EXTERNAL CO2 LEVELS INFLUENCE ENERGY YIELDING METABOLIC PATHWAYS UNDER HYPOXIA IN THE LEECH, HIRUDO-MEDICINALIS

Medicinal leeches, Hirudo medicinalis L., were exposed to hypocapnic (nominal pCO2 0.0 kPa) or hypercapnic (pCO2 1.3 kPa) hypoxia (pO2 2.0 kPa) for up to 4 days. Changes in blood concentrations and tissue content of organic acids were measured. In the early phase of hypercapnic hypoxia leeches accumulated mainly succinate, and the normally high malate concentrations in blood and tissue were reduced concomitantly. Between 2 and 24 h of hypoxia the animals started to produce propionate which is the major end-product of this type of hypoxia. Only low concentrations of lactate were found in blood and tissue. During the initial phase of hypocapnic hypoxia an accumulation of succinate was also observed, but at a higher rate than during hypercapnic hypoxia. Subsequently, propionate was also produced. Between 24 and 48 h of hypocapnic hypoxia the concentrations of succinate in blood and tissue decreased rapidly and lactate appeared as an end-product of anaerobic metabolism. The results indicate that, in leeches under "environmental" hypoxia, the external concentration of carbon dioxide affects the pathway of carbohydrate metabolism resulting in either succinate/propionate production or in lactate accumulation.