DIFFERENTIAL SENSITIVITY TO MG2-BLOCK AND TUBOCURARINE-BLOCK OF FROG NEUROMUSCULAR-JUNCTIONS IN SUMMER AND WINTER()

Several parameters of transmitter release in neuromuscular junctions w ere compared in 'winter' and 'summer' frogs. In low Ca2+/high Mg2+-blo ck, the quantal content m of endplate potentials was similar for both groups. In curarized junctions, however, endplate potentials were abou t twice as large in winter as in summer, and facilitation was lower an d depression higher. This indicates that transmitter release is higher in winter junctions. This is not reflected in Mg2+-block, suggesting that strong calcium deprivation may suppress release in winter more st rongly than in summer junctions.