Kinetic analysis of the action of chemical modulators on neuromuscular transmission.

Abstract

The kinetics of the direct action of 4-aminopyridine (4AP) and streptomycin (SM) on the mechanism of transmitter release were studied by recording the endplate potentials from curarized frog muscles using conventional microelectrode techniques, and by calculating the fractional release from the store of available acetylcholine quanta from the experiments with tetanic rundown during short train stimulations. To explain the tremendous facilitatory effect of 4AP on the fractional release and the antagonistic interaction of SM thereupon, it was postulated that 4AP and SM modify the transmitter output by combining with the Ca-dependent process X; i.e., 4AP and SM compete for the occupancy of the X site. A combination of 4AP or SM presumably modifies allosterically the action of the Ca-X complex, resulting in a profound augmentation of the evoked release of the available transmitter with the former and its depression with the latter. The theoretically derived equations from the above assumptions agreed reasonably well with the results obtained. Aminopyridines have long been known to have a variety of actions on physiological functions. They have been shown to reduce the K conductance of the axonal membrane, exert an anticurare action on the neuromuscular junction (SoBEK et al., 1968 ; BOWMAN et al., 1977 ; HARVEY and MARSHALL, 1977), increase catecholamine release in a variety of tissues, stimulate respiratory activity (FASTIER and MC-DOWALL, 1958), and produce strychnine-like convulsions (FASTIER and MCDOWALL 1958 ; HARVEY and MARSHALL, 1977). The amplitude of the twitch tension of a muscle fiber is potentiated by these compounds (FASTIER and MCDOWALL, 1958 ; BOWMAN et al., 1977 ; HARVEY and MARSHALL, 1977). In particular, 4-aminopyridine (4AP) and 3,4-diaminopyridine (34DAP) have a profound effect on the neuromuscular junction. By enhancing enormously the release of transmitter from the motor nerve terminal (DURANT and MARSHALL, 1980), 4AP and 34DAP antagonize the d-tubocurarine (dTC) and antibiotic-induced neuromuscular blockades, and also relieve the long-lasting muscle paralysis produced by Botulinum

Catechol and noradrenaline facilitate the neuromuscular transmission via different mechanisms.

“…It should be pointed out that the pharmacological dose-release relationship of PC obtained in the present paper is identical in many respects to the case of "chemical potentiation or accelerated evoked release" (KATZ and MILEDI, 1979;MAENO et al, 1981) of 4-aminopyridine (4AP). The amount of dTC required to suppress the EPP potentiated by 1 mM PC (55-75 /M) was comparable to that of 1 mM 4AP (66-75 /M).…”

“…This suggests that the presynaptic action of NA cannot be explained from the above scheme. The accelerated evoked release by PC found in the present paper (KA=35 µM and PA= 0.76) was somewhat less than that of 4AP (KA=8.5 µM and PA=0.92: MAENO et al, 1981), but it was considerably more potent than TEA (KA=0.5 mM and PA=0.90: MAENO et a!.,1982). Thus, the present data reveal that measurement of the fractional release provides an important clue in determining the mechanisms of actions of various release modifying agents.…”

“…When t is small (e. g. 0.1-0.3 sec) a-kt can be approximated to unity, and we obtain Pt =P. Consequently, we are able to estimate a fairly accurate value of P. It has already been shown by MAENO et al (1981) that P serves as a useful index in analyzing the transmitter output. The amount of a' tubocurarine (dTC) in each test solution was adjusted so that the first EPP in a train was of almost the same amplitude in a series of experiment.…”

“…Furthermore, the antagonistic interaction of streptomycin (SM) on the dose-release relation of PC (Maeno, unpublished) was the same as that observed with 4AP . However, peculiar multiple EPP responses seen in the presence of 4AP (LUNDH,1978;MAENO et al, 1981) or tetraethylammonium (TEA) (KOKETSU, 1958;MAENO et al, 1982) were rarely recorded in PC-Ringer. Relatively slow development of facilitatory presynaptic effect found with PC (15-30 min to reach the quasiequilibrium state) suggested that PC acts intracellularly as does 4AP.…”

“…To explain the action of chemicals which accelerate remarkably the tetanic rundown of EPPs, MAENO et al (1981) have postulated two types of evoked release of transmitter. Under ordinary conditions, the transmitter output is proportional to the amount of Ca-bound X (Ca4X); i. e. 4Ca+ X Ca4X Pca' regular evoked release,…”