Myoglobin enhances cardiac performance in antarctic icefish species that express the protein

Channichthyid icefishes of Antarctica are unique among adult vertebrates. All icefish species lack hemoglobin and red blood cells in their circulating blood. All icefishes examined to date also lack the intracellular oxygen-binding protein myoglobin (Mb) in their oxidative skeletal muscles. However, some icefish species do express Mb in their heart ventricles. It is unknown whether Mb in those species in which it is present represents an evolutionary relic or has functional significance. To address this problem, we compared mechanical performance of isolated, perfused hearts from two species of icefish in which Mb is either present (Chionodraco rastrospinosus) or is absent (Chaenocephalus aceratus). Hearts were challenged with increasing afterload (2.5-4.0 kPa) under conditions of defined basal flow (approximately 100 ml.min-1.kg-1), in both the presence and absence of 5 mM sodium nitrite, a Mb poison. Unlike hearts from C. aceratus, which were unable to maintain a constant cardiac output under pressure loading, those from C. rastrospinosus retained a constant flow up to 3.5 kPa afterload. At the upper range of power outputs, hearts of Mb-lacking C. aceratus display greater oxygen utilization than those of Mb-containing C. rastrospinosus. Poisoning of Mb significantly impaired the ability of C. rastrospinosus hearts to face pressure loading without reduction in flow, whereas those of C. aceratus were refractory to the treatment. The results strongly support a functional role for Mb in the former species.