[Myofibril breakdown during spreading damage. II. A calcium-free medium].

Peculiarities of Zenker's degeneration (ZD) have been investigated in fast muscle fibres of the frog incubated in a Ringer solution free of Ca++ (R--Ca) with a normal or increased (by 100 mM) concentration of KCl. ZD in these solutions is distinguished by a 10--90 minutes delay of the appearance of the primary contraction knot and cessation of ZD development in the majority of fibres after formation of several (1--5) contraction knots. In the presence of 0.5 mM EDTA in R--Ca, after a few typical contraction knots are formed, fibres commonly fall into large fragments that retain cross-striation. Contracted or super-contracted state of sarcomeres in detached contraction knots and at the necrosis boundary, as well as an increasing lysis of contactile material and proliferation of fibre membrane structures in the region of ZD arrested boundary, are characteristic of ultrastructural changes during ZD in calcium-free solutions.