Antagonists and activators at calcium channels. Effects in the gastrointestinal tract

Protective effects of Ca2+ channel blockers against methyl mercury toxicity

The protective effects of Ca2+ channel blockers against the toxicity of methyl mercury were examined by both in vivo and in vitro experiments. In the in vivo study we first examined the effects of the Ca2+ channel blockers (20 mg/kg/day), flunarizine, nifedipine, nicardipine, and verapamil against the toxic level of methyl mercury treatment (5 mg/kg/day of methyl mercuric chloride for 12 consecutive days). However, there was a difference in potency of the effects among the reagents. All the Ca2+ channel blockers prevented a decrease in body weight and/or the appearance of the symptoms of neurological disorders in the rats treated with methyl mercury. In the next experiment, we examined flunarizine at different levels of supplementation (1, 25 and 50 mg/kg/day). Flunarizine in a dose-dependent manner prevented a decrease in body weight, appearance of the symptoms of neurological disorder and mortality in the rats treated with methyl mercury. Flunarizine treatment (25 mg/kg/day) for the first 5 days did not affect mercury distribution among the tissues, suggesting that the mechanism of protection against methyl mercury-induced toxicity may be attributed to its own pharmacological effect. In the in vitro study we examined the effect of flunarizine (0, 0.5, 5 and 50 microM) using primary cultures of cerebellar granular cells in 96-well culture plates. Viable cell numbers were estimated 1 and 3 days after treatment with methyl mercury. The estimated 50% lethal concentration (LC50) of methyl mercury was higher in plates treated with 5 and 50 microM of flunarizine both on days 1 and 3, indicating that flunarizine protected the primary cultured cerebellar granular cells against the toxicity of methyl mercury. As such, Ca2+ channel blockers protected against the toxicity of methyl mercury both in vivo and in vitro, suggesting that Ca2+ plays an important role in the mechanisms of methyl mercury toxicity.

Inhibitory effects of calcium channel antagonists on motor dysfunction induced by intracerebroventricular administration of paraquat

This study reports the effects of Ca2+ channel blockers (Ca antagonists) on intraneuronal Ca2+ ([Ca2+]i) movements and on the disturbance of rotarod performance produced in rats by intracerebroventricular administration of paraquat. Paraquat (50 nmol) produced a decrement in rotarod performance which was present at 30 min. and maximal at 60 min. and was not associated with overt behavioural changes; larger doses of paraquat (100-400 nmol intracerebroventricularly) produced paresis and convulsions which severely disrupted rotarod behaviour. The disruption of rotarod performance after paraquat (50 nmol intracerebroventricularly) was significantly reduced by giving Ca antagonists (flunarizine, verapamil and nicardipine) not only intraperitoneally 15 min. after paraquat but also intracerebroventricularly immediately before paraquat. The order of pharmacological potency was flunarizine > or = verapamil > nicardipine. In contrast, intracerebroventricular administration of Bay K 8644, a Ca agonist, enhanced the disruption of rotarod performance caused by paraquat (50 nmol). In in vitro studies, paraquat markedly potentiated the rapid increase in [Ca2+]i levels evoked by 50 mM KCl in rat brain synaptosomal fraction, although paraquat alone produced a small prolonged rise in [Ca2+]i levels which had a slow onset. The above results suggest that paraquat induced neurotoxicity is associated with increased [Ca2+]i levels in brain neuronal cells, and that paraquat might effect on membrane activity instability.

Effects of calcium channel blockers on pharmacologically induced contractions of rainbow trout (Oncorhynchus mykiss) intestine

Calcium depletion/replacement studies were carried out to examine the role of calcium in contraction of trout intestinal smoot muscle in vitro. Three chemically distinct calcium channel blockers were used to determine whether voltage operated calcium channels (VOCs) were involved in calcium entry with either agonist or depolarization-induced contractions. Contractions induced by depolarizing intestinal smooth muscle with potassium were totally dependent on extracellular calcium, whereas receptor-mediated responses to 5-hydroxytryptamine (5-HT) and carbachol also relied on calcium derived from intracellular stores. The calcium channel blockers, verapamil, nitrendipine, and diltiazem, all shifted the calcium-response curve for potassium to the right, supporting the existence of VOCs in trout intestinal smooth muscle. The calcium-response curve for 5-HT was also shifted to the right, suggesting that 5-HT can induce calcium uptake into the smooth muscle via VOCs, in addition to mobilizing intracellular calcium. Verapamil also appeared to block 5-HT receptors directly. Carbachol-induced contractions were only reduced by diltiazem at low concentrations of calcium (0.1-1 mM), suggesting that diltiazem has some other mechanisms of action than the other calcium channel blockers. Activation of muscarinic receptors may induce calcium entry through channels other than the VOCs, in addition to mobilizing intracellular calcium.

Bay K 8644, modifier of calcium transport and energy metabolism in rat heart mitochondria: a new intracellular site of action

1. The dihydropyridine Ca2+ channel agonist Bay K 8644 (10-200 microM) produced a concentration-dependent increase in State 4 respiration in the rat heart mitochondria with the highest concentration (200 microM) increasing the rate from 33.1 +/- 0.7 to 187.0 +/- 13.3 ng atoms O2 consumed min-1 mg-1 protein. 2. Bay K 8644 (200 microM) reduced State 3 respiration from 247.2 +/- 11.7 to 174.4 +/- 0.06 ng atoms O2 min-1 mg-1 protein, reduced the respiratory control index (RCI) from 5.3 +/- 0.45 to 1.1 +/- 0.03 and reduced the ADP:O ratio from 2.75 +/- 0.03 to 1.3 +/- 0.15. 3. A similar, but smaller, stimulation of State 4 respiration was seen with nitrendipine (25-200 microM), the rate increasing from 22.6 +/- 1.0 to 33.1 +/- 1.8 ng atoms O2 consumed min-1 mg-1 protein in the presence of 200 microM nitrendipine. 4. Bay K 8644 (10-60 microM) increased the total Ca2+ uptake into rat heart mitochondria, the total increasing from 248.8 +/- 8.4 to 406.9 +/- 17.6 ng Ca2+ mg-1 protein at 60 microM Bay K 8644 (EC50 = 18.9 +/- 1.4 microM). 5. Bay K 8644 (10-60 microM) produced a concentration-dependent reduction in the Ca2+ influx rate (IC50 = 52.5 +/- 2.8 microM). Similar effects were seen with (+)-Bay K 8644 and (-)-Bay K 8644. 6. Nitrendipine (40-120 microM) stimulated Ca2+ efflux from mitochondria preloaded with the ion; the efflux rate increasing from 2.9 +/- 0.05 to 114.2 +/- 6.2 nmol Ca2+ min-1 mg-1 protein (EC50 = 57.3 +/- 1.3 microM). 7. These data indicate dihydropyridine-induced changes in the activity of the mitochondrial Na+/Ca2 . antiporter pathway; nitrendipine causing stimulation and Bay K 8644 causing inhibition.

Antiulcer activity of the calcium antagonist propyl-methylenedioxyindene--II. Effects on acid secretion and gastric emptying in rats

1. Propyl-methylenedioxyindene (pr-MDI) is an intracellularly acting calcium antagonist which protects against cold/restraint-induced stress ulcers in rats. The doses of pr-MDI which produce antiulcer activity (10-30 mg/kg i.p.) are significantly lower than those which exhibit cardiovascular effects. 2. Two potential mechanisms for the antiulcer action of pr-MDI were investigated in this study: the effects on hydrochloric acid secretion and on gastric motility (gastric emptying). 3. Bethanechol-induced hydrochloric acid secretion in acutely pylorus-ligated rats was significantly obtunded by pr-MDI (30 mg/kg i.p.), but the effect was significantly weaker than that produced by verapamil (16 mg/kg i.p.) or cimetidine (10 mg/kg i.p.). Since 30 mg/kg pr-MDI produces greater antiulcer activity than the very high dose of 16 mg/kg verapamil, it is unlikely that inhibition of acid secretion plays more than a contributory role in the antiulcer mechanism of action of pr-MDI. 4. pr-MDI (10-30 mg/kg i.p.) produced a dose-dependent slowing of gastric emptying in rats fed a methylcellulose/Phenol Red test meal, and this effect correlated well with the antiulcer action. Verapamil (16 mg/kg i.p.) did not affect gastric emptying. 5. The results indicate that a reduction of gastric motility plays a major role in the mechanism of the antiulcer action of pr-MDI.

Antiulcer activity of the calcium antagonist propyl-methylenedioxyindene--I. Effect on cold/restraint stress-induced ulcers in rats

1. Propyl-methylenedioxyindene (pr-MDI) is an intracellularly acting calcium antagonist with H2-receptor blocking properties. Stimulus-secretion coupling is inhibited by much lower concentrations of pr-MDI than is excitation-contraction coupling. 2. Since the processes leading to gastric ulceration are calcium-dependent, the aim of this study was to determine if pr-MDI could provide useful antiulcer activity at doses below those required to produce cardiovascular effects. 3. The antiulcer activity of pr-MDI (10-30 mg/kg) was examined in the cold (4 degree C)/restraint (3 hr) stress-induced ulcer model in male rats, and compared with the effects of the H2-blocker cimetidine (10-30 mg/kg) and the calcium channel blocker verapamil (11-32 mg/kg). The drugs were administered intraperitoneally 10 min prior to the cold/restraint stress. 4. All three drugs significantly reduced the number of ulcers and the cumulative length of ulcerated stomach surface in a roughly dose-dependent and equivalent manner. However, whereas the antiulcer doses of verapamil were extremely high, those of pr-MDI were one-sixth to one-half of its antiarrhythmic ED50 in rodents.

Antiulcer activity of the calcium antagonist propyl-methylenedioxyindene. IV. Effects on gastric lesions in rats induced by cold-restraint stress and thyrotropin-releasing hormone

Propyl-methylenedioxyindene (pr-MDI; 30 mg/kg, i.p.), an intracellular calcium antagonist, significantly reduced the number and size of erosions per stomach induced by cold-restraint stress by 69% and 86%, respectively. Our previous findings indicate that the antiulcer activity of pr-MDI is highly correlated with its inhibitory effect on gastric motor activity. Since central TRH is suggested as the brain mediator responsible for cold-restraint stress gastric ulcers in rats, the inhibitory action of pr-MDI was evaluated in the TRH-induced gastric lesion model. Pr-MDI (30 mg/kg) did not reduce the gastric erosions induced by intracisternal administration of 100ng RX77368, a stable thyrotropin-releasing hormone (TRH) analogue, even though it abolished the RX77368-induced stimulation of gastric emptying, gastric acidity, and acid output. Since pr-MDI (30 mg/kg, i.p.) significantly inhibited the stimulation of gastric motility by both cold-restraint stress and TRH, but only cold-restraint stress-induced gastric erosions were effectively reduced by the drug, the present findings suggest a possible dissociation between the ulcerogenic mechanisms of cold-restraint stress and intracisternal administration of TRH.