Physiological basis for swimming endurance differences between LDH-B genotypes of Fundulus heteroclitus

Adenosine triphosphate levels in erythrocytes are correlated with LDH-B genotype in Fundulus heteroclitus. Adenosine triphosphate is the fish's allosteric modifier of hemoglobin oxygen affinity. Since oxygen delivery to muscle affects swimming performance, fish of each homozygous LDH-B phenotype were swum to exhaustion at 10 degrees or 25 degrees C to determine whether in vitro differences attributed to the LDH-B allelic isozymes were manifest in vivo. At 10 degrees C, the critical swimming speed of the LDH-BaBa phenotype was 3.6 body lengths per second, whereas that of the LDH-BbBb phenotype was 4.3 body lengths per second. At 25 degrees C there were no differences between LDH-B phenotypes in erythrocyte adenosine triphosphate levels, blood oxygen affinity, or swimming performance.