The cellular basis of cardiotonic steroid action

Antinociceptive Synergistic Interaction Between Clonidine and Ouabain on Thermal Nociceptive Tests in the Rat

The antinociceptive effect produced by spinal injection of clonidine (an alpha(2)-adrenergic agonist) is mediated by a cholinergic mechanism. We aimed in the current study to evaluate the antinociceptive interaction between intrathecally administered ouabain, an inhibitor of Na(+), K(+)-ATPase, and clonidine. We used rats chronically implanted with lumbar intrathecal catheters to examine the ability of intrathecal clonidine and ouabain and the mixtures of clonidine-ouabain to alter tail-flick latency. To characterize the interaction, isobolographic analysis was performed. Intrathecal clonidine (0.5-10 microg) and ouabain (0.1-5 microg) produced significant dose- and time-dependent antinociception in the tail-flick tests. The median effective dose (ED(50)) values for intrathecally administered ouabain and clonidine were 2.3 microg and 4.7 microg, respectively. The experimental point for the ouabain-clonidine combination decreased significantly (P < .05) below the lines of additivity. Isobolographic analysis exhibited a synergistic interaction after the coadministration of ouabain and clonidine. No motor impairment was observed in the animals after intrathecal administration of the combination of ouabain and clonidine or clonidine alone. Intrathecal pretreatment with atropine but not yohimbine blocked the antinociceptive effect of ouabain and attenuated its interaction with spinal clonidine. These results suggest that the synergistic interaction of ouabain and clonidine were probably mediated, at least in part, via an enhancement of cholinergic transmission in the spinal nociceptive processing system.

PERSPECTIVE

Although intrathecal clonidine produces pronounced analgesia, antinociceptive doses of intrathecal clonidine produce several side effects, including hypotension, bradycardia, and sedation. This article presents antinociceptive synergistic interaction between clonidine and ouabain on thermal nociceptive tests in the rat.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions

The Na(+)/Ca(2+) exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca(2+) from the cell. Two basic properties characterize them. 1) Their activity is not predicted by thermodynamic parameters of classical electrogenic countertransporters (dependence on ionic gradients and membrane potential), but is markedly regulated by transported (Na(+) and Ca(2+)) and nontransported ionic species (protons and other monovalent cations). These modulations take place at specific sites in the exchanger protein located at extra-, intra-, and transmembrane protein domains. 2) Exchange activity is also regulated by the metabolic state of the cell. The mammalian and invertebrate preparations share MgATP in that role; the squid has an additional compound, phosphoarginine. This review emphasizes the interrelationships between ionic and metabolic modulations of Na(+)/Ca(2+) exchange, focusing mainly in two preparations where most of the studies have been carried out: the mammalian heart and the squid giant axon. A surprising fact that emerges when comparing the MgATP-related pathways in these two systems is that although they are different (phosphatidylinositol bisphosphate in the cardiac and a soluble cytosolic regulatory protein in the squid), their final target effects are essentially similar: Na(+)-Ca(2+)-H(+) interactions with the exchanger. A model integrating both ionic and metabolic interactions in the regulation of the exchanger is discussed in detail as well as its relevance in cellular Ca(i)(2+) homeostasis.

Functional proteins involved in regulation of intracellular Ca(2+) for drug development: pharmacology of SEA0400, a specific inhibitor of the Na(+)-Ca(2+) exchanger

The Na(+)-Ca(2+) exchanger (NCX) is involved in regulation of intracellular Ca(2+) concentration. A specific inhibitor of NCX has been required for clarification of the physiological and pathological roles of NCX. We have developed 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400), a highly potent and selective inhibitor of NCX. SEA0400 in the concentration range that inhibits NCX exhibits negligible affinities for the Ca(2+) channels, Na(+) channels, K(+) channels, noradrenaline transporter, and 14 receptors; and it does not affect the activities of the store-operated Ca(2+) channel, Na(+)-H(+) exchanger, and several enzymes including Na(+),K(+)-ATPase and Ca(2+)-ATPase. Furthermore, recent studies show that SEA0400 attenuates ischemia-reperfusion injury in the brain, heart, and kidney and radiofrequency lesion-induced edema in rat brain. These findings suggest that NCX plays a key role in ischemia-reperfusion injury and may be a target molecule for treatment of reperfusion injury-related diseases.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Role of the Na(+)-Ca2+ and Na(+)-H+ antiporters in prolactin release from anterior pituitary cells in primary culture

2',4'-dimethylbenzamilamiloride (DMB), a somewhat selective inhibitor of the Na(+)-Ca2+ exchanger, in concentrations of 10, 30 and 100 microM did not produce any significant effect on baseline prolactin release from anterior pituitary cells in primary culture. When prolactin secretion was stimulated by the inhibitor of the Na(+)-K(+)-ATPase, ouabain, that activates the Na(+)-Ca2+ exchanger as a Ca(2+)-influx pathway, DMB was able to produce inhibition of prolactin secretion. 5-(N,N-hexamethylene) amiloride (HMA), another amiloride analog which specifically inhibits the Na(+)-H+ antiporter and has no inhibitory activity on the Na(+)-Ca2+ exchanger, at the concentrations of 0.1, 1 and 10 microM, did not affect basal prolactin release whereas it significantly reduced prolactin release stimulated by thyrotropin releasing hormone (TRH) (1 microM). These results suggest that the Na(+)-Ca2+ antiporter is involved in the process of prolactin release elicited by the inhibition of the Na(+)-K(+)-ATPase whereas the Na(+)-H+ antiporter is involved in the prolactin secretion elicited by TRH.

A significant fraction of calcium transients in intact guinea pig ventricular myocytes is mediated by Na(+)-Ca2+ exchange

Ca2+ mobilization elicited by simulation with brief pulses of high K+ were monitored with confocal laser scanned microscopy in intact, guinea pig cardiac myocytes loaded with the calcium indicator fluo-3. Single wavelength ratioing of fluorescence images obtained after prolonged integration times revealed non-uniformities of intracellular Ca2+ changes across the cell, suggesting the presence of significant spatial Ca2+ gradients. Treatment with 20 microM ryanodine, an inhibitor of Ca2+ release from the SR, and 10 microM verapamil, a calcium channel blocker, reduced by 42% and 76% respectively the changes in [Ca2+]i elicited by membrane depolarization. The overall spatial distribution of [Ca2+]i changes appeared unchanged. Ca2+ transients recorded in the presence of verapamil and ryanodine (about 20% of the size of control responses), diminished in the presence of 50 microM 2-4 Dichlorbenzamil (DCB) or 5 mM nickel, two relatively specific inhibitors of the Na+/Ca2+ exchange mechanism. Conversely, when the reversal potential of the Na+/Ca2+ exchange was shifted to negative potentials by lowering [NA+]o or by increasing [Na+]i by treatment with 20 microM monensin, the amplitude of these Ca2+ transients increased. Ca2+ transients elicited by membrane depolarization and largely mediated by reverse operation of Na(+)-Ca2+ exchange could be recorded in the presence of ryanodine, verapamil and monensin. These finding suggest that in intact guinea pig cardiac cells, Ca2+ influx through the Na+/Ca2+ exchange mechanism activated by a membrane depolarization in the physiological range can be sufficient to play a significant role in excitation-contraction coupling.

Effects of androgens and antiandrogens on the inotropism induced by ouabain and isoproterenol on the left atrium of the rat in vitro

1. The effect of androgens 5 beta- and 5 alpha-dihydrotestosterone (DHT, 10(-9) M), and the antiandrogens cyproterone acetate (CPA, 10(-8)-10(-6) M), chlormadinone acetate (CMA, 10(-8)-10(-6) M), medroxyprogesterone acetate (MPA, 10(-8)-10(-6) M), spironolactone (SPI, 10(-5) M), flutamide (F, 10(-5) M) and cimetidine (C, 10(-5) M), on inotropic positive effect induced by ouabain (10(-8)-10(-5) M) and isoproterenol (10(-8)-10(-6) M), on electrically stimulated left atria of rat, has been assayed. 2. Ouabain (10(-6) M) did not modify the inotropic effect of isoproterenol (10(-8)-10(-6) M). 3. The androgens 5 beta- and 5 alpha-DHT (10(-9) M) and the antiandrogens SPI (10(-6) M), F (10(-5) M) and C (10(-5) M) inhibit the inotropic effect of ouabain and isoproterenol on electrically stimulated left atria of the rat. 4. The antiandrogens CPA, MPA and CMA to 10(-7) M, inhibit the inotropic effect of ouabain. The CPA (10(-8)-10(-6) M) inhibit, in a dose-dependent way the positive inotropic effect of isoproterenol. MPA and CMA (10(-8)-10(-6) M) also inhibit the inotropic effect isoproterenol but the inhibitory effect is greater with 10(-8) M than 10(-6) M of both drugs. 5. Taken together, our results suggest that steroidal hormones could modulate the cardiac contractility through interference with Na-pump in a non-digitalic site and/or with intracellular mediators in left atrium.

The Na+-Ca++ exchanger in central nerve endings: The relationship between its pharmacological blockade and dopamine release from tuberoinfundibular hypothalamic neurons

2', 4'-Dimethylbenzamiloride (DMB), an inhibitor of Na(+)-Ca++ antiporter dose-dependently (10-100 microM) inhibited Na(+)-dependent 45Ca++ efflux from brain synaptosomes. This compound was also able to stimulate basal release of [3H]DA from superfused TIDA neurons. Another amiloride analogue, 5-N-methyl-N-guanidinocarbonylmethylamiloride (MGCMA, 100-300 microM), which lacks of inhibitory properties on the Na(+)-Ca++ antiporter, failed to modify basal [3H]DA release from TIDA neurons. In addition, when the antiporter operates as a Ca(++)-influx pathway, DMB dose-dependently inhibited Na(+)-dependent 45Ca++ uptake in brain synaptosomes, whereas it did not prevent K(+)-induced 45Ca++ uptake, which reflets the activation of voltage-operated Ca++ channels. Finally DMB inhibited ouabain-induced [3H]DA release, which depends on the activation of the Na(+)-Ca++ exchanger due to the inhibition of the Na+/K(+)-ATPase pump.

Effects of an endogenous ouabainlike compound on heart and aorta

An endogenous ouabainlike compound (OLC) has been purified from human plasma, and mass spectrometry has shown it to be indistinguishable from plant-derived ouabain. This human OLC was tested for its effects on evoked tension in guinea pig left atria and aortic rings. The tissues were incubated at 37 degrees C in bicarbonate-buffered physiological salt solution gassed with 95% O2-5% CO2. In atria stimulated electrically at 1 Hz, 85 and 170 nM human OLC increased peak active force to 177 +/- 15% and 313 +/- 32% of control, respectively (n = 3), with little effect on the duration of contraction. On washout of the OLC, peak systolic force returned to the control level with a half-time of 4.3 +/- 0.5 minutes. Similar results were obtained with 160 nM plant-derived ouabain: peak systolic force increased to 310 +/- 31% of control (n = 4) and returned to the control level with a half-time of 3.8 +/- 0.2 minutes during washout. In aortic rings, neither 170 nM human OLC nor 160 nM plant ouabain (30-minute treatments) affected resting (unstimulated) tension, but they increased the contractions evoked by histamine (0.2-1.0 microM) to 156 +/- 13% (n = 4) and 143 +/- 6% (n = 4) of control responses, respectively. The mean half-time for washout of the OLC and plant ouabain-induced augmentation of histamine-evoked tension exceeded 35 minutes. These data show that human OLC has cardiotonic and vasotonic actions qualitatively and quantitatively similar to those observed with plant ouabain.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of increased digitalis tolerance in streptozotocin-induced diabetic rat myocardium

Our earlier studies revealed that the inotropic and cardiotoxic responses to ouabain are depressed significantly in chronically-induced diabetic rats (Navaratnam S and Khatter JC, Arch Int Pharmacodyn 301: 151-164, 1989). In the present study, we examined the Na(-)-Ca2+ exchange mechanism in sarcolemmal membrane vesicles (right-side out orientation) isolated from chronically-induced diabetic rat hearts. The apparent initial rates of Na(+)-dependent 45Ca2+ uptake were substantially lower in vesicles from 6- and 12-week diabetic rat hearts when compared to non-diabetic controls. These rates were reduced further in vesicles of 24-week diabetic rat hearts. Associated with the progressive reduction in initial rates was also a progressive reduction in the maximum amount of 45Ca2+ accumulated by the vesicles. A kinetic analysis revealed a significant reduction in the maximum initial rate of 45Ca2+ uptake (Vmax) in vesicles of 24-week diabetic rat hearts. Affinity for 45Ca2+, however, was the same for both diabetic and control groups. The efflux rate of 45Ca2+ was also depressed in these vesicles, and they retained significantly more 45Ca2+ than controls after 2-4 min of initiation of Na(+)-dependent 45Ca2+ efflux. These data demonstrate that the trans-sarcolemmal Ca2+ flux through Na(+)-Ca2+ exchange is depressed and may explain the observed increase in digitalis tolerance of the myocardium in diabetic rats.

Sodium current-induced release of calcium from cardiac sarcoplasmic reticulum

The role of sodium-calcium exchange at the sarcolemma in the release of calcium from cardiac sarcoplasmic reticulum was investigated in voltage-clamped, isolated cardiac myocytes. In the absence of calcium entry through voltage-dependent calcium channels, membrane depolarization elicited release of calcium from ryanodine-sensitive internal stores. This process was dependent on sodium entry through tetrodotoxin-sensitive sodium channels. Calcium release under these conditions was also dependent on extracellular calcium concentration, suggesting a calcium-induced trigger release mechanism that involves calcium entry into the cell by sodium-calcium exchange. This sodium current-induced calcium release mechanism may explain, in part, the positive inotropic effects of cardiac glycosides and the negative inotropic effects of a variety of antiarrhythmic drugs that interact with cardiac sodium channels. In response to a transient rise of intracellular sodium, sodium-calcium exchange may promote calcium entry into cardiac cells and trigger sarcoplasmic calcium release during physiologic action potentials.

Comparative ability of digoxin and an aminosugar cardiac glycoside to bind to and inhibit Na+,K+-adenosine triphosphatase. Effect of potassium

We compared the abilities of digoxin and aminogalactose digitoxigenin (ASI-222) to bind to, or inhibit, purified dog heart Na+,K+-ATPase in the presence of 1, 10, or 80 mM potassium chloride. Changing the potassium concentration from 1 to 10 mM increased the dose producing 50% inhibition of enzyme activity (IC50) by 9- and 2.5-fold for digoxin and ASI-222 respectively. Raising the potassium concentration to 80 mM increased the IC50 for digoxin 3-fold but did not alter significantly the IC50 for ASI-222. Equilibrium binding studies showed that this enzyme exhibited a single class of specific binding sites for both digoxin and ASI-222. Raising the potassium concentration did not affect the maximum number of binding sites (Bmax) but increased the apparent dissociation constant (KD) for digoxin. Potassium differentially affected the affinity and number of binding sites for ASI-222; raising the potassium concentration from 1 to 10 mM did not affect the Bmax or the KD, but raising it to 80 mM increased both. The effect of i.v. infusion of potassium chloride upon cardiac upon cardiac arrhythmias produced by i.v. infusion of digoxin or ASI-222 in anesthetized dogs was also determined. Infusion of potassium chloride reversed the cardiac arrhymias due to digoxin to normal rhythm, but not those due to ASI-222. In conclusion, the interaction of digoxin and the polar digitalis agent, ASI-222, with dog heart Na+,K+-ATPase was differentially affected by potassium. These agents also also produced cardiac arrhythmias, which were differentially affected by potassium.

Ouabain distinguishes between nicotinic and muscarinic receptor-mediated catecholamine secretions in perfused adrenal glands of cat

1. The effect of ouabain on catecholamine (adrenaline and noradrenaline) secretion induced by agents acting on cholinoceptors was studied in perfused cat adrenal glands. Acetylcholine (ACh) (5 x 10(-7) to 10(-3) M), pilocarpine (10(-5) to 10(-3) M) and nicotine (10(-6) to 5 x 10(-5) M) caused dose-dependent increases in catecholamine secretion. Both ACh and nicotine released more noradrenaline than adrenaline and the reverse was the case for pilocarpine. 2. Ouabain (10(-5) M) enhanced catecholamine secretion induced by ACh (10(-5) M), pilocarpine (10(-3) M) and nicotine (3 x 10(-6) M) during perfusion with Locke solution. The ratio of adrenaline to noradrenaline was not affected by ouabain. 3. In the absence of extracellular Ca2+, ACh and pilocarpine, but not nicotine, still caused a small increase in catecholamine secretions, which were enhanced by treatment with ouabain (10(-5) M) plus Ca2+ (2.2 mM) for 25 min. The effect of ouabain was much more significant on noradrenaline secretion than on adrenaline secretion. The enhanced response was blocked by atropine (10(-6) M) but not by hexamethonium (5 x 10(-4) M). 4. Nifedipine (2 x 10(-6) M) inhibited the responses to pilocarpine and nicotine. The treatment with ouabain (10(-5) M) reversed only the response to pilocarpine and resulted in a significant increase in the proportion of noradrenaline released. 5. It is suggested that ouabain enhances evoked catecholamine secretions by facilitating Ca2+ entry through nicotinic receptor-linked Ca2+ channels and by increasing the intracellular Ca2+ pool linked to muscarinic receptors.

Inhibition by taurine of Na(+)-Ca2+ exchange in sarcolemmal membrane vesicles from bovine and guinea pig hearts

1. Taurine, but not GABA, beta-alanine and glycine, inhibited Na(+)-dependent Ca2+ uptake in bovine cardiac sarcolemmal membrane vesicles in a dose-dependent manner. 2. The inhibition of Na(+)-dependent Ca2+ uptake was noncompetitive with respect to Ca2+ concentration. 3. The inhibitory effect of taurine on the exchange was also observed in cardiac sarcolemmal vesicles prepared from guinea pig, but not from rat. 4. Taurine did not affect Na(+)-dependent Ca2+ efflux nor ATP-dependent Ca2+ uptake in the bovine cardiac membranes.

Possible mechanisms of positive inotropic action of synthetic human calcitonin gene-related peptide in isolated rat atrium

The effect of synthetic human calcitonin gene-related peptide (hCGRP) on the isolated and electrically driven left atria of rats were investigated. The peptide at concentrations of 3 X 10(-9)-3 X 10(-7) M produced positive inotropic effects on the left atria in a dose-dependent manner. Verapamil (10(-5) M) and adenosine (10(-4) M) reduced the positive inotropic effect of hCGRP at concentrations of 3 X 10(-9) and 3 X 10(-8) M, but not at that of 3 X 10(-7) M. Ouabain (5 X 10(-5) M) inhibited the effect of hCGRP in concentrations of 3 X 10(-7) and 3 X 10(-8) M, but not in that of 3 X 10(-9) M. Simultaneous pretreatment with verapamil (10(-5) M) and ouabain (5 X 10(-5) M) suppressed the positive inotropy by hCGRP at all concentrations tested. On the other hand, tetrodotoxin (10(-6) M) potentiated only the positive inotropic effect of 3 X 10(-7) M hCGRP. Metoprolol (10(-7) M) and theophilline (10(-3) M) did not affect the inotropic effect of hCGRP. These results suggest that the positive inotropic effect of hCGRP is not mediated by beta-adrenoceptors but by two distinct mechanisms of action, which was inhibited by verapamil but not by ouabain (facilitation of Ca++ influx in lower concentrations of hCGRP) and which was blocked by ouabain but not by verapamil and potentiated by tetrodotoxin (inhibition of Na+/Ca++ exchange mechanism at higher concentrations of hCGRP).