the well-known, delayed active tension development. In contrast, fully activated preparations do not show superimposed oscillations in tension. Force-controlled quick stretches or quick releases induce equivalent oscillations in length which are more distinct than the tension oscillations, however. These oscillations are also demonstrated to be due to stretch or release-induced changes in the Ca++ fluxes between SR vesicles and the "sarcoplasm". (7) This stretch induced release of Ca++ from the SR could be one reason for the multiple-phase course of active and passive isometric tension development in vivo following quick changes in length, and could also partly explain the prestretch dependence of tension development which cannot be explained from the degree of overlapping of the contractile filaments alone...