The isolation by Shimomura, Johnson and Saiga1 of a protein, aequorin, that emits light in the presence of micromolar concentrations of ionised calcium opened up new possibilities for the study of ionised calcium inside cells2–5. It is a relatively simple matter to introduce aequorin into large cells by microinjection, and the rate of light emission gives a direct measure of intracellular free calcium. During an investigation into the action of certain anaesthetics on squid axons, we noticed that these agents always increase the light output from intracellular aequorin. Subsequent analysis has now revealed that this does not result from a rise in ionised calcium inside the axon but seems to reflect a direct effect of the anaesthetic agent on the aequorin molecule. The agents studied all produce greater activation of the light emitting reaction at a constant level of ionised calcium. These rather simple observations have several important biological implications: (1) aequorin might be an interesting model system for studying interaction between anaesthetics and proteins, (2) other Ca-sensitive proteins might behave like aequorin, their affinity for calcium being subject to modulation, and (3) such modulation of endogenous Ca-binding proteins might contribute to the mechanism of anaesthesia.
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