Thermal Dependence and Acclimation of Fast Start Locomotion and Its Physiological Basis in Rainbow Trout (Oncorhynchus mykiss)

Rainbow trout (family Salmonidae, Oncorhynchus mykiss) were acclimated to 5° and 20° C for 4 wk. The acclimatory response of the C-start (fast-escape swimming) and the molecular, biochemical, and contractile properties of fast-twitch muscle were investigated. With peptide mapping, no change was noted in the expression of myosin heavy chains in fast-twitch muscles with thermal acclimation. The myofibrillar ATPase activity of fast-twitch muscle isolated from individuals of each acclimation group was determined at 5° and 20° C The activity of myofibrillar ATPase was highly temperature dependent (Q10 = 2.9). Acclimation temperature had no significant effect on ATPase activity measured at 5° or 20° C The thermal stability of the myofibrillar ATPase, however, was significantly increased following a period of acclimation to a higher temperature. Fast-twitch muscle contraction time (force onset to 50% relaxation) decreased from 32 ms to 12 ms with an acute change in temperature from 5° to 20° C (Q10 = 1.9). Acclimation to low temperature resulted in only a small increase in twitch contraction speed (11%). After specimens were exposed to either 5° or 20° C, C-starts were elicited at both acclimation temperatures. The linear speed (m/s) and angular velocity (degrees/s) attained by the fish at each temperature were both temperature-dependent (Q10 = 1.2). The total distance moved in 40 ms and the angular velocity of the fish at 20° C increased slightly (24% and 15%, respectively) following a period of acclimation to the higher temperature. Acclimation temperature had no statistically significant effect on maximum linear velocity. Thus, trout are capable of only minor acclimatory adjustments in response to exposure to high temperature. Further, it appears that this acclimatory response has not been facilitated by the differential expression of temperature-specific myofibrillar protein isoforms, as has been previously found in other polyploid fish species.