Organic anion concentrations in the blood of Hirudo medicinalis were determined in samples from individual animals using reverse-phase high-pressure liquid chromatography (HPLC) and ultraviolet detection. Quantitatively important anions were malate, α-ketoglutarate, succinate, lactate and fumarate, constituting about 70% of total blood anions. Malate had the highest concentration, 34–68 mequiv I−1 blood, which could (depending on metabolic state) exceed the level of blood Cl (approx. 36 mequiv I−1). Organic acid concentrations in the blood were considerably higher than in the tissue.

Blood organic acid concentrations changed more with POO2 than with temperature. They were unaffected by short periods of aerobic exercise, but stress due to handling and prolonged restraint led to a drastic increase of blood lactate and succinate levels, while malate, fumarate and a-ketoglutarate levels decreased.

After feeding on hypertonic, Cl-rich meals, the Cl concentration in leech blood increased far more than the cation concentrations. This was not compensated by a decrease of organic acid levels. The regulatory mechanisms for inorganic and organic ion homeostasis function independently.

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