Comparison of the inhibitory effects of mexiletine and lidocaine on the calcium current of single ventricular cells

Effects of mexiletine and lidocaine on inward calcium current (ICa) of single ventricular myocytes from guinea pigs were studied using tight seal whole cell clamp method. Mexiletine at the concentrations of 10, 30 and 100 microM decreased ICa by 23.0, 28.9 and 55.4%, respectively, while lidocaine decreased it by 8.9, 16.8 and 25.2%. At all concentrations tested, a potency for ICa inhibition in mexiletine was significantly greater than that in lidocaine (p less than 0.05). The results suggest that mexiletine has, at therapeutic concentrations, a considerable blocking action on the Ca channels other than well-known action on the Na channels.     

Phenytoin preferentially inhibits L-type calcium currents in whole-cell patch-clamped cardiac and skeletal muscle cells

The effect of the anticonvulsant diphenylhydantoin (phenytoin) was tested on the inward calcium currents of whole-cell patch-clamped cells from rat and human muscles and from frog atrium. A concentration of 10 microM phenytoin was required to obtain a threshold inhibitory effect and, even with high concentrations (100 microM), the inhibition was not complete. In skeletal muscle (rat and human cells in culture), phenytoin (30 microM) exerted a more potent effect on the high-threshold calcium current (ICa,L inhibition: 53 +/- 6% mean +/- SDn-1) rather than on the low-threshold one (ICa,T inhibition: 16 +/- 10%). Similar results were obtained on dissociated frog atrial cells. These data are to be contrasted with those previously reported on neuronal cells, where specific inhibition of ICa,T was reported. Thus, the action of phenytoin appears to be different in muscle and nerve so that phenytoin does not appear to be a specific inhibitor of ICa,T.     

Electrocardiographic dose-dependent changes in prophylactic doses of dosulepine,lithium and citalopram

Tricyclic antidepressant drugs dosulepine (TCA), serotonin selective reuptake inhibitor (SSRI) and prophylactic agent with antidepressant effect lithium carbonicum (Li) have different cardiovascular side-effects. We compared them in the prophylactic therapy of periodic affective disorder in remission with TCA, SSRI and Li. Our previous papers confirmed the most prominent effects of heart electric field parameters in TCA patients (Slavícek et al., 1998). In the present work we studied for the first time the dose-dependent changes of ECG, body surface potential maps (BSPM - parameter DIAM 30, 40) in 43 TCA dosulepine, 40 SSRI citalopram and 30 Li outpatients (Hamilton scale: HAMD?10; age 40+/-5 years; treated for depressive disorders or bipolar disorders). The daily doses of dosulepine were 50-250 mg, citalopram 20-80 mg, Li plasma levels 0.66+/-0.08 meq/l. The electrocardiogram (ECG), vectorcardiogram (VCG), and BSPM were measured and calculated by the Cardiag 112.1 diagnostic system. The results have shown a relation between the dose of dosulepine and extremum (maximum and minimum) of depolarization isoarea map in dosulepine, but not in citalopram patients. The repolarization BSPM changes were most pronounced in SSRI patients. Lithium in long-term prophylaxy (1-22 years) caused only minimal ECG BSPM changes. The present results correspond with our previous observations.     

Effect of antidepressants on ATP-dependent calcium uptake by neuronal endoplasmic reticulum.

This study investigated the effect of tricyclic and atypical antidepressants on adenosine triphosphate (ATP) dependent calcium uptake by the endoplasmic reticulum of lysed synaptosomes from rat brain cortex. Tricyclic antidepressants (imipramine, desipramine, clomipramine, amitriptyline) exhibited no effect in the lower range (0.06 to 2 microM) of drug concentrations, and a concentration-dependent inhibition of calcium uptake in the upper range (6 to 200 microM). A concentration-dependent inhibition was observed for atypical antidepressants (mianserin, desmethylmianserin, venlafaxine, desmethylvenlafaxine, fluoxetine) in both the lower and the upper range of drug concentrations. Since no stimulation of calcium uptake was observed in either concentration range, it appears that the tricyclic and atypical antidepressants tested are not capable of normalizing, through their effect on the endoplasmic reticulum, an overactive calcium signal. which is possibly implicated in the etiology of affective disorders. Also, although only marginal inhibition of calcium uptake is expected at brain concentrations of tricyclics and mianserin-desmethylmianserin that are likely to be encountered during clinical use, a more substantial inhibition could occur with fluoxetine.     

Bunazosin, an alpha-adrenoceptor antagonist, blocks calcium current in guinea-pig ventricular myocytes.

1. Effect of bunazosin, an alpha 1-adrenoceptor antagonist, upon the slow inward Ca2+ current (ICa) was studied in single ventricular myocytes of the guinea-pig using a whole-cell patch-clamp technique. 2. Bunazosin (10-100 microM) decreased ICa in a concentration-dependent manner with an IC50 of 60 microM during depolarization to + 10 mV from the holding potential of -40 mV. 3. As for the inactivation process of ICa, the steady-state inactivation (f infinity) curve was shifted toward more negative potentials from -12 mV to -17 mV and -21 mV at f infinity = 0.5, by 30 microM and 70 microM bunazosin. 4. Inhibition of ICa by the compound (10 microM) was also dependent on stimulation frequency, with greater block induced at 2 Hz (50%) than at 0.33 Hz (13%). 5. It is concluded that bunazosin possesses a direct Ca2+ channel-blocking (class 4) action in a rate-dependent fashion.     

Inhibitory actions of the selective serotonin re-uptake inhibitor citalopram on HERG and ventricular L-type calcium currents

Using whole-cell patch clamp recording of heterologous HERG-mediated currents in transfected mammalian cells, we observed that the selective serotonin re-uptake inhibitor citalopram blocks HERG with an IC(50) of 3.97 microM. This is slightly less potent than fluoxetine in our system (IC(50) of 1.50 microM). In isolated guinea pig ventricular cardiomyocytes citalopram inhibited L-type calcium current (I(Ca,L)). The voltage dependence of I(Ca,L) inactivation in the presence of 100 microM citalopram was shifted significantly leftward. As a result, the I(Ca,L) 'window' in citalopram was found to be (a) smaller and (b) leftward-shifted compared to control. The effects of citalopram on both calcium current amplitude and the I(Ca,L) 'window' may help to explain citalopram's good cardiac safety profile, given its propensity to block HERG at excessive dosages.     

Effects of capsaicin on molluscan neurons: a voltage clamp study

The effects of capsaicin (CAP) on membrane ionic currents of identified and non-identified neurons were investigated by use of the single electrode clamp (SEC). CAP (300 microM, 22 degrees C, pH 7.4) caused a 25-50% reduction of the inward current and a 50-80% reduction of the outward current in normal or Na-free (Tris) solution. The Na current (INa) was moderately decreased (about 10%) in LPa2 neuron, but a 50% reduction of the peak Ca current (ICa) was observed. The action of CAP on ICa varied from cell to cell but an enhanced inactivation of the fast calcium current was found in all neurons studied. CAP (150 microM, 10 min) highly attenuated the long-lasting component of the inward current in LPa2 recorded in Na-free (TEA) Ba solutions. CAP attenuated the fast outward current (IA) and voltage-dependent outward current (IK) in 100 and 300 microM concentrations for the half blocking dose (ID50) in LPa2 neuron, respectively. CAP decreased the slow outward tail currents but hardly influenced the leakage current (IL). We suggest that the acute action of CAP coupled with a series of events in the neuronal membrane can modify the conductance via electrically excitable calcium, potassium and sodium channels differentially.     

A cellular mechanism for nitric oxide-mediated cholinergic control of mammalian heart rate

The biochemical signaling pathways involved in nitric oxide (NO)- mediated cholinergic inhibition of L-type Ca2+ current (ICa[L]) were investigated in isolated primary pacemaker cells from the rabbit sinoatrial node (SAN) using the nystatin-perforated whole-cell voltage clamp technique. Carbamylcholine (CCh; 1 microM), a stable analogue of acetylcholine, significantly inhibited ICa(L) after it had been augmented by isoproterenol (ISO; 1 microM). CCh also activated an outward K+ current, IK(ACh). Both of these effects of CCh were blocked completely by atropine. Preincubation of the SAN cells with L-nitro- arginine methyl ester (L-NAME; 0.2-1 mM), which inhibits NO synthase (NOS), abolished the CCh-induced attenuation of ICa(L) but had no effect on IK(ACh). Coincubation of cells with both L-NAME and the endogenous substrate of NOS, L-arginine (1 nM), restored the CCh- induced attenuation of ICa(L), indicating that L-NAME did not directly interfere with the muscarinic action of CCh on ICa(L). In the presence of ISO the CCh-induced inhibition of ICa(L) could be mimicked by the NO donor 3-morpholino-sydnonimine (SIN-1; 0.1 mM). SIN-1 had no effect on its own or after a maximal effect of CCh had developed, indicating that it does not inhibit ICa(L) directly. SIN-1 failed to activate IK(ACh), demonstrating that it did not activate muscarinic receptors. Both CCh and NO are known to activate guanylyl cyclase and elevate intracellular cGMP. External application of methylene blue (10 microM), which interferes with the ability of NO to activate guanylyl cyclase, blocked the CCh-induced attenuation of ICa(L). However, it also blocked the activation of IK(ACh), suggesting an additional effect on muscarinic receptors or G proteins. To address this, a separate series of experiments was performed using conventional whole-cell recordings with methylene blue in the pipette. Under these conditions, the CCh-induced attenuation of ICa(L) was blocked, but the activation of IK(ACh) was still observed. Methylene blue also blocked the SIN-1-induced decrease in ICa(L). 6-anilino-5,8-quinolinedione (LY83583; 30 microM), an agent known to decrease both basal and CCh-stimulated cGMP levels, prevented the inhibitory effects of both CCh and SIN-1 on ICa(L), but had no effect on the activation of IK(ACh) by CCh. In combination, these results show that CCh- and NO-induced inhibition of ICa(L) is mediated by cGMP.(ABSTRACT TRUNCATED AT 400 WORDS)     

Modulation of voltage-dependent Ca channel current by arachidonic acid and other long-chain fatty acids in rabbit intestinal smooth muscle.

The effects of arachidonic acid (AA) and other long-chain fatty acids on voltage-dependent Ca channel current (ICa) were investigated, with the whole cell patch clamp method, in longitudinal smooth muscle cells of rabbit ileum. 10-30 microM AA caused a gradual depression of ICa. The inhibitory effect of AA was not prevented by indomethacin (10 microM) (an inhibitor of cyclooxygenase) or nordihydroguaiaretic acid (10 microM) (an inhibitor of lipoxygenase). 1-(5-Isoquinolinesulfonyl)-2-methylpiperazine (H7; 25-50 microM) or staurosporine (2 microM) (inhibitors of protein kinase C) did not block the AA-induced inhibition of ICa, and application of phorbol ester (a protein kinase C activator) (phorbol-12,13-dibutyrate, 0.2 microM) did not mimic the AA action. Some other cis-unsaturated fatty acids (palmitoleic, linoleic, and oleic acids) were also found to depress ICa, while a trans-unsaturated fatty acid (linolelaidic acid) and saturated fatty acids (capric, lauric, myristic, and palmitic acids) had no inhibitory effects on ICa. Myristic acid consistently increased the amplitude of ICa at negative membrane potentials. The present results suggest the possible role of AA, and perhaps other fatty acids, in the physiological and/or pathological modulation of ICa in smooth muscle.     

Effects of N-n-butyl haloperidol iodide on L-type calcium channels and intracellular free calcium in rat ventricular myocytes.

The ability of N-n-butyl haloperidol iodide (F2) to cause vasodilation, and thereby produce a cardioprotective effect, has been well documented. The aim of this study was to investigate whether F2 might act as a Ca2+ antagonist. Myocytes were obtained from rat heart, and the whole-cell patch-clamp technique was used to record Ca2+ current. Laser scanning confocal microscopy was used to measure intracellular free calcium ([Ca2+]i). The results obtained from this study demonstrate that F2 reduced calcium current (ICa) in a concentration-dependent manner with an IC50 of 1.19 micromol/L, upshifted the current-voltage curve of ICa, shifted the inactivation kinetics of ICa leftward, and slowed down the recovery of ICa from inactivation. F2 decreased the fluorescent intensity of [Ca2+]i elevation induced by KCl with an IC50 of 1.61 micromol/L, and had no effects on the intracellular calcium release induced by caffeine and inositol-1,4,5-trisphosphate. These findings indicate that F2 may act as a calcium antagonist, which could account for its cardiovascular benefits.     

Imipramine and citalopram facilitate amyloid precursor protein secretion in vitro

Comorbid depression of Alzheimer's disease (AD) is a common mood disorder in the elderly and a broad spectrum of antidepressants have been used for its treatment. Abeta peptides and other derivatives of the amyloid precursor protein (APP) have been implicated as central to the pathogenesis of AD. However, the functional relationship of APP and its proteolytic derivatives to antidepressant therapy is not known. In this study, Western blotting was used to test the ability of the tricyclic antidepressant (TCA) imipramine or the selective serotonin reuptake inhibitor (SSRI) citalopram to change the release of APP and the protein kinase C (PKC) content. Both antidepressants increased APP secretion in primary rat neuronal cultures. Imipramine or citalopram enhanced the level of secreted APP by 3.2- or 3.4-fold, respectively. Increases in PKC level were observed only after imipramine treatment. These in vitro data suggest that both TCA and SSRI are able to interfere with the APP metabolism. Imipramine promotes the non-amyloidogenic route of APP processing via stimulatory effects on PKC. We propose that PKC is not involved in the mechanism underlying the effects of citalopram on the APP metabolism. Since the secreted APP is not further available for the pathological cleavage of beta- and gamma-secretases, antidepressant medication might be beneficial in AD therapy.     

Strychnine decreases the voltage-dependent Ca2+ current of bothAplysia and frog ganglion neurons

1. The effects of strychnine on the voltage-dependent Ca2+ current (ICa) were studied in physically isolated Aplysia neurons and enzymatically dissociated frog sensory neurons of the dorsal root ganglion. Neurons were studied under the internal perfusion and the voltage clamp condition. 2. Strychnine decreased the ICa with threshold concentrations for effect at 1 to 10 microM. The depression of ICa increased with strychnine dose without effects on the current-voltage relation of ICa. The effects of low concentrations of strychnine were reversible, but recovery was incomplete at higher concentrations. The potency of strychnine was about 10 times less than that of diltiazem, an organic Ca2+ antagonist. At 100 microM the ICa of Aplysia neurons was reduced to about half of the control. This concentration of strychnine also reduced the peak amplitude of ICa of frog sensory neurons. 3. These results indicate that, in addition to its actions on transmitter responses and on Na+ and K+ currents, strychnine has effects on ICa that have not previously been appreciated.     

Tyrosine kinase inhibitors block calcium channel currents in vascular smooth muscle cells.

Selective inhibitors of tyrosine kinases, tyrphostin 23 and genistein, produced concentration-dependent inhibition of voltage-operated calcium channel currents in vascular smooth muscle cells isolated from rabbit ear artery. The potency of these two structurally dissimilar inhibitors was similar to that reported for their action as inhibitors of tyrosine kinases. Daidzein, an inactive analogue of genistein, had little inhibitory effect on calcium channel currents at concentrations below 300 microM consistent with an action of these agents at a tyrosine kinase. However, tyrphostin 1, a reportedly less active tyrphostin derivative, also inhibited calcium channel currents with a potency similar to tyrphostin 23. These findings suggest that voltage-operated calcium channels in vascular smooth muscle may be modulated by endogenous tyrosine kinase(s) which display different sensitivities to inhibitors compared with the epidermal growth factor (EGF) receptor. Alternatively the possibility of direct blocking actions of these inhibitors at voltage-operated calcium channels cannot be excluded.     

Intracellular Ca(2+) regulates responsiveness of cardiac L-type Ca(2+) current to protein kinase A: role of calmodulin

The goal of this study was to determine whether the protein kinase A (PKA) responsiveness of the cardiac L-type Ca(2+) current (ICa) is affected during transient increases in intracellular Ca(2+) concentration. Ventricular myocytes were isolated from 3- to 4-day-old neonatal rats and cultured on aligned collagen thin gels. When measured in 1 or 2 mM Ca(2+) external solution, the aligned myocytes displayed a large ICa that was weakly regulated (20% increase) during stimulation of PKA by 2 microM forskolin. In contrast, application of forskolin caused a 100% increase in ICa when the external Ca(2+) concentration was reduced to 0.5 mM or replaced with Ba(2+). This Ca(2+)-dependent inhibition was also observed when the cells were treated with 1 microM isoproterenol, 100 microM 3-isobutyl-1-methylxanthine, or 500 microM 8-bromo-cAMP. The responsiveness of ICa to PKA was restored during intracellular dialysis with a calmodulin (CaM) inhibitory peptide but not during treatment with inhibitors of protein kinase C, Ca(2+)/CaM-dependent protein kinase, or calcineurin. Adenoviral-mediated expression of a CaM molecule with mutations in all four Ca(2+)-binding sites also increased the PKA sensitivity of ICa. Finally, adult mouse ventricular myocytes displayed a greater response to forskolin and cAMP in external Ba(2+). Thus Ca(2+) entering the myocyte through the voltage-gated Ca(2+) channel regulates the PKA responsiveness of ICa.     

Serotonin decreases a background current and increases calcium and calcium-activated current in pedal neurons ofHermissenda

The effects of serotonin (5-HT) on membrane potential, membrane resistance, and select ionic currents were examined in large pedal neurons (LP1, LP3) of the mollusk Hermissenda. Calcium (Ca) action potentials were evoked in sodium-free artificial seawater containing tetramethylammonium, tetraethylammonium, and 4-aminopyridine (0-Na, 4-AP, TEA ASW). They failed at stimulation rates greater than 0.5/sec and were blocked by cadmium (Cd). Under voltage clamp the calcium current (ICa) responsible for them also failed with repeated stimulation. Thus, ICa inactivation accounts for refractoriness of the Ca action potential. The addition of 10 microM 5-HT to 0-Na, 4-AP, TEA ASW produced a slight depolarization and increased excitability and input resistance. Under voltage clamp the background current decreased. The voltage-dependent inward, late outward, and outward tail currents, sensitive to Cd, increased. ICa inactivation persisted. Under voltage clamp with Ca influx blocked by Cd, the addition of 10 microM 5-HT decreased the remaining current uniformly over membrane potentials of -10 to -100 mV. Thus, 5-HT reduces a background current that is active within the physiological range of the membrane potential, voltage insensitive, independent of Ca influx, noninactivating, and not blocked by 4-AP or TEA.     

Regulation of the cardiac calcium channel by protein phosphatases

The calcium current (ICa) through the L-type channel in cardiac ventricular cells is enhanced by phosphorylation of a channel protein [Kameyama, M., Hofmann, F. & Trautwein, W. (1985) Pflügers Arch. Eur. J. Physiol. 405, 285-293]. We investigated the possible contribution of the 'catalytic subunits' of protein phosphatase 1 and 2A in the down-regulation of the cardiac calcium channel. Single guinea-pig ventricular myocytes were voltage clamped and the following results were obtained. (1) Intracellular perfusion of the myocyte with the catalytic subunits of protein phosphatase 1 (2 microM) as well as 2A (2.3 microM) completely abolished the increase of ICa induced by isoprenaline (0.05 microM) but did not decrease the basal level of ICa. Alkaline and acid phosphatases were without detectable effect. (2) Cell dialysis with the modulator of protein phosphatase 1 (inhibitor-2) under control conditions (without addition of isoprenaline) caused a slow significant increase of ICa. (3) The time course for the wash-out of the isoprenaline effect was considerably prolonged in the presence of high concentrations of inhibitor-2. (4) Perfusion of the myocyte under basal conditions with adenosine 5'-[gamma-thio]triphosphate led to a slow increase of ICa. Additional superfusion of the cell with a threshold concentration of isoprenaline (0.01 microM) resulted in a rapid increase of ICa which could not be washed out during at least 10 min. From these results we make the following conclusions. (1) The calcium channel from guinea-pig myocytes is regulated by phosphorylation-dephosphorylation. (2) The catalytic subunits of the protein phosphatases 1 as well as 2A, purified from rabbit skeletal muscle, catalyse the down-regulation of the channel. (3) Indirect evidence suggests that endogenous protein phosphatase 1 contributes only partially to the dephosphorylation of the calcium channel in the intact myocyte.     

pH changes in the invaginating synaptic cleft mediate feedback from horizontal cells to cone photoreceptors by modulating Ca2+ channels

Feedback from horizontal cells (HCs) to cone photoreceptors plays a key role in the center-surround-receptive field organization of retinal neurons. Recordings from cone photoreceptors in newt retinal slices were obtained by the whole-cell patch-clamp technique, using a superfusate containing a GABA antagonist (100 microM picrotoxin). Surround illumination of the receptive field increased the voltage-dependent calcium current (ICa) in the cones, and shifted the activation voltage of ICa to negative voltages. External alkalinization also increased cone ICa and shifted its activation voltage toward negative voltages. Enrichment of the pH buffering capacity of the extracellular solution increased cone ICa, and blocked any additional increase in cone ICa by surround illumination. Hyperpolarization of the HCs by a glutamate receptor antagonist-augmented cone ICa, whereas depolarization of the HCs by kainate suppressed cone ICa. From these results, we propose the hypothesis that pH changes in the synaptic clefts, which are intimately related to the membrane voltage of the HCs, mediate the feedback from the HCs to cone photoreceptors. The feedback mediated by pH changes in the synaptic cleft may serve as an additional mechanism for the center-surround organization of the receptive field in the outer retina.     

Selective coupling of different muscarinic acetylcholine receptors to neuronal calcium currents in DNA-transfected cells

Acetylcholine (ACh) can inhibit calcium currents (ICa) in nerve cells by activating muscarinic ACh receptors (mAChR). There are several different genetic subtypes of mAChR. It is not known which subtype(s) are responsible for ICa inhibition. To resolve this issue, we measured ICa inhibition by ACh with patch-clamp recording, by using Ba2+ as charge carrier, in clones of NG108-15 neuroblastoma x glioma hybrid cells transfected with DNA for mAChRI, II, III and IV. Control (non-transfected) cells showed a mean maximum inhibition of peak ICa of 12.8 +/- 1.8% (n = 36) at 1 mM ACh. No consistent increase in inhibition was detected in vector-transfected cells, or in cells transformed to express mAChRI or mAChRIII. In contrast, inhibition was significantly increased in clones transformed to express mAChRII or mAChRIV. Inhibition was not correlated with the number of muscarinic receptors as determined by 3H-quinuclidinyl benzilate binding. Inhibition in both control and transfected cells was prevented by pretreatment with pertussis toxin (PTx). Inhibition persisted in the presence of extracellular or intracellular dibutyryl cyclic AMP, and hence is not because of inhibition of adenylate cyclase. We conclude that the inhibition of neuronal ICa is mediated preferentially by mAChRII and mAChRIV, via a PTx-sensitive GTP-binding protein.     

Tricyclic antidepressants inhibit voltage-dependent calcium channels and Na(+)-Ca2+ exchange in rat brain cortex synaptosomes

We have used a resting (5 mM K+) or depolarizing (60 mM K+) choline-based medium, and a nondepolarizing sodium-based or choline-based medium, to characterize the inhibitory potential of tricyclic antidepressants against the voltage-dependent calcium channels or the Na(+)-Ca2+ exchange process, respectively, in synaptosomes from rat brain cortex. Imipramine, desipramine, amitriptyline, and clomipramine inhibited net K(+)-induced 45Ca uptake with similar IC50 values (26-31 microM), and this uptake was also inhibited by diltiazem with an IC50 of 36 microM; these results indicate an inhibition of voltage-dependent calcium channels by tricyclic antidepressants. The net uptake of 45Ca induced by Na(+)-Ca2+ exchange was also inhibited by the four tricyclic antidepressants tested, but not by diltiazem; imipramine (IC50 = 94 microM) was a more potent inhibitor of this process than desipramine (IC50 = 151 microM), and the IC50 values of amitriptyline (107 microM) and clomipramine (97 microM) were similar to that of imipramine. Some degree (approximately 25%) of brain calcium channel blockade could be present at the steady-state concentrations of tricyclic antidepressants expected to occur therapeutic use of these compounds to treat depression or panic disorder.     

Clinical potency of calcium channel blockers in asthma may be related to their inhibition of receptor-mediated phasic responses in vitro.

The calcium channel blockers (CCB) have been clinically effective in exercise-induced asthma. The completeness of protection with the CCB might be related specifically to inhibition of Ca2+ influx or release. To examine this hypothesis, the rank order of potency of inhibition of the CCB, nicardipine, diltiazem and verapamil on the steady-state and kinetic parameters of the phasic and tonic responses to the muscarinic receptor agonist carbachol (10 microM) and KCl (40 mM) in the intact isolated guinea-pig trachea was determined. The Ca2+ channel agonist Bay K 8644 was also examined for its effects on intracellular Ca2+. Nicardipine abolished the KCl response at both 0.1 microM and 1 microM concentrations. The amplitude of the KCl response was inhibited equally by 1 microM diltiazem (61% inhibition) and 1 microM verapamil (68% inhibition). The rate constant of onset of the KCl response was similarly inhibited 60% by diltiazem and 66% by verapamil. Nicardipine abolished the carbachol phasic response at the 1 microM concentration. The amplitude of the phasic response was inhibited equally by 0.1 microM nicardipine (61.3% inhibition), 1 microM diltiazem (64.5% inhibition) and 1 microM verapamil (71% inhibition). The rate constant of decay of the phasic response was inhibited equally by 0.1 microM nicardipine (43% inhibition) and 1 microM diltiazem (29% inhibition). The rate constant of onset of the phasic response was unaffected by nicardipine, diltiazem and verapamil. Only 1 microM nicardipine inhibited the amplitude and rate constant of onset of the tonic response. The only effect of Bay K 8644 (1 microM) was to increase the phasic response amplitude. The CCB demonstrate a similar order of potency for inhibition of the phasic responses and clinical efficacy of the CCB in exercise-induced asthma (nicardipine > verapamil > diltiazem).(ABSTRACT TRUNCATED AT 250 WORDS)