MYOGLOBIN ENHANCES CARDIAC-PERFORMANCE IN ANTARCTIC ICEFISH SPECIES THAT EXPRESS THE PROTEIN

Channichthyid icefishes of Antarctica are unique among adult vertebrat es. All icefish species lack hemoglobin and red blood cells in their c irculating blood. All icefishes examined to date also lack the intrace llular oxygen-binding protein myoglobin (Mb) in their oxidative skelet al 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 significanc e. To address this problem, we compared mechanical performance of isol ated, perfused hearts from two species of icefish in which Mb is eithe r present (Chionodraco rastrospinosus) or is absent (Chaenocephalus ac eratus). Hearts were challenged with increasing afterload (2.5-4.0 kPa ) under conditions of defined basal flow (similar to 100 ml . min(-1). kg(-1)), in both the presence and absence of 5 mM sodium nitrite, a M b poison. Unlike hearts from C. aceratus, which were unable to maintai n a constant cardiac output under pressure loading, those from C. rast rospinosus retained a constant flow up to 3.5 kPa afterload. At the up per range of power outputs, hearts of Mb-lacking C. aceratus display g reater oxygen utilization than those of Mb-containing C. rastrospinosu s. Poisoning of Mb significantly impaired the ability of C. rastrospin osus hearts to face pressure loading without reduction in flow, wherea s those of C. aceratus were refractory to the treatment. The results s trongly support a functional role for Mb in the former species.