A Calcium Aspect of the Effect of Noradrenaline on the Resting Membrane Potential in Somatic Muscle Cells of the Earthworm

A hyperpolarizing effect of noradrenaline (NA) on muscle cells of the earthworm caused by activation of the membrane ion pumps is eliminated in a Ca-free medium, in the case of replacement of Na⁺ by Mn²⁺, or when verapamil or chlorpromazine have been added to the bath solution. A decrease or an increase in the Ca²⁺ concentration in the solution, as well as caffeine application, do not influence the resting membrane potential (RMP) of muscle cells. It is supposed that signal transmission from the membrane adrenoreceptors to the ion pump of earthworm muscle cells by NA is provided via entry of extracellular Ca²⁺ ions into the cell with subsequent involving of Ca²⁺ acceptor proteins similar to calmodulin in vertebrate animals.