Force generation among cells in the relaxing heart.

When an isolated bundle of mammalian heart cells has been soaked for several hours in a solution containing 3 mM EGTA, a calcium chelator, 2 important changes in the tissue occur: (1) the surface membrane becomes very permeable to small ions and molecules so that the contractile proteins can be directly activated by Ca ions added to the bathing solution; and (2) all intercalated discs open. In this preparation direct Ca activation can produce the transmission of even greater forces among the cells than can be achieved by electrical stimulation of the intact bundle in the presence of catecholamines, high Ca and low Na. Some structure other than the intercalated discs must be transmitting the force. A careful examination of the ultrastructure of the mammalian myocardium has revealed a highly organized network of microfilaments which run between the basement membranes of adjacent cells and from the basement membranes to the collagen fibers. This hitherto undescribed network has all the structural appearances of a major force-bearing structure, and it is probably the primary pathway for force transmission during systole as well as the restoration to the diastolic configuration during relaxation.