Calcium Mechanism of Norepinephrine Activation of Ionic Pump in Somatic Cells of Lumbricus Terrestris Earthworm Muscle Wall

Blockade of norepinephrine-induced hyperpolarization in earthworm muscle cells was observed in a calcium-free medium, after substitution of Ca2+ with Mn2+, and in the presence of verapamil or chlorpromazine to the incubation saline. Changes in Ca2+ concentration in the saline and addition of caffeine had no effect on the resting potential of muscle cells. It was hypothesized that signal transduction from norepinephrine-activated membrane adrenoceptors to the ionic pump in earthworm muscle cells depends on influx of extracellular Ca2+ and subsequent involvement of Ca2+-accepting proteins similar to calmodulin in vertebrates.