Functional adaptations in hemoglobins from ectothermic vertebrates.

Abstract

Hemoglobin (Hb) increases the O2 carrying capacity of the blood of ectothermic vertebrates by about twenty times compared to physically dis­ solved O2. This drastically raises the blood O2 capacitance coefficient ({30,= .leo /.lpo) and permits corresponding reductions in the cardiac output (Qh) 2 2 • required for a given convective O2 transfer (V 0) as predicted by the Fick • • 2 equation (V 0, = Qh . (30, . .lp 0,). The O2 transporting role of Hb, however, also depends on its qualitative properties, namely (a) its intrinsic O2 binding properties and (b) its interaction with factors that modulate these properties inside the red cells. Ectotherm erythrocytes contain a full complement of cellular apparatus (including nucleus, mitochondria, and endoplasmic reticu­ lum), exhibit high metabolic activity compared to mammalian erythrocytes, and greater potential for feedback regulation of tissue O2 supply via cellular metabolites that affect Hb-02 affinity. The molecular properties of Hb have been dealt with in recent treatises (1, 77, 84, lO7). This review focuses on the physiological function of ectotherm Hb in the red blood cells, particularly the interand intraspecific adaptations to exogenous and endogenous factors like ambient hypoxia (low O2 tension), temperature, activity, and dormancy, and illustrates these with representative examples.