The effects of samarium on the contractile responses of the guinea-pig taenia coli to acetylcholine, potassium, histamine and barium.

The role of calcium in supporting the phasic contractions of the isolated guinea-pig taenia coli induced by acetylcholine, potassium, histamine or barium has been studied using the lanthanid samarium (Sm3+). A carefully selected concentration of Sm3+ (6 X 10(-5)M) depressed to a same extent the phasic responses of the tissue to submaximal concentrations of the agonists used whilst maximum responses to histamine and barium were unaffected. Calcium and barium-induced responses of the depolarized taenia coli were not inhibited by 6 X 10(-5)M Sm3+. The rate of loss of responsiveness of the depolarized tissue to acetylcholine was increased, that to histamine was not altered while that to barium was decreased by 6 X 10(-5)M Sm3+. These findings support the concept that phasic responses of the guinea-pig taenia coli to submaximal concentrations of the agonists used are dependent on a Sm3+-sensitive calcium store(s) probably located at the cell membrane. However, maximum responses to histamine and barium seem to be supported, at least partially, by a Sm3+-insensitive depot(s) probably located at cytoplasmic compartments.