Abstract
The effects of the mistletoe toxins, Viscotoxins A 3 and B, on excitation and contraction processes in isolated preparations of cardiac, smooth, and skeletal muscle were investigated. In low concentrations (1–10 μg/ml), Viscotoxin B reduced the isometric twitch and produced contracture and progressive depolarization of rabbit papillary muscle. These changes were almost completely reversed by increasing the extracellular calcium concentration from 2.0 to 4.0 mM. In single frog muscle fibres, Viscotoxins A 3 and B, 1 μg/ml, caused a progressive depolarization of the cell membrane; associated with this change, a transient contracture developed. The depolarization could be prevented by an increase in extracellular calcium concentration from 1.8 to 12 mM. Rabbit aortic strips were less sensitive to the Viscotoxins than heart and skeletal muscle, but in a concentration of 200 μg/ml, Viscotoxin A 3 produced a slowly developing contracture that could not be reversed by calcium. It is suggested that the Viscotoxins, by binding to the cell membrane, displace bound calcium from a membrane site.
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