Sensitivity of hemoglobins to intracellular effectors: Primitive and derived features

Abstract

Abstract The sensitivity of hemoglobin‐O 2 binding to urea and Cl − , effectors in higher vertebrates and also blood constituents that vary with salinity in elasmobranchs, was examined in the highly euryhaline cownose ray Rhinoptera bonasus . The response was compared with that in a closely related stenohaline species, as well as several other elasmobranchs, teleosts, and simpler hemoglobin‐containing animals. Although the O 2 binding of R. bonasus red blood cells and stripped hemoglobin is not sensitive to urea, trimethylamine oxide, or Cl − , neither is the hemoglobin of the stenohaline butterfly ray. Cl − lowers O 2 affinity of the hemoglobins of three species of stenohaline sharks; urea only slightly increases O 2 affinity of one of these hemoglobins and another is insensitive. In contrast, the hemoglobins of two highly euryhaline teleosts are sensitive to urea as well as Cl − ; urea raises and Cl − lowers O 2 affinity. Thus, while urea insensitivity does appear to protect the respiratory properties of the O 2 carrier in the elasmobranchs, Cl − sensitivity appears to be unrelated to changes in the blood. Since the red blood cells and/or hemoglobins of simpler animals such as annelids and molluscs are also insensitive to urea and Cl − , insensitivity in the elasmobranchs is more likely to represent a primitive, inherited feature than one specifically evolved along with their distinctive form of osmoregulation. The limited information available suggests that ATP lowers O 2 affinity of the shark but not ray hemoglobins, which may reflect the origin of intracellular modulation within the elasmobranchs.