aconitine and dihydroouabain

The rate of non-quantal acetylcholine (ACh) release was estimated at the mouse neuromuscular junction by observing the effect of (+)-tubocurarine on endplate membrane potential or current in preparations pretreated with an irreversible anti-acetylcholinesterase (anti-AChE). Voltage clamping was an effective method for measuring non-quantal release. Non-quantal release was markedly inhibited by 10 microM aconitine. Non-quantal release was not significantly increased by 10 microM dihyroouabain (DHO). (It has been reported that ouabain increases the leak). Non-quantal release was roughly doubled following exposure to hypertonic solution or to elevated K(+)-solution. This is in accord with the hypothesis that the leak is by way of ACh transporters incorporated into the terminal membrane following exocytosis, but other interpretations remain to be tested.

Topics: Acetylcholine; Aconitine; Animals; Diaphragm; Female; In Vitro Techniques; Kinetics; Male; Membrane Potentials; Mice; Motor Endplate; Neuromuscular Junction; Ouabain; Quantum Theory; Tubocurarine

1. The inotropic and electrophysiological effects of aconitine were measured in the isolated, isometrically contracting guinea-pig papillary muscle during the prearrhythmic phase of alkaloid action. 2. In muscles stimulated continually at 1 Hz, 1 mumol/l aconitine produced a positive inotropic effect that reached 38 +/- (SEM) 9% immediately before the onset of arrhythmia (n = 3). 3. If aconitine (0.5 mumol/l) was applied to non-stimulated (resting) muscles for 30 min and 1-Hz stimulation resumed thereafter, the arrhythmia occurred after 724 +/- 101 beats. Prolongation of the rest exposure to 2 h did not significantly diminish the number of prearrhythmic beats. Thus, the onset of aconitine action is critically determined by muscle activity (rather than by time), and a 30-min aconitine application to the resting muscle suffices for complete equilibration of the tissue. 4. Using the preequilibration-at-rest procedure, the positive inotropic effect of aconitine (0.25 - 4 mumol) was found (a) to be absent in the rested-state contraction, (b) to grow with both number of subsequent beats and alkaloid concentration, and (c) to reach a similar prearrhythmic maximum at all concentrations. This maximum amounted to about 1/4 of the maximum positive inotropic effect of dihydroouabain. It was not influenced by reserpine pretreatment of the guinea pig. 5. Aconitine (1 mumol/l) delayed the repolarization phase of the action potential by establishing a secondary plateau at approximately -60 mV. This effect paralleled the positive inotropic effect and, like the positive inotropic effect, was abolished by 10 mumol/l tetrodotoxin (TTX). In partially depolarized muscles ([K]0 = 24 mmol/l) aconitine (8 mumol/l) produced a TTX-sensitive increase in amplitude and rate of rise of the rested-state contraction; this indicates a voltage-dependent effect on some resting Na channels. 6. While delaying the late repolarization phase, aconitine markedly shortened the early repolarization at levels positive to -40 mV, reduced the overshoot and decreased the maximum rat of depolarization of the action potential. Slow action potentials ([K]0 = 24 mmol/l; 10 mumol/l TTX) were insensitive to aconitine. 7. We conclude that the well known property of aconitine to prolong the Na influx during the action potential leads to a positive inotropic effect, thus confirming the importance of Na influx for the regulation of myocardial contractility. The exact mechanism of an additional effect by which ac

Topics: Aconitine; Aconitum; Action Potentials; Animals; Electric Stimulation; Female; Guinea Pigs; In Vitro Techniques; Ion Channels; Male; Myocardial Contraction; Ouabain; Papillary Muscles; Reserpine; Sodium; Stimulation, Chemical; Tetrodotoxin