New selective inhibitors of the transmembrane Ca conductivity in mammalian myocardial fibres. Studies with the voltage clamp technique

Revisiting ageless antiques; synthesis, biological evaluation, docking simulation and mechanistic insights of 1,4-Dihydropyridines as anticancer agents

The historic DHP nucleus was serendipitously discovered by Arthur Hantzsch about 130 years ago and is still considered a hidden treasure for various pharmacological activities. Twenty-one DHP analogues were synthesized using the expedient one pot Hantzsch synthesis for screening as anticancer agents. Initially, the in vitro anti-proliferative single dose against a panel of 18 cancer cell lines showed that compounds 11b and 8f were the superlative candidates regarding their antitumor effect (GI% mean = 66.40% and 50.42%, correspondingly) compared to cisplatin (GI% mean = 65.58%) and doxorubicin (GI% mean = 74.56%). Remarkably, compound 11b showed a remarkable MDA-MB-468 anticancer activity (GI%=80.81%), higher than cisplatin (64.44%) and doxorubicin (76.72%), as well as strong antitumor activity against lung cancer A549 (GI%= 83.02%), more powerful than both cisplatin and doxorubicin. Compound 11b exhibited an exceptional anticancer activity against lung cancer cell line (A549) as its GI₅₀ in nanomolar was (540 nM) with a 9-fold increase greater than cisplatin (GI₅₀ = 4.93 µM) and with a selectivity index = 131 to cancer cells over normal cells. Further mechanistic investigations proved that DHPs anticipate simultaneously TOPI and RTKs (VEGFR-2, HER-2 and BTK) which can stimulate BAX/BAK and the executioner caspases via rtPCR studies.

Enhanced Late Na and Ca Currents as Effective Antiarrhythmic Drug Targets

While recent advances clarified the molecular and cellular modes of action of antiarrhythmic drugs (AADs), their link to suppression of dynamical arrhythmia mechanisms remains only partially understood. The current classifications of AADs (Classes I, III, and IV) rely on blocking peak Na, K and L-type calcium currents (I_Ca,L), with Class II with dominant beta receptor blocking activity and Class V including drugs with diverse classes of actions. The discovery that the calcium and redox sensor, cardiac Ca/calmodulin-dependent protein kinase II (CaMKII) enhances both the late Na (I_Na-L) and the late I_Ca,L in patients at high risk of VT/VF provided a new and a rational AAD target. Pathological rise of either or both of I_Na-L and late I_Ca,L are demonstrated to promote cellular early afterdepolarizations (EADs) and EAD-mediated triggered activity that can initiate VT/VF in remodeled hearts. Selective inhibition of the I_Na-L without affecting their peak transients with the highly specific prototype drug, GS-967 suppresses these EAD-mediated VT/VFs. As in the case of I_Na-L, selective inhibition of the late I_Ca,L without affecting its peak with the prototype drug, roscovitine suppressed oxidative EAD-mediated VT/VF. These findings indicate that specific blockers of the late inward currents without affecting their peaks (gating modifiers), offer a new and effective AAD class action i.e., “Class VI.” The development of safe drugs with selective Class VI actions provides a rational and effective approach to treat VT/VF particularly in cardiac conditions associated with enhanced CaMKII activity such as heart failure.

Anacardium occidentale Linn. (Anacardiaceae) stem bark extract induces hypotensive and cardio-inhibitory effects in experimental animal models

Anacardium occidentale Linn. (Anacardiaceae) is a plant largely used in Africa for the treatment of different diseases. In Côte d'Ivoire it's commonly used for the treatment of hypertension. The present study was carried out in order to assess the effects of Anacardium occidentale extract (ANOE) on cardiovascular parameters in animal models. A mercury manometer kymograph of Ludwig was used to measure the blood pressure of normotensive rabbits in control conditions (normal physiological solution) and under the influence of ANOE. The contractile activity of an isolated rat heart was also measured in control conditions and under the influence of ANOE in different physiological media using a modified Langendhorff (1895) apparatus. The aqueous Anacardium occidentale (ANOE) bark extract applied intravenously in different doses (12, 40, 90, and 167 mg/kg b.w.), produced a significant dose-dependent decrease in blood pressure of previously normotensive rabbits (up to 89% vs control). Atropine (1 mg/ml) pre-treatment failed to reverse the hypotensive effects elicited by the extract. ANOE applied to isolated rat heart preparations in different concentrations (0.01, 0.1, 1.0, and 10 µg/ml) induced negative inotropic and chronotropic effects. Atropine pre-treatment of heart preparations (0.1 µg/ml) failed to reverse the negative effects induced by ANOE. The extract's action on heart contractile activity studied in modified culture media further confirmed its cardio-inhibitory effects. ANOE induced strong hypotensive and cardio-inhibitory effects in animal models.

Positive chronotropic responses induced by alpha 1-adrenergic stimulation of normal and "ischemic" Purkinje fibers have different receptor-effector coupling mechanisms

We studied the mechanisms underlying the increase in automaticity induced by alpha 1-adrenergic stimulation of normal and "ischemic" canine Purkinje fibers. Fibers were superfused with a control Tyrode's solution, followed by an ischemic superfusate that included 10 mM KCl, 5 mM NaHCO3, Po2 of 10-25 mm Hg, and pH 6.7. To exclude beta-adrenergic actions, propranolol was added to all solutions. In the presence of phenylephrine, normal automaticity at high membrane potentials usually decreased, whereas the incidence of abnormal automaticity during ischemia was increased from a control value of 10% to 30%. Block of an alpha 1-receptor subtype with chloroethylclonidine in the presence of phenylephrine caused normal automaticity to increase in all fibers studied and significantly increased abnormal automaticity to 70%. The alpha-adrenergic-induced increase in automaticity did not occur in ischemic fibers from animals pretreated with pertussis toxin (PTX), which ADP-ribosylated and functionally inactivated the 41-kd family of GTP regulatory proteins. In contrast, the use of PTX enhanced the increase in automaticity induced by phenylephrine in normally polarized Purkinje fibers. Ryanodine, which blocks sarcoplasmic reticulum Ca2+ release, attenuated the increase in normal automaticity in nonischemic fibers but had no effect on abnormal automaticity in ischemic fibers. The increase in abnormal automaticity was, however, blocked by the alpha 1 subtype blocker WB 4101, which also blocks the increase in automaticity in normal fibers. In conclusion, the increase in abnormal automaticity in ischemic Purkinje fibers depends on a WB 4101-sensitive alpha 1-adrenergic receptor subtype whose actions are transduced by a PTX-sensitive 41-kd G protein and are not blocked by ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-independent GABA release from striatal slices: the role of calcium channels

We have investigated the role of Ca2+ and Ca2+ channels in the modulation of GABA release. Brain slices prepared from rat striatum were preincubated with [3H]GABA, superfused with Krebs bicarbonate buffer, and exposed to electrical field stimulation (2 Hz for 3 min). Tritium efflux was measured as an index of GABA release. Both resting and evoked efflux were greatly accelerated by deleting Ca2+ from the medium and adding EGTA (1 mM). However, when the concentration of Mg2+ in the buffer was elevated to 10 mM, no effect of the Ca2(+)-deficiency was observed on resting release and its impact on evoked overflow was diminished. Moreover, addition of verapamil (10 microM), a Ca2+ channel blocking agent, reduced evoked overflow even in the absence of external Ca2+, while 4-aminopyridine (10 microM), a K+ channel inhibitor, enhanced GABA efflux in normal buffer but had no effect in the absence of Ca2+. Finally, we have shown previously that nipecotic acid, an inhibitor of high affinity GABA transport, increases GABA overflow in normal buffer, but blocks it in Ca2(+)-free buffer. Collectively, these results suggest that Ca2+ channels may play two roles in the regulation of depolarization-induced GABA release. Firstly, these channels permit a depolarization-induced influx of Ca2+ which then promotes GABA release. In addition, these channels influence GABA release through a mechanism that does not involve external Ca2+. Although the precise nature of this latter involvement is unclear, we propose that the Ca2+ channels serve to permit an influx of Na+, which in turn promotes Ca2(+)-independent release through an influence on the high affinity GABA transport system.

Blue light promotes ionic current influx at the growing apex ofVaucheria terrestris

Irradiation of the growing apex of the algaVaucheria terrestris Götz var.terrestris with blue light (BL), which causes a transient acceleration of growth, also causes a large transient increase in inwardly directed current, which was monitored with a vibrating probe. The growing apex is normally the site of an inward current, and the surface of the non-growing, basal part of the coenocytic cell the site of an outward current. Irradiation of the apex causes only a slight increase in current efflux at the basal part of the cell. The BL-promoted current influx at the apex (BLCI) usually starts within 10 s after the onset of irradiation, preceding the light-growth response. With BL pulses shorter than 3 min, the BLCI reaches a maximum in about 3 min, and then declines to its original value over the next 3 min. If the BL pulse is longer than 3 min, the BLCI continues until the light is turned off. The threshold energy of the BLCI with broad-band BL is 2-5 J·m(-2), i.e. smaller than for both the light-growth response and phototropic response. The maximum BLCI reaches a value of approx. 5 μA·cm(-2), equivalent to an influx of 50 pmol·cm(-2)·s(-1) of monovalent cations. The effect of red light (RL) is completely different from that of BL: it either causes increases in the inward current of less than 0.3 μA·cm(-2), or a transient decrease of current. Furthermore, the direction of the RL-induced change is always the same at the apex and trunk, indicating the participation of photosynthesis. Our results indicate that the BLCI is kinetically and spatially related to the light-growth response and the phototropic bending ofVaucheria. It seems to be a necessary step for the phototropic bending.

Electrophysiological alternans and restitution during acute regional ischaemia in myocardium of anaesthetized pig

1. Alternate long and short action potential durations, or electrical alternans, has only been sporadically observed in ischaemic myocardium in situ. We systematically studied alternans in the latter to characterize the phenomenon, relate it to ventricular arrhythmia and suggest possible mechanisms. 2. Sixteen Landrace pigs were anaesthetized (Azaperone, N2O and O2), ventilated and the hearts exposed. A branch of the left coronary artery was ligated. Left intraventricular and systemic pressures were monitored. Monophasic action potentials were recorded simultaneously with up to five suction electrodes in and around the proposed ischaemia area. 3. A computer measured the duration of every action potential, at several phases of repolarization, throughout the first hour of ischaemia. This allowed the systematic study of the alternans. Measurements during defined stimulus protocols were also made for the construction of electrical restitution curves. 4. Alternans was found in all recordings within the ischaemic area and in two-thirds of those in the 'border' area. There was no alternans in non-ischaemic areas. 5. The alternans, when action potential duration was plotted for every beat, appeared as an oscillation which was pleomorphic. It could be: (a) stable for hundreds of beats; (b) switched or triggered (by one extraneous beat having a different cycle length) between one stable state with high and one with low or absent alternans; (c) damped; (d) undamped to take a crescendo form, sometimes preceding ventricular fibrillation. 6. The alternans in general showed an ill-defined peak incidence between about 200 to 1500 beats after the onset of ischaemia, and a clearer late peak at about 3000 beats. These periods occurred at about 2-7 min and 15-40 min, corresponding to so-called phase 1A and 1B arrhythmia respectively. Only the late peak was seen with triggered alternans. 7. The electrical restitution curve for the action potential duration during ischaemia when compared with curves, constructed with data from non-ischaemic myocardium, showed a progressive depression in plateau, a reduction in magnitude and was flattened at 1 h. However, there was a reversal or reduction in decline at about 15-45 min. 8. We propose that electrical alternans is a distinctive electrophysiological characteristic of ischaemic myocardium which may be causally related to ventricular arrhythmia and fibrillation, and that at least two mechanisms contribute to the alternans: (i) electrical restitution of the action potential and (ii) changes in intracellular calcium cycling.

Verapamil inhibits serotonin uptake of endothelial cells in culture by a mechanism unrelated to Ca2+ channel blockade

Verapamil inhibited Na+-dependent uptake of serotonin (5-HT) by bovine pulmonary artery endothelial cells in culture both exposed to room air and stimulated by prior exposure to anoxia. The effect of verapamil occurred even in the absence of Ca2+ from the assay medium. Although absence of Ca2+ from the medium moderately reduced 5-HT uptake, stimulation of uptake was nevertheless observed for cells previously exposed to anoxia. Verapamil altered the Km, but not the Vmax, of 5-HT uptake. There was no change in 45Ca2+ uptake or release by cells previously exposed to anoxia as compared to those exposed to room air and verapamil did not influence 45Ca2+ fluxes by either set of cells. It is concluded that verapamil inhibits 5-HT uptake by endothelial cells through a mechanism other than Ca2+ channel blockade; the results are consistent with competitive inhibition of a 5-HT carrier. The stimulatory effect of anoxia on 5-HT uptake does not occur through a change in Ca2+ fluxes.

The Ca++ permeability of the apical membrane in neuromast hair cells

The mechanosensitivity of eel (Anguilla anguilla) neuromasts was measured by the impulse responses of single afferent nerve fibers to mechanical stimuli. It is dependent on the potential across the skin and on the ions in the water outside the apical membrane of the sensory cells. The mechanosensitivity decreases to zero when the skin is polarized by 10-100 mV cathodal DC (skin surface negative); it increases with increasing (10-60 mV) anodal DC and remains remarkably constant with higher polarization (Fig. 1). The mechanosensitivity increases with increasing concentrations of Ca++ outside the apical membrane of the sensory cells. Na+ and K+ have no influence. Addition of La , Co++, Mg++, D 600 and A-QA 39 inhibits the mechanosensitivity; the degree of inhibition varies with the inhibitor and the ratio [Ca++]/[inhibitor], indicating that the inhibition is competitive (Figs. 2, 3). We conclude that the apical membrane is specifically permeable to Ca++ ('late Ca channel') and that the inward receptor current through the apical membrane is carried by Ca++. Streptomycin also inhibits mechanosensitivity by competing with Ca++. With streptomycin, however, anodal polarization reduces, rather than increases, the mechanosensitivity (Fig. 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of D-600 on sodium current in squid axons

The effects of the calcium antagonist D-600 (methoxyverapamil) on the excitatory inward sodium current, INa, of internally perfused squid giant axons were studied under voltage-clamp conditions. We observed little or no effect of the drug when it was added to the external solution at concentrations of 10-200 microM. Furthermore, it did not produce a frequency, or use-dependent block of INa when repetitive voltage-clamp pulses were used at rates of 2-5 Hz. However, it did produce use-dependent blockade of INa when it was placed internally at a concentration of 200 microM. These results in conjunction with other studies suggest that D-600 is a selective blocker of calcium channels in squid axons when the drug is placed in the external solution. Its effects, when placed in the internal solution, are similar to those of permanently charged local anesthetic derivatives, which also produce use-dependent block of INa.

The inhibition of Ca uptake in cardiac membrane vesicles by verapamil

Cardiac membrane vesicles take up Ca2+ in response to Na+ gradient (high inside) and negative inside membrane potential. Both components of the Ca2+ uptake, the Na+ gradient dependent uptake and the membrane potential dependent uptake are inhibited by verapamil; the action is dose-dependent and the concentrations of verapamil required to inhibit the Ca2+ uptake to 50% of its maximal value are 50 and 60 microM respectively. In the concentration ranges tested (50-750 microM Ca2+), the inhibitory effect of verapamil could not be antagonized by increasing the Ca2+ concentration of the medium. Introducing verapamil into the vesicles by rapid freezing and slow thawing of the vesicles had the same inhibitory effect as adding the same concentration of verapamil on the outside of the vesicles. Adding verapamil to both sides of the vesicle membrane led to higher inhibition of Ca2+ uptake. It is proposed that addition of verapamil can cause a change in cardiac membranes which is manifested by a decrease in the driving membrane potential and Ca2+ transport.

Effects of verapamil on contraction and relaxation of cultured chick embryo ventricular cells during calcium overload

The calcium channel blocking agents verapamil and nifedipine have been reported to lessen abnormalities of left ventricular relaxation in patients with severe left ventricular hypertrophy or coronary artery disease. Whether these effects in human beings are related in part to a direct effect on myocardial calcium metabolism is difficult to determine because of complicating drug influences on ventricular loading via systemic arterial vasodilation, on myocardial blood supply via coronary artery vasodilation and on reflex changes in sympathetic tone. For this reason, the effects of verapamil were investigated in a cellular model of impaired relaxation using spontaneously contracting tissue cultured monolayers of chick embryo ventricular cells exposed to high external calcium ([Ca]o). Under control conditions ([Ca]o, 0.9 mM), verapamil (2 X 10(-8)M) induced a 57 +/- 8% decrease in amplitude of cell contraction monitored with a phase contrast microscope video motion detector system. Elevation of [Ca]o from 0.9 to 8.0 mM resulted in a decrease in amplitude and velocity of contraction and a decrease in velocity of relaxation associated with an upward shift in diastolic cell wall position, suggesting a failure of normal myofilament dissociation. These abnormalities were completely reversible on reperfusion with [Ca]o, 0.9 mM. On re-exposure of the cells to [Ca]o, 8.0 mM, in the presence of verapamil, there was an increase in amplitude of contraction (0.56 +/- 0.11 to 1.03 +/- 0.09 micron, p less than 0.01) and velocity of relaxation (4.97 +/- 0.89 to 9.94 +/- 0.87 micron/s, p less than 0.01) compared with exposure to [Ca]o, 8.0 mM, alone, and an attenuation of the upward shift in diastolic cell wall position.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of rat mast cell degranulation by verapamil

Calcium antagonists, e.g. verapamil, prevent exercise-induced asthma. This protective effect may proceed from inhibition of contraction of bronchial smooth muscle, release of mediators by primary effector cells, e.g. mast cells, or both. Therefore, we studied the inhibitory effect of increasing concentrations of verapamil on both in vitro antigen-induced degranulation and ionophore A23187-induced release of labelled serotonin by rat peritoneal mast cells. There was a dose-dependent inhibition by verapamil of both ovalbumin-induced degranulation of mast cells passively sensitized by incubation with mice IgE-rich serum and ionophore-induced release of tritiated serotonin by mast cells previously incubated with (3H)-5HT; the 50% inhibiting concentration was 1.4 X 10(-4) mol I-1 and 5.2 X 10(-5) mol I-1, respectively. An attractive explanation of our results is that verapamil inhibits the antigen-induced release of mediators by mast cells through its calcium antagonist effect. Our results also suggest that the preventing effect of calcium antagonists on asthma may be multi-factorial since other authors have clearly shown that these drugs inhibit contraction of guinea-pig tracheal smooth muscle in vitro.

Kinetics of acetylcholine-activated cation channel blockade by the calcium antagonist D-600 in Aplysia neurons

The effects of the calcium channel blocker D-600 on the cation channels activated by acetylcholine (ACh) was studied in voltage-clamped Aplysia neurons by voltage-jump relaxation analysis. D-600 blocked the steady-state ACh current in a highly voltage-dependent manner, the degree of antagonism increasing with membrane hyperpolarization. In the presence of D-600 the current relaxations following hyperpolarizing command steps became biphasic. The time constants of ACh-induced current relaxations (tau f), which approximate the mean channel lifetime, were reduced in a voltage-dependent manner, the degree of reduction of tau f increasing with increasing membrane potential. In addition to the acceleration of tau f, a slow, inverse kinetic component (tau s) of the relaxation appeared in the presence of D-600. The rate of this inverse kinetic component was accelerated either by increasing the agonist or antagonist dose or by increasing the membrane potential. These results suggest that D-600 acts to antagonize the acetylcholine response through a blockade of the open state of the transmitter-activated cation channel. Possible kinetic schemes for this interaction are discussed.

Origin of calcium ions involved in the generation of a slow afterhyperpolarization in bullfrog sympathetic neurones

The origin of Ca2+ that activates the Ca2+-dependent K+ conductance which is responsible for the slow afterhyperpolarization (a.h.p.) following an action potential was studied in bullfrog sympathetic ganglia. The decay phase of the a.h.p. was a graded function of the extracellular Ca2+, and showed a voltage sensitivity opposite to that of the Ca2+-dependent K+-current reported previously (Pallotta et al. 1981), indicating that it reflected the time course of an increase in intracellular free Ca2+. An a.h.p. of longer duration was generated in cells which showed more pronounced rhythmic hyperpolarizations induced by intracellular Ca2+ release. The duration of the a.h.p. recorded with electrodes filled with K3-citrate [a.h.p. (citrate)], which favors Ca2+ release, was longer than the a.h.p. recorded with KCl-filled electrodes [a.h.p. (C1)]. D-600 (50-100 microM) drastically reduced the a.h.p. (C1), but had less effect on the a.h.p. (citrate). Caffeine which facilitates Ca2+ release prolonged the a.h.p. (C1), but had less effect on the a.h.p. (citrate). The a.h.p. (citrate) showed a greater sensitivity to a low temperature than the a.h.p. (C1). Mn2+ (1-3 mM) depressed both types of a.h.ps to the same extent. These results suggest that the origin of intracellular Ca2+ for a.h.p. (C1) is mainly Ca2+ influx during an action potential, while that for the a.h.p. (citrate) is both Ca2+ entry and intracellular Ca2+ release, although the effect of Mn2+ is difficult to explain fully.

Calcium antagonists and calmodulin inhibitors block cytokinin-induced bud formation in Funaria

The plant hormone cytokinin stimulates nuclear migration followed by an asymmetric cell division in target cells of the protonema of the moss Funaria hygrometrica, leading to bud formation. The role of calcium in this developmental event was investigated by examining the effects of various calcium antagonists on the cytokinin-induced division. Calcium-free medium (buffered with EGTA), the extracellular Ca2+ antagonist La3+ (lanthanum), and the Ca2+ channel inhibitors D 600 and verapamil all block bud formation. These inhibitions are partially reversed by washing the cells or by raising the extracellular [Ca2+]. The Ca2+ ionophore A23187 partially reversed the effects of D 600 and verapamil. Bud formation is also inhibited by the intracellular Ca2+ antagonist TMB-8 (8-diethylamino)octyl 3,4,5-trimethoxybenzoate HCl), and this inhibition is partially reversed by washing or raising the extracellular [Ca2+]. The cross walls of both the filaments and bud initial cells formed during TMB-8 exposure exhibit a distorted morphology. High concentrations of TMB-8 block nuclear migration. The calmodulin inhibitor trifluoperazine stops cytokinin-induced budding more effectively than the related compound chlorpromazine. Low concentrations of these two compounds do not affect nuclear migration; however, the target cell does not enter mitosis. These results support the hypothesis that a rise in intracellular calcium mediates cytokinin-induced bud formation in Funaria. It is concluded that the proposed cytokinin-induced rise in intracellular calcium may be effected in part by the activation of calmodulin. The essential source of Ca2+ appears to be extracellular, because blocking Ca2+ uptake with Ca2+ transport inhibitors can block both nuclear migration and subsequent division.

The inhibition of Na-dependent Ca uptake by verapamil in synaptic plasma membrane vesicles

This study on the effect of verapamil on the Na+-dependent Ca2+ uptake system (Na+-Ca2+ exchanger) of synaptic plasma membrane (SPM) vesicles showed that verapamil inhibited the Na+-dependent Ca2+ uptake in a dose dependent manner. A 50% inhibition was obtained with 175 microM verapamil. Calcium uptake in SPM vesicles is dependent on the Na+ gradient across the membrane and on the membrane potential. Both components of the Ca2+ uptake system were inhibited by verapamil. Na+-coupled GABA uptake in SPM vesicles was also inhibited by verapamil in a similar dose-dependent fashion. It seems therefore that verapamil inhibits Na+ gradient driven processes in synaptic plasma membrane vesicles.

Does intracellular release of Ca2+ participate in the afterhyperpolarization of a sympathetic neurone?

The afterhyperpolarization (AHP) of an action potential in the bullfrog sympathetic ganglion cell was highly sensitive to anions (a factor affecting Ca2+ release) filled in a recording electrode; it was slower for citrate ion than for Cl-. The AHP recorded with a 'KCl-electrode' was suppressed drastically by D-600 (Ca2+-antagonist) and prolonged significantly by caffeine (promoting Ca2+ release), while the AHP recorded with a 'K3-citrate-electrode' was affected only slightly by these agents. Thus, these results suggest that Ca2+ entry during an action potential is the main origin of Ca2+ for the AHP recorded with a 'KCl-electrode', and favour the idea that the intracellular release of Ca2+ by an action potential as well as the Ca2+ influx participates in the mechanism of the AHP recorded with a 'K3-citrate-electrode'.

Hemodynamic effects of verapamil in children and adolescents with hypertrophic cardiomyopathy

The acute hemodynamic effects of verapamil were evaluated in nine children with hypertrophic cardiomyopathy. Verapamil, 0.1 mg/kg, was administered as an i.v. bolus over 2 minutes, followed by a 20-minute continuous infusion of 0.007 mg/kg/min. Hemodynamic measurements were obtained at rest in nine patients and at maximal supine bicycle exercise in seven before and 15 minutes after verapamil. At rest, verapamil increased the mean cardiac output from 3.3 +/- 0.9 to 3.7 +/- 0.9 l/min/m2 (+/- SD) (p less than 0.02) and decreased left ventricular end-diastolic pressure from 19.3 +/- 8.1 to 14.5 +/- 6.9 mm Hg (p less than 0.006). In six patients with resting left ventricular outflow tract obstruction, the systolic pressure gradient decreased from 17.5 +/- 7.2 to 5.2 +/- 4.5 mm Hg (p less than 0.04). Repeat supine bicycle exercise testing after verapamil showed increases in total work performed (1743 +/- 1284 to 3168 +/- 1643 kg-m, p less than 0.006) and maximal cardiac index during exercise (6.5 +/- 1.3 to 7.8 +/- 1.8 l/min/m2, p less than 0.05), and decreases in maximal exercise left ventricular end-diastolic pressure (29.1 +/- 10.1 to 19.3 +/- 10.4 mm Hg, p less than 0.002) and left ventricular systolic outflow tract gradient (31.2 +/- 10.5 to 1.75 +/- 1.7 mm Hg, p less than 0.04). These results suggest that verapamil may be an effective therapeutic agent for the treatment of hypertrophic cardiomyopathy in children.

Effect of TMB-8 on the pulmonary vasoconstrictor action of prostaglandin F2 alpha and the thromboxane mimic, U 46619

1 TMB-8 (8-(N,N-diethylamino)octyl-3,4,5 trimethoxybenzoate HCl), an intracellular calcium antagonist, had no direct action on the pulmonary vasculature of the perfused canine lung lobe preparation. 2 The pulmonary pressor response to the thromboxane mimic, U46619, was not affected by TMB-8. 3 The vasopressor response to prostaglandin F2 alpha (PGF 2 alpha) was significantly attenuated but not completely blocked by TMB-8. 4 We conclude that the pulmonary pressor response to PGF 2 alpha is dependent on both intracellular and extracellular calcium pools for contraction and that U46619 facilitates either solely extracellular calcium influx or mobilizes an intracellular calcium pool not inhibited by TMB-8.

Pharmacological basis for the therapeutic applications of slow-channel blocking drugs

Excitation-contraction coupling in cardiac muscle as well as in smooth muscle is mediated by the transmembrane fluxes of calcium. In the case of cardiac muscle, this transfer occurs through the slow-inward channel. Agents that selectively inhibit the myocardial slow-channel also block calcium entry in smooth muscle cells, particularly in arteries. Thus, such selective inhibitors of the slow channel, exemplified by verapamil, nifedipine, and diltiazem, produce a marked negative inotropic effect in cardiac muscle; in whole animals or in man, such a propensity is largely nullified or even reversed by the profound vasodilator effects of these compounds. The drugs known as calcium antagonists are chemically heterogeneous and they may exhibit associated pharmacological properties such as noncompetitive sympathetic inhibition while having varying potencies for inhibiting the calcium influx in smooth muscle and in the heart and nodal tissues. These similarities and differences influence the net electrophysiologic and hemodynamic effects of calcium antagonists in man. Electrophysiologically, the main effect is a depressant one on the AV node in which most agents of the class lengthen AV conduction and enhance refractoriness, a property that is relevant in the termination of PSVT and to the slowing of the ventricular response in atrial flutter and fibrillation. The effective refractory periods of atrial, ventricular, and His-Purkinje tissues or the bypass tracts are not altered by calcium antagonists, but conduction may be improved in ischemic tissues. On the surface ECG, the only effect is the short-term lengthening of the PR interval with no change in the QRS or Q-Tc intervals. The sinus frequency is variably affected relative to the competing influences of the direct effect, reflex response to hypotension and of sympathetic antagonism. The sinus node recovery time is affected little normally, but may be prolonged dramatically in the sick sinus syndrome. Hemodynamically, as a class of drugs, calcium antagonists produce a complex interplay of simultaneous changes in preload, afterload, contractility, coronary flow, and heart rate. The net hemodynamic effect that becomes apparent will be dependent on the agent used, on the cardiac condition and the level of ventricular function present, on the intactness of the autonomic nervous system, and on the route of drug administration. An appreciation of the electrophysiological and hemodynamic actions of calcium antagonists relative to their individual pharmacologic properties permits the rational choice of the appropriate agent in the control of a wide spectrum of cardiocirculatory disorders.

The effects of verapamil, quiescence, and cardioplegia on calcium exchange and mechanical function in ischemic rabbit myocardium

The effects of verapamil (1 mg/liter, 2 x 10(-6) mol/liter), quiescence, and cardioplegia (K+ 16 mmol/liter, Mg2+ 16 mmol/liter) on calcium exchange and mechanical function during ischemia and reperfusion have been investigated in the rabbit interventricular septum at 32 degrees C. Calcium influx and efflux were recorded continuously with 47Ca2+ and 45Ca2+. After 60 minutes of total ischemia and reperfusion for 30 minutes, there was a net calcium gain of 4.9 mmol/kg dry tissue. Verapamil given before total ischemia reduced net calcium gain to 1.5 mmol/kg dry tissue (n = 5, P less than 0.03). When given only on reperfusion after total ischemia, or 10 minutes before reperfusion during low flow ischemia, verapamil did not affect calcium exchange. Cardioplegia begun 10 minutes before total ischemia reduced net calcium gain to 1.0 +/- 0.26 mmol/kg dry tissue (n = 6, P less than 0.001). Cardioplegia during the first 10 minutes of reperfusion, or lack of electrical stimulation during reperfusion, did not reduce calcium gain. Net calcium gain correlated with the maximum rise in resting tension and with the recovery of developed tension. In control experiments neither verapamil nor cardioplegia altered influx or efflux of slowly exchanging calcium. The cardioprotective effects of cardioplegia and the calcium channel blocker verapamil appear to be due to a reduction of myocardial work rather than to any specific direct action on calcium fluxes across the myocardial cell membrane.

Clinical pharmacokinetics of verapamil in patients with atrial fibrillation

The pharmacokinetic parameters and oral bioavailability of the antiarrhythmic drug verapamil were determined in six patients with atrial fibrillation. Plasma samples were taken following i.v. injection of verapamil 10 mg (Isoptin 2 ml), and oral verapamil 80 mg (Isoptin 2 tablets of 40 mg). Verapamil and its N-demethylated metabolite, norverapamil, were analyzed to 1 ng/ml plasma by gas chromatography-mass spectrometry using deuterated standards. Following intravenous injection, the disposition of verapamil followed a biexponential pattern with a fast distribution phase and a slower elimination of phase (t 1/2 beta = 5.79 h), corresponding to a plasma clearance of 0.26 l/kg/h. After oral administration, only an elimination phase was evident, with the same elimination rate (t 1/2 beta = 5.53 h). The oral bioavailability was 10.5% +/- 7.5%. The norverapamil formed after i.v. and oral administration of verapamil had plasma half-lives of 5.86 h and 6.77 h, respectively. The elimination of verapamil in patients with atria fibrillation was decreased compared to that in healthy young volunteers and the oral bioavailability was lower. Very good correlation between the percentage reduction in heart rate and the log plasma concentration of verapamil was found in every patient during the elimination phase, irrespective of the route of administration. There was also a high correlation when the plasma concentration -- effect data from the patients were pooled (r = 0.59, n = 71; p less than 0.0005).

The complex structure of mammalian heart contraction

Guinea pig papillary muscle contraction at 25 degrees C and 2 mg/liter norepinephrine (NE) consists of two components (C1, C2). Their parameters were determined in an analysis of contraction after abrupt load shifts. The C1 amplitude increased by 81 +/- 22% (N = 9) when stimulation frequency was raised from 0.1 to 1.0 Hz; its time to peak was constant at 169 +/- 6 msec (N = 12). C1 was abolished by 2 mM caffeine. The amplitude of C2 was constant between 0.1 and 0.5 Hz but decreased to 19 +/- 10% (N = 4) at 1.0 Hz; its time to peak correlated with the duration of action potential (DAP) (r = 0.98) and decreased from 518 +/- 12 msec at 0.1 Hz to 314 +/- 20 msec at 1.0 Hz (N = 12). C2 was eliminated by 5 mg/liter verapamil. Increase of DAP by a current sent through the muscle in the single sucrose gap technique gave rise to a C3 with time to peak equal to the new DAP. Mammalian heart muscle contraction in the presence of NE is supposed to be the sum of components elicited by Ca release (and after-release) from sarcoplasmic reticulum and a tonic component evoked by Ca release from sarcolemmal binding sites.

The effects of anaesthetics on the uptake and release of amino acid neurotransmitters in thalamic slices

1 The effect of thiopentone, methohexitone, urethane and ketamine on the uptake and release of gamma-aminobutyric acid (GABA) and D-aspartate by rat thalamic slices has been investigated. 2 A high, supra-anaesthetic concentration of methohexitone increased the uptake of both D-aspartate and GABA. 3 None of the anaesthetics used had any detectable effect upon the spontaneous release of either amino acid. 4 Urethane and ketamine had no effect upon the K+-stimulated release of either amino acid. 5 Methohexitone and thiopentone produced a biphasic dose-response on the K+-stimulated release of both amino acids; low concentrations enhanced release, high concentrations depressed release. 6 Bicuculline hydrochloride and picrotoxin both significantly reduced the barbiturate-induced enhancement of K+-stimulated amino acid release, but did not significantly alter the depression of K+-stimulated release at higher barbiturate concentrations. 7 Baclofen, either alone (1 microM to 1 mM), or tested against the barbiturates, had no detectable effect.

Neuronal and glial release of [3H]GABA from the rat olfactory bulb

GABA uptake and release mechanisms have been shown for neuronal as well as glial cells. To explore further neuronal versus glial components of the [3H]gamma-aminobutyric acid ([3H]GABA) release studies were performed with two different microdissected layers of the olfactory bulb of the rat: the olfactory nerve layer (ONL), consisting mainly of glial cells, and the external plexiform layer (EPL) with a high density of GABAergic dendritic terminals. In some experiments substantia nigra was used as a GABAergic axonal system and the trigeminal ganglia as a peripheral glial model. Spontaneous release of [3H]GABA was always lower in neuronal elements as compared with glial cells. A veratridine-evoked release was observed from the ONL but not from the trigeminal ganglia. Tetrodotoxin (TTX) abolished the veratridine-evoked release from the ONL, which also showed a partial inhibition when high magnesium concentrations were used in a Ca2+-free solution. beta-Alanine was strongly exchanged with [3H]GABA from the ONL of animals with the olfactory nerve lesioned and from animals with no lesion; but only a small heteroexchange was found from the external plexiform layer. The beta-alanine heteroexchange was able to deplete the releasable GABA store from the ONL of lesioned animals. In nonlesioned animals and the external plexiform layer, the veratridine-stimulated release of [3H]GABA was not significantly reduced after the beta-alanine heteroexchange. Stimulation of the [3H]GABA release by high concentrations of potassium elicited a higher release rate from axonal terminals than from dendrites or glia. Neurones and glia showed a similar inhibition of [3H]GABA release when a high magnesium concentration was added to a calcium-free solution. When D-600 was used as a calcium-flux blocker no inhibition of the release was observed in glial cells, whereas an almost complete blockage was found in both neuronal preparations (substantia nigra and EPL). These results provide further evidence for differential release mechanisms of GABA from CNS neurones and glial cells.

Ca-K bi-ionic action potential in squid giant axons

Under intracellular perfusion with a solution containing K+ as the sole cation species, squid giant axons were found to be capable of developing all-or-none action potentials when immersed in a medium in which CaCl2 was the only electrolyte. The adequate range of ion concentration for demonstrating this capability was mentioned. The reversal potential level measured by the voltage-clamp technique varied directly with the logarithm of the concentration of extracellular Ca-ion; the proportionality constant was close to RT/2F. The action potential observed under this Ca-K bi-ionic condition could not be suppressed by addition of tetrodotoxin or saxitoxin to the external medium. The external Ca-ion could be replaced with Co- or Mn-ion without eliminating the capability of the axons to develop action potentials. D-600 could not suppress the inward current observed under the voltage-clamp condition, but 4-aminopyridine could suppress it. The experimental findings were interpreted based on the current channel hypothesis and on the macromolecular theory.

Periodic responses in squid axon membrane exposed intracellularly and extracellularly to solutions containing a single species of salt

A periodic membrane potential change was found to occur in squid giant axons which were internally and externally perfused with solutions of an identical composition and were hyperpolarized by passing a sustained inward current. The solution contained Co2+ or Mn2+ as the sole cation species at a concentration of 1-10 mM. The amplitude of the response was roughly 100 mV. The current intensity and the ion concentration had large effects on the response. The voltage-clamp technique revealed an N-shaped I-V characteristic of the membrane system. The membrane emf of the resting and excited states was almost the same but the membrane conductance was increased in the excited states. The response was suppressed with 4-aminopyridine reversibly but unchanged with tetrodotoxin or D-600. Those unusual ionic conditions did not deprive axons of their ability to produce ordinary action potentials in physiological solutions. The experimental conditions employed and the results obtained were very close to those for some of the artificial membrane models. Applicability of the physico-chemical theories developed for these models is discussed.

Potassium and caffeine contractures in fast and slow muscles of the chicken

1 K+ contractures, caffeine contractures and electrical properties were studied in slow (posterior latissimus dorsi; p.l.d.) and fast (anterior latissimus dorsi; a.l.d.) chicken muscles. 2. P.l.d. K+ contractures show a transient increase of tension that relaxes spontaneously. Contractures in a.l.d. show an initial component followed by a maintained tension. 3. A.l.d. K+ contractures of similar amplitude and time course were reproduced at 4 min intervals. In p.l.d., the interval needed for full recovery is about 30 min. In Cl-free saline p.l.d. and a.l.d. K+ contractures can be reproduced at 4 min intervals. 4. The time course of repolarization after a short exposure to 160 mM-KCl was much slower in p.l.d. than in a.l.d. In Cl-free saline the time course of repolarization becomes faster in p.l.d. 5. The membrane resistance was not modified in a.l.d. and was increased in p.l.d. by Cl-free saline. The calculated Cl- conductance in p.l.d. was about 70% of the total membrane conductance. 6. In a.l.d., Mn2+, D600 and external Ca2+ reduction greatly diminishes the maintained phase of the K+ contracture leaving the initial phase almost unmodified. Under similar conditions p.l.d. K+ contractures were slightly reduced. 7. P.l.d. caffeine contractures (10-40 mM) were not maintained and they were not modified by Ca-free saline, Cd2+, Co2+, Mn2+ and D600. 8. A.l.d. caffeine contractures (2-15 mM) were maintained and were highly dependent on external Ca2+. In addition they were greatly reduced by Cd2+, Co2+, Mn2 and D600. 9. It is suggested that caffeine contractures of a.l.d. are elicited by a Ca2+ entry into the muscle from the external fluid.

Effect of verapamil on the pulmonary vasoconstrictor action of prostaglandin F2 alpha and a synthetic PGH2 analogue

1 The vasopressor response to prostaglandin F2 alpha (PGF2 alpha) and to ((15S)-hydroxy-11 alpha, 9 alpha-(epoxymethano)-prosta-5Z, 13E-dienoic acid) (U-46619) in the canine isolated lung lobe was significantly attenuated following the administration of verapamil. 2 The pressor response to arachidonic acid (AA) was not affected by the presence of verapamil. 3 The pulmonary pressor effect of PGF2 alpha and U-46619 is dependent, at least in part, on Ca2+ influx into vascular smooth muscle cells. 4 The pulmonary pressor response to AA cannot be attributed to PGF2 alpha or to endoperoxide intermediates but to some other product dependent on intracellular calcium stores.

The selective inhibition of serotonin-induced contractions of rabbit cerebral vascular smooth muscle by calcium-antagonistic dihydropyridines. An investigation of the mechanism of action of nimodipine

I studied the role of calcium in the activation of isolated rings of saphenous and basilar arteries of the rabbit by comparing the effect of calcium withdrawal with the effect of the calcium antagonist nimodipine [isopropyl(2-methoxyethyl)-1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridinedicarboxylate]. Serotonin-induced contractions of both vessels were inhibited quickly by incubation in calcium-free solution, showing the contractions of both vessels to be dependent on extracellular calcium. Potassium-induced contractions of both vessels were inhibited markedly by nimodipine (basilar: ID50 = 1.7 X 10(-10) mol/liter; saphenous: ID50 = 2.6 X 10(-10) mol/liter), showing depolarization-induced calcium influx (probably through "potential sensitive channels", PSCs) to be sensitive to nimodipine. In the basilar artery the sustained, tonic phase of serotonin-induced contractions (probably through "receptor operated channels," ROCs) was potently inhibited (ID50 = 7.3 X 10(-10) mol/liter) by nimodipine. However, the initial short-lived phase of this contraction of the basilar artery was relatively unaffected (ID50 = 2 X 10(-6) mol/liter), indicating that the inhibitory action of nimodipine on tonic contractions of the basilar artery was not due to antagonism at the serotonin receptor level. In contrast, in the saphenous artery the serotonin-induced contractions were unaffected by nimodipine in doses up to 2.4 X 10(-5) mol/liter. It is postulated that the selective inhibition of the sustained tonic contraction of the basilar artery is due to a selective inhibition by nimodipine of calcium movement through ROCs in this vessel. Agonist-induced activation of ROCs in peripheral blood vessels does not seem to be affected by calcium antagonists.

Is ouabain-sensitive rubidium or potassium uptake a measure of sodium pump activity in isolated cardiac muscle?

(1) The significance of the specific (ouabain-sensitive) 86Rb+ or 42K+ uptake by cardiac muscle preparations which are not 'sodium-loaded' was studied. (2) In left atrial preparations of guinea-pig heart, resting 86Rb+ uptake was relatively low. It was markedly increased by electrical stimulation. This stimulated uptake was further enhanced by isoproterenol and inhibited by verapamil. (3) In rat atria, the resting 86Rb+ uptake was somewhat higher than in guinea-pig atria, and the increase in uptake caused by electrical stimulation was smaller. In guinea-pig right ventricular papillary muscle, the resting uptake was highest among those tissues studied, and the response to electrical stimulation was smallest. In the latter tissue, verapamil produced only a minimal inhibition of the specific 86Rb+ uptake. (4) The effect of the frequency of electrical stimulation of 86Rb+ uptake paralleled its influence on the force of contraction, suggesting the involvement of intracellular sodium in both events. (5) In both left atrial and right papillary muscle preparations of guinea-pig heart, specific 42K+ uptake observed with 5.8 mM K+ was relatively high, and was increased only slightly by electrical stimulation. This electrical stimulation, however, increased ouabain-induced inhibition of 42K+ uptake, suggesting that the stimulation increases the amount of Na+ available to the sodium pump. (6) When the K+ concentration was 1 mM, the resting 42K+ uptake was low, and could be enhanced by electrical stimulation. (7) Thus, in cardiac muscle preparations which are not sodium loaded, the specific 86Rb+ or 42K+ uptake can be used to estimate the rate of sodium influx, which is equivalent to the rate of sodium efflux under steady-state conditions, provided that neither Rb+ nor K+ is in excess compared to the Na+ available to the pump. If Rb+ or K+ is in excess, its specific uptake may not reflect changes in transmembrane Na+ movement.

[Electrophysiological classification of newer antiarrhythmic drugs (author's transl)]

Newer antiarrhythmic drugs are tentatively classified on the basis of their influence on the transmembrane action potential, the fast sodium inward current and its kinetics, and the slow inward current which is primarily carried by calcium ions. Disopyramide is probably a quinidine-like antiarrhythmic drug, while the effects of aprindine, lorcainide, and tocainide resemble those of lidocaine. Mexiletine and propafenon cannot be classified definitely on the basis of the hitherto available electrophysiological data. Besides their effects on the fast inward current these substances may have additional calcium antagonistic properties.

Verapamil therapy: a new approach to pharmacologic treatment of hypertrophic cardiomyopathy

The hemodynamic and clinical effects of verapamil administration were examined in patients with hypertrophic cardiomyopathy. Infusion of 0.021 mg/kg/min of verapamil decreased the basal left ventricular outflow tract gradient from 94 +/- 14 to 49 +/- 14 mm Hg, while cardiac index increased from 2.5 +/- 0.2 to 2.8 +/- 0.2 L/min/sq m. Orally administered, 80 or 120 mg four times daily (QID), increased exercise capacity relative to placebo by 26% (1.6 +/- 0.5 minute; p less than 0.005) and propranolol, 40 or 80 mg QID, increased it by 21 +/- 8% (1.3 +/- 0.5 minute, p less than 0.025). Nine patients considered their symptomatic status "best" while receiving verapamil, seven while receiving placebo, and only three while receiving propranolol. Seventy-three patients have been discharged from the hospital receiving long-term verapamil therapy, and 49 have reported continuing symptomatic benefit with many showing improved exercise capacity up to two years later. Thus, verapamil can improve exercise capacity and symptomatic status in certain patients with hypertrophic cardiomyopathy, thereby providing physicians with a new therapeutic agent for the treatment of this disorder.