Activity and expression of nitric oxide synthase in pork skeletal muscles

• SM exhibited NOS activity after 3 d of storage at 4 °C.
• The degradation of nNOS in LT muscle was due to activation of calpain.
• nNOS positively correlated with type IIA/X muscle fiber in pork skeletal muscles.
• Sarcolemma and cytoplasm localization of nNOS was found in pork LD muscle.

The objective of this study was to investigate the biochemical changes of nitric oxide synthase (NOS) in pork skeletal muscles during postmortem storage. Longissimus thoracis (LT), psoas major (PM) and semimembranosus (SM) muscles of pork were removed immediately after slaughter and stored under vacuum condition at 4 °C for 0, 1 and 3 d. Results showed that all three muscles exhibited NOS activity until 1 d while SM muscle retained NOS activity after 3 d of storage. The content of nNOS in SM muscle was stable across 3 d of storage while decreased intensity of nNOS was detected at 1 and 3 d of aging in PM and LT muscles due to the degradation of calpain. Immunostaining showed that nNOS was located at not only sarcolemma but also cytoplasm at 0 and 1 d of storage. Our data suggest that postmortem muscles possess NOS activity and nNOS expression depends on muscle type.