Postmortem alterations in the pH range of myofibrillar ATPase activation/inactivation

A histochemical assay for myofibrillar adenosine triphosphatase (mATPase) activity is routinely utilized in the delineation of fiber types in healthy human skeletal muscle. Each fiber type has a specific pH range of mATPase stability (activation). Outside of this pH range, mATPase activity is labile (inactivated), no reaction product is formed, and the fibers remain unstained. The aim of the present study was to carefully investigate the pH stability/lability of mATPase in postmortem muscles. To this end, vastus lateralis muscle samples were obtained approximately 0.5, 1, 2, 3, and 4 days after death, as well as control samples from a healthy young man and woman. Serial cross sections of the muscle samples were assayed for mATPase activity throughout preincubation pH ranges of 4.15-4.7 and 10.2-10.5 in increments of 0.05 pH units. Myosin heavy chain analysis (as well as a regression analysis comparing fiber type area and relative myosin heavy chain content) verified the mATPase-based fiber types. The pH ranges of mATPase stability/lability for the control samples were as previously reported, and support the use of preincubation pH values of 4.3, 4.6, and 10.4 for the delineation of fiber types in normal human muscle. For the postmortem samples, both quantitative and qualitative changes altered the pH ranges of mATPase activation/inactivation. Quantitative changes consisted of a time-dependent loss of mATPase activity that was inhibited in all fibers outside the pH range of 4.15-10.50. In addition, qualitative changes caused "shifts to the left" in mATPase stability within the fast fiber types (IIA and IIB). As such, complete inhibition of mATPase activity did not occur until preincubation at pH 4.45 and pH 4.30 for fiber types IIA and IIB, respectively. For the postmortem vastus lateralis muscle samples, optimal preincubation pH values for mATPase-based fiber type delineation were pH 4.30, 4.45, and 10.35. The reason for these qualitative changes in mATPase stability is not known. However, postmortem changes such as increased lactate production and marked acidification may play a role.