Abstract
We have studied the effects of Mn2+ on the contractile response induced by Bay K 8644, a dihydropyridine Ca2+ agonist, on guinea-pig taenia coli. Mn2+ (5 mM) completely inhibited Bay K 8644 (10(-6) M)-induced rhythmic contraction and contracture to baseline values in normal Ca2+ medium; thereafter, the contraction progressively increased to about 90% of the K+ (60 mM)-induced tonic response. In Ca2+-free medium in the presence of Bay K 8644 Mn2+ also evoked contraction and a concomitant increase in Mn2+ influx into the cytoplasm. These results suggest that during the opening of voltage-dependent Ca2+ channels activated by Bay K 8644, Mn2+ can enter cytoplasm through the channels and induce contraction in taenia coli.
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