Organic Ca Antagonists Research Articles

Magnesium and cardiovasvular regulatory factor

Magnesium ions (Mg(2+)) modulate cardiovascular cell function by suppressing Ca(2+) influx. Mg(2+) inhibit ion channels, including voltage-dependent Ca(2+) channels in cardiac myocytes, non-selective cation channels in smooth muscle cells and capacitative Ca(2+) channels in endothelial cells. Although the inhibitory action of Mg(2+) is not so efficacious as organic Ca antagonists, Mg(2+) exert cardioprotective action by antiarrhythmic action, decreasing cell injury, including cell death, induced by ischemia-reperfusion and suppression of myocardial stunning. On the other hand, depletion of intracellular Mg(2+) is an important factor to aggravate various types of cardiovascular disorder. Mg(2+) play a crucial role in pharmacological therapy in not only cardiovascular medicine, but also in cardiac and brain surgery and in the field of anesthesiology and obstetrics.

Effect of organic calcium antagonists and magnesium on the development of metrazol kindling

In experiments on rats it was shown that i.p. administration of finoptin (verapamil), magnesium sulfate or ryodipine (an 1,4-dihydropyridine) 15 min before each daily injection of pentylenetetrazole (PTZ) in a subconvulsive dose (30 mg/kg i.p.) significantly (for 10-12 days delayed the development of pentylenetetrazole-induced kindling and attenuated kindled seizure reaction as compared with the controls. In animals sensitive to PTZ which were selected on the test of their reaction to previous single PTZ injection in a dose of 40 mg/kg finoptin, magnesium or finoptin + magnesium resulted in suppression of kindling development at late stages (after 2-week administration of drugs together with PTZ). After the withdrawal of the drugs there was a tendency to an increase of seizure reaction to the testing PTZ dose (30 mg/kg). The enhanced seizure susceptibility to test PTZ dose has being persisted during all observation period (8 months). Finoptin administered 15 min prior to PTZ had no effect on the severity of seizure reaction of fully kindled animals which had not received the drugs. The results obtained show that organic Ca-antagonists and magnesium delay the development of kindling induced seizure susceptibility, but cannot completely prevent it. The results also suggest that mechanisms of the chronic epileptogenesis (development of kindling induced seizure susceptibility) and those of the acute convulsive reaction to the epileptogen are not similar.

Effects of organic and inorganic Ca antagonists on rat platelet arachidonic acid metabolism in the presence of Ca2+, Sr2+ and Ba2+

The effects of Ca-antagonists on the thrombin-induced mobilization of radiolabeled arachidonate preincorporated into rat platelets as well as the subsequent formation of labeled cyclooxygenase and lipoxygenase products were analyzed in the presence of either Ca2+ or Ca2+-substitutes, Sr2+ and Ba2+. Results indicate that following thrombin stimulation (0.2 U/ml) in the presence of Ca2+, nitrendipine (Nit), Cd2+ or Mn2+ reduced the release of arachidonate and the biosynthesis of thromboxane B2. Inhibition of arachidonic acid release and metabolism were also obtained by both Nit and Cd2+ in the presence of Sr2+ and Ba2+. Results from studies with a semi-purified phospholipase A2 fraction prepared from rat platelets indicated that the activity was almost unaffected by Nit or Cd2+. From these findings, we concluded that inhibition of platelet-induced release and metabolism of arachidonic acid by the Ca-antagonists tested require intact platelets. These data support the hypothesis of an interaction of these agents at an unknown surface membrane level.

Low-sodium contractures indicating sarcolemmal na/ca-exchange in the frog heart

1. 1. In the frog heart, Ca 2+ enters the cell by the slow inward current ( I si ) and by an electrogenic, carrier-mediated, and passive Na-out/Ca-in-exchange. 2. 2. The latter reverses to Na-in/Ca-out-exchange during depolarization and thereby controls relaxation. 3. 3. The exchange ratio is 3 Na + for 1 Ca 2+. 4. 4. The Na/Ca-exchange is not inhibited by organic Ca-antagonists in frog myocardium, indicating that the initiation of the heart beat may mainly depend on I si. 5. 5. This is not necessarily in contradiction with the Na-Ca-antagonism, since there also exists an antagonism between Na + and Ca 2+ in the slow channel. 6. 6. However, the contractures caused by a decrease of Na 0 + are mediated by the Na/Ca-exchange.

Ca-channel blockers and the electrophysiology of synaptic transmission of the guinea-pig olfactory cortex

Slices of guinea-pig olfactory cortex have been used to compare the potency of various Ca-blockers on the electrophysiology of synaptic transmission. Listed in the order of potency, the divalent cations Cd 2+, Ni 2+, Mn 2+, Co 2+, La 3+ and Mg 2+ depressed synaptic transmission. The organic Ca-blockers, nifedipine or nimodipine or verapamil and diltiazem were ineffective up to 0.01 nmol/l. Verapamil, D600 or diltiazem (0.1–0.3 mmol/l) depressed both synaptic transmission and the sodium-mediated presynaptic action potential. These results reaffirm the idea that “organic Ca-antagonist’ do not block all Ca-channels in brain and the high Cd 2+ sensitivity suggests the Ca-channels in post- and presynaptic membranes have dissimilar pharmacological profiles.

Effects of the calcium channel blocker diltiazem on the excitability of mouse oocytes

AbstractEffects of the organic Ca antagonist diltiazem on Ca channels were studied in ovulated and unfertilized oocytes of the mouse by using intracellular recording techniques. The resting potential was not affected by diltiazem. The threshold level of the Ca action potential shifted slightly toward positive voltages with diltiazem concentration, but the shift was not statistically significant. The overshoot and maximum rate of rise of the Ca action potential were inhibited by the drug in a dose‐dependent manner, but higher amounts of diltiazem were necessary to cause similar blocking effects on Ca channels in mouse oocytes than in other differentiated cells. Increases of external concentration of Ca2+ antagonized the degree of diltiazem inhibition. However, the sequence of block of Ca2+, Sr2+, and Ba2+ currents was different for diltiazem vs Cd2+. It is suggested that diltiazem inhibition can not be explained by simple competitive scheme, ie, antagonism between diltiazem and permeant cations does not occur at the same binding site associated with the Ca channel in mouse oocytes.

Action potentials dependent on monovalent cations in developing mouse embryos

Action potentials were examined using intracellular recording techniques to study the ionic mechanisms of excitability in oocytes and embryos of the mouse from the 1-cell through to the 16-cell stages of development. At all stages examined, action potentials dependent on monovalent cations (Na + or Li +) were observed under Ca 2+-free conditions, and the maximum rate of rise (MRR) of the Na action potential was larger than that of the Li action potential at a given concentration of monovalent cations. Both the Na and Li action potentials were insensitive to tetrodotoxin, and they were blocked by inorganic (Co 2+, Cd 2+, Mn 2+, La 3+) and organic (diltiazem) Ca antagonists. These properties were exactly the same as those of the Ca channels present in the membranes of the mouse embryos. In addition, competition was observed between permeant monovalent and divalent cations: the overshoot and MRR of the Na or Li action potentials were reduced in the presence of Ca 2+. These results suggest that Na + or Li + go through the Ca channels when the external Ca 2+ concentration was very low, and that the Ca channels are more permeable to Na + than to Li +. Separate Na channels could not be detected or induced at any stages of development.

Effects of calcium antagonists on release of [3H]noradrenaline in rabbit aorta

The effects of several organic Ca antagonists and putative calmodulin antagonists on the release of [3H]noradrenaline from rabbit aorta were examined. A 65.4 mM K solution and electrical stimulation induced a Ca-dependent increase in the 3H-efflux from the adventitial layer (containing nerve terminals) but not from the media-intimal layer. Verapamil, D600, N-(6-aminohexyl)-5-chloro-l-naphthalenesulphonamide (W-7) and N2-dansyl-L-arginine-4-t-butylpiperidine amide (TI 233), in the concentration of 10−5 M, did not inhibit the increase in 3H-efflux. A high concentration (2 × 10−4 M) of verapamil induced a Ca-independent increase in the 3H-efflux from the adventitia but not from the media-intimal layer. D600, diltiazem, W-7 and TI233, in the concentration of 2 × 10−4 M, also increased the 3H-efflux; 65.4 mM K, added in the presence of 2 × 10−4 M verapamil or W-7, produced no further increase in 3H-efflux. The 3H-efflux evoked by electrical stimulation was only partially inhibited by 2 × 10−4 M verapamil. It is suggested that the evoked release of noradrenaline in rabbit aorta is relatively insensitive to organic Ca antagonists and calmodulin antagonists, and that high concentrations of these inhibitors themselves release noradrenaline by a mechanism independent of external Ca.

Voltage-clamp analysis of a calcium-mediated potassium conductance in cockroach (Periplaneta americana) central neurones.

The electrical properties of motoneurone cell bodies in the metathoracic ganglion of the cockroach, Periplaneta americana, have been studied by the voltage-clamp technique. Most experiments were carried out on a single identified cell (cell 28), the cell body of which, as for most other insect motoneurones, is electrically inexcitable. For comparison, some experiments were also carried out on dorsal unpaired median (d.u.m.) cells, the cell bodies of which are excitable. The two cell types differed only in that the d.u.m. cells developed a transient net inward current when depolarized towards zero membrane potential. This current was reduced but not abolished by Ca-free saline. Both cell types had an N-shaped current-voltage relation, as typically seen for molluscan and other neurones, but the location of the falling phase of the relation showed an unusually strong time dependence. The N-shape was abolished by prolonged exposure to Ca-free saline, suggesting it to be due to a K conductance that was activated by the entry of Ca ions through voltage-dependent channels. An outward current was also elicited by ionophoretic injection of Ca ions. The reversal potential of this current varied with the saline K concentration, in the manner expected if the current was carried by K ions. The Ca-mediated K current was blocked by La ions and by the organic Ca antagonist D-600. A series of double-pulse experiments on voltage-clamped cell bodies of cell 28 suggested that very short periods of Ca entry were sufficient to activate the K conductance fully. These experiments also suggested that the K conductance activation was due to intracellular Ca accumulation rather than to its being directly linked to the inward Ca current. The activation of the K conductance by intracellular Ca ions was made more effective by cell membrane depolarization. The Ca-mediated conductance did not inactivate significantly under conditions in which substantial inactivation has been observed in other neurones. The physiological significance of the electrical properties of cell 28 is discussed.

Enhancement of phenoxybenzamine-induced irreversible blockade by organic Ca Antagonists

Organic Ca antagonists such as D600, diltiazem and nicardipine enhanced the irreversible blockade by phenoxybenzamine (POB) of the histamine response in guinea pig taenia caecum. The isotonic contractile response of the muscle was tested after incubation with POB (5 × 10 −6 M, 10 min) with or without an organic Ca antagonist. The blockade by POB was greater in muscle treated with an organic Ca antagonist and the effect lasted for more than 7 h.

Enhancement of phenoxybenzamine-induced irreversible blockade by organic Ca antagonists of the histamine response in guinea pig taenia caecum

Structurally different organic Ca antagonists such as D600, diltiazem and nicardipine enhanced the irreversible blockade by phenoxybenzamine (POB) of the histamine response in guinea pig taenia caecum. The isotonic contraction of the muscle to histamine was tested after incubation with POB (5xlO-6M, 10 min) with or without an organic Ca antagonist. The blockade by POB was greater in muscles treated with an organic Ca antagonist at concentrations that singly depressecene contraction to histamine to less than 50% of the control (D600, 5×10-7 M; di 1 tiazem, 1×10−5M;and nicardipine,3xlO-8M), and the effect lasted for more than 8 hrs;but the blockade was unchanged at lower concentrations of Ca antagonists. Mn ion had no such enhancing effect. The response to 40 mM K ion was not affected. The organic Ca antagonists alone at these concentrations did not affect the contraction to histamine after washing for 3 hrs. D557(desmethoxyverapamil) and 1-cis-isomer of diltiazem were less effective in enhancing than D600 & diltiazem, respectively. Antihistamines failed to protect the histamine receptor from the enhanced effect of POB. Only high doses of diphenhydramine which have high membrane stabilizing action could protect only the response to the maximal dose of histamine. Tritiated-POB binding to the intact muscle strips was significantly increased by nicardipine (3×10−°M). As the enhancing effect is parallel with the Ca antagonistic actions of the drugs used, it is concluded that the effect originated from the blockade of the Ca channel. We speculate as follows:The organic Ca antagonists stabilize the Ca channel in resting conformation, which makes some components of the receptor system more susceptible to POB. Eur.J.Pharmacol. 87 319 (1983)

Differences in actions of organic and inorganic Ca-antagonists on the Ca current of Helix neurone

Differences in actions of organic and inorganic Ca-antagonists on the Ca current were investigated in snail (Helix aspersa) neurones perfused intracellularly and voltage-clamped by a suction pipette technique. Organic Ca-antagonists used were diltiazem, verapamil, nifedipine, nicardipine and KB-944; and inorganic Ca-antagonists used were cobalt, manganese and cadmiun. Results are summarized as follows: 1) The organic Ca-antagonists showed a time-dependent inhibition on the current amplitude during a depolarizing pulse lasting for 1 to 2 sec and a use-dependent depressant effect on the peak amplitude of the current. Double-pulse experiments showed that the organic Ca-antagonists also accelerated the current inactivation. Inorganic Ca-antagonists did not exhibit the accelerating actions on the current inactivation. 2) Diltiazem and verapamil showed almost equivalent inhibitory effects on the divalent cation currents carried by Ba, Ca and Mn ions which were permeant through the Ca channel. On the other hand, an inorganic Ca-antagonist such as cobalt ions greatly suppressed the peak amplitude of the Ba current, but slightly suppressed that of the Mn current. These results suggested that the site of action of organic Ca-antagonists partly differed from those of inorganic Ca-antagonists.

Effects of calcium antagonists on release of 3H-norepinephrine in rabbit aorta

Effects of several organic Ca antagonists and putative calmodulin antagonists on the release of 3H-norepinephrine from rabbit aorta were examined. 65.4mM K solution increased the 3H-efflux from the aorta previously loaded with 3H-norepinephrine, and the increment was inhibited by removing Ca in the medium. Verapamil, D600, N-(6-aminohexyl)-5-chioro-l-naphthalenesulphonamide (W-7), and N2-dansyl-L-arginine-4-t-butyIpiperidine amide (Tl 233), in the concentration of 10-5M, did not inhibit the 65.4mM K-induced increase in the 3H-efflux. Verapamil rather potentiated the efflux. High concentration (2 X 10-4M) of verapamil and W-7 increased the 3H-efflux. The increment was not dependent on external Ca. 65.4mM K, added in the presence of 2 X 10-4M verapamil or W-7, did not produce further increase in the 3H-efflux. D600, diltiazem, prenylamine and Tl 233, in the concentration of 2 X 10-4M, also increased the 3H-efflux. From these results, it is suggested that adrenergic nerve terminals are relatively insensitive to the inhibitory effects of organic Ca antagonists and calmodulin antagonists and that high concentration of these inhibitors themselves release norepinephrine by a mechanism independent of external Ca.

Comparison of the inhibitory effects of organic Ca antagonist and nitric oxide-compound on smooth muscles.

Effects of an organic Ca antagonist (verapamil), a nitric oxide-compound (sodium nitroprusside) and a metabolic inhibitor (antimycin A) on smooth muscle contractions were compared. 1) Verapamil dose-dependently suppressed the K-induced contraction, but had little effect on the norepinephrine-induced contraction in rabbit aorta. In guinea-pig taenia coli, verapamil inhibited both the histamine- and the K-induced contractions dose-dependently. Effect of verapamil was antagonized by raising the concentration of external Ca. Verapamil inhibited the K-induced increase in 45Ca uptake at the concentration needed to inhibit the K-induced contraction. 2) Sodium nitroprusside inhibited the norepinephrine-induced contraction in aorta, but had no effect on the K-induced contraction in aorta and the histamine- and the K-induced contractions in taenia coli. The norepinephrine-induced contraction in rat aorta was also inhibited by sodium nitroprusside, and this inhibitory effect was antagonized by methylene blue-pretreatment. 3) Antimycin A did not suppress the K-induced contraction in aorta, although this inhibitor strongly inhibited the K-induced contraction in the absence of added glucose. In guinea-pig taenia coli, the K-induced contraction was inhibited by antimycin A, and the inhibitory effect was antagonized by excess glucose. Antimycin A inhibited the K-induced increase in 45Ca uptake at the concentration lower than that needed to inhibit the K-induced contraction in aorta. Antimycin A also decreased both the ATP content and the oxygen consumption of the muscles. In conclusion, we found that verapamil, sodium nitroprusside and antimycin A exert differential effects on smooth muscle contractions.

Effects of organic Ca-antagonists on the Na current in rabbit dipersed ventricular cells.

Effects of organic Ca-antagonists on the Ma current were investigated in rabbit dispersed ventricular cells by the suction pipette method. Drugs used were diltiazem (DL), verapamil (VR), nifedipine (NF) and nicardipine (NC). The Na current remained unaffected in the presence of MF and NC at concentrations below 0.1 μM, but VR and DL at a concentration of 0.1 μM started to reduce the maximal peak amplitude (MPA) of the Na current. Higher concentrations of the Ca-antagonists induced a decrease in the MPA of the Ma current, and the effects mere dnse-dependent. The current-voltage relationships showed that the current amplitude was reduced at all membrane potentials without any change in the potential at which the maximal inward current was induced. The effects of DL (1 μM), VR (1 μM), NF (1G μM) and NC (10 μM) on the steady state inactivation and the reactivation process of the Ma current were investigated. The Ca-antagonists shifted the midpoint of the steady-state inactivation curve in the hyperpoiarizing direction by 4 to 7 mV and made the curve slightly steeper. The half reactivation time in the control was 24 to 27 msec, but the time for half-reactivation in the presence of these agents was significantly prolonged to about 40 msec or more. The results allowed interpretation of mechanisms by which DL and VR exert antiarrhythmic actions at clinical doses.

Effects of calmodulin antagonists on tension and cellular calcium content in depolarized vascular and intestinal smooth muscles.

1 Several putative calmodulin antagonists have been examined for their inhibitory action on muscle tension and cellular Ca content in the K-depolarized vascular and intestinal smooth muscles. 2 The 65.4 mM K-induced sustained contraction in the media-intimal layer of rabbit aorta and the 45.4 mM K-induced sustained contraction in guinea-pig taenia coli were inhibited by the calmodulin antagonists, prenylamine, chlorpromazine, N2-dansyl-L-arginine-4-t-butylpiperadine amide (No. 233), and N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide (W-7), and also by the organic Ca antagonists, verapamil and diltiazem. 3 The cellular Ca content in rabbit aorta and guinea-pig taenia coli as measured by a modified lanthanum technique increased in the high-K solutions. The increments were inhibited by these antagonists at concentrations similar to those required to inhibit the K-induced contractions. However, W-7 did not change (in aorta) or only slightly decreased (in taenia coli) the K-induced increase in the cellular Ca content. 4 A high concentration (2 X 10(-4)M) of W-7 increased the resting cellular Ca content without increasing the muscle tension in aorta. The increment was inhibited by verapamil, sodium nitroprusside or hypoxia (N2 aeration). 5 It is suggested that the inhibitory effects of prenylamine, chlorpromazine and No. 233 may be attributed mainly to the Ca antagonistic effect whereas W-7 may inhibit the process beyond the transmembrane Ca influx.

Calcium Action Potentials in the Skeletal Muscle Fibres of the Stick Insect Carausius Morosus

ABSTRACT The ionic requirements for the generation of action potentials in the ventral longitudinal muscle fibres of the stick insect, Carausius morosus, were investigated. Ca-free Ringer rapidly and reversibly abolished the action potential. In the presence of tetraethylammonium (TEA) ions (to suppress outward currents) the overshoot of the action potential changed 26 mV for a 10-fold change in [Ca]o. The maximum rate of rise of the action potential (measured in TEA Ringer) showed saturation at high [Ca]o. Cobaltous ions (20 mm) and the organic Ca antagonist D 600 (5 × 10−4 g/ml) reversibly inhibited the action potential ; the inhibitory effect of 1 mm-La3+ was irreversible. Barium and strontium, but not magnesium, were able to substitute for calcium as charge carriers. These results suggest that an inward movement of Ca2+ underlies the action potential of Carausius fibres.

Synaptic modulation of calcium-dependent potassium conductance in myenteric neurones in the guinea-pig.

1. Ganglion cells of the myentric plexus of the guinea-pig small intestine were studied with intracellular recording methods. 2. Electrical stimulation of the interganglionic connectives elicited slow synaptic excitation (slow e.p.s.p.) that was associated with an increase in the input resistance of the cell. The slow e.p.s.p.s continued for several seconds after termination of stimulation, and they occurred only in neurones in which prolonged hyperpolarizing after-potentials followed an action potential. 3. Superfusion of the neurones with solutions containing either 1-5 mM-Mn2+ or 16 mM-Mg2+ and 1 mM-Ca2+ mimicked the slow e.p.s.p. The common characteristics of Mn2+, Mg2+ and the slow e.p.s.p. were: (a) depolarization of the membrane potential, (b) increased input resistance of the cell, (c) augmented excitability, (d) blockade of post-pike hyperpolarizing potentials and (e) reversal potential between -70 and -75 mV. 4. Analyses based on the 'constant field equation' indicated that the permeability ratios of K+ to other permeant ionic species were reduced when Ca2+ influx was blocked by Mn2+ or Mg2+. 5. The organic Ca antagonist D-600 did not affect the neurones. 6. The results suggest that slow synaptic modulation of excitability within the myenteric plexus involves a reduction of both resting GK and post-spike GK which is secondary to suppression of Ca2+ influx by the neurotransmitter for the slow e.p.s.p.

An ionophoretic model for the study of calcium-calcium exchange

A Pressman cell was used to study the Br-X537A-mediated translocation of 45Ca from one aqueous phase into another across an immiscible organic phase. In this system, an increase in the concentration of 40Ca in one chamber (mimicking the extracellular compartment) resulted in an increased efflux of 45Ca added in trace amount to the other chamber (simulating the cytosolic compartment). Such a Ca-Ca exchange process was also observed when the concentration of ionophore was increased, and could be blocked by the organic Ca-antagonist suloctidil. However, no Ca-Ca exchange occurred when the downhill rate of 40Ca translocation was increased by use of a pH gradient. This ionophoretic model also suggests that a low intracellular concentration of Ca tends to maintain Ca influx at a low rate, and that the asymetrical distribution of Ca facilitates rather than impeeds the ionophoretic extrusion of intracellular Ca against its chemical gradient.

Effects of manganese and other agents on the calcium uptake that follows depolarization of squid axons.

1. The Ca-sensitive photoprotein aequorin was injected into squid axons and the light response to stimulation or depolarizing voltage clamp pulses recorded.2. The effects of Mn(2+), Co(2+), Ni(2+), La(3+) and of the organic Ca antagonists D-600 and iproveratril on the early tetrodotoxin-sensitive and late tetrodotoxin-insensitive components of the light response were studied.3. The late tetrodotoxin-insensitive component can be blocked, reversibly, by concentrations of Mn, Co and Ni that reduce but do not block the tetrodotoxin-sensitive component. The late component can also be blocked by La(3+) and the organic Ca antagonists D-600 and iproveratril.4. Mn(2+), Co(2+), Ni(2+) and the drug D-600 all reduce the Na currents, but have little effect on either outward or inward K currents. Tetraethylammonium blocks the outward K current but has no appreciable effect on the tetrodotoxin-insensitive entry of Ca.5. Concentrations of Mn between 5 and 50 mM substantially reduce the light output during a train of action potentials; they also slightly reduce the rate of rise of the action potential.6. On pharmacological grounds it is concluded that the tetrodotoxin-insensitive component of Ca entry does not represent Ca ions passing through the K permeability channels. There must exist a potential-dependent late Ca channel that is distinct from the well known Na and K channels of the action potential. A possible function for this late Ca channel in the coupling of excitation to secretion is discussed.