Molecular control of inositol pentaphosphate in red blood cells of pirarucu ( <i>Arapaima gigas</i> )

The oxygen affinity of fish hemoglobins is regulated by organic phosphates, adjusting oxygen uptake and delivery. While adenosine triphosphate (ATP) and guanosine triphosphate (GTP) are common regulators, inositol pentaphosphate (IP 5 ) is unique to certain fish and birds. The pirarucu ( Arapaima gigas (Schinz, 1822)), an air-breathing Amazonian fish, transitions from gill respiration to air-breathing, accompanied by changes in Hb-O2 affinity regulation. This study examined IP 5 biosynthesis in pirarucu erythrocytes across development. Organic phosphate levels (ATP, GTP, IP 5 ) and expression of genes involved in inositol phosphate metabolism ( ipmk, itpk-1, ip3k) were analyzed in aquatic (9.86 ± 0.38 cm) and air-breathing (60.2 ± 3.8 cm) specimens. Air-breathing pirarucu had higher erythrocyte IP 5 and lower ATP and GTP. Gene expression analysis showed increased ipmk, itpk-1, and ip3k, suggesting regulatory rather than molecular changes. Phylogenetic analysis confirmed conservation of these genes. This ontogenetic increase in IP 5 , along with the upregulation of genes involved in its synthesis, highlights a genetic and cellular adaptation that may contribute to improved oxygen regulation during aerial respiration in A. gigas.