The eurythermal myofibrillar protein complex of the mummichog (Fundulus heteroclitus): adaptation to a fluctuating thermal enviroment

Summary

Mummichogs from salt marshes of the Atlantic coast of Maine (USA) experience a mean seasonal temperature range of −1 °C to +15 °C. However, during summer tidal cycles they may experience rapid temperature changes between 15 °C and 30 °C. Observations of animals in the wild suggest that swimming capability is maintained during acute temperature fluctuations which would substantially impair contractile function in other fishes.

Myofibrils were isolated from the fast epaxial musculature and APTase activity determined in a medium of 40 mM imidazole, pH 7.2 (20 °C), 50 mM KCl, 5 mM ATP, 6 mM MgCl₂, 5 mM EGTA, 0.4 mg/ml protein (I=0.124) in the presence (pCa 5.15) and absence (pCa 7.15) of 5 mM CaCl₂. More than 90% of the ATPase activity was calcium dependent over the temperature range 0–35 °C. Arrhenius plots of Mg²⁺ Ca²⁺ myofibrillar ATPase activity showed a discontinuity in slope at 12.5 °C. Values for activation enthalpy (ΔH≠) of the ATPase were 28,100 cal/mole in the range 0–12.5 °C and 11,900 cal/mole between 12.5–35 °C. The pCa's required to give onehalf maximal ATPase activity were 6.39 (0.407 μM), 6.18 (0.661 μM), and 6.24 (0.575 μM) at 5, 15 and 25 °C, respectively. Thermal denaturation of Ca²⁺-sensitivity and ATPase activity proceed essentially in parallel. At 0.5 mg/ml in standard incubation medium the temperature required to denature 50% of activity over 30 min is 40–41 °C. Identical results were obtained for groups of fish acclimated for 6 weeks to either 5 °C, 15 °C or 25 °C.

The higher temperature dependence and relatively low ATPase activity at very cold temperatures suggest a relatively torpid state for overwintering fish. Compared to other fishes previously studied, results with mummichogs indicate that both Ca²⁺ regulatory function and catalytic activity of the myofibrillar complex has low temperature sensitivity over the range of 12–35 °C. These factors apparently obviate the need for acclimatory modifications of the contractile proteins.