Stimulation of frequency of MEPPs at the frog neuromuscular junction by extracellular EGTA

Exposure of the frog neuromuscular junction at 17 degrees C or 23 degrees C to salines with low [Ca2+], buffered with EGTA, cause mepp frequency to fall after 4--6 min to about 20% of the control rate. Results obtained in the presence of verapamil suggest that this fall is a consequence of a lower Ca2+-influx coupled with the action of the extracellular EGTA in promoting Ca2+-efflux from the terminals. These findings confirm the suggestion that [Ca2+]i has a major role in determining mepp frequency. At 13 degrees C, the fall in mepp frequency after addition of EGTA is preceded by a transient (1--2 min) rise in mepp rate which is not present at 17 degrees C or in the presence of verapamil. This transient acceleration in spontaneous release is believed to be because EGTA promotes the emptying of a Ca2+-reservoir on or beneath the inner face of the membrane, thus causing a rapid Ca2+-efflux via the Ca2+-sensitive sites that trigger exocytosis of transmitter. The significance of the sensitivity of the response to temperature is discussed. The suppressive effect of higher temperatures can be reversed to some extent by hyperosmotic salines, an effect that may reflect the action of hypertonicity on the plasmalemma. It is concluded that the characteristics of the release system may change markedly at about 16 degrees C. Ca2+-EGTA buffers are widely used; it is suggested that extracellular EGTA can also modify [Ca2+]i in cellular systems.