LANTHANIDES AND EXCITATION-CONTRACTION COUPLING IN GUINEA-PIG ILEAL LONGITUDINAL SMOOTH MUSCLE.

The response of guinea-pig ileal longitudinal smooth muscle to acetylcholine and related muscarinic stimulants are of considerable interest because both the phasic (initial, fast) and tonic (later, slow) components of the contractile response show a very steep dependence upon Ca**2** plus and are very sensitive to La**3** plus . Hence, a comparative survey of the lanthanide series with a range of ionic radii very similar to that of Ca**2** plus (Ca**2** plus equals 0. 99A, La**3** plus equals 1. 006A, Lu**3** plus equals 0. 85A), which adopt the same 8-coordination, are subject to the same steric effects as Ca**2** plus and whose chemistry is in general very similar to that of the group IIA cations, is of interest.