A calmodulin-like ca receptor in the ca channel

NO donors stimulate noradrenaline release from rat hippocampus in a calmodulin-dependent manner in the presence of L-cysteine

Nitrogen oxides (NO) such as nitric oxide have been suggested to potentiate neurotransmitter release in a variety of neuronal cells. In this study, we showed that NO donors stimulate the release of noradrenaline (NA) from rat hippocampus both in vivo and in vitro. Co-addition of NO donors (sodium nitroprusside [SNP] or S-nitroso-N-acetylpenicillamine [SNAP]) and thiol compounds (dithiothreitol [DTT] or L-cysteine) stimulated [3H]NA release from prelabeled hippocampal slices. Microdialysis in freely moving rats was used to ascertain the role of NO in control of NA release from the hippocampus in vivo. Co-addition of SNAP and L-cysteine stimulated endogenous NA release within 30 min. The concentration of NA peaked between 30-60 min to almost 3 times basal level. Another thiol compound, glutathione, had no effect on [3H]NA release in the presence of SNP or SNAP. In the presence of SNAP, the effect of L-cysteine was much higher than that of the D-isomer, although SNAP did not show stereospecificity. The effect of SNAP/L-cysteine was rapid and the maximal increase in [3H]NA release was attained 0-1 min after application, which was similar in time course to the effect of KCI. Unlike the release by KCI, SNAP/L-cysteine-stimulated NA release was independent of extracellular CaCl2. However, pretreatment with the calmodulin antagonists W-7 or trifluoperazine significantly reduced the SNAP/L-cysteine-stimulated [3H]NA release. Formation of nitric oxide and activation of guanylate cyclase by nitric oxide were not responsible for SNAP/L-cysteine-stimulated NA release. These findings suggest that NO donors stimulate NA release from the hippocampus in the presence of thiol compounds such as L-cysteine in vivo and in vitro in a calmodulin-dependent, Ca(2+)-and cyclic GMP-independent manner. The physiological roles of thiol compounds such as L-cysteine or glutathione as intermediates of NO are discussed.

The effects of terbium on the accumulation of cisplatin in human ovarian cancer cells

In this investigation, we report a relationship between the terbium (Tb3+) binding protein and the accumulation of cisplatin in human ovarian cancer cells. The number of Tb3+ binding sites in cisplatin-resistant C13+ cells is significantly greater by 79% than those in cisplatin-sensitive 2008 cells. Exposure to Tb3+ also increased the cellular accumulation of cisplatin. The accumulation of cisplatin as a function of the Tb3+ concentration in the C13+ cells (0.85%/microM Tb3+) was significantly greater than the accumulation of cisplatin in 2008 cells with respect to Tb3+ (0.46%/microM Tb3+). The number of Tb3+ binding sites in revertant RH4 cells was similar to that in 2008 cells. The RH4 cells were less sensitive to the stimulatory effects of Tb3+ than the C13+ cells. Our results show that the Tb3+ binding protein correlates with cisplatin resistance, and the receptor binding of Tb3+ increases the accumulation of cisplatin in cisplatin-resistant cells.

A role for calcium/calmodulin kinase(s) in the regulation of GABA exocytosis

A possible role for protein kinases in the regulation of GABA exocytosis in nerve endings was investigated. The effect on the release of the radioactive neurotransmitter ([3H]GABA) from mouse brain synaptosomes of several protein kinase inhibitors was estimated after treatment with 37 mM K+ in the absence of external Na+, a condition under which [3H]GABA release is completely Ca2+ dependent. Among the inhibitors one group inhibit the kinases by the catalytic site (i.e. staurosporine and H7) and others (TFP, sphingosine and W7) act on the regulatory site of protein kinases. The compounds of the second group, which are reported to inhibit calmodulin dependent events and the increase in cytosolic Ca2+ (Cai) induced by high K+ depolarization, were the most efficient inhibitors of [3H]GABA release. The selective inhibitor of CaMPK II, KN-62, also markedly diminished [3]GABA release as well as the increase in Cai induced by high K+. The kinase inhibitors from the first group that are unable to diminish the increase in Cai induced by high K+ were also less efficient inhibitors of [3H]GABA release even at high concentrations. The present results indicate that at the doses tested all the drugs inhibit to some extent the release of the Ca2+ dependent fraction of [3H]GABA perhaps by inhibiting a CaMPK II mediated phosphorylation step triggered by depolarization and facilitated by the elevation of Cai. In addition, the second group of antagonists and KN-62 inhibit the elevation of Cai to high K+ thus exhibiting a higher efficiency on [3H]GABA release than the first group of antagonists.

Blockers of Ca2+ channels in the plasmalemma of perfused Characeae cells

Ionic currents in the plasmalemma of perfused Nitella syncarpa cells identified as currents through Ca2+ channels were registered for the first time. The effect of 1,4-dihydropyridine derivatives (nifedipine, nitrendipine, riodipine) and phenylalkylamines (verapamil, D600) as well as the agonist CGP-28392 on the Ca2+ channels in the plasmalemma of perfused cells of Nitellopsis obtusa and Nitella syncarpa have been studied. A blocking effect of 1,4-dihydropyridine derivatives and phenylalkylamines on the plasmalemma Ca2+ channels has been detected. Phenylalkylamines have been found to block both inward and outward Ca2+ currents. The activating effect of the agonist CGP-28392 on the Ca2+ channels of plasmalemma has been shown.

Sphingosine, W-7, and trifluoperazine inhibit the elevation in cytosolic calcium induced by high K+ depolarization in synaptosomes

A possible role for protein kinases in the regulation of free cytosolic Ca2+ levels in nerve endings was investigated by testing the effect of several kinase inhibitors on the increase in cytosolic Ca2+ (monitored with the Ca(2+)-sensitive dye fura-2) induced by depolarization with 15 or 30 mM K+. The ability of various drugs to inhibit the cytosolic Ca2+ response appeared to correlate with their reported mechanism of action in inhibiting protein kinases. W-7 and trifluoperazine, drugs reported to inhibit calmodulin-dependent events, were effective inhibitors of the increase in cytosolic Ca2+ induced by high K+ depolarization, as was sphingosine, a drug that inhibits protein kinase C by binding to the regulatory site, but which also inhibits calcium/calmodulin kinase. On the other hand, drugs that inhibit protein kinases by binding to the catalytic site, such as H-7 (1 mM), staurosporine (1 microM), and K252a (1 microM), were ineffective. Activation of protein kinase C, which is blocked by each of these drugs, does not appear to be essential to the maintenance of elevated cytosolic Ca2+ in depolarized synaptosomes. All of the drugs, including sphingosine, that functionally inhibit the depolarization-induced elevation in cytosolic Ca2+ have in common the ability to bind to calmodulin. Because the drugs that inhibit protein kinases by competing with ATP binding at the active catalytic site did not block the response in this system, we suggest that a calmodulin or a calmodulin-like binding site participates in the regulation of Ca2+ increases after depolarization.

Mechanisms of antagonistic action of internal Ca2+ on serotonin-induced potentiation of Ca2+ currents in Helix neurones

The influence of internal Ca2+ ions has been investigated during intracellular perfusion of isolated neurones from pedal ganglia of Helix pomatia in which serotonin (5-HT) induces a cyclic-adenosine-monophosphate-(cAMP)-dependent enhancement of high-threshold Ca2+ current (ICa). Internal free Ca2+ ([Ca2+]i) was varied between 0.01 and 10 microM by addition of Ca(2+)-EGTA [ethylenebis(oxonitrilo)tetraacetate] buffer. Elevation of [Ca2+]i depressed the 5-HT effect. The dose/effect curve for the Ca2+ blockade had a biphasic character and could be described by the sum of two Langmuir's isotherms for tetramolecular binding with dissociation constants Kd1 = 0.063 microM and Kd2 = 1 microM. Addition of calmodulin (CM) antagonists (50 microM trifluoperazine or 50 microM chlorpromazine), phosphodiesterase (PDE) antagonists [100 microM isobutylmethylxanthine (IBMX) or 5 mM theophylline] and protein phosphatase antagonists [2 microM okadaic acid (OA)] in the perfusion solution caused "anticalcium" action and modified the Ca2+ binding isotherm. Using the effect of OA and IBMX, two components of the total Ca2+ inhibition were separated and evaluated. In the presence of one of these blockers tetramolecular curves with Kd1 = 0.04 microM and Kd2 = 0.69 microM were obtained describing the activation of the retained unblocked enzyme--PDE or calcineurin (CN) correspondingly. The sum of these isotherms gave a biphasic curve similar to that in control. Leupeptin (100 microM), a blocker of Ca(2+)-dependent proteases did not influence the amplitude of 5-HT effect, indicating that channel proteolysis is not involved in the depression. Our findings show that the molecular mechanism of Ca(2+)-induced suppression of the cAMP-dependent upregulation of Ca2+ channels is due to involvement of two Ca(2+)-CM-dependent enzymes: PDE reducing the cAMP level, and CN causing channel dephosphorylation. No other processes are involved in the investigated phenomenon at a Ca2+ concentration of less than or equal to 10 microM.

Light and electron microscopic localization of alpha subunits of GTP-binding proteins, G(o) and Gi, in the cerebral cortex and hippocampus of rat brain

Antibodies that recognize alpha subunits of G(o), Gi2 and Gi3 were used to evaluate their association with synaptic junctions. G(o), but not Gi, was concentrated within perikaryal and dendritic cytoplasm of a small population of bipolar neurons. All three G-proteins were associated with the intracellular surface of dendritic, axonal and astrocytic plasma membranes and postsynaptic densities (PSDs). However, association with PSDs was more prevalent for the two Gi's than for G(o) while the association with terminals forming putatively excitatory synapses was more prevalent for G(o) and Gi3 than for Gi2. Thus, neuromodulators may modulate the release of excitatory transmitters via activation of presynaptic Gi3 and G(o) and also regulate the opening of Ca2+ and/or K+ channels via activation of Gi's and G(o) at PSDs.

Inhibition of membrane currents and rises of intracellular Ca2+ in PC12 cells by CGS 9343B, a calmodulin inhibitor

The calmodulin inhibitor 1,3-dihydro-1-[1-((4-methyl-4H,6H-pyrrolo[1,2-a]-[4,1]benzoxazepin - 4-yl)methyl)-4-piperidinyl]-2H-benzimidazol-2-one maleate (CGS 9343B) caused a reversible block of voltage-activated Ca2+, Na+, and K+ currents in differentiated rat pheochromocytoma (PC12) cells. The drug also inhibited nicotinic acetylcholine receptor (nAChR) channel currents but not inward currents evoked by extracellular ATP. Depolarization-induced intracellular Ca2+ transients were almost completely inhibited in growth cones and cell bodies by CGS 9343B. Our results suggest actions of CGS 9343B on ion fluxes unrelated to calmodulin inhibition.

Interaction of the dihydropyridine calcium antagonist, CD-349, with calmodulin

The characteristics of the binding of the 1,4-dihydropyridine Ca2+ antagonist, 2-nitratopropyl 3-nitratopropyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine 3,5-dicarboxylate (CD-349), to calmodulin (CaM) and the effect of CD-349 on the Ca2+/CaM-dependent enzyme, cyclic GMP (cGMP) phosphodiesterase (PDE), were investigated. CD-349 showed a Ca2(+)-dependent binding to CaM, in equilibrium column binding studies. CD-349 inhibited the [3H]CD-349 binding to CaM, at a concentration producing a 50% inhibition (IC50) of 2.4 microM, whereas the CaM antagonist, trifluoperazine hydrochloride (TFP), stimulated the [3H]CD-349 binding to CaM. Scatchard plot analysis of the binding of CD-349 to CaM revealed that the apparent dissociation constant (Kapp) of CD-349 was 2.1 microM and the maximal number of binding sites (Bmax) of CD-349 was 1.0 nmol/nmol CaM. In the presence of TFP, the Kapp and Bmax values of CD-349 binding to CaM were changed to 1.1 microM and 1.5 nmol/nmol CaM respectively. Although the CaM antagonists, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) and TFP, decreased and increased, respectively, the fluorescence intensity caused by 2-p-toluidinylnaphthalene-6-sulfonic acid (TNS)-CaM binding, CD-349 only slightly decreased the fluorescence of TNS bound CaM. CD-349 inhibited both basal and Ca2+/CaM-activated cGMP PDE activity. However, CaM did not competitively antagonize the CD-349-induced inhibition of the Ca2+/CaM-activated PDE activity. In addition, the kinetic study showed that CD-349 inhibited both basal and Ca2+/CaM-activated cGMP PDE activity, competitively with cGMP, with almost the same inhibition constant (Ki). These results suggest that CD-349 binds to CaM, with Ca2+ dependency, at sites differing from those which bind to the CaM antagonist. The inhibitory activity of CD-349 on Ca2+/CaM-dependent PDE does not seem to be due to a CaM antagonistic action.

Calmodulin defects cause the loss of Ca2(+)-dependent K+ currents in two pantophobiac mutants of Paramecium tetraurelia

Two behavioral mutants of Paramecium tetraurelia, pantophobiacs A1 and A2, have single amino acid defects in the structure of calmodulin. The mutants exhibit several major ion current defects under voltage clamp: (i) the Ca2(+)-dependent K+ current activated upon depolarization of Paramecium is greatly reduced or missing in both mutants, (ii) both mutants lack a Ca2(+)-dependent K+ current activated upon hyperpolarization, and (iii) the Ca2(+)-dependent Na+ current is significantly smaller in pantophobiac A1 compared with the wild type, whereas this current is slightly increased in pantophobiac A2. Other, minor defects include a reduction in peak amplitude of the depolarization-activated Ca2+ current in pantophobiac A2, increased rates of voltage-dependent inactivation of this Ca2+ current in both pantophobiac A1 and pantophobiac A2, and an increase in the time required for the hyperpolarization-activated Ca2+ current to recover from inactivation in the pantophobiacs. The diversity of the pantophobiac mutations' effects on ion current function may indicate specific associations of calmodulin with a variety of Ca2(+)-related ion channel species in Paramecium.

Calmodulin discriminates between the two enantiomers of the receptor-operated calcium channel blocker SK&F 96365: a study using 1H-NMR and chiral HPLC

1H nuclear magnetic resonance at 360 MHz shows that SK&F 96365 (1-(beta-[3-(p-methoxyphenyl)-propyloxy]-p-methoxyphenethyl)-1H- imidazole hydrochloride), an antagonist of mammalian receptor-operated calcium channels, interacts with the calcium-binding regulatory protein calmodulin (CaM). This may be inferred by a number of chemical shift changes in the spectrum of the calcium-saturated protein induced by addition of the compound. Moreover, two well-resolved singlets corresponding to the 2-proton of the SK&F 96365 imidazolium moiety are observed in the spectrum over a wide range of protein:compound ratios. Separation of rac SK&F 96365 into its two enantiomers by high-performance liquid chromatography on a cellulose tris (4-methylbenzoate) column enabled us to show that the doubling of this NMR signal in the presence of CaM is due to a propensity of the protein to distinguish between the two optical isomers of the compound.

CGS 9343B and W7 (calmodulin antagonists) inhibit KCl-induced increase in cytosolic free calcium and insulin secretion of RINm5F cells

CGS 9343B:1,3-Dihydro-1-[1-[4-methyl-4H,6H-pyrrolo[1,2-a]-[4,1] benzoxazepin-4-yl)methyl)-4-piperidinyl]-2H-benzimidazol-2-o ne maleate and W7:N-6(aminohexyl)-5-chloro-1-naphthalenesulfonamide) are calmodulin antagonists with different specificities. The effects of CGS 9343B and W7 on cytosolic free calcium concentration ([ Ca2+]i) and insulin release were investigated in rat insulinoma cells (RINm5F). As measured with the Quin-2 technique, preincubation with CGS 9343B (0.3-10 microM) and W7 (5-50 microM) concentration dependently decreased KCl (25 mM)-mediated accumulation of cytosolic calcium. Both, CGS 9343B (10 microM) and W7 (50-100 microM) almost abolished the alanine- and KCl-induced increase in [Ca2+]i and significantly inhibited KCl (25 mM)- and alanine (10 mM)-mediated insulin release. W5 (100 microM), the chlorine-deficient analogue of W7 with decreased affinity for calmodulin, did not inhibit the KCl-induced increase in [Ca2+]i and enhanced basal and KCl-mediated insulin release by 56% and 189%, respectively. Our data suggest that CGS 9343B and W7 inhibit the depolarization-induced calcium uptake and subsequent increase in [Ca2+]i.

Modulation of calcium current by calmodulin antagonists

The short-term effects of bath applied calmodulin antagonists--chlorpromazine, trifluoperazine and calmidazolium (R24571)--on potential-dependent calcium channels in the membrane of intracellularly perfused snail neurons were studied in voltage clamp conditions. All the drugs affected the calcium inward current peak value, the effects being reversible and dependent on the concentration used. Submicromolar concentrations (0.1-1 microM) increased the current amplitude (the maximal effect was on the average 20% at 0.5 microM), whereas higher concentrations inhibited the current. Analysis of the dose-effect curve for the blockade suggests positive cooperativity in the interaction of the drugs with the channel; experimental data on chlorpromazine action (10-100 microM) are well approximated by a binding curve for two molecules with the effective Kd = 70 microM. The efficiency of the blockade depended neither on the current-carrying cations (calcium or barium) nor on the intracellular introduction of 10 mM EGTA. The presence of calmodulin antagonists influenced the blockade of the calcium current by inorganic blockers: 50 microM chlorpromazine decreased the Kd value from 90 to 50 microM for the current blockade by Cd ions. It is suggested that calmodulin antagonists interact with two sites in the calcium channel, with high and low binding affinity (responsible for enhancement and inhibition of the current, respectively). The interaction induces changes in binding of penetrating cations in the channel, thereby producing modulation of the calcium current amplitude.

Bepridil, myocardial accumulation kinetics and dynamic effects in the isolated rabbit heart

Both myocardial uptake and disposition of bepridil in the isolated rabbit heart showed two-compartment characteristics which possibly reflects the existence of superficial and deep binding sites. Terminal accumulation and disposition half-lives were 218 and 196 min., respectively. The half-times of the initial distributory processes were about 33 min. At a drug concentration in the perfusion liquid of 0.54 micrograms ml-1 (1.27 microM) the average concentration of bepridil in the myocardium at steady state was about 489 micrograms g-1 (1161 microM) with 43% referable to the deepest, presumably intracellular compartment. Increasing bepridil concentrations from 3 to 2333 ng ml-1 (7-5542 nM) in the perfusion liquid caused a terminal decrease in coronary flowrate to 58% of the mean control flowrate. Amplitude and velocity of myocardial contraction both decreased in a biphasic way to about 28.6 and 13.6%, respectively. Apparent dynamic steady states developed within about 20 min. Inhibitory Em-values related to the first phase were 39.8 and 53.2%, and to the second phase 97.7 and 98.5%, respectively. Heart beating frequency also decreased biphasically to 53.9% and showed inhibitory Em-values of 17.2 and 47.5% related to the two phases. Myocardial oxygen consumption decreased to 55.6%. The electrocardiographic PQ- and QRS-intervals increased to 147 and 133%, respectively. The frequency-corrected QT-interval also increased significantly from 100 to 123%. Our findings demonstrate a slow and very pronounced accumulation of bepridil in the rabbit heart. Biphasic and very marked negative inotropic and chronotropic effects and a less than proportional decrease in oxygen consumption developed much faster.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel blocker inhibition of the calmodulin-dependent effects of thyroid hormone and milrinone on rabbit myocardial membrane Ca2+-ATPase activity

The Ca2+-ATPase activity of rabbit myocardial membranes is stimulated in vitro by L-thyroxine and by milrinone, a bipyridine. These effects are concentration dependent and calmodulin requiring. The calcium channel blockers nifedipine and verapamil have been reported to have anti-calmodulin effects in other assay systems. In this study we have examined the effects of nifedipine and verapamil on rabbit myocardial membrane Ca2+-ATPase activity, in the absence (basal activity) and presence of exogenous L-thyroxine (T4), 10(-10) M, and milrinone, 10(-7) M. Basal enzyme activity was inhibited by a minimum of 10(-6) M nifedipine (IC50 of 3.4 X 10(-5) M) and 10(-5) M verapamil (IC50 of 1.5 X 10(-4) M). Both calcium antagonists inhibited enzyme stimulation by T4 and milrinone, with half-maximal inhibition of T4 and milrinone effects, respectively, at 2.9 X 10(-5) M and 9.0 X 10(-6) M nifedipine and 3.0 X 10(-5) M and 5.2 X 10(-5) M verapamil. The addition of exogenous purified calmodulin, 40 ng/micrograms membrane protein, in the presence of 10(-5) M nifedipine or verapamil restored T4-stimulated enzyme activity. Nifedipine and verapamil, each at a concentration of 10(-6) M, significantly inhibited binding of radioiodinated calmodulin to rabbit heart membranes in vitro. These studies provide evidence that nifedipine and verapamil have an anti-calmodulin effect in this myocardial enzyme system. Through interaction with calmodulin, the channel blockers inhibit thyroid hormone and milrinone stimulation of myocardial membrane Ca2+-ATPase.

Heterogeneity of conductance states in calcium channels of skeletal muscle

The single channel conductance of the dihydropyridine (DHP)-sensitive calcium channel from rabbit skeletal muscle transverse tubules was analyzed in detail using the planar bilayer recording technique. With 0.1 M BaCl2 on both sides of the channel (symmetrical solutions), the most frequent conductance is 12 pS, which is independent of holding potential in the range of -80 to +80 mV. This conductance accounts for approximately 80% of all openings analyzed close to 0 mV. Two additional channels of conductance 9 and 3 pS are also present at all positive potentials, but their relative occurrence close to 0 mV is low. All channels depend on the presence of agonist Bay K 8644 and are inhibited by the antagonist nitrendipine. The relative occurrence of 9 and 3 pS can be increased, and that of 12 pS decreased, by several interventions such as external addition of cholesterol, lectin (wheat germ agglutinin), or calmodulin inhibitor R24571 (calmidazolium). The 9- and 3-pS channels are also conspicuous at positive potentials larger than +40 mV. We suggest that 9- and 3-pS channels are two elementary conductances of the same DHP-sensitive Ca channel. Under most circumstances, these two conductances are gated in a coupled way to generate a channel with a unitary conductance of 12 pS. Interventions tested, including large depolarizations, probably decompose or uncouple the 12-pS channel into 9 and 3 pS.

Verapamil, diltiazem and nifedipine interactions with calmodulin stimulated (Ca2+ + Mg2+)-ATPase

The functional interactions of the three prototype Ca2+ antagonists, verapamil, diltiazem and nifedipine, were examined in relation to the calmodulin regulated plasma membrane Ca2+ pump ATPase. For this we used low ionic strength derived, calmodulin depleted, human red cell ghost membranes. Exogenously added calmodulin activated basal (Ca2+ + Mg2+)-ATPase in a concentration-dependent manner. Half-maximal activation by 6 nM calmodulin was antagonized by 10(-3) M verapamil and 10(-3) M diltiazem 25.1 and 12.1% respectively. The inhibition appeared to be specific for calmodulin activation since basal activity was not affected by these agents. Nifedipine had no effects on basal or calmodulin stimulated (Ca2+ + Mg2+)-ATPase activity. Unlike dihydropyridine modulation of verapamil and diltiazem binding at high affinity channel sites, nifedipine in this system did not alter the inhibitory responses of verapamil and diltiazem. The calmodulin directed antagonism of the two drugs was shown to be strictly additive over a full range of calmodulin concentrations and appeared to change predominantly the Vmax and, to a lesser degree, the affinity of calmodulin for the (Ca2+ + Mg2+)-ATPase. It is concluded that this model system provides evidence for additional functional discrepancies among the various classes of Ca2+ antagonists.

Pharmacodynamic properties of felodipine

This overview presents the pharmacodynamic properties of felodipine as studied in animal experiments with emphasis on results from our laboratory. Felodipine is 100-fold more potent in causing inhibition of spontaneously active vascular smooth muscle than myocardium in vitro. This vascular selectivity is significantly greater than that of nifedipine (potency ratio 15). Verapamil, D-600, La3+ and reduction of [Ca2+]o lack selectivity. The cellular mechanisms underlying this variable selectivity are not clear at present. In conscious spontaneously hypertensive rats (SHR), the plasma concentration required for 20% reduction of mean arterial pressure is approximately 10 nmol. Mean arterial pressure is lowered dose-dependently as a result of reduced peripheral vascular resistance accompanied by increased cardiac output due to transient tachycardia and increased stroke volume. There was rapid resetting of the baroreflex set point but unaltered sensitivity after felodipine and hydralazine in SHR. Therefore, the reflexogenic increase in heart rate and plasma renin activity subsided within 3 to 5 hours of continuous felodipine administration in SHR. In addition, there was a uniform dilatation of the peripheral vascular beds after felodipine administration. During long term treatment of SHR with felodipine, progression of left ventricular and vascular wall hypertrophy was prevented. Within the 'therapeutic dose range', the only primary effect observed in addition to arterial vasodilation is diuresis/natriuresis caused by a renal tubular action.

Phenytoin: mechanisms of its anticonvulsant action

Phenytoin is a major anticonvulsant drug that is very effective in controlling a wide variety of seizure disorders while impairing neurological function little, if at all. Early work suggested the hypothesis that the drug's effects were due to a selective block of high-frequency neuronal activity. This theory is reevaluated in the light of accumulated observations on the effects of phenytoin in many neuronal and synaptic preparations. Most of these observations can be explained by a use- and frequency-dependent suppression of the sodium action potential by phenytoin, with a consequent filtering out of sustained high-frequency neuronal discharges and synaptic activity. The molecular mechanism for this is a voltage-dependent blockade of membrane sodium channels responsible for the action potential. Through this action, phenytoin obstructs the positive feedback that underlies the development of maximal seizure activity, while normal brain activity, proceeding at lower neuronal firing rates, is spared its depressant action. Other mechanisms of action that may contribute to the drug's efficacy and selectivity are also discussed.

Terbium fluorescence studies of cisplatin binding to GH3/B6 pituitary tumor cells

Terbium (Tb3+) fluorescence was used to investigate the interaction of cisplatin with GH3/B6 pituitary tumor cells. The binding of cisplatin to GH3/B6 cells quenched the fluorescence intensity of bound Tb3+. The IC50 for cisplatin inhibition of Tb3+-GH3/B6 fluorescence was determined to be 190 microM. Cisplatin was found to non-competitively inhibit the cellular binding of Tb3+, causing a dramatic decrease in the maximum number of high-affinity Tb3+ binding sites (by 33%), without markedly affecting their binding affinity. The half-life for the cellular binding of cisplatin was calculated to be 2.7 min. It was suggested that the plasma membrane of GH3/B6 cells contain a specific protein receptor for binding cisplatin.

Slow calcium channel blockers and calmodulin. Effect of felodipine, nifedipine, prenylamine and bepridil on cardiac sarcolemmal calcium pumping ATPase

The effect of four slow Ca2+ channel blockers (felodipine, nifedipine, prenylamine and bepridil) that possess the ability to bind to calmodulin (CaM) section and to inhibit myosin light chain kinase (MLCK) on CaM-regulated Ca2+ pumping ATPase of cardiac sarcolemma (SL) and brain cyclic AMP phosphodiesterase (PDE) was studied. The ability of these drugs to inhibit Ca2+ pumping ATPase correlated with their inhibitory effect on CaM-activated Ca2+-dependent PDE. Nifedipine was unable to inhibit markedly both enzymes. Prenylamine also was a weak inhibitor, which was unexpected because of its CaM binding potency. Felodipine (10-50 microM) and bepridil (50 microM) markedly reduced activities of SL Ca2+ pumping ATPase and PDE. Striking differences were, however, demonstrated when Ca2+ and CaM concentrations, respectively, were increased. Previously it was reported that inhibition of the SL Ca2+ pumping ATPase by the CaM antagonist calmidazolium could be overcome by increasing Ca2+ concentrations (J. M. J. Lamers and J. T. Stinis, Cell Calcium 4, 281-294, 1983). Felodipine (10-50 microM) in the present study, appeared to be equipotent with calmidazolium in reducing Ca2+ pumping ATPase, but increasing Ca2+ up to 12.2 microM could not counteract this effect. Felodipine (2-10 microM) also inhibited brain PDE noncompetitively with respect to CaM contrary to the competitive effectors calmidazolium and bepridil. On the other hand, bepridil (10-20 microM) decreased or increased Ca2+ pumping ATPase activity depending on the Ca2+ concentration (0.29 and 12.2 microM, respectively) used. These findings suggest at least two types of CaM antagonists, which can be discriminated on basis of their inhibition patterns of PDE and heart SL Ca2+ pumping ATPase.

A calcium requirement for electric field-induced cell shape changes and preferential orientation

C3H/10T1/2 mouse embryo fibroblasts stimulated by a steady electric field (10 V/cm) for 30 min exhibited lamellar retraction on the sides facing the electrodes. Some cells elongated and preferentially oriented with their long axis perpendicular to the field direction. Depletion of external calcium or blockage of calcium influx with lanthanum or the calcium channel blocker D-600 resulted in a reduction of the field-induced response. When external calcium was elevated stepwise from 0 to 10 mM, the field-induced response increased correspondingly. Electric stimulation in the presence of the calcium ionophore A23187 resulted in an increase of spindle-shaped cells with no preferential orientation. This response was blocked by calcium depletion and lanthanum, but not by D-600. The anticalmodulin drug W-13 inhibited the field-induced responses observed in normal buffer as well as in the presence of A23187. Some cell death resulted from prolonged electric field exposure, and the mortality was reduced by calcium depletion, lanthanum or D-600, but was not affected by W-13. We postulate that local calcium influx through channels opened by the electric field produces areas of high intracellular calcium which stimulate the cytoskeletal network to induce lamellar retraction. Prolonged field-induced calcium influx may eventually overcome the cell's mitochondrial calcium-buffer system, leading to necrotic calcification.

Neuroleptic drugs attenuate calcium influx and tension development in rabbit thoracic aorta: effects of pimozide, penfluridol, chlorpromazine, and haloperidol

This study was designed to determine whether neuroleptic drugs have calcium channel blocking activity in isolated rings of rabbit thoracic aorta. The results confirm previous findings that pimozide and penfluridol are calcium channel blockers. However, the data do not support the conclusion that these agents are selective for the voltage-sensitive calcium channel. The results also show that both haloperidol and chlorpromazine (which represent different classes of neuroleptic drugs) are also calcium channel blockers in vascular smooth muscle.

Is renin secretion governed by the calcium permeability of the juxtaglomerular cell membrane?

Using cell cultures rich in renal juxtaglomerular cells we found that a change of the intracellular c-AMP concentration is not a prerequisite for an alteration of the renin secretion rate. Modulators of renin secretion including activators of the adenylate cyclase, however, altered the calcium permeability of the cellular plasma membrane in a way that stimulators of renin secretion lowered the calcium permeability and vice versa. Our results suggest that renin secretion is controlled by the intracellular calcium concentration and not by c-AMP. We postulate that modulators of renin secretion act by changing the calcium permeability of the cell membrane.