Voltage- and time-dependent action of histrionicotoxin on the endplate current of the frog muscle.

Abstract

A B S T R A C T Histrionicotoxin, a toxin isolated from skin secretions of a Colombian arrow poison frog, Dendrobates histrionicus, decreased the amplitude and timecourse of the endplate current, and altered the voltage dependence of the halfdecay time. In addition, the toxin produced a characteristic nonlinearity in the current-voltage relationship of the endplate current when 3-s voltage conditioning steps were used. Reduction in time of the conditioning steps to 10 ms made the current-voltage relationship linear. The decrease in peak amplitude of the endplate current (epc) produced by histrionicotoxin measured during long hyperpolarizing conditioning steps was fitted by a single exponential function. The calculated rate constants ranged from 0.03 to 0.14 s -1 and varied with membrane potential at hyperpolarizing levels. The voltage-and time-dependent action of histrionicotoxin does not require an initial activation of receptors by acetylcholine (ACh). The characteristic of the current-voltage relationship can be accounted for by the observed voltage and time dependency of the attenuation of the endplate current amplitude in the presence of histrionicotoxin during long conditioning steps. These effects of histrionicotoxin on the peak amplitude, and on the voltage and time dependence of the epc were concentration-dependent and slowly reversible upon washing out the toxin. Thus, the voltage-and time-dependent action of histrionicotoxin at the endplate is related to an increase in the affinity between the toxin and the ACh receptor-ionic channel complex. This increase in affinity is postulated to be due to a conformational change of the macromolecule in the presence of histrionicotoxin which is demonstrated to be relatively slow, i.e., on the order of tens of seconds.

Differentiation of the open and closed states of the ionic channels of nicotinic acetylcholine receptors by tricyclic antidepressants.

“…The elimination ofthe nonlinearity in the I/V relationship ofthe epc (1 and 2 ,M) when the membrane potential was changed from the holding potential at -50 mV to the extremes of +50 mV and -150 mV, using 3- (1981) 5243 by using voltage sequence B suggests that the nonlinearity observed when sequence A is used is independent of AcCho receptor activation. In this respect, the results produced by these agents are qualitatively similar to the effects of histrionicotoxin and phencyclidine on epc amplitude (5,6). reps remained unaltered whether voltage sequence A, B, or C was used.…”

“…Because our initial experiments with the tricyclic antidepressants (10, 11) disclosed a voltage-and time-dependent effect on epc amplitude, we decided to investigate these voltage-and time-dependent effects by using various membrane potential conditioning sequences (6,20). Voltage sequence A was similar to that used to examine the effect ofhistrionicotoxin on epcs in frog sartorius muscles (6). The sequence consisted of 10-mV steps from an original holding potential of -50 mV.…”

“…The blockade of ionic channels in their open conformation by these agents can yield a voltage-dependent decrease in the endplate current (epc) time constant of decay (Tep,) (4). Under certain conditions, however, the voltage-and time-dependent decrease of peak amplitude and consequent hysteresis loop in the current/voltage (I/V) relationship of the epc produced by these agents involves the ionic channel in its closed conformation (4)(5)(6)(7)(8). Recent studies have provided strong evidence that TepC may be altered by blockade of the closed channel (7,8).…”

The interaction between hexamethonium and tubocurarine on the rat neuromuscular junction

“…At higher concentrations (400 gM and above) the decay phase was often split into a fast and a slow exponential component, as found by Milne & Byrne (1981) (Adams, 1977;Masukawa & Albuquerque, 1978;Tiedt et al, 1979;Adams & Feltz, 1980a, b) and on autonomic ganglion cells (Ascher et al, 1979;Rang, 1982) (Jenkinson, 1960;Colquhoun et al, 1979) and gallamine (Colquhoun & Sheridan, 1982) has been shown to be insensitive to membrane potential. Colquhoun & Rang (1976) Pennefather & Quastel, 1981).…”

The ionic channel of the nicotinic acetylcholine receptor is unable to differentiate between the optical antipodes of perhydrohistrionicotoxin

“…These matched pairs of muscles were used because the toxin washes out of muscle so slowly that tests of the two enantiomers on the same muscle are impossible. To prevent twitch, glycerol shock was used [12-141. The technique for EPC experiments and analysis is described elsewhere [13,14]. It has been shown [6] that 10pM Hiz-HTX reduces EPC peak amplitude to about 50% of the control value.…”