Statistical parameters of transmitter release at frog neuromuscular junctions treated with guanidine or tetraethylammonium

Abstract

Transmitter release at Mg++ -depressed frog neuromuscular junctions can be described using binomial statistics. The number of quanta (m) released by the nerve terminal action potential is directly proportional to the mean probability (p) that a quantum will be released and the number of quanta available for release (n). Guanidine or tetraethylammonium (TEA) increased m and n, but had no effect on p. At junctions depressed by d-tubocurarine, both compounds enhanced the amplitude of the initial end-plate potential, caused an accelerated rate of fade of end-plate potential amplitudes and raised the steady-state level of end-plate potential amplitude. This finding was interpreted to mean that guanidine and TEA increased transmitter mobilization and may be related to the increase by the compounds of the parameters n. If so, then the data support the idea that n represents the number of quanta available for release rather than the number of transmitter release sites in the terminal membrane. Neither compound affected the power relationship between [Ca++]o and transmitter release. When plotted on a double logarithmic basis, the slope of the line relating [Ca++]o to increased transmitter release was 3.7, a relationship not altered by the drugs. Thus, the compounds had no effect on the fundamental interaction between Ca++ and transmitter release sites. In contrast to [Ca++]o, the power relationship between increased transmitter release and the concentration of drug in the bathing solution was 0.69 for guanidine and 0.84 for TEA. Because of this finding, it was concluded that the compounds increased transmitter release by mechanisms other than or in addition to increasing Ca++ conductance.