When motor nerve terminals are slightly depolarized with increased [K+]o, progressive increases in [Ca2+]o raise min.e.p.p. frequencies until a maximum is reached; further increases then produce a depression (Cook & Quastel, 1973; Matthews & Wickelgren, 1977). 2. Increases in [Mg2+]o also produce the depression. 3. It has been suggested that the depression results from this sequence of events: (a) the divalent cations screen the fixed negative surface charges on the outer face of the nerve terminal, which (b) decreases the negativity of the surface potential, which (c) increases the voltage gradient within the membrane itself, which (d) tends to shut depolarization-gated channels for Ca2+ entry, which (e) decreases min.e.p.p. frequency. 4. In agreement with the interpretation, in frog neuromuscular junctions slightly depolarized with 11 mM-[K+]o, min.e.p.p. frequency is a monotonically increasing function of [Ca2+]o, as long as the sum of [Ca2+]o plus [Mg2+]o is kept constant. 5. The decrease in min.e.p.p. frequency caused by raising [Mg2+]o by 5 mM can be counterbalanced by raising [K+]o by about 9 mM. Using the Grahame equation (1947), assuming that the elevated divalent cations act solely by screening and have no effect on conductance, the negative surface charge is estimated to be roughly 1 electronic charge/75 A2.
