Differential sensitivity to Mg(2+)-and tubocurarine-block of frog neuromuscular junctions in summer and winter.

Several parameters of transmitter release in neuromuscular junctions were compared in "winter' and "summer' frogs. In low Ca2+/high Mg(2+)-block, the quantal content m of endplate potentials was similar for both groups. In curarized junctions, however, endplate potentials were about twice as large in winter as in summer, and facilitation was lower and depression higher. This indicates that transmitter release is higher in winter junctions. This is not reflected in Mg(2+)-block, suggesting that strong calcium deprivation may suppress release in winter more strongly than in summer junctions.