Minimal disuse muscle atrophy and seasonal alterations in the calcium handling system in skeletal muscle of hibernating brown bears

John Davidson Hershey

doi:10.7273/000005868

Previous studies on wild black bears (Ursus americanus) have shown skeletal muscle morphology, composition and overall force generating capacity do not differ drastically between seasons despite prolonged inactivity during hibernation. The first goal of these studies was to compare the amount of muscle atrophy in captive brown bears (U. arctos) to that observed in wild black bears and measure seasonal differences in twitch characteristics. Samples from the biceps femoris muscle were collected during the summer and winter; protein concentration, fiber type composition and fiber cross-sectional area were measured along with twitch characteristics. Protein concentration of samples decreased 8.2% during winter compared to summer. Myosin heavy chain (MHC) isoforms I, IIa, and IIx were identified and the relative proportions did not change between seasons. Fiber cross-sectional area and the relative proportion of fast and slow fibers also remained unchanged between seasons. There were no significant changes in twitch contraction time or half-fall time between seasons while the half-rise time in the twitch contractions increased 53% in winter relative to summer samples. Increased half-rise time without corresponding changes in the proportion of MHC isoforms during hibernation was unexpected given the amount of inactivity during hibernation and may indicate altered calcium handling during contractions in the myocyte. The second goal of these studies, therefore, was to determine if seasonal alterations were present in the calcium handling system of skeletal muscle in brown bears. We hypothesized that bears undergo seasonal alterations in the calcium handling system, specifically the elements related to the sarcoplasmic reticulum (SR) including the type 1 ryanodine receptor (RyR1), SR associated calcium ATPases (SERCA1 and 2) and calsequestrin (CSQ). In the biceps femoris muscle from summer to winter, RyR1 levels decreased by 52%, SERCA1 and 2 levels decreased by 25% and 15%, respectively; CSQ levels increased 16% during hibernation. These data are consistent with the seasonal changes observed with twitch dynamics. Furthermore, changes in calcium dynamics may impact the cell signaling pathways involved with phenotypic expression of contractile proteins during hibernation and could help minimize disuse muscle atrophy.

Minimal disuse muscle atrophy and seasonal alterations in the calcium handling system in skeletal muscle of hibernating brown bears

Files and links (1)

Abstract

Metrics

Details