Description: The in vitro oxygen-binding characteristics of haemocyanin were investigated in whole blood of two species of pelagic squid, Illex illecebrosus and Loligo pealei. pH-independent Haldane coefficients (ΔHCO3−/ΔHcyO2) (where HcyO2 is haemocyanin-bound oxygen) slightly smaller than —1 were found in both species. Oxygen-linked CO2 binding was not present. Buffer values ranged between 5 and 5.8 m mol l−1pH unit−1. For further analyses a pH/saturation diagram was selected to show the effect of pH on oxygen binding at constant POO2 in a continuous plot. The slopes of the resulting oxygen isobars (ΔHcyO2/ΔpH or ΔS/ΔpH) (where S is oxygen saturation) depend on pH. The diagram allows evaluation of both the Bohr coefficients (ΔlogP50/ΔpH) and the Hill coefficients (n50) at specific pH values. It provides an integrated illustration of the importance of the Bohr effect and cooperativity for oxygen binding. In accordance with Wyman's linkage equation, Bohr and Haldane coefficients are found to be identical. Both are pH-independent between pH7 and 8. The changing slopes of the oxygen isobars are likely to reflect changes in cooperativity with pH. Maximum values of n50 coincide with maximum steepness of the oxygen isobars in the physiological range of pH and POO2. Assuming that the haemocyanin acts as a buffer for venous POO2, this maximum in pH sensitivity and its decrease in the higher and lower pH ranges are discussed in the light of the maintenance of pigment function in vivo.

Description: In the eurythermal cuttlefish Sepia officinalis, performance greatly depends on hearts that ensure systemic oxygen supply over a broad range of temperatures. We therefore aimed to identify adjustments in energetic cardiac capacity and underlying mitochondrial function supporting thermal acclimation that could be critical for the cuttlefish's competitive success in variable environments. Two genetically distinct cuttlefish populations were acclimated to 11°C, 16°C and 21°C, respectively. Subsequently, skinned and permeabilised heart fibres were used to assess mitochondrial functioning by means of high-resolution respirometry and a substrate-inhibitor protocol, followed by measurements of cardiac citrate synthase activity. In cuttlefish hearts, thermal sensitivity of mitochondrial substrate oxidation was high for proline and pyruvate but low for succinate. Oxygen efficiency of catabolism rose from 11°C to 21°C via shifts to oxygen-conserving oxidation of proline and pyruvate as well as via reduced proton leak. Acclimation to 21°C decreased mitochondrial complex I activity in Adriatic cuttlefish and increased complex IV activity in English Channel cuttlefish. However, compensation of mitochondrial capacities did not occur during cold acclimation to 11°C. Moreover, cold adapted English Channel cuttlefish had larger hearts with lower mitochondrial capacities than warm adapted Adriatic cuttlefish. The changes observed for substrate oxidation, mitochondrial complexes, proton leak or heart weights improve energetic efficiency and essentially seem to extend tolerance to high temperatures and reduce associated tissue hypoxia. We conclude that cuttlefish sustain cardiac performance and thus, systemic oxygen delivery over short and long-term changes of temperature and environmental conditions by multiple adjustments in cellular and mitochondrial energetics.