Abstract
The calcium precipitation method was used in an attempt to localize myofibrillar adenosine triphosphatase (ATPase) activity within the sarcomeres of single, unfixed, glycerol-extracted fibers from the pectoralis major muscle of leghorn chicks. ATPase was apparently present in only the A-I overlap region of unshortened fibers, and was apparently present in only the contraction bands of shortened fibers. The results are shown to be self-contradictory in view of the known submicroscopic morphology of the sarcomeres. However, the usual classical controls provided no evidence of artifactual localization. In a novel control, separate solutions of calcium and phosphate were caused to diffuse simultaneously into a fiber. The resulting precipitate was localized in patterns indistinguishable from those obtained from incubation of shortened or unshortened fibers in ATP. These patterns result from the localization of precipitate in regions of high protein concentration. Caution in interpreting the results of cytochemical procedures even though controls provide no evidence of artifactual localization is urged.