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Abstract
The Ca-sensitive photoprotein aequorin has been used to record repetitive free Ca transients in single rat liver cells responding to either ATP or ADP. The time-course of individual transients is longer when ATP is the agonist. If both agonists operate through the same receptor, then curtailment of the receptors' activity occurs more rapidly when ADP is the agonist, and we infer that an agonist-occupied receptor linked to a GTP-liganded G protein is the moiety responsible for activating a phosphatidylinositol bisphosphate-specific phospholipase C. The alternative explanation is that ATP and ADP act through different receptors.