Characteristics of resting tension of myocardium and localization of its elements

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Abstract
The resting tension characteristics of the isolated rat papillary muscle preparation were studied. Resting tension was found to be functionally independent of active tension and its maintenance appeared to be a passive process. Stress relaxation and shifts in resting tension to new and stable levels were, however, observed. Microscopic studies on muscles fixed in situ did not support the concept of an extracellular stromal origin for the high resting tensions. The simultaneous and equal reductions in both active and resting tensions, which resulted from the addition of proteolytic enzymes, suggested a cellularly based resting tension system. Extraction of contractile proteins by salt solutions of high ionic strength permanently reduced resting tension. Since actomyosin-extracting solutions permanently reduced resting tension levels, whereas myosin-extracting solutions did not, actin would appear to be more directly involved. The evidence thus suggests that parallel elasticity is, together with tension generation and series elasticity, a property of components of the contractile system.