Calcium channel antagonists in the treatment of asthma

The pathophysiologic processes that contribute to airway obstruction in asthma involve Ca2+-dependent excitation-contraction and stimulus-secretion coupling mechanisms. The emergence of new compounds that specifically inhibit Ca2+ flux across membrane ionic channels has stimulated widespread interest in the therapeutic potential of these agents in asthma. Studies with these agents in relevant in vitro test systems and animal models, however, have yielded conflicting results and have thus far failed to furnish strong support for a therapeutic role. In human studies, these agents have been found to inhibit exercise-induced bronchospasm, but their ability to inhibit the effects of other stimuli and to dilate airways is equivocal. In general, clinical trials with currently approved drugs--diltiazem, nifedipine, and verapamil--are limited by potency, formulation, and side effects of these agents. What future role, if any, Ca2+ channel antagonists will have in the treatment of asthma is likely to depend on the development of newer agents with greater tissue selectivity at the level of airway smooth muscle and mast cells.

Use of calcium channel blocking agents in the management of childhood asthma

The function of the cells involved in the genesis of bronchial asthma is calcium dependent. The "Ca2+ hypothesis of asthma" suggests that calcium channel blocking agents may have a beneficial effect in asthma. The clinical trials with the available classic calcium antagonists in childhood asthma are very few and their clinical application remains to be determined. Sodium cromoglycate and ketotifen, which have been in clinical use for many years, can be considered as calcium channel blockers and seem to have a significant role in the prophylaxis of bronchial asthma.

Calcium channel antagonist binding sites labeled by 3H-nimodipine in human brain

In vitro binding of 3H-nimodipine to human brain membranes is demonstrated in this study. This binding was specific and saturable, and had an apparent affinity constant (KD) of 0.27 nM. The maximal number of binding sites for 3H-nimodipine was 5.8 pmoles/gm wet weight of human frontal cortex. The binding was shown to be dependent on calcium, with half-maximal stimulation obtained at 3 X 10(-5) M CaCl2. Other 1,4-dihyropyridine calcium antagonists were shown to be competitive antagonists of 3H-nimodipine binding. In contrast, the calcium antagonists, verapamil and diltiazem, had complex interactions with 3H-nimodipine binding. These results represent the first identification of 3H-calcium antagonist binding sites in human brain, and they confirm that various calcium antagonist drugs may differ with respect to both their potency and their molecular site of action.

Action of papaverine and verapamil on norepinephrine and calcium evoked contraction of vascular and non vascular smooth muscle

On isolated rat vas deferens both papaverine and verapamil show a non competitive antagonism against norepinephrine and a competitive antagonism against Ca2+. Verapamil is nearly 100 times more active than papaverine. On the main pulmonary artery and on the thoracic aorta of the rabbit, verapamil shows a competitive antagonism against both norepinephrine and Ca2+ but it is more effective (almost 100 times) against Ca2+ than norepinephrine. Papaverine also shows a competitive antagonism against norepinephrine but a non competitive antagonism against Ca2+. The contrasting results obtained on rat vas deferens and rabbit vessels might be due to: 1) mechanism(s) of action of the agonist; 2) properties of the biological object as far concerns receptor activation and mechanism(s) of excitation-contraction coupling; 3) mechanism(s) of action of spasmolytic drugs. All these factors act in a cooperative way in determining the quality and the quantity of the observed responses.

Calcium entry blocking drugs, 'calcium antagonists' and vascular smooth muscle function

The group of drugs known as "calcium antagonists' is under extensive investigation in experimental animals and man and a re-evaluation of their pharmacological properties is overdue. Recent proposals to adopt the more specific nomenclature of calcium entry blockers for some of these compounds (Vanhoutte & Bohr, 1981) should be supported since there is much confusion in the literature with this class of compound. In this review, which concentrates on vascular smooth muscle, only nifedipine, verapamil, their close chemical analogues and diltiazem are recognised as being relatively selective calcium entry blocking drugs. Whilst definitive evidence for calcium entry blockade must include the demonstration of a selective inhibition of Ca2+-influx into a tissue over a range of concentrations also inhibiting contraction, it is nevertheless possible to define several simple pharmacological criteria which may aid in the identification of such activity. These criteria include the selective antagonism of K+ and Ca2+-induced contractions, relative to those of noradrenaline in suitable vascular smooth muscle preparations and a selective inhibition of alpha 2- as opposed to alpha 1-adrenoreceptor mediated pressor responses in, for example, pithed rat preparations. Recent pharmacological and biochemical studies have identified 3 major subgroups of "calcium antagonist' drugs but the compounds within each subgroup varies with the technique adopted. It is therefore suggested that a combination of both pharmacological and ligand-binding studies be used for purposes of classification. Which mechanism, if any, of inhibiting calcium entry is therapeutically most desirable remains an important question for future research.

Nifedipine in bronchial asthma

In 11 patients with bronchial asthma and regular overnight falls in PEFR of greater than 15%, we demonstrated significant bronchodilation restricted only to large airways after the administration of a single dose of 10 mg of nifedipine. A statistically significant increase in PEFR, SGaw, and FEV1 was noticed at 1 and especially 2 hr after nifedipine administration. During 4 days of nifedipine treatment (10 mg t.i.d.), the overnight fall in mean PEFR was statistically significant (p less than 0.02) and less than the mean fall of PEFR during the 4 days of placebo treatment. Thus nifedipine does modify the basal bronchial tone of patients with asthma and diminishes the circadian swing of airway resistance.

Demonstration of central nervous system tolerance to ethanol in mice: consequent effects on smooth muscle in vitro

Mice were given ethanol (9 g/kg) or saline (57 ml/kg) daily in three divided doses for periods of 1 and 4 days to study the effects of such ethanol (ETOH) exposure on central nervous system (CNS) and peripheral smooth muscle (vasa deferentia) function. After 1 day, ETOH-treated mice were functionally tolerant to the hypothermic (3 g/kg, intraperitoneal), but not to the hypnotic (3.25 g/kg, intraperitoneal) effect of ETOH. Functional tolerance to both CNS effects of ETOH was demonstrated in mice after the 4-day ETOH exposure. Norepinephrine (NE) and high K+-depolarizing solutions each elicited dose-dependent contractions in the mouse vasa deferens preparation in vitro that consisted of a phasic and tonic component. The tonic components of the NE and K+ responses were more dependent upon extracellular Ca2+ (Ca2+ext) than the phasic components. Addition of ETOH (120 to 480 mM) or the Ca2+ channel-antagonist nifedipine (1 X 10(-9) to 3 X 10(-7) M) to the preparation selectively inhibited the tonic component of the NE and K+ responses, suggesting that both agents acted to inhibit the responses by interfering with the translocation of Ca2+ext. Vasa deferentia isolated from ETOH-treated mice did not exhibit altered reactivity to NE in the phasic or tonic component of the response. The isolated smooth muscle from mice centrally tolerant to ETOH did not appear to be tolerant to inhibitory effect of ETOH or cross-tolerant to the inhibitory effect of nifedipine on stimulant-induced contractions.

Calcium antagonists and calmodulin: effect of verapamil, nifedipine and diltiazem

The effect of three calcium antagonists (verapamil, nifedipine and diltiazem) on the calcium-induced activation of phosphodiesterase (a calmodulin dependent process) was investigated. Therapeutically relevant concentrations of verapamil, nifedipine and diltiazem were used. In the presence of calmodulin, phosphodiesterase activity was stimulated by calcium in the range 4 X 10(-6)-2.5 X 10(-5) M. Diltiazem (10(-6) M), verapamil (10(-6) M) and nifedipine (10(-7) and 10(-6) M) had no influence on phosphodiesterase activity in the presence or absence of calmodulin at any concentration of calcium employed. By contrast trifluoroperazine abolished the Ca2+ activation of the phosphodiesterase enzyme. From this it is concluded that while the interaction of calcium antagonists with calmodulin may be of interest in the study of the mode of action of calmodulin, it probably does not contribute to their vasodilator activity.

Effects of calcium and verapamil on vesicourethral smooth muscle of rabbits

Isolated smooth muscle strips from the rabbit bladder body, bladder base, and proximal urethra were contracted with ionic calcium (Ca2+) alone and with the calcium-selective ionophore A23187, acetylcholine, norepinephrine, adenosine triphosphate (ATP), and direct electrical stimulation. The effects of Ca2+ and the calcium entry blocker verapamil on spontaneous muscle activity and on contractions induced by these agonists were examined. Ca2+ -free Tyrode's solution and verapamil, 1 x 10(-7)M and above, relaxed all of the vesicourethral smooth muscle strips. In addition verapamil, 1 x 10(-8) to 1 x 10(-6) M depending on the particular stimulant employed, noncompetitively inhibited smooth muscle contractions elicited by Ca2+, acetylcholine, norepinephrine, ATP, and direct electrical stimulation. It was concluded that transmembrane Ca2+ influx was important not only in the maintenance of tone and spontaneous phasic muscle activity, but also for the activation of contractions induced by all of the stimulants tested. The data also suggest that intracellular Ca2+ fraction(s) participate in the contractile responses to acetylcholine and norepinephrine challenge, but not to contractions evoked by ATP or electricity.

Calcium, the control of smooth muscle function and bronchial hyperreactivity

The Ca2+ requirements for excitation-contraction coupling in smooth muscle may be satisfied from both intracellular and extracellular sources, the relative extent of use of which is both tissue- and stimulant-dependent. Extracellular Ca2+ is apparently mobilized through two separate pathways, receptor operated (ROC) and potential dependent (PDC) Ca2+ channels. The latter process is sensitive to the Ca2+-channel antagonists, a heterogeneous group of compounds including verapamil, nifedipine and diltiazem. Ca2+ mobilization in respiratory smooth muscle is reviewed. The available evidence for this multiple stimulant-sensitive system indicates that both intra- and extracellular sources of Ca2+ are used. Data from bovine, canine and guinea pig tracheal muscle indicate, from studies of Ca2+-dependence of response and Ca2+ channel antagonist sensitivity, that the extent of use of extracellular Ca2+ lies in the order K+ greater than histamine greater than or equal to 5-hydroxytryptamine greater than acetylcholine. The bronchodilator activity of the Ca2+ channel antagonists is noted. Bronchial hyperreactivity is characterized by an increased sensitivity to a variety of stimulants including cold air, exercise, histamine and acetylcholine. The possible origins of this defect are noted. It is suggested that a defect in Ca2+ mobilization or in the receptor - Ca2+ mobilization coupling process at the level of the smooth muscle may constitute an important underlying cause of bronchial hyperreactivity. Potential analogies to reactivity changes seen in hypertensive vascular smooth muscle are noted.

The binding of [3H]nitrendipine to receptors for calcium channel antagonists in the heart, cerebral cortex, and ileum of rats

The binding properties of the calcium channel antagonist, [3H]nitrendipine, were investigated in homogenates of the rat cerebral cortex, heart and ileum. The specific component of [3H]nitrendipine binding was consistent with mass-action behavior and was characterized by a high affinity dissociation constant in the range of 0.1-0.3 nM. A variety of other calcium channel antagonists inhibited the binding of [3H]nitrendipine with Ki's that agree generally with the ability of these drugs to block contractions of cardiac and smooth muscle. The inhibition of [3H]nitrendipine binding by other dihydropyridines was consistent with competitive antagonism whereas the inhibition caused by verapamil and D600 resembled negative heterotropic cooperativity. Consistent with this latter postulate was the observation that the kinetics of [3H]nitrendipine binding are altered by verapamil, with both the association rate and the dissociation rate being increased. La+3 and several divalent cations caused an inhibition of [3H]nitrendipine with the rank order of potency being Cd+2 greater than La+3 greater than Ni+2 greater than Co+2 = Mn+2 greater than Mg+2 = Ba+2 greater than Ca+2.

Some calcium and lysosome antagonists inhibit 3H-spiperone binding to the porcine anterior pituitary

The dopamine antagonist 3H-spiperone binds to dopamine receptors in crude membrane preparations of the porcine anterior pituitary. In competition studies, several calcium channel antagonists (bepridil, D600 and verapamil) and a lysosomal enzyme inhibitor (chloroquine) displaced spiperone binding in a dose related manner. Because dopamine receptor activity modulates prolactin release from the anterior pituitary, it is suggested that the previously observed effects of some of these agents on prolactin release or storage may have been initiated at the level of the dopamine receptor or through physical modifications of the plasma membrane rather than the calcium channel or lysosome.

Pharmacokinetics of diltiazem in selected animal species and human beings

The absorption, distribution and elimination of diltiazem hydrochloride in rodent and canine species are reviewed. The drug is well absorbed but undergoes first pass metabolism after oral administration. Diltiazem is extensively distributed, and 52 to 81 percent is bound to serum protein, depending on the species studied. Diltiazem is metabolized in the liver by several pathways; deacetylation, N-demethylation, and O-demethylation are the primary degradative steps. The metabolites are excreted in urine and feces, indicating that biliary excretion occurs. There is some evidence for enterohepatic cycling. Diltiazem is rapidly eliminated (t 1/2 = 2.24 hours) in beagle dogs, and the relatively short half-life appears to be a result of the high level of plasma clearance (46.1 +/- 4.8 ml/min/per kg body weight). A comparison of the plasma diltiazem clearance with hepatic blood flow in the dog indicates that the drug is eliminated at a rate dependent on hepatic blood flow.

Short-term efficacy of oral verapamil in rest angina. A double-blind placebo controlled trial in CCU patients

To determine the efficacy and safety of oral verapamil in patients with rest angina admitted to the Coronary Care Unit (CCU), a double-blind placebo-controlled trial was undertaken. Of the 65 patients with rest angina screened for the study, 15 met the inclusion criteria (at least two episodes of chest pain associated with ST-T segment changes per 24 hours) during single-blind placebo phase (Day 1). Patients were then randomized to receive either placebo or verapamil (80 mg every 6 hours) on Day 2. Protocol was designed such that those who did not respond to the placebo (nonresponders) received verapamil, 80 mg every 6 hours, whereas verapamil nonresponders received increased doses (120 mg every 6 hours) on Day 3. Those who did respond (responders) continued to receive their medication. Similar action was taken on Day 4, depending on chest pain frequency and clinical evaluation. The study drug was unblinded on Day 4. At the end of the four-day period, 13 patients were receiving verapamil (nine patients, 80 mg every 6 hours, and four patients, 120 mg every 6 hours) and all but one were responders. One patient received placebo all through the period of the study and was also considered to be a responder. In the remaining one patient evidence of myocardial necrosis developed after he received a single dose of verapamil (80 mg on Day 2). Except for the prolongation of PR interval in two patients while taking verapamil, no side effects from verapamil therapy were observed. These data demonstrate the efficacy of oral verapamil in reducing episodes of myocardial ischemia in the majority of all patients with rest angina.