Fig. 4. Myoglobin enhances the performance of icefish hearts. Cardiac output was measured in hearts from Mb-containing Chionodraco rastrospinosus and Mb-lacking Chaenocephalus aceratus at their inherent rhythms. The afterload against which the heart pumped perfusate was incrementally increased to describe the decay in heart performance as a function of pressure challenge. (A) When pumping oxygenated Ringers solution, Mb-containing hearts (Mb+, filled circles) were able to maintain cardiac output to greater afterload challenges than could Mb-lacking hearts (Mb–, open circles). (B) When 5 mmol l^–1 NaNO₂, which selectively poisons Mb function, was incorporated into the perfusate, mechanical performance of Mb-containing hearts was decremented so dramatically that they were outperformed by Mb-lacking hearts, which were refractory to the treatment. (Adapted from Acierno et al., 1997.)