The effects of Cd<sup>2+</sup> on spontaneous contraction, and the contractures induced by 0mM Na<sup>+</sup>, 60mM K<sup>+</sup> and 10<sup>-6</sup> M acetylcholine, 1mM caffeine were studied in order to elucidate diverse actions of Cd<sup>2+</sup> on the Ca<sup>2+</sup> mobilization related with contractility in the antral circular muscle of guinea pig stomach. Cd<sup>2+</sup> inhibited the spontaneous contraction in a does dependent manner (10<sup>-6</sup> M 10<sup>-4</sup> M). Cd<sup>2+</sup> (3 X 10<sup>-5</sup>M) suppressed 60 mM K<sup>+</sup> induced contracture composed or a phasic and a tonic response and the increased tonic response by the increased external Ca<sup>2+</sup> concentration. Cd<sup>2+</sup> also suppressed acetylcholine induced contracture composed of repetitive phasic and a tonic component and the increased tonic response by the increased external Ca<sup>2+</sup> concentration. Caffeine in the concentration of 1mM evoked contracture but Cd<sup>2+</sup> suppressed the contracture. Cd<sup>2+</sup> suppressed the amplitude of the Na<sup>+</sup> tee contracture dose dependently and the amplitude of Na<sup>+</sup> free contracture almost decreased to 20% of control amplitude in the concentration of 10<sup>-4</sup> M Cd<sup>2+</sup>. From the above results, it is suggested that Cd<sup>2+</sup> may inhibit not only Ca<sup>2+</sup> influx via voltage sensitive, receptor operated Ca<sup>2+</sup> channel and Na/ca exchange but also intracellular Ca<sup>2+</sup> release from the sarcoplasmic reticulum in the antral circular muscle of guinea pig stomach.
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