Fig. 2.

Effect of high-K solution on quantal transmitter release in a csp mutant preparation. Acsp mutant preparation was warmed to 32°C until nerve impulse-evoked responses failed completely (as in Fig.1A). The initial record (32°C normal K) shows mejps that persist under these circumstances. The bath solution was then replaced with high-K solution (32°C high K), and mejp frequency was not detectably altered. (However, the membrane potential dropped from −45 to −28 mV.) When this preparation was cooled back to 22°C for 15 min, one sees a large increase in mejp frequency (22°C high K), although the membrane potential of the muscle fiber remains depolarized (at −27 mV).