Topochemical Factors in Potentiation of Contraction by Heavy Metal Cations

In addition to the previously studied Zn²⁺, low concentrations (about 0.5 mM) of Be²⁺, Ba²⁺, Cd²⁺, Ni²⁺, Cu²⁺, Pt⁴⁺ and, outstandingly, 0.5 µM of UO₂²⁺, potentiate the twitch of frog sartorius and toe muscles by prolonging the active state of contraction. The degree of potentiation is a roughly S-shaped function of p(metal²⁺), suggesting that each metal binds to a ligand of the muscle fiber, representative apparent affinity constants being: UO₂²⁺, 5 x 10⁶; Zn²⁺, 2.8 x 10⁵; and Cd²⁺, 2 x 10⁴. UO₂²⁺ potentiation effects are rapidly reversed by PO₄, and Zn²⁺ and Cd²⁺ effects by EDTA, PO₄, and cysteine. The rapidity of these reversals by the nonpenetrating EDTA and PO₄, and the fact that heavy metal ions evidently potentiate by prolonging the action potential, indicate that the metal potentiators exert their primary action at readily accessible (i.e. plasma and T tubular) membrane sites. The relatively slow kinetics of development of potentiation, and the even slower reversal of it in pure Ringer's solution, indicate that the metal ions are bound to connective tissue, as well as to muscle fibers. The binding effects at the readily accessible membrane sites evidently impairs delayed rectification and thus modifies the action potential and excitation-contraction coupling so as to cause potentiation. SH is excluded, and PO₄ and imidazole are possibilities, as the membrane ligand binding the potentiating metal ions.