ABSTRACT
The information available on metabolism of invertebrate red blood cells (RBC) is scant. Some metabolic and physiological characteristics of RBCs of the annelid bloodworm Glycera dibranchiata and the blood clam Noetia ponderosa are reported. 1–4 Since 1989 we and our teams have studied the aerobic and anaerobic metabolism of the RBC from the blood clam Scapharca inaequivalvis. 5–8 The blood clams belong to the primitive family Arcidae, Mollusca.
Annelid and molluscan RBCs are nucleated and appear to differentiate metabolically from vertebrate (un)nucleated RBCs by an increased permeability to substrate, their magnitude of amino acid catabolism, and a higher aerobic metabolism. They probably have a relatively lower activity of the pentose phosphate pathway than vertebrate RBCs. Radiolabeled CO2 production has been observed by adding radioactive glucose and glutamate to RBCs from G. dibranchiata and N. ponderosa. 1–4
For G. dibranchiata and N. ponderosa a lower anaerobic capacity than vertebrate RBC is assumed. Rapid hemolysis of the annelid RBCs occurs when either whole sera or washed cells are exposed to a P02 of zero or to KCN. Also RBCs of the blood clam lyse when exposed to anoxia in vitro for periods longer than 15–30 min. 1–4 S. inaequivalvis RBC membranes appear to be much stronger and osmotically less fragile than the previous species. When freshly sampled blood is equilibrated for up to 6 h with a constant stream of N2 gas or air in the presence of either H2S or HCN, less than 5% lyses occurs. 5 , 7 Typical anaerobic end products of euryoxic marine invertebrate cells accumulate (malate, succinate, acetate, propionate, alanine, strombine/alanopine), whereas aspartate is utilized. 6 , 8
With regard to energy metabolism the RBCs of S. inaequivalvis behave in general as typical molluscan body cells. They are, however, distinct concerning the lack of phosphagen and absence of glycogenolysis. The low glycogen content and the absence of glycogen phosphorylase indicate that this compound cannot serve as anaerobic fuel.
Unlike RBC-lacking bivalves, blood clams do not switch off aerobic metabolism at very low ambient oxygen concentration of the seawater. Blood taken from N. ponderosa exposed to very low oxygen tension (approximately 0.3 ppm) still contains significant amounts of oxygen which may serve to prevent cell lyses. 2 , 3 To assess the critical oxygen tension of S. inaequivalis, changes in metabolite levels in RBC in response to low oxygen tensions were examined. Small but significant increases in succinate and L-alanine levels were seen when animals were exposed to 2.1 ppm oxygen. No D-lactate or octopine is detectable in RBC. ATP content of the RBC is maintained at a high level with exposure to oxygen tensions as low as 1.70 ppm. 9
