Adenosine uptake sites in dog heart and brain; interaction with calcium antagonists

Physiological and pharmacological roles of vascular nucleoside transporters

Adenosine modulates various vascular functions such as vasodilatation and anti-inflammation. The local concentration of adenosine in the vicinity of adenosine receptors is fine tuned by 2 classes of nucleoside transporters: equilibrative nucleoside transporters (ENTs) and concentrative nucleoside transporters (CNTs). In vascular smooth muscle cells, 95% of adenosine transport is mediated by ENT-1 and the rest by ENT-2. In endothelial cells, 60%, 10%, and 30% of adenosine transport are mediated by ENT-1, ENT-2, and CNT-2, respectively. In vitro studies show that glucose per se increases the expression level of ENT-1 via mitogen-activating protein kinase-dependent pathways. Similar results have been demonstrated in diabetic animal models. Hypertension is associated with the increased expression of CNT-2. It has been speculated that the increase in the activities of ENT-1 and CNT-2 may reduce the availability of adenosine to adenosine receptors, thereby weakening the vascular functions of adenosine. This may explain why patients with diabetes and hypertension suffer greater morbidity from ischemia and atherosclerosis. No oral hypoglycemic agents can inhibit ENTs, but an exception is troglitazone (a thiazolidinedione that has been withdrawn from the market). ENTs are also sensitive to dihydropyridine-type calcium-channel blockers, particularly nimodipine, which can inhibit ENT-1 in the nanomolar range. Those calcium-channel blockers are noncompetitive inhibitors of ENTs, probably working through the reversible interactions with allosteric sites. The nonsteroidal anti-inflammatory drug sulindac sulfide is a competitive inhibitor of ENT-1. In addition to their original pharmacological actions, it is believed that the drugs mentioned above may regulate vascular functions through potentiation of the effects of adenosine.

Inhibition of human equilibrative nucleoside transporters by dihydropyridine-type calcium channel antagonists

Dihydropyridine-type calcium channel antagonists, in addition to having a vasodilatory effect, are known to inhibit cellular uptake of nucleosides such as adenosine. However, the nucleoside transporter subtypes involved and the mechanism by which this occurs are not known. Therefore, we have studied the inhibitory effects of dihydropyridines on both human equilibrative nucleoside transporters, hENT-1 and hENT-2, which are the major transporters mediating nucleoside transport in most tissues. Among the dihydropyridines tested, nimodipine proved to be the most potent inhibitor of hENT-1, with an IC(50) value of 60+/-31 muM, whereas nifedipine, nicardipine, nitrendipine, and felodipine exhibited 100-fold less effective inhibitory activity. Nifedipine, nitrendipine, and nimodipine inhibited hENT-2 with IC(50) values in the micromolar range; however, nicardipine and felodipine had no significant effect on hENT-2. Removal of the 4-aryl ring or changing the nitro group at the 4-aryl ring proved not to be detrimental to the inhibitory effects of dihydropyridines on hENT-1, but resulted in a drastic decrease in their inhibitory effects on hENT-2. Kinetic studies revealed that nimodipine and nifedipine reduced V(max) of [(3)H]uridine transport without affecting K(m). The inhibitory effects of nimodipine and nifedipine could be washed out. In addition, nimodipine and nifedipine inhibited the rate of NBTGR-induced dissociation of [(3)H]NBMPR from hENT-1 cell membrane. We conclude that dihydropyridines are non-competitive inhibitors of hENT-1 and hENT-2, probably working through reversible interactions with the allosteric sites. The inhibitory potencies of dihydropyridines may be associated with the structure of the 4-aryl ring, as well as the ester groups at the C-3 and C-5 positions.

A Ca channel blocker, benidipine, increases coronary blood flow and attenuates the severity of myocardial ischemia via NO-dependent mechanisms in dogs

OBJECTIVES

This study was undertaken to examine whether a dihydropyridine Ca channel blocker, benidipine, increases cardiac NO levels, and thus coronary blood flow (CBF) in ischemic hearts.

BACKGROUND

Benidipine protects endothelial cells against ischemia and reperfusion injury in hearts.

METHODS AND RESULTS

In open chest dogs, coronary perfusion pressure (CPP) of the left anterior descending coronary artery was reduced so that CBF decreased to one-third of the control CBF, and thereafter CPP was maintained constant (103+/-8 to 42+/-1 mmHg). Both fractional shortening (FS: 6.1+/-1.0%) and lactate extraction ratio (LER: -41+/-4%) decreased. Ten minutes after the onset of an intracoronary infusion of benidipine (100 ng/kg/min), CBF increased from 32+/-1 to 48+/-4 ml/100g/ min during 20 min without changing CPP (42+/-2 mmHg). Both FS (10.7+/-1.2%) and LER (-16+/-4%) also increased. Benidipine increased cardiac NO levels (11+/-2 to 17+/-3 nmol/ml). The increases in CBF, FS, LER and cardiac NO levels due to benidipine were blunted by L-NAME. Benidipine increased cyclic GMP contents of the coronary artery of ischemic myocardium (139+/-13 to 208+/-15 fmol/mg protein), which was blunted by L-NAME.

CONCLUSION

Thus, we conclude that benidipine mediates coronary vasodilation and improves myocardial ischemia through NO-cyclic GMP-dependent mechanisms.

Evidence for physiologically active axonal adenosine receptors in the rat corpus callosum

Several neurotransmitter receptors have been identified on axons, and emerging evidence suggests that central axonal conduction may be modulated by neurotransmitters. We have recently demonstrated the presence of extra-synaptic adenosine Al receptors along rat hippocampal axons. We now present immunocytochemical evidence for Al receptors on rat corpus callosum axons and show that these receptors actively modulate axon physiology. Using rat brain coronal slices, we stimulated the corpus callosum and recorded the evoked extracellular compound action potential. The lipid-soluble, Al-specific adenosine receptor agonist cyclopentyladenosine, dose-dependently decreased the compound action potential amplitude, an effect reversed by the specific Al antagonist 8-cyclopentyl-1, 3-dipropylxanthine. These data provide the first direct evidence that axonal Al adenosine receptors modulate axon physiology in the adult mammalian brain. Influencing axonal transmission is a potentially powerful mechanism of altering information processing in the nervous system.

Acute effects of progesterone on glucose metabolism in rat adipocytes: are they modulated by endogenous adenosine?

Progesterone rapidly inhibits glucose oxidation of isolated rat adipocytes. Because this inhibition is triggered by endogenous adenosine, the present study was designed to examine the effect of the steroid on cyclic adenosine monophosphate (cAMP) accumulation, its relation to lipolysis, and the possible participation of adenosine. The results strongly indicate that physiological concentrations of progesterone increase the release of adenosine by isolated adipocytes, with maximal release at the end of a 20-minute incubation. Progesterone decreased both cAMP levels and lipolysis in quiescent adipocytes or in adipocytes stimulated by isoproterenol. The increase of endogenous adenosine may explain the decline of cAMP and glycerol levels observed with progesterone. The effects of the steroid on lipolysis disappeared when adenosine was hydrolyzed by adenosine deaminase (ADA). On the other hand, in the absence of endogenous adenosine, the effect of progesterone on the cAMP level was decreased only in isoproterenol-stimulated cells. The inhibitory effects of progesterone on cAMP and glycerol production seem not to be related directly to the adenosine A1 receptor, for selective A1 receptor antagonists (8-cyclopentyl-1,3-dipropylxanthine [DPCPX] and CP 68,247) did not counteract these effects. However, mechanisms mediated by guanyl nucleotide binding proteins cannot be excluded. The decrease of cAMP and of lipolysis may be related to a stimulation of phosphodiesterases (PDEs). When PDEs I [Ca(2+)-calmodulin-regulated PDE family) were blocked by a selective inhibitor (CP 41,757), the progesterone inhibitory effect persisted, suggesting that PDEs I are not regulated by the steroid. On the other hand, the progesterone effect on cAMP accumulation but not on lipolysis disappeared in the presence of a selective inhibitor of the PDE IV family (cAMP-dependent-specific family). Ro 20.1724. When the specific inhibitor of PDE I or PDE IV was combined with ADA, the progesterone effect on cAMP disappeared. Taken together, these results suggest that the progesterone inhibitory action on cAMP levels was not mediated through A1 receptors or through activation of PDE I, but may be related to PDE IV activities. The progesterone effect on lipolysis seemed not to be directly related to changes in cAMP levels; an effect on PDE III activities in relation with the increase of adenosine release cannot be excluded.

Ionic dependence of adenosine uptake into cultured astrocytes

Adenosine uptake in cultured astrocytes is dependent on various ions and energy metabolism. The Na(+)-gradient plays an important role, since nigericin, ouabain, amiloride and substitution of Na+ with choline inhibited adenosine uptake. The proton-gradient was of importance, since carbonylcyanide m-chlorophenylhydrozone (CCCP) and omeprazole also inhibited adenosine uptake. Furthermore, adenosine uptake was dependent on Cl- anion. Substitution of Cl- with isethionate, as well as DIDS or furosemide inhibited adenosine uptake. Adenosine uptake was also sensitive to Ca2+ gradient, removal of extracellular Ca2+ and calcimycin inhibited adenosine uptake. Adenosine uptake was not dependent on extracellular K+ and was not affected by valinomycin. Although, K(+)-channel openers (BRL 34195 and nicorandil) as well as the K(+)-channel antagonist, glyburide, inhibited adenosine uptake, the inhibitory effect of BRL 34915 was not antagonized by glyburide. Rotenone and 2,4-dinitrophenol also inhibited adenosine uptake. Ionic dependence and metabolic energy dependence of adenosine uptake suggest that uptake is primarily an active process.

Adenosine inhibits divalent cation influx across human neutrophil plasma membrane via surface adenosine A2 receptors

Adenosine and its analogues inhibited increases in divalent cation influx stimulated by platelet-activating factor (PAF) and formyl-methionyl-leucyl-phenylalanine (FMLP) in a dose-dependent fashion. This effect was antagonized by theophylline, an adenosine receptor antagonist. When extracellular adenosine was removed by adenosine deaminase, the effect of adenosine was completely abolished. Two adenosine analogues with different affinities for adenosine receptor subtypes, 5'-N-ethylcarboxamideadenosine (NECA) and L-N6-phenylisopropyladenosine (PIA), also inhibited divalent cation influx, NECA being more potent than PIA. These results suggest that adenosine and its analogues inhibit divalent cation influx across neutrophil plasma membranes via surface adenosine A2 receptors. Adenosine had little effect on the initial peaks of intracellular free calcium rises induced by chemoattractants, but it inhibited the subsequent rise in free calcium. Since calcium influx through the divalent cation channels or neutrophil plasma membranes is responsible for maintaining free calcium concentration following the initial peaks, we suggest that adenosine modulates neutrophil function by interfering with this calcium influx.

Possible involvement of pertussis toxin-sensitive G proteins and D2 dopamine receptors in the A1 adenosine receptor-adenylate cyclase system in rat cerebral cortex

To identify the involvement of dopamine receptors in the transmembrane signaling of the adenosine receptor-G protein-adenylate cyclase system in the CNS, we examined the effects of pertussis toxin (islet-activating protein, IAP) and apomorphine on A1 adenosine agonist (-)N6-R-[3H]phenylisopropyladenosine ([3H]PIA) and antagonist [3H]xanthine amine congener ([3H]XAC) binding activity and adenylate cyclase activity in cerebral cortex membranes of the rat brain. Specific binding to a single class of sites for [3H]XAC with a dissociation constant (KD) of 6.0 +/- 1.3 nM was observed. The number of maximal binding sites (Bmax) was 1.21 +/- 0.13 pmol/mg protein. Studies of the inhibition of [3H]XAC binding by PIA revealed the presence of two classes of PIA binding states, a high-affinity state (KD = 2.30 +/- 1.16 nM) and a low-affinity state (KD = 1.220 +/- 230 nM). Guanosine 5'-(3-O-thio)triphosphate or IAP treatment reduced the number of the high-affinity state binding sites without altering the KD for PIA. Apomorphine (100 microM) increased the KD value 10-fold and decreased Bmax by approximately 20% for [3H]PIA. The effect of apomorphine on the KD value increase was irreversible and due to a conversion from high-affinity to low-affinity states for PIA. The effect was dose dependent and was mediated via D2 dopamine receptors, since the D2 antagonist sulpiride blocked the phenomenon. The inhibitory effect of PIA on adenylate cyclase activity was abolished by apomorphine treatment. There was no effect of apomorphine on displacement of [3H]quinuclidinyl benzilate (muscarinic ligand) binding by carbachol. These data suggest that A1 adenosine receptor binding and function are selectively modified by D2 dopaminergic agents.

Dihydropyridines alter adenosine sensitivity in the rat hippocampal slice

1. The effects of adenosine and a range of adenosine analogues, which are resistant to uptake processes, were studied in the presence of dihydropyridines and verapamil on the population spike potential recorded from the CA1 area of the hippocampal slice. 2. Nifedipine and Bay K 8644, a calcium channel antagonist and activator respectively, enhanced the inhibitory action of adenosine in a concentration-dependent manner. This was in contrast to their effect on adenosine analogues where the inhibition of the population potential was significantly attenuated. Similar interactions between the adenosine compounds and the dihydropyridines were also displayed in studies on spontaneous epileptiform activity in the CA3 region. 3. This effect of nifedipine and Bay K 8644 was not shown by the dihydropyridines, nimodipine or nitrendipine, or by the phenylalkylamine, verapamil. 4. Addition of the adenosine uptake blocker dipyridamole reversed the action of nifedipine on adenosine, so that inhibition by adenosine was now attenuated by nifedipine in a similar manner to that observed with the adenosine analogues. 5. These results can be explained with reference to binding studies that show displacement of adenosine analogues from the adenosine receptor by dihydropyridines. An action at the adenosine uptake site by the dihydropyridines explains the enhancement of adenosine inhibition. 6. The possible sites for this interaction are discussed.

Widening potential for Ca2+ antagonists: non-L-type Ca2+ channel interaction

In addition to their well characterized interaction with the alpha 1 subunit of the voltage-dependent L-type Ca2+ channel, certain Ca2+ antagonists have been reported to modulate an increasing number of cellular functions as diverse as extrusion of cytotoxic substances, or cleavage of cAMP by phosphodiesterase. Some of these interactions (such as the reversal of multidrug resistance by Ca2+ antagonists for the treatment of lymphoma patients) have already been exploited clinically; some (such as protection of ischemic tissue by Ca2+ antagonists interacting with mitochondrial sites) open new therapeutic issues. In this survey of the non-L-type channel Ca2+ antagonist target structures known to date, Gerald Zernig evaluates the available data and emphasizes common characteristics shared by the seemingly diverse target structures. Research on these sites might help to understand yet unexplained effects of Ca2+ antagonists and possibly lead to the development of novel drugs with higher selectivity for non-L-type Ca2+ channel structures.

Diltiazem enhances and flunarizine inhibits nimodipine's antiseizure effects

The dihydropyridine calcium channel antagonist, nimodipine has antiepileptic and anticonvulsive properties that are thought to be mediated through neuronal calcium channel blockade. The dihydropyridine binding site can be positively and negatively allosterically regulated by the benzothiazepines and the phenylalkylamines/piperazines, respectively. We investigated this binding interaction at the physiologic level by examining the effects of diltiazem (a benzothiazepine) and flunarizine (a piperazine) on the antiseizure activity of nimodipine. Seizures were induced with pentylenetetrazole in awake rats with chronically implanted EEG electrodes. Calcium channel antagonists were administered intracerebroventricularly 30 min after pentylenetetrazole at doses given at 15 min intervals. Diltiazem and flunarizine alone lacked antiseizure properties. The calculated ED50 values for nimodipine were: nimodipine alone = 135 micrograms; nimodipine + diltiazem (100 micrograms) = 67 micrograms. Nimodipine + flunarizine (10 micrograms) completely suppressed nimodipine's antiseizure activity. These findings may reflect the interaction observed among these agents at binding sites associated with the calcium channel and supports the idea that dihydropyridines mediate their antiseizure actions through neuronal calcium channel antagonism.

In vivo and in vitro evidence of an adenosine-mediated mechanism of calcium entry blocker activities

Drugs such as dipyridamole (200 micrograms/kg/min), an adenosine uptake inhibitor, and theophylline (300 micrograms/kg/min), an adenosine receptor antagonist, respectively increased and decreased postischemic hyperemia in normal subjects, as well as in POAD patients. Moreover, dipyridamole pretreatment was able to antagonize the reduction of peak flow induced by nifedipine, and the potentiating effect of flunarizine on postischemic hyperemia was affected significantly by theophylline, thus suggesting a possible interference of calcium entry blocker drugs with the endogenous adenosine system. In a cellular model (polymorphonuclear leukocytes--PMN) the inhibitory effect of calcium entry blockers on stimulated functions (degranulation and free radical production) was highly antagonized by theophylline. Finally, a 1H-NMR spectroscopy study showed a binding interaction between adenosine and flunarizine on the cell membrane. An adenosine-receptor coupling to the calcium entry blocker channels is suggested.

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.

Ca++ antagonists and ACAT inhibitors promote cholesterol efflux from macrophages by different mechanisms. I. Characterization of cellular lipid metabolism

The effects of the slow Ca++ channel blocker, nifedipine, and ACAT inhibitor, octimibate, on the cholesterol metabolism of cholesterol-loaded macrophages were compared. We demonstrated that apolipoprotein A-I containing high density lipoproteins (HDL) bind to specific receptor sites on macrophages, are internalized, take up cholesterol, and are then released from the cells as native lipoproteins. The ACAT inhibitor enhances HDL receptor activity and promotes HDL-mediated cholesterol efflux from cultured mouse peritoneal macrophages. In contrast, the Ca++ antagonist increases acetyl LDL-mediated cholesterol influx, abolishes the increase in HDL binding induced by cholesterol accumulation, enhances apo E synthesis, and promotes cholesterol efflux by a mechanism independent of the presence of HDL in the surrounding medium. Concomitantly, a decrease in nucleoside transporter activity, an increase in intracellular ATP hydrolysis, adenosine and cyclic AMP concentration, and a stimulation of the activities of acid and neutral cholesteryl ester hydrolase and ACAT indicated that protein kinase A-catalyzed phosphorylation reactions might be involved in the increase in cholesterol efflux. The Ca++ antagonist-induced efflux occurred only with lysosomal-associated cholesterol, while the ACAT inhibitor acted on the formation of cytoplasmic lipid droplets. The secreted lipoprotein particles contained 68% unesterified cholesterol and 21% phospholipids, 8% esterified cholesterol, and 3% triglycerides. The phospholipid components were: 72% phosphatidylcholine, 22% sphingomyelin, and 6% phosphatidylserine, phosphatidylinositol, and phosphatidylethanolamine. We conclude that macrophages release cholesterol in two ways: 1) an HDL-mediated release of unesterified cholesterol increasing upon ACAT inhibition, and 2) an HDL-independent secretion of cholesterol which can be amplified by Ca++ antagonists.

Interactions between calcium channel compounds and adenosine systems in brain of rat

A number of organic ligands of calcium channels were investigated for possible actions on several aspects of adenosine systems in the cerebral cortex of rat. The principle findings of the present study were that a number of antagonists of calcium channels and the agonist compound Bay K 8644 inhibited binding to adenosine receptors, binding to nucleoside transporters, and the accumulation of [3H]adenosine with a low microM potency. 2-Nitrophenyl dihydropyridine derivatives were more potent than 3-nitrophenyl dihydropyridine or non-dihydropyridine ligands of calcium channels at inhibiting binding to adenosine receptors. Dihydropyridine ligands of calcium channels were more potent than non-dihydropyridine ligands of calcium channel in inhibiting the binding of [3H]nitrobenzylthioinosine to cortical membranes or inhibiting the accumulation of [3H]adenosine into synaptoneurosomes. However, unlike the case of adenosine receptors, no distinction between 2-nitrophenyl and 3-nitrophenyl dihydropyridine derivatives was observed. In addition, the non-dihydropyridine ligand of calcium channels, diltiazem was a weak inhibitor of the accumulation of [3H]adenosine. These results demonstrate that organic ligands of calcium channels, particularly dihydropyridine compounds, can interact with several aspects of adenosine systems.

The pharmacology of nisoldipine

Nisoldipine is a calcium antagonist that specifically blocks the slow or voltage-dependent calcium channel up to the highest concentrations. This mode of action has been confirmed in pharmacological studies on isolated organs, electrophysiological and binding studies, and by the measurement of transmembrane calcium transport. As with other dihydropyridine calcium antagonists, an interaction with intracellular calcium reservoirs and calmodulin seems to be of minor importance. The drug exhibits higher potency, longer duration of action, and a higher binding affinity in vitro and in vivo than nifedipine. In contrast to its vasodilating and spasmolytic activity, its negative inotropic effect occurs in vitro only after higher concentrations than after nifedipine. In whole animals a secondary positive inotropic effect occurs regularly owing to sympathetic counter-regulation. The influence of nisoldipine on cardiac stimulus formation and conduction is also very slight in anesthetized animals, and is completely eliminated in awake animals and humans by counter-regulation up to very high doses. The cardiac anti-ischemic action of nisoldipine has been demonstrated in various ischemia models and is probably based predominantly on its afterload-reducing properties in addition to its spasmolytic effect on the coronary arteries. Various other suspected effects, for which there are isolated indications, e.g., inhibition of thromboxane synthesis, preload reduction, interaction with the transport of adenosine, and normalization of the sarcolemmal Na+, K(+)-ATPase activity, are probably of subordinate importance. Its antihypertensive effect is explained primarily by lowering of the peripheral resistance. There are, however, some indications that nisoldipine exerts certain effects over and above pure vasodilation. The prevention of postischemic calcium overloading in the renal tubule epithelium and the natriuretic effect are probably of importance in the therapeutic action. Clinically, nisoldipine was found more potent and prolonged in its action in comparison with nifedipine. In comparative studies, nisoldipine, 10 mg once a day, was found equieffective with nifedipine 10 mg three times or 20 mg twice a day in angina or hypertension, respectively.

Inhibition of radioligand binding to A1 adenosine receptors by Bay K8644 and nifedipine

Two dihydropyridine compounds, Bay K8644 (a calcium entry activator) and nifedipine (a calcium entry blocker), were found to inhibit the binding of [3H]phenylisopropyladenosine ([3H]PIA) to A1 adenosine receptors in rat cerebral cortex membranes with comparable potencies (IC50 10-30 microM). Scatchard analyses indicated that both Bay K8644 and nifedipine inhibited the binding of [3H]PIA by increasing the KD but without significant effect on the Bmax. When tested at 100 microM, neither Bay K8644 nor nifedipine showed a significant effect on [3H]-p-aminoclonidine ([3H]PAC; alpha 2-adrenergic receptor), [3H]dihydroalprenolol ([3H]DHA; beta-adrenergic receptor), [3H]spiperone (dopamine receptor), and [3H]nitrobenzylthioinosine [( 3H]NBMPR; nucleoside transporter) binding. In the presence of 10 mM Mg2+, the ability of 2-chloroadenosine (2-Cl-Ad, an A1 adenosine receptor agonist) to displace [3H]PIA binding was increased. Conversely, the potencies of 1,3-diethyl-8-phenylxanthine (DPX; an A1 receptor antagonist), Bay K8644 and nifedipine in inhibiting [3H]PIA binding were unchanged. It is suggested that both Bay K8644 and nifedipine may act as antagonists of adenosine A1 receptors, in addition to their well-known effects on calcium channels.

Effects of Ca2+-channel antagonists on nucleoside and nucleobase transport in human erythrocytes and cultured mammalian cells

Lidoflazine strongly inhibited the equilibrium exchange of uridine in human erythrocytes (Ki approximately 16 nM). Uridine zero-trans influx was similarly inhibited by lidoflazine in cultured HeLa cells (IC50 approximately to 80 nM), whereas P388 mouse leukemia and Novikoff rat hepatoma cells were three orders of magnitude more resistant (IC50 greater than 50 microM). Uridine transport was also inhibited by nifedipine, verapamil, diltiazem, prenylamine and trifluoperazine, but only at similarly high concentrations in both human erythrocytes and the cell lines. IC50 values ranged from about 10 microM for nifedipine and about 20 microM for verapamil to more than 100 microM for diltiazem, prenylamine and trifluoperazine. The concentrations required for inhibition of nucleoside transport are several orders higher than those blocking Ca2+ channels. Lidoflazine competitively inhibited the binding of nitrobenzylthioinosine to high-affinity sites in human erythrocytes, but did not inhibit the dissociation of nitrobenzylthioinosine from these sites on the transporter as is observed with dipyridamole and dilazep.

[3H]dipyridamole binding to guinea pig brain membranes: possible heterogeneity of central adenosine uptake sites

The binding of [3H]dipyridamole ([3H]DPR) to guinea pig brain membranes is described and compared to that of [3H]nitrobenzylthioinosine ([3H]NBI). The binding of [3H]DPR is saturable, reversible, and specific with pharmacologic evidence indicating that this ligand is binding to the adenosine uptake site. Compared to [3H]NBI the binding of [3H]DPR is of higher capacity (Bmax = 208 +/- 16 fmol/mg protein for [3H]NBI and 530 +/- 40 fmol/mg protein for [3H]DPR) and lower affinity (KD = 0.35 +/- 0.02 nM for [3H]NBI and 7.6 +/- 0.7 nM for [3H]DPR). The adenosine uptake inhibitors are the most potent inhibitors of binding (Ki of 10(-8)-10(-7) M) whereas adenosine receptor ligands such as cyclohexyladenosine, 2-chloroadenosine, and various methylxanthines are several orders of magnitude less potent (Ki 10(-5)-10(-2). The inhibition of [3H]DPR binding by NBI is biphasic, with only 40% of binding being susceptible to inhibition of NBI concentrations less than 10(-5) M. The tissue distribution of [3H]DPR binding parallels that of [3H]NBI although in most cases significantly more sites are observed with [3H]DPR. Calcium channel blocking agents such as nifedipine, nimodipine, and verapamil are also inhibitors of [3H]DPR binding with potencies in the micromolar range. The data are consistent with [3H]DPR being a useful additional ligand for the adenosine uptake site and provide evidence that multiple uptake binding sites exist of which only about 40% are NBI-sensitive.

Antiatherogenic properties of calcium antagonists

A generalized accumulation of cholesterol, calcium and matrix materials (collagen, elastin and proteoglycans) occurs in an age-dependent manner in major arteries. Human atherogenesis is a disease of arteries characterized by a focal accumulation of fibrous matrix elements, lipids and calcium at lesion sites. Studies in cholesterol-fed animal models have indicated that calcium competitors and chelating agents can reduce calcium, lipid and matrix accumulation in arterial lesions and reduce the extent of lesion formation. These agents generally alter soft and hard tissue calcium pools or have deleterious side-effect profiles. Antiatherogenic studies with calcium antagonists (which have been shown to be safe in human clinical studies) have created confusion because of conflicting results. It is apparent, however, that high doses of calcium antagonists can significantly decrease atherogenic lesion development in cholesterol-fed rabbits. The antiatherogenic effects of calcium antagonists may be the result of changes in intracellular calcium pools within smooth muscle cells, which may lead to alterations in cellular metabolic activity or may be due to activities not related to calcium channel effects. Several mechanisms involving regulation of lipoprotein receptor synthesis, lipoprotein uptake or degradation, cholesterol ester hydrolytic activity and arterial matrix synthesis are discussed as potential sites of activity for calcium antagonists. A dihydropyridine channel antagonist, PN 200-110 (isradipine), has been shown to be a very potent antiatherogenic agent in the rabbit and also to be a potent inhibitor of smooth muscle cell matrix synthesis.

[3H]dipyridamole binding to nucleoside transporters from guinea-pig and rat lung

Membranes from guinea-pig lung exhibited high-affinity binding of [3H]dipyridamole, a potent inhibitor of nucleoside transport. Binding (apparent KD 2 nM) was inhibited by the nucleoside-transport inhibitors nitrobenzylthioinosine (NBMPR), dilazep and lidoflazine and by the transported nucleosides uridine and adenosine. In contrast, there was no detectable high-affinity binding of [3H]dipyridamole to lung membranes from the rat, a species whose nucleoside transporters exhibit a low sensitivity to dipyridamole inhibition. Bmax. values for high-affinity binding of [3H]dipyridamole and [3H]NBMPR to guinea-pig membranes were similar, suggesting that these structurally unrelated ligands bind to the NBMPR-sensitive nucleoside transporter with the same stoichiometry.

Novel interactions of cations with dihydropyridine calcium antagonist binding sites in brain

The effects of monovalent (Na+, Li+, K+, Rb+) and divalent (Ca2+, Mg2+, Mn2+) cations on dihydropyridine calcium antagonist binding sites in brain and cardiac membranes were investigated using a low ionic strength buffer (5 mM Tris-HCl, pH 7.4), and the dihydropyridine, [3H]-nitrendipine. At 25 degrees C, the monovalent cations Na+, Li+, and K+ (100 mM) but not Rb+ significantly decreased the apparent dissociation constant (KD) but had no effect on the maximum binding site capacity (Bmax) of [3H]-nitrendipine in brain. The divalent cations Ca2+, Mg2+, and Mn2+ (2 mM) significantly increased the Bmax, but did not affect the KD of [3H]-nitrendipine. The effects of cations were concentration-dependent (EC50 monovalent cations 10-25 mM; EC50 divalent cations 50-200 microM) and demonstrated brain region selectivity. The effect of Ca2+, but not Mg2+ or Mn2+ on [3H]-nitrendipine binding was described by a two-site model. At 25 degrees C, neither mono- nor divalent cations altered the characteristics of [3H]-nitrendipine binding to rat cardiac membranes. At 37 degrees C, Na+ (100 mM) but not K+ (100 mM) significantly increased the Bmax of [3H]-nitrendipine in rat brain membranes. Ca2+ (2 mM) significantly increased the Bmax of [3H]-nitrendipine binding to rat brain membranes to a greater extent than at 25 degrees C. Both Na+ and K+ had no effect on [3H]-nitrendipine binding to cardiac membranes, while Ca2+ (2 mM) significantly decreased the KD of [3H]-nitrendipine. It is suggested that the selective effects of mono- and divalent cations on [3H]-nitrendipine binding to rat brain and cardiac membranes may be associated with differences in the calcium current blocking activity of dihydropyridine calcium antagonists in brain and cardiac tissues.

Cardiac sarcoplasmic reticulum contains a low-affinity site for phenylalkylamines

The distribution of the bovine cardiac binding sites for the organic calcium-channel blockers was studied. Crude microsomal membranes were separated into three fractions, which contained mainly membranes derived from sarcolemma, 'junctional' sarcoplasmic reticulum containing transversal tubuli, and free sarcoplasmic reticulum. The high-affinity binding site for the dihydropyridines, determined in the presence of nitrobenzylthioinosine, was enriched 12-fold and 17-fold in sarcolemma and junctional sarcoplasmic reticulum. The binding sites for the phenylalkylamines, determined with [3H]verapamil or [3H](-)desmethoxyverapamil, were enriched 1.5-3.4-fold in sarcolemma and junctional sarcoplasmic reticulum but 6-10-fold in free sarcoplasmic reticulum. The phenylalkylamine-binding site, present in free sarcoplasmic reticulum, was partially destroyed by chymotrypsin or phospholipase A2 and C treatment. Specific binding was proportional to the concentration of the added membrane protein. The binding of (-)desmethoxyverapamil was half-maximally inhibited by 6.5 mM calcium chloride and was optimal in the presence of 5 mM EGTA. In three out of five preparations (-)desmethoxyverapamil bound to a single site with an apparent Kd value of 191 +/- 42.8 nM and a density of 34.5 +/- 7.7 pmol/mg protein. In two out of five preparations an additional high-affinity site (Kd approximately 0.67 nM) was detected. The low-affinity site bound other phenylalkylamines, but stereospecific binding of phenylalkylamines was not observed. Binding of phenylalkylamines to the low-affinity site was inhibited by some but not all calmodulin 'antagonists'. Furthermore dihydropyridines did not affect the binding of (--)desmethoxyverapamil suggesting that the low-affinity site differs considerably from the high-affinity sarcolemmal site. These results suggest that free sarcoplasmic reticulum contains a binding site for phenylalkylamines at a relative high density, which is not related to the high-affinity site present in the voltage-dependent calcium channel.

Solubilization of the bovine cardiac sarcolemmal binding sites for calcium channel blockers

Nonionic and ionic detergents were used to solubilize the bovine cardiac sarcolemmal binding sites for nimodipine and (-)desmethoxyverapamil in the absence of added ligand. Only Chaps, digitonin and sucrose monolauryl ester were able to solubilize the binding sites in a form that bound radioligands. About 45% of each of the membrane-bound high-affinity site was solubilized by 0.4% Chaps (w/v) in the presence of 48% (w/v) glycerol. The solubilized binding sites were destroyed by trypsin or by a 10-min incubation at 50 degrees C. Calcium stimulated nimodipine binding slightly at 0.3 mM and inhibited (-)desmethoxyverapamil binding completely with an IC50 of 1.2 mM. Nimodipine binding was reduced by 20% in the presence of EGTA. The solubilized receptors sedimented in sucrose density gradients with an apparent s20,w of 21 S. An identical sedimentation value was obtained for the cardiac sarcolemmal and skeletal transverse tubulus receptor which were prelabeled with nitrendipine and solubilized by digitonin. Solubilization reduced the affinity of nimodipine for its high-affinity site slightly from 0.35 nM to 1.2 nM and that for its low-affinity site from 33 nM to 130 nM. Solubilization did not affect significantly the specific density of these sites. Binding of nimodipine to the low-affinity site was completely abolished by 0.1 microM nitrobenzylthioinosine. After solubilization only the high-affinity site for (-)desmethoxyverapamil could be measured with tenfold reduced affinity (Kd = 15.3 nM) but unchanged specific density. Binding to the solubilized high-affinity site for nimodipine and (-)desmethoxyverapamil was stereospecific and showed a similar rank order as the particulate binding sites. Binding of nimodipine was inhibited allosterically by phenylalkylamines. Similarly, (+)PN200-110 inhibited allosterically (-)desmethoxyverapamil binding. d-cis-Diltiazem stimulated nimodipine binding at 20 degrees C 1.2-fold, reduced the dissociation rate from 0.018 min-1 to 0.0083 min-1 and had no effect on the association rate (0.173 min-1. nM-1). The Kd calculated from the rate constants was 0.1 nM and in close agreement with the value of 0.49 nM measured under equilibrium conditions in the presence of nitrobenzylthioinosine. In contrast, desmethoxyverapamil increased the dissociation rate of nimodipine to 0.03 min-1. The association and dissociation rate constants for (-)desmethoxyverapamil were 0.024 min-1. nM-1 and 0.025 min-1, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of nifedipine and felodipine on cerebral blood flow during anoxic episodes

Cerebral blood flow (CBF) in the rat was monitored by a venous outflow technique with an extracorporeal circulation, which allows for the continuous recording of flow over periods of several hours. Brief periods of anoxia increase the rate of flow. The dihydropyridine calcium antagonists did not affect basal flow rate and depressed the increase in CBF elicited by anoxia. These findings may have significant implications for the therapeutic use of dihydropyridine calcium antagonists in brain ischaemia.

Nifedipine: more than a calcium channel blocker

Nifedipine exhibits a greater incidence of side effects than the other currently marketed calcium channel antagonists. In addition to those effects attributable to calcium channel blockade, nifedipine produces side effects similar to the effects of adenosine. It is probable that nifedipine exerts part of its physiological actions through potentiation of adenosine. Adenosine, an endogenous calcium channel blocker, modifies synaptic events throughout the nervous system and causes sedation, smooth and skeletal muscle relaxation, anticonvulsion, hypotension and hypothermia, all reversible by caffeine or theophylline administration. Nifedipine inhibits adenosine uptake from, and release into, the extracellular space and binds at an adenosine receptor. Both nifedipine and adenosine interact with benzodiazepine binding sites. Interaction between nifedipine and adenosine should be kept in mind when treating patients with nifedipine.

Basic and clinical aspects of adenosinergic neuromodulation

Adenosine and the methylxanthines have marked and opposite effects on behavior both of which are now thought to be mediated by cell surface adenosine receptors present in brain. These receptor sites have now been characterized using simple radioreceptor ligand binding techniques. Pharmacologic, autoradiographic and behavioral studies involving adenosine and the methylxanthines strongly suggest a neuromodulatory role for adenosine and indicate that adenosinergic neurons constitute an important central nervous system depressant system. A key component of the adenosinergic system is the adenosine uptake site which represents the inactivation mechanism for receptor mediated adenosine action. The adenosine uptake site can be identified as distinct from the adenosine receptor using a specific ligand. The two key components of the adenosine system, i.e., the receptor and uptake site, can therefore be studied using simple binding techniques. This should facilitate the development of new drugs specific for each system. Adenosine agonists can be expected to have sedative, anticonvulsant and anxiolytic actions whereas adenosine antagonists such as caffeine have stimulant and anxiogenic properties. Adenosine uptake blockers should have pharmacologic actions similar to adenosine agonists. The adenosinergic system, therefore, offers unique opportunities for developing new and potentially useful clinical agents.

Solubilization of an adenosine uptake site in brain

Procedures are described for the solubilization of adenosine uptake sites in guinea pig and rat brain tissue. Using [3H]nitrobenzylthioinosine [( 3H]NBI) the solubilized site is characterized both kinetically and pharmacologically. The binding is dependent on protein concentration and is saturable, reversible, specific, and high affinity in nature. The KD and Bmax of guinea pig extracts are 0.13 +/- 0.02 nM and 133 +/- 18 fmol/mg protein, respectively, with linear Scatchard plots obtained routinely. Similar kinetic parameters are observed in rat brain. Adenosine uptake inhibitors are the most potent inhibitors of [3H]NBI binding with the following order of potency, dilazep greater than hexobendine greater than dipyridamole. Adenosine receptor ligands are much less potent inhibitors of binding, and caffeine is without effect. The solubilized adenosine uptake site is, therefore, shown to have virtually identical properties to the native membrane site. The binding of the adenosine A1 receptor agonist [3H]cyclohexyladenosine [( 3H]CHA) to the solubilized brain extract was also studied and compared with that of [3H]NBI. In contrast to the [3H]NBI binding site [3H]CHA binds to two apparent populations of adenosine receptor, a high-affinity site with a KD of 0.32 +/- 0.06 nM and a Bmax of 105 +/- 30 fmol/mg protein and a lower-affinity site with a KD of 5.50 +/- 0.52 nM and Bmax of 300 +/- 55 fmol/mg protein. The pharmacology of the [3H]CHA binding site is consistent with that of the adenosine receptor and quite distinct from that of the uptake [( 3H]NBI binding) site. Therefore, we show that the adenosine uptake site can be solubilized and that it retains both its binding and pharmacologic properties in the solubilized state.

Characterization of the binding sites for nimodipine and (-)-desmethoxyverapamil in bovine cardiac sarcolemma

The bovine cardiac sarcolemmal binding sites for the dihydropyridine nimodipine and the phenylalkylamine (-)-desmethoxyverapamil were studied. The density of the nimodipine and (-)-desmethoxyverapamil binding sites increased 8.3-fold and 3.4-fold with the sarcolemma. The binding sites for both compounds were destroyed by trypsin. Nimodipine bound in the presence of 1 mM free calcium to a high-affinity and a low-affinity site with apparent Kd values of 0.35 +/- 0.09 nM (n = 9) and 33 +/- 6.0 nM (n = 9) and with apparent densities of 0.3 +/- 0.05 pmol/mg (n = 9) and 8.2 +/- 1.0 pmol/mg (n = 9). The binding to the high-affinity site was abolished by 1 mM EGTA. The binding sites were specific for dihydropyridines. The (-)-isomers of several phenylalkylamines inhibited nimodipine binding by an apparent allosteric mechanism. (-)-Desmethoxyverapamil bound in the presence of 5 mM EGTA to a high-affinity and a low-affinity site with apparent Kd values of 1.4 +/- 0.3 nM (n = 6) and 171 +/- 26 nM (n = 6) and with apparent densities of 0.16 +/- 0.02 pmol/mg (n = 6) and 13.6 +/- 2.7 pmol/mg (n = 6). The binding to both sites was inhibited by calcium with a half-maximal concentration of 4.3 mM. The binding sites were specific for the other phenylalkylamines and had a higher affinity for the (-)-isomers than for the (+)-isomers. Nimodipine inhibited the binding of (-)-desmethoxyverapamil by an apparent allosteric mechanism. d-cis-Diltiazem inhibited non-competitively the binding of (-)-[3H]desmethoxyverapamil with a Ki of 3.7 microM. Diltiazem up to concentrations of 10 microM did not affect the amount of nimodipine bound at equilibrium at 20 degrees C. However, but in agreement with this result, diltiazem decreased threefold at 20 degrees C the dissociation and association rates for the high-affinity nimodipine receptor. These rates were only marginally affected at 4 degrees C and 37 degrees C. d-cis-Diltiazem reversed in a competitive manner the inhibition of nimodipine binding elicited by the addition of (-)-desmethoxyverapamil with a Ka value of 1.6 microM. The amount of nimodipine bound was inhibited by 50% by the adenosine uptake inhibitors nitrobenzylthioinosine and hexobendine with apparent median inhibitory concentrations of 1 nM and 3 nM, respectively. Nitrobenzylthioinosine completely abolished binding of nimodipine to the low-affinity site, but did not affect binding to the high-affinity site.(ABSTRACT TRUNCATED AT 400 WORDS)

Human red-blood-cell Ca2+-antagonist binding sites. Evidence for an unusual receptor coupled to the nucleoside transporter

The human red blood cell ghost Ca2+-antagonist binding sites were characterized with (+/-)-[3H]nimodipine. The labelled 1,4-dihydropyridine bound in a non-cooperative, reversible manner with a Kd of 52 nM at 25 degrees C to 9.65 pmol sites/mg ghost protein. The stereochemistry of the binding domain was evaluated with the optically pure enantiomers of chiral 1,4-dihydropyridines. In contrast to the 1,4-dihydropyridine-selective receptors on Ca2+ channels in electrically excitable tissues, the (+) enantiomer of nimodipine and the (-) enantiomer of the benzoxadiazol 1,4-dihydropyridine (PN 200-110) were bound with higher affinity than the respective optical antipodes. The human red blood cell ghost [3H]nimodipine-labelled sites also interacted with the inorganic Ca2+-antagonist La3+ (increase in the number of binding sites), and were allosterically regulated by the optical enantiomers of the phenylalkylamine-type Ca2+-antagonists (e.g. verapamil, desmethoxyverapamil, methoxyverapamil). The benzothiazepines d- or l-cis-diltiazem were without effect. Nucleosides (adenosine approximately equal to inosine greater than cytidine) were inhibitory at the nimodipine-labelled site, as were the nucleoside uptake inhibitors dipyridamole, hexobendine, dilazep, nitrobenzylthioinosine and nitrobenzylthioguanosine. The binding sites have essential sulfhydryl groups, show trypsin sensitivity, but are relatively heat stable. When nitrobenzylthioinosine was employed as a covalent probe to inactivate the red blood cell ghost nucleoside carrier, [3H]nimodipine binding was irreversibly lost. (+)-Nimodipine greater than (-)-nimodipine inhibited [14C]adenosine transport into human red blood cells. A good correlation between IC50 values for inhibition of [3H]nimodipine binding and IC50 values for inhibition of [14C]adenosine uptake was found for 18 compounds. Sheep red blood cells (which lack the nucleoside transporter) had no detectable [3H]nimodipine binding sites. It is concluded that the Ca2+-antagonist receptor sites of the human erythrocyte are coupled to the nucleoside transporter.

High and low affinity states of the dihydropyridine and phenylalkylamine receptors on the cardiac calcium channel and their interconversion by divalent cations

Drug receptors associated with Ca2+-channels in isolated chick heart membranes were found to exist in high and low affinity states. When assays were conducted in the presence of EDTA most of the receptors detected with the dihydropyridines (+)[3H]PN 200-110 or [3H]nitrendipine appeared to be in the lower affinity state. Inclusion of either Mg2+ or Ca2+ in the binding reactions resulted in the disappearance of the lower affinity state and the conversion of the receptors to a single high affinity state. Similar results were obtained with the phenylalkylamine derivative [3H]desmethoxyverapamil (D888). The results suggest that both the dihydropyridine and phenylalkylamine receptors on the cardiac Ca2+-channel can exist in interconvertible high and low affinity states in vitro, and that the proportion of receptors in each affinity state can be altered by the absence or presence of divalent cations.