Upregulation of the Ca_v 1.1-ryanodine receptor complex in a rat model of critical illness myopathy

The processes that trigger severe muscle atrophy and loss of myosin in critical illness myopathy (CIM) are poorly understood. It has been reported that muscle disuse alters Ca²⁺ handling by the sarcoplasmic reticulum. Since inactivity is an important contributor to CIM, this finding raises the possibility that elevated levels of the proteins involved in Ca²⁺ handling might contribute to development of CIM. CIM was induced in 3- to 5-mo-old rats by sciatic nerve lesion and infusion of dexamethasone for 1 wk. Western blot analysis revealed increased levels of ryanodine receptor (RYR) isoforms-1 and -2 as well as the dihydropyridine receptor/voltage-gated calcium channel type 1.1 (DHPR/Ca_V 1.1). Immunostaining revealed a subset of fibers with elevation of RYR1 and CaV 1.1 that had severe atrophy and disorganization of sarcomeres. These findings suggest increased Ca²⁺ release from the sarcoplasmic reticulum may be an important contributor to development of CIM. To assess the endogenous functional effects of increased intracellular Ca²⁺ in CIM, proteolysis of a-fodrin, a well-known target substrate of Ca²⁺-activated proteases, was measured and found to be 50% greater in CIM. There was also selective degradation of myosin heavy chain relative to actin in CIM muscle. Taken together, our findings suggest that increased Ca²⁺ release from the sarcoplasmic reticulum may contribute to pathology in CIM.