1. The abilities of dinitrophenol, NaCl, Ruthenium Red and the Ca2+-selective ionophore A23187 to release 45 Ca2+ from isolated hepatocytes and liver mitochondria (incubated at 37 degrees C in the presence of 0.1 microM-free Ca2+, Mg2+, ATP and phosphate ions) were compared with the action of adrenaline on 45Ca2+ release from isolated hepatocytes. The effects of adrenaline were most closely described by those of the ionophore A23187. 2. In isolated hepatocytes, a release of 45Ca2+ and stimulation of O2 utilization similar to that induced by adrenaline was observed in the presence of 500 and 20 microM-arachidonic acid respectively. The effect of arachidonic acid on 45Ca2+ release was not specific for this unsaturated fatty acid. 3. Inhibitors of arachidonic acid metabolism, including indomethacin and eicosa-5,811,14-tetraynoic acid, did not block the effects of adrenaline on 45Ca2+ or glucose release from isolated hepatocytes. 4. The ability of adrenaline to stimulate 45Ca2+ release from isolated hepatocytes was rapidly reversed after the subsequent addition of phenoxybenzamine to the cell suspension, and was completely blocked by 0.5 mM-dibucaine. 5. The results are consistent with the action of a Ca2+-selective ionophore in the mechanism by which adrenaline induces the release of Ca2+ from mitochondria in the liver cell and indicate that it is unlikely that arachidonic acid or a metabolite of arachidonic acid is involved in this process.

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Monolayer cultures of rat aorta smooth muscle cells synthesized the anti-aggregatory substance prostacyclin via the cyclooxygenase pathway from 14C-labeled arachidonic acid. The product was identified both by bioassay and by mass spectrometry. Labeled cells produced prostacyclin only when exposed to the initiator thrombin: treatment with therapeutic concentrations of aspirin (0.2 millimolar) for 30 minutes completely destroyed the cells' ability to synthesize prostacyclin. Prostacyclin synthesis from exogenous arachidonic acid recovered fully within 1 to 2 hours by a cycloheximide-sensitive process. Thrombin responsivness, which was permanently impaired in confluent nondividing cultures, recovered substantially and within 24 hours only when cells were stimulated to divide by subculturing. These results indicate that resting vascular cells can rapidly synthesize new cyclooxygenase, but that aspirin destroys additional components of the prostacyclin system which can only be replaced during cell division.

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