ZINC IONS BLOCK THE 2ND BUT NOT THE FIRST PHASIC RESPONSE TO REPETITIVE APPLICATION OF CARBACHOL AND HISTAMINE IN GUINEA-PIG TAENIA CECI

In the presence of Zn2+ (0.3 mM), carbachol (10(-6) M) or histamine (1 0(-5) M) induced the phasic response in guinea-pig taenia caeci while the tonic response was markedly inhibited. However, when the muscles w ere kept in Zn2+-containing medium following the first stimulation wit h either carbachol or histamine, neither application of carbachol nor of histamine elicited another phasic contraction. Caffeine (25 mM) did not induce contraction in the presence of Zn2+. After the washing out of caffeine in the presence of Zn2+, however, the muscle did then dev elop the phasic response on the application of carbachol or histamine. In conclusion, Zn2+ did not affect the carbachol or histamine-induced Ca2+ release from the storage sites. However, when Zn2+ was continuou sly present, Ca2+ was not supplied to the storage sites. Furthermore, carbachol and histamine mobilized a common cellular Ca2+ store, but th ey activated Ca2+ release channels different from the ones activated b y caffeine in the Ca2+ storage sites.