The influence of polyvalent cations on the activation of end plate receptors has been studied in vitro on the sartorius muscle of the frog. In the absence of extracellular calcium, the sensitivity of the receptors to depolarizing quaternary ammonium salts was markedly reduced. Maximum receptor activation occurred in those fibers equilibrated in 1.8 mM calcium Ringer solution, with the response being reduced as the calcium concentration was raised or lowered. Magnesium was less efficient than calcium in regulating the sensitivity of the end plate receptors, the maximum receptor response occurring in those fibers equilibrated in 8 mM magnesium Ringer solution. In the presence of lanthanum the end plate response to carbamylcholine or acetylcholine was enhanced. Lanthanum increased the conductance change produced by carbamylcholine both in polarized and in potassium-depolarized fibers. The application of 10-2 mM lanthanum to the end plate increased MEPP's amplitude, rise time, and half-fall time by 19, 54, and 45%, respectively. The results suggest that polyvalent cations influence postjunctional membrane receptor processes in addition to their well-documented prejunctional action.

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