1. Plaice caught by trawl net and plaice exercised in laboratory tanks all show high levels of lactic acid (33–44 mmol/kg) in the anaerobic swimming muscle. During exhausting exercise 2 moles of lactate are formed from 1 mole of glycogen glucose. After an 8 h rest 50–80% of the muscle glycogen is restored.

2. Blood lactate levels remain low (0.5-2 mmol/1) in the majority of plaice caught by trawl. In a small number of plaice, peak levels over 5 mmol/1 are reached 2-4 h after capture. Low blood lactate levels could be guaranteed in all fish exercised 24 h after the stress of capture and in tank-adapted fish exercised and injected with the β-adrenergic stimulating drug, isoxsuprine hydrochloride. The blood lactate in plaice, tank-adapted for more than 8 days and then exercised, may reach peak levels up to 5 mmol/l 2-4 h later.

3. High blood lactate levels were obtained by injecting the β-adrenergic block propranolol to stressed exercised fish. The α-adrenergic block did not have this effect. All plaice with blood lactate levels reaching 5–12 mmol/l died.

4. The results indicate that the muscle cells regulate the release or nonrelease of their lactate load to the blood stream and increases in the blood circulating to the muscle do not influence this release. The non-release mechanism may be actived by a catecholamine circulated in the blood stream following a stress.

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