Tetrodotoxin and manganese ions: effects on electrical activity and tension in taenia coli of guinea pig.

Tetrodotoxin, at concentrations up to 5 x 10(-6) gram per milliliter, has no effect on the spontaneous discharge in the smooth muscle of taenia coli. However, the spontaneous discharge is abolished by Mn(++) at a concentration of 0.5 millimole per liter. The contraction induced by immersing the muscle in isotonic KCl solution is also suppressed in the presence of Mn(++). Because Mn(++) is a specific suppressor of the spike induced by Ca(++) and tetrodotoxin is an inhibitor of the spike induced by Na(+), we suggest that Ca(++) is a charge carrier in the production of spike potential in the smooth muscle and that the entry of intervening Ca(++) through the membrane acts as a trigger for the contraction of smooth muscle.