Abstract
Effects of protamine and lanthanum on contractile responses of isolated guinea pig ileum segment or ileum longitudinal muscle strip plus relaxtion responses of the isolated guinea pig taenia coli have been demonstrated herein. Protamine and lanthanum inhibited nonspecifically contractile responses of the ileum to various agonists such as acetylcholine, serotonin, bradykinin, potassium chloride and barium chloride and caused a depression of the maximum response in the acetylcholine dose-response curve. Nonspecific inhibitory actions were similar to papaverine action since the inhibitory actions were overcome easily with addition of calcium ion to the organ bath fluids. Protamine and lanthanum, however, inhibited to a greater degree contractile responses by such agonists as serotonin and bradykinin that act through receptors rather than through membrane. Both protamine and lanthanum blocked the relaxation response by isoproterenol in taenia coli without reducing the resting tone of the muscle. From these findings, it is presumed that protamine and lanthanum have the same mechanism or site of action, that is, they replace the surface site calcium ion on the cell membrane, thus blocking the utilization of calcium which is essential for a receptor-agonist interaction.
