Ceveratrum alkaloids: progress in understanding their membrane and inotropic actions

Prevention by specific chemical classes of alpha 1-adrenoceptor antagonists of veratrine-contractures in rat left atria independently of alpha 1-adrenoceptor blockade

1. The putative direct protective effects of a series of chemically diverse alpha 1-adrenoceptor antagonists against veratrine alkaloid-induced tetanic contractures in rat isolated left atria have been investigated. 2. Atria were mounted in organ baths containing normal, oxygenated physiological salt solution (20 ml, pH 7.4), for isometric tension recording. Atria were electrically driven at 4 Hz and were maintained at 34 degrees C. Veratrine (100 micrograms ml-1) was applied to the atria to elicit tetanic (diastolic) contracture. 3. Concentration-dependent protective effects against veratrine-contractures, in the absence of negative inotropic responses, were observed with the quinazoline congeners, prazosin and doxazosin and with the benzodioxane-related compounds, WB 4101 and its thio analogue, benoxathian. IC50 concentrations and apparent Hill coefficients of all four drugs ranged from 0.27 to 0.93 microM, and from 0.86 to 1.09, respectively, and are consistent with interaction at a single site. 4. In contrast, no protective activity versus veratrine-contractures was observed with corynanthine, 5-methyl-urapidil, phenoxybenzamine, phentolamine or chloroethylclonidine (10 microM). 5. Contractures were prevented by prazosin at concentrations 2-3 log units higher than those which antagonized methoxamine-evoked inotropic responses. In addition, concomitant alpha 1-adrenoceptor occupancy by high concentrations of methoxamine (100 microM), phentolamine (10 microM, inactive per se in preventing contracture), or both drugs together, failed, in each case, to modify significantly the protective effects of prazosin or WB 4101 against veratrine-contractures. 6. Our findings demonstrate that alpha 1-adrenoceptor antagonists which prevent veratrine-contractures belong to specific chemical classes of the quinazoline- and benzodioxane-type. The mechanism by which these drugs afford protection is apparently independent of an interaction with defined alpha 1-adrenoceptors.

Veratrine-induced tetanic contracture of the rat isolated left atrium. Evidence for novel direct protective effects of prazosin and WB4101

An investigation has been made of the putative direct myocardial protective effects of the alpha 1-adrenoceptor antagonists, prazosin and WB4101, against tetanic contractures of rat isolated left atria following modified Na+ channel function and consequent Ca2+ loading elicited by veratrine. Veratrine evoked concentration-dependent, reversible, tetanic contractures which were critically dependent upon the external Ca2+ concentration. Tetrodotoxin (TTX), prazosin, WB 4101 and R 56865 (0.1-10 microM) prevented tetanic contracture elicited by veratrine (100 micrograms/ml) at concentrations which were significantly lower than those which decreased active tension development. The apparent Hill coefficients (nH) obtained for TTX, prazosin, WB 4101 and R 56865 were comparable (range 0.79-0.93), and are consistent with a single site of action. In contrast, the class 1 antiarrhythmic agents, quinidine and lidocaine, elicited no significant inhibition of veratrine-induced contracture at 30 microM, but almost completely abolished the contractures at 100 microM. The nH values for quinidine and lidocaine were found to be significantly greater than unity (3.1 and 2.6, respectively). The L-type Ca2+ channel blockers, diltiazem, nicardipine, nifedipine and verapamil only weakly prevented tetanic contracture, whilst markedly, and concentration-dependently, decreasing active tension development. Neither atropine (10 microM) nor propranolol (1 microM) significantly modified either veratrine-induced contractures or active tension development. In conclusion, evidence is presented of novel, direct protective effects of prazosin and WB 4101 against tetanic contracture following modified Na+ channel function and Ca2+ loading provoked by veratrine. The precise mechanisms involved are unclear at present, but appear to be distinct from blockade of atrial alpha 1-adrenoceptors or L-type Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Veratridine-induced intoxication: an in vitro model for the characterization of anti-ischemic compounds?

Due to the complexity of ischemia-induced cellular dysfunction many different pharmacological approaches have been tested to improve cellular ischemia resistance. However, despite the importance of [Na+]i for ischemia-induced dysfunction, only very few studies have investigated the contribution of the Na+ channel to ischemia-induced failure of intracellular ion homeostasis. Since an activation of Na+ channels by veratridine also results in a failure of intracellular ion homeostasis, the veratridine- and ischemia-induced alterations of cellular function were compared. Moreover, despite the difference in the electrophysiological changes induced by veratridine and ischemia, the possible involvement of a slowly inactivating, less selective Na+ channel in both veratridine- and ischemia-induced cellular dysfunction is discussed. As a conclusion it is suggested that veratridine intoxication could be a helpful in vitro method for the characterization of putative anti-ischemic compounds.

Veratridine activates a silent sodium channel in rat isolated aorta

To investigate the existence of silent Na+ channels, isolated rat aorta was treated with veratridine (0.1 mM) and the resulting Ca2+ uptake was determined. After 30-min incubation the total tissue uptake of Ca2+ and Ca2+ uptake increased from 2.325 +/- 0.017 to 2.614 +/- 0.080 nmol/mg wet weight (ww) and from 162.6 +/- 9.7 to 218.1 +/- 13.0 pmol/mg ww, respectively. The veratridine-induced Ca2+ uptake was blocked by tetrodotoxin (1 microM; to 17 +/- 5%) but not altered by amiloride (10 microM-1 mM). Activation of Na+/Ca2+ exchange by Na+ removal increased Ca2+ uptake from 74.2 +/- 4.5 to 97.3 +/- 5.3 pmol/mg ww, which was suppressed by amiloride (10 microM-1 mM). Nifedipine (10 nM) and verapamil (0.1 microM) at concentrations at which depolarization-induced Ca2+ uptake was diminished did not attenuate veratridine-induced Ca2+ uptake. Phenytoin at 0.1 mM reduced the Ca2+ uptake induced by veratridine or by depolarization. R 56865 (0.1 microM) and R 59494 (1 microM), novel anti-ischemic compounds inhibiting slowly inactivating Na+ channels, suppressed the veratridine-induced but not the depolarization-induced Ca2+ uptake. Guanidinium uptake was increased by veratridine (0.1 mM) from 371.2 +/- 7.2 to 574.8 +/- 45.9 pmol/mg ww. These results suggest that the rat aorta possesses a Na+ channel which is electrically silent under normal conditions but could be activated by veratridine.

Protective effect of bepridil against veratrine-induced contracture in rat atria

In isolated stimulated rat atria, superfusion with veratrine caused a marked contracture (VIC) which was absent in calcium-free medium and which was inhibited by tetrodotoxin (IC50VIC of 1.38 microM). Lowering the extracellular calcium concentration from 2.5 to 0.5 or 0.1 mM reduced the veratrine-induced contracture and delayed its onset. Superfusion of bepridil (1-10 microM) for 60 min before and during veratrine exposure markedly slowed the onset of contracture, reduced the maximum response (IC50VIC = 2.11 microM) and facilitated recovery upon washout of the alkaloid. The direct negative inotropic effect (NIE) of bepridil (IC50NIE = 10.96 microM) resulted in an VIC/NIE ratio of 5.19 for this drug. The protective effects of bepridil were rate-independent and were not modified by the presence of atropine (1.4 microM) and propranolol (0.3 microM) in the medium. Diltiazem, verapamil and nifedipine only reduced veratrine-induced contracture at concentrations much higher than those producing a negative inotropic effect, giving them negative NIE/VIC ratios of 0.31, 0.08 and 0.08 respectively. Like bepridil, flunarizine had a positive NIE/VIC ratio (15.87, IC50VIC = 3.71 microM). The lack of effect of the quaternary derivative of bepridil CERM 11888 indicated that intracellular sites of action may be involved in the activity of bepridil on veratrine-induced contracture. Given that veratrine-induced changes may mimic some of the pathological changes occurring in ischaemia, the results suggest that bepridil and flunarizine may be more effective than L-type, slow calcium ion-channel blockers in protecting against calcium overload during ischaemia and reperfusion injury.

Paradox response of frog muscle membrane to changes in external potassium

Applying conventional microelectrode technique the anomalous behaviour of membrane potential in response to changes in [K+]o was demonstrated in normal and cevadine-treated muscles bathed in Cl- -free medium. Partial repolarization of the cevadine-depolarized membrane and reappearance of the slow membrane potential oscillation (SMPO) were induced by elevating [K+]o from 2.5 mM to 10-20 mM. Both effects were reversed by return to 2.5 mM [K+]o. The K-induced repolarization was markedly reduced by 20 mM Cs+, but not by 0.1 mM ouabain, 1 mM 4-aminopyridine, or 1 mM diethyl-pyrocarbonate. The elevation of [K+]o failed to repolarize muscle fibers that had been depolarized only to a small extent. No K-induced repolarization has been observed in Cl- -containing fluid. In cevadine-free experiments the omission of potassium from the extracellular space in Cl- -free solution hyperpolarized some of the fibers, while depolarized others. Strong electrical stimuli applied in zero K-zero Cl solution turned all the fibers into depolarized state; on returning to 2.5 mM [K+]o complete repolarization was achieved in most of the fibers. It has been concluded that the paradox response of the muscle membrane to changes in [K+]o can be attributed to the K-dependent conductance changes of the inward rectifier K channel providing an explanation for the plateau-formation of SMPO and for the existence of two stable levels of membrane potential of the skeletal muscle bathed in Cl- -free medium.

Inhibitory effects of noradrenaline and dopamine on calcium influx and neurotransmitter glutamate release in mammalian brain slices

Noradrenaline and dopamine (0.1-100 microM) inhibited 45Ca2+ uptake and glutamate release induced by veratrine (25 microM) in cortical and striatal slices but were without effect when added alone. Each parameter was inhibited in a dose-dependent manner by noradrenaline in cortical slices (IC50 = 0.05 microM) and by dopamine in striatal slices (IC50 = 0.08 microM). Noradrenaline (0.01-100 microM) was without influence on veratrine-induced 45Ca2+ influx or glutamate release in the striatal preparation, and likewise dopamine was inactive in cortex slices. The use of adrenoceptor antagonists suggests that the action of noradrenaline is mediated by the alpha 2-receptor which is thought to be adenylate cyclase linked. Dopamine appeared to be acting through the D-2 receptor.

Common anticonvulsants inhibit Ca2+ uptake and amino acid neurotransmitter release in vitro

Phenytoin (PHT), phenobarbital (PB), and carbamazepine (CBZ) dose-dependently inhibited veratrine-stimulated calcium influx and evoked amino acid neurotransmitter release in rat cortical slices at relatively low concentrations. The action on Ca2+ influx was in the clinical effective dose range for these anticonvulsant drugs, whereas the action on amino acid release was mostly well above this range. Neither ethosuximide (ESM) nor sodium valproate (VPA) had any effect on the veratrine-stimulated Ca2+ influx. Stimulated amino acid release was not affected by ESM, whereas VPA specifically inhibited the release of aspartate in preference to glutamate and GABA at concentrations well within the clinically effective dose range. The actions of VPA and ESM on other parameters measured were detectable only at very high concentrations, which are likely to be irrelevant in defining their clinical mode of action.

The positive inotropic effect of aconitine

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 aconitine reduces the overshoot and shortens the plateau phase of the action potential awaits further study.