A Quantitative Study of the Ca²⁺/Calmodulin Sensitivity of Adenylyl Cyclase in Aplysia, Drosophila, and Rat

Abstract: Studies in Aplysia and Drosophila have suggested that Ca²⁺/calmodulin‐sensitive adenylyl cyclase may act as a site of convergence for the cellular representations of the conditioned stimulus (Ca²⁺ influx) and unconditioned stimulus (facilitatory transmitter) during elementary associative learning. This hypothesis predicts that the rise in intracellular free Ca²⁺ concentration produced by spike activity during the conditioned stimulus will cause an increase in the activity of adenylyl cyclase. However, published values for the Ca²⁺ sensitivity of Ca²⁺/calmodulin‐sensitive adenylyl cyclase in mammals and in Drosophila vary widely. The difficulty in evaluating whether adenylyl cyclase would be activated by physiological elevations in intracellular Ca²⁺ levels is in part a consequence of the use of Ca²⁺/EGTA buffers, which are prone to several types of errors. Using a procedure that minimizes these errors, we have quantified the Ca²⁺ sensitivity of adenylyl cyclase in membranes from Aplysia, Drosophila, and rat brain with purified species‐specific calmodulins. In all three species, adenylyl cyclase was activated by an increase in free Ca²⁺ concentration in the range caused by spike activity. Ca²⁺ sensitivity was dependent on both calmodulin concentration and Mg²⁺ concentration. Mg²⁺ raised the threshold for adenylyl cyclase activation by Ca²⁺ but also acted synergistically with Ca²⁺ to activate maximally adenylyl cyclase.