Thermal Stability of Contractile Proteins in Bat Wing Muscles Explains Differences in Temperature Dependence of Whole-Muscle Shortening Velocity.

AbstractMuscle contractile properties are dependent on temperature: cooler temperatures generally slow contractile rates. Contraction and relaxation are driven by underlying biochemical systems, which are inherently sensitive to temperature. , a small Neotropical bat, experiences large temperature differentials among body regions, resulting in a steep gradient in temperature along the wing. Although the bats maintain high core body temperatures during flight, the wing muscles may operate at more than 10°C below body temperature. Partially compensating for these colder operating temperatures, distal wing muscles have lower temperature sensitivities in their contractile properties, including shortening velocity, relative to the proximal pectoralis. Shortening velocity is correlated with the activity of myosin ATPase, an enzyme that drives the cross-bridge cycle. We hypothesized that the thermal properties of myofibrillar ATPase from the pectoralis and forearm muscles of the bat wing would correlate with the temperature sensitivity of those muscles. Using myofibrillar ATPases from the proximal and distal muscles, we measured enzyme activity across a range of temperatures and enzyme thermal stability after heat incubation across a range of time points. We found that forearm muscle myofibrillar ATPase was significantly less thermally stable than pectoralis myofibrillar ATPase but that there was no significant difference in the acute temperature dependence of enzyme activity between the two muscles.