Acute temperature effects on metabolic rate, ventilation, diffusive water exchange, osmoregulation, and acid-base...

The Pacific hagfish (Eptatretus stoutii) is a representative of the most basal extant craniates, and is a marine osmoconformer with an extremely low metabolic rate (MO₂ = 475 µmol O₂/kg/h at 12 °C). We investigated potential physiological trade-offs associated with compensatory changes in gill ventilation and perfusion when 12 °C-acclimated hagfish were acutely exposed to 7 °C or 17 °C, as reflected in diffusive unidirectional water flux ([Formula: see text], measured with tritiated water: ³H₂O), net ammonia flux (J_amm), and plasma ion and acid-base status. [Formula: see text] was high (~ 1.4 L/kg/h at 12 °C) in comparison to marine teleosts and elasmobranchs. MO₂ increased linearly with temperature (R² = 0.991), and was more sensitive (Q₁₀ = 3.22) in the 12-7 °C range than either J_amm (1.86) or [Formula: see text] (1.35), but the pattern reversed from 12 to 17 °C (Q₁₀s: MO₂ = 2.77, J_amm = 2.88, [Formula: see text] = 4.01). Heart rate, ventilatory index (a proxy for total ventilation), and coughing frequency also increased but with different patterns. At 17 °C, plasma [Ca²⁺] and [Mg²⁺] decreased, although osmolality increased, associated with elevations in plasma [Na⁺] and [Cl^-]. Blood pH and PCO₂ were unaffected by acute temperature changes while [HCO₃^-] increased. Hyperoxia (PO₂ > 300 Torr) attenuated the increase in [Formula: see text] at 17 °C, did not affect J_amm, and had diverse effects on plasma ion and acid-base status. Our results suggest a clear osmorespiratory compromise occurring for the diffusive water fluxes as a result of acute temperature changes in this osmoconformer.