Abstract
Electrophysiological recordings from freshly-dissociated smooth muscle cells from toad stomach revealed that substance P enhances one of two types of Ca2+ currents. That is, substance P enhances the slowly inactivating, high-threshold current but not the fast inactivating, low-threshold current. Acetylcholine has the same effect, but the acetylcholine action is blocked by atropine whereas the substance P action is not, indicating that the two agents act at different receptor sites. Thus, substance P, like acetylcholine, has a dual excitatory action on the smooth muscle cells employed in these studies, enhancing a specific type of Ca2+ current, as demonstrated here, and suppressing a voltage-sensitive K+ conductance, as previously described [Sims, S.M., Walsh, J.V., Jr. & Singer, J.J. (1986) Am. J. Physiol. 251, C580-C587].