A calmodulin-binding protein called "caldesmon" was purified from chicken gizzard muscle as the major calmodulinbinding protein in this tissue. Its molecular weight estimated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis was 150,000, and two ofthese polypeptides constituted the native molecule. Caldesmon is an actin-binding protein also, binding F-actin reversibly in the presence or absence of Ca2 . The interaction of caldesmon with F-actin was abolished by the binding ofcalmodulin with the caldesmon. Because the interaction between caldesmon and calmodulin was Ca2+-dependent but the interaction between caldesmon and F-actin was not, Ca2+ acts as a flip-flop switch between the formations of two complexes, caldesmon calmodulin and caldesmon-F-actin: increasing the formation of the former complex at increased Ca2+ level and the formation of the latter complex at decreased Ca21 level. The equilibrium of the formations ofboth complexes was achieved at a Ca2+ concentration near 1 JIM.

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The application of electrical pulses to slices of guinea pig cerebral cortex led to an increase in the levels of adenosine 3',5'-phosphate (cyclic 3',5'-AMP) of more than 11-fold within 10 min. This effect of electrical pulses was severely reduced in the presence of theophylline. Cyclic 3',5'-AMP accumulation in slices was increased in the presence of norepinephrine and histamine about 1.5-fold and six-fold, respectively; the effect of electrical pulses was augmented in the presence of maximal amounts of either amine. For these and other reasons, the accumulation of cyclic 3',5'-AMP induced by electrical stimulation cannot be ascribed to the release and action of either histamine or norepinephrine.

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