The ultrahistochemical picture of the so-called reversed ATPase in the gastrocnemius muscle of the rat.

The ultrahistochemical localization of the "reversed" ATPase activity was investigated. Red muscle fibres showed permanent sarcomere contraction, enzymatic activity in the inner membrane and matrix of mitochondria, and large, osmiophilic, probably calcium-containing structures within mitochondria and on their outside. White muscle fibre sarcomeres were relaxed, and activity within their sarcoplasmic reticulum was marked, but slight in the mitochondria. The relaxed state of the sarcomere in the white muscle fibres is supposed to be connected with inactivation of myofibrillar ATPase by acid preincubation, whereas red muscle contraction indicates that acid preincubation does not inactivate their myofibrillar ATPase. That the product of its activity failed to become visible in the sarcomeres is probably due to imperfection of the method. Two sub-types of red muscle fibres were distinguished: those showing only enzymatic activity in mitochondria, and those containing large intra- and extramitochondrial osmiophilic structures. The origin and composition of these structures is difficult to explain. A relation seems to exist between their presence within mitochondria and outside.