MECHANISM OF CADMIUM‐INDUCED CONTRACTION IN ILEAL LONGITUDINAL MUSCLE OF GUINEA‐PIG

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Abstract
1 The mechanism of cadmium (Cd2+)-induced contraction was studied in isolated ileal longitudinal muscle of guinea-pig. 2 CdCl2 (1 × 10−8 to 1 × 10−4 m) caused a transient contraction which subsided within approximately 6 min of application. The contraction was reproducible and dependent on the concentration. The dose-response curve was bell-shaped. A maximal response was observed at concentrations of 5 × 10−6 to 1 × 10−5 m. 3 The contractile effect was inhibited to some degree at 20°C or by tetrodotoxin (0.1 μg/ml), hyoscine (0.1 μg/ml) or hexamethonium (10 μg/ml), but completely inhibited by Ca2+-removal from the medium. 4 Cd2+ increased the output of [14C]-acetylcholine biosynthesized from [14C]-choline by the preparation depending on the concentration. The increase terminated within the first 6 min and was reduced by tetrodotoxin (0.1 μg/ml) or by removal of Ca2+ from the medium. 5 Both the contractile and transmitter releasing effects of Cd2+ were dependent on the concentration of external Ca2+. Strontium ions were able to replace Ca2+ for Cd2+-induced transmitter release. 6 It is suggested that Cd2+ contracts ileal longitudinal muscle through a release of cholinergic transmitter from the parasympathetic nerve terminals, which is dependent on external Ca2+. It also has a smaller hyoscine-resistant contractile effect, presumably due to a direct action on smooth muscle cells.

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