Ultrastructure of the contraction-relaxation cycle of glycerinated rabbit psoas muscle: II. The ultrastructure of glycerinated fibers relaxed in EDTA and ATP following ATP-induced contraction

Abstract

Studies of ultrastructural changes of glycerinated rabbit psoas muscle allowed to relax in an ATP-EDTA solution following ATP-induced contraction as well as of rigid fibers plasticized in the same solution have shown that the contraction bands found in tissue fixed during contraction are completely reversible if more than 1 minute is allowed for relaxation. With times less than 1 minute, contraction band disappearance occurs only in those sarcomeres which have been significantly reextended. It also was observed that fibers which were relaxed for as long as 15 minutes were able to recontract to as great an extent as those relaxed for 30 seconds, that is to a length less than or equal to that obtained in the first contraction. The ultrastructural pattern of recontracted sarcomeres is the same as that seen in tissue fixed during the first contraction, with contraction bands reappearing at the A—I-boundary or Z-line. The above observations further support the conclusion that the presence of contraction bands is the only ultrastructural feature peculiarly diagnostic of contracted tissue, while varying degrees of interdigitation can be found in sarcomeres fixed in all the physiological states: rigid, plastic, contracted, and relaxed.