Allosteric modification of α-bungarotoxin binding by the ‘calcium channel antagonist’ verapamil

Methoxyverapamil reduction of nicotine-induced catecholamine release involves inhibition of nicotinic acetylcholine receptor currents

The mechanism by which the putative Ca2+ channel blocker methoxyverapamil (D600) inhibits nicotine-induced catecholamine release was investigated in bovine adrenal chromaffin cells and in neurons from paravertebral sympathetic ganglia of chick embryos. We found D600 to prevent catecholamine release evoked by 30 s applications of nicotine with a significantly higher potency than the release induced either by 30 s K+ depolarizations or by electrical field stimulation of sympathetic neurons. Like the use-dependent action of D600 upon Ca2+ channels, the magnitude of inhibition of the K(+)-evoked secretion depended on the duration of stimulation (10 s to 5 min). Data on catecholamine release were supplemented by patch-clamp recordings. We found whole-cell currents in chromaffin cells evoked by (extrapolated) 0.5 s applications of nicotine to be significantly more sensitive to D600 than Ca2+ currents induced by a 0.5 s depolarization from -80 to 0 mV. In both instances, the potency of D600 depended on the duration of the (nicotinic and depolarizing) stimuli. Our data suggest that D600 inhibits nicotine-induced catecholamine release by reducing nicotinic acetylcholine receptor currents rather than voltage-gated Ca2+ currents. Hence, in chromaffin cells as well as in sympathetic neuronal preparations, D600 does not appear to be a suitable tool to investigate the part voltage-activated Ca2+ currents play in cellular events induced by nicotine.

Inhibition by calcium channel blockers of the binding of platelet-activating factor to human neutrophil granulocytes

The inhibitory action of calcium channel blockers on platelet-activating factor (PAF)-induced activation of human polymorphonuclear granulocytes (PMNL) and on the binding of [3H]PAF to neutrophils was studied. Verapamil and diltiazem inhibited PAF (10(-8)-10(-15) M)-induced degranulation and superoxide production in a dose-dependent manner, with pA2 values of 5.6 and 6.1 for verapamil and 5.9 and 6.2 for diltiazem, respectively. Both channel blockers inhibited the specific binding of [3H]PAF to PMNL in dose-dependent fashion, with Ki values of 3.9 +/- 0.6 X 10(-5) M and 3.2 +/- 0.4 X 10(-5) M for verapamil and diltiazem, respectively. Scatchard analysis of the binding data revealed that both calcium channel blockers decreased the receptor binding affinity and slightly increased the number of high-affinity PAF receptors, whereas they did not affect the binding affinity and number of low-affinity receptors. These results indicate that calcium channel blockers can inhibit neutrophil responses elicited by PAF by a mechanism other than inhibition of calcium influx and suggest that the PAF receptor may be closely associated with calcium channels.

Organic calcium channel antagonists provoke acetylcholine receptor autodesensitization on train stimulation of motor nerve

The effects of nicardipine and other organic Ca2(+)-channel antagonists on the responses induced by indirect train stimulation (3 s, 50-100 Hz) were studied in mouse phrenic nerve diaphragm preparations. Nicardipine at 1-10 microM, which alone did not affect single or tetanic contractions or the amplitude of evoked endplate potentials and spontaneous miniature endplate potentials, caused tetanic contraction to fade completely in the presence of 0.3 microM neostigmine or 50 microM diisopropylfluorophosphate. In combination with these anticholinesterases, nicardipine caused a severe run-down and shortening of endplate potentials in 1-2 s. This effect on endplate potentials was dependent on stimulus frequency and on extracellular Ca2+. The effect was accelerated by intracellular injection of Ca2+, but retarded by injection of EGTA. The amplitudes of miniature endplate potentials and the evoked endplate depolarization were also depressed during repetitive stimulation. On termination of repetitive stimulation, all postsynaptic responses, including evoked endplate potentials, miniature endplate potentials and single twitches, recovered to pre-train level in 3-10 s. These results suggest that the postsynaptic nicotinic receptors had lost the functional activity during repetitive stimulation. The time-courses of the aforementioned changes initiated by repetitive stimulation were similar to the fast phase of desensitization induced by acetylcholine. The irreversible action of alpha-bungarotoxin on acetylcholine receptor was attenuated in the presence of nicardipine and neostigmine if repetitive stimulation was applied. The same effects were observed with other organic Ca2(+)-channel antagonists (diltiazem, verapamil and nifedipine) as well as agonist (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyr idine- 5-carboxylate, BAY K8644), but not with Mn2+, theophylline or caffeine. It is inferred that organic Ca2(+)-channel antagonists interact directly with acetylcholine receptor ion channel, enhance its autodesensitization liability and thus cause extinction of endplate potentials on repetitive stimulation.

Verapamil, neuromuscular transmission and the nicotinic receptor

The effect of verapamil on neuromuscular transmission was studied in the frog by analysing ionophoretic endplate current (iEPC) trains and the growth and decay phases of miniature endplate currents (mepcs). In addition, single channel data on the interaction of verapamil with the nicotinic acetylcholine receptor were obtained from cultured embryonic chick skeletal muscle cells. Verapamil caused both open and closed channel blockade in the iEPC trains. Mepc amplitude was decreased at low micromolar concentrations, and at higher concentrations there was also accelerated mepc decay indicating open channel blockade. The latter effect could not be explained by a sequential channel occlusion mechanism. Analysis of the mepc rising phase showed that low micromolar concentrations of the drug decreased the pool of receptors which could be activated. Single channel data confirmed the specific interaction of verapamil with the nicotinic receptor, showing closed channel blockade at low concentrations, and at higher levels the shortening of open channel lifetime. It is suggested that both forms of blockade may involve desensitization processes.

Nicotinic agonists regulate alpha-bungarotoxin binding sites of TE671 human medulloblastoma cells

The TE671 human medulloblastoma cell line expresses a variety of characteristics of human neurons. Among these characteristics is the expression of membrane-bound high-affinity binding sites for alpha-bungarotoxin, which is a potent antagonist of functional nicotinic acetylcholine receptors on these cells. These toxin binding sites represent a class of nicotinic receptor isotypes present in mammalian brain. Treatment of TE671 cells during proliferative growth phase with nicotine or carbamylcholine, but not with muscarine or d-tubocurarine, induced up to a five-fold increase in the density of radiolabeled toxin binding sites in crude membrane fractions. This effect was blocked by co-incubation with the nicotinic antagonists d-tubocurarine and decamethonium, but not by mecamylamine or by muscarinic antagonists. Following a 10-13 h lag phase upon removal of agonist, recovery of the up-regulated sites to control values occurred within an additional 10-20 h. These studies indicate that the expression of functional nicotinic acetylcholine receptors on TE671 cells is subject to regulation by nicotinic agonists. Studies of the murine CNS have consistently indicated nicotine-induced up-regulation of nicotinic acetylcholine receptors, thereby supporting the identification of the toxin binding site on these cells as the functional nicotinic receptor. Although a mechanism for this effect is not apparent, nicotine-induced receptor blockade does not appear to be involved.