Skeletal muscle sarcoplasmic reticulum glycogen status influences Ca2+ uptake supported by endogenously synthesized ATP

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Abstract
The purpose of this investigation was to determine whether there is a link between sarcoplasmic reticulum (SR) glycogen status and SR Ca2+ handling. In this investigation, skeletal muscle SR was purified from female Sprague-Dawley rats (200–250 g). Glycogen was extracted from the SR purified from one hindlimb, whereas the SR purified from the contralateral limb served as control. Before removal of the tissue, the animals were anesthetized with an intraperitoneal injection of ketamine (80 mg/kg) and xylazine (10 mg/kg). Both α-amylase treatment (AM) and removal of EDTA from the homogenization and storage buffers reduced the amount of glycogen associated with the SR ( P < 0.05). AM treatment reduced the glycogen phosphorylase content of SR ( P < 0.05). In contrast, creatine kinase (CK) and pyruvate kinase (PK) contents were increased after both glycogen extraction protocols ( P < 0.05). Under exogenous ATP conditions, both AM and EDTA-free (EF) treatments resulted in an increase in Ca2+-stimulated ATPase activity when normalized to sarco(endo)plasmic reticulum calcium-ATPase (SERCA) content ( P < 0.05). CK and PK-supported SR Ca2+ uptake was decreased ( P < 0.05) in the AM group when normalized to SERCA and CK or SERCA and PK content, respectively. AM was more effective than the EF for extracting glycogen associated with purified SR. Glycogen extraction alters the yield of purified SR proteins and must be taken into account when investigating SR calcium handling. Removal of glycogen from purified SR causes a change in Ca2+-handling properties as measured by ATPase and uptake activities.

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