Binding of propranolol and gentamicin to small unilamellar phospholipid vesicles. Contribution of ionic and hydrophobic forces

Development of acidic phospholipid containing immobilized artificial membrane column to predict drug-induced phospholipidosis potency

Drug-induced phospholipidosis (DIPLD) represents a big concern for both regulatory authorities and pharmaceutical companies in drug discovery. Many researches pointed out that the negatively charged intralysosomal lipids play an important role in the formation of DIPLD. To better mimic this negatively charged lipid surface, a novel immobilized artificial membrane (IAM) column was prepared via in situ copolymerization of 12-methacryloyl n-dodecylphosphocholine (MDPC) and 12-methacryloyl n-dodecylphosphoric acid (MDPA). By introducing MDPA, the surface of the resulting monolithic column can be maintained negatively charged over a broad pH range. Scanning electron microscopy, elemental analysis and nano-HPLC experiments were carried out to characterize the physicochemical properties and chromatographic performance of the obtained monolithic IAM column. The results of ζ-potential and retention mechanism studies indicate that both hydrophobic and electrostatic interactions contribute greatly to the retention of cation analytes owing to the existence of the negatively charged MDPA under acidic conditions. To better assess the DIPLD potency of drug, the molar ratio between MDPC and MDPA in the monolithic column was carefully optimized. The results show that the poly(MDPC₇₀PA₃₀-co-EDMA) column has the best predictability with only two false-positives (donepezil, flecainide) in qualitative analysis of 61 drugs.

Preferential Adsorption of l-Histidine onto DOPC/Sphingomyelin/3β-[N-(N',N'-dimethylaminoethane)carbamoyl]cholesterol Liposomes in the Presence of Chiral Organic Acids

We investigated the effect of organic acids such as mandelic acid (MA) and tartaric acid (TA) on the adsorption behavior of both histidine (His) and propranolol (PPL) onto liposomes. A cationic and heterogeneous liposome prepared using 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/sphingomyelin (SM)/3β-[N-(N',N'-dimethylaminoethane)carbamoyl]cholesterol (DC-Ch) in a ratio of (4/3/3) showed the highest adsorption efficiency of MA and TA independent of chirality, while neutral liposome DOPC/SM/cholesterol = (4/3/3) showed low efficiency. As expected, electrostatic interactions were dominant in MA or TA adsorption onto DOPC/SM/DC-Ch = (4/3/3) liposomes, suggesting that organic acids had adsorbed onto SM/DC-Ch-enriched domains. The adsorption behaviors of organic acids onto DOPC/SM/DC-Ch = (4/3/3) were governed by Langmuir adsorption isotherms. For adsorption, the membrane polarities slightly decreased (i.e., membrane surface was hydrophilic), but no alterations in membrane fluidity were observed. In the presence of organic acids that had been preincubated with DOPC/SM/DC-Ch = (4/3/3), the adsorption of l- and d-His onto those liposomes was examined. Preferential l-His adsorption was dramatically prevented only in the presence of l-MA, suggesting that the adsorption sites for l-His and l-MA on DOPC/SM/DC-Ch = (4/3/3) liposomes are competitive, while those for l-His and d-MA, l-TA, and d-TA are isolated.

Alterations in endo-lysosomal function induce similar hepatic lipid profiles in rodent models of drug-induced phospholipidosis and Sandhoff disease

Drug-induced phospholipidosis (DIPL) is characterized by an increase in the phospholipid content of the cell and the accumulation of drugs and lipids inside the lysosomes of affected tissues, including in the liver. Although of uncertain pathological significance for patients, the condition remains a major impediment for the clinical development of new drugs. Human Sandhoff disease (SD) is caused by inherited defects of the β subunit of lysosomal β-hexosaminidases (Hex) A and B, leading to a large array of symptoms, including neurodegeneration and ultimately death by the age of 4 in its most common form. The substrates of Hex A and B, gangliosides GM2 and GA2, accumulate inside the lysosomes of the CNS and in peripheral organs. Given that both DIPL and SD are associated with lysosomes and lipid metabolism in general, we measured the hepatic lipid profiles in rodent models of these two conditions using untargeted LC/MS to examine potential commonalities. Both model systems shared a number of perturbed lipid pathways, notably those involving metabolism of cholesteryl esters, lysophosphatidylcholines, bis(monoacylglycero)phosphates, and ceramides. We report here profound alterations in lipid metabolism in the SD liver. In addition, DIPL induced a wide range of lipid changes not previously observed in the liver, highlighting similarities with those detected in the model of SD and raising concerns that these lipid changes may be associated with underlying pathology associated with lysosomal storage disorders.

On the Nature of Physiologically-Based Pharmacokinetic Models -A Priori or A Posteriori? Mechanistic or Empirical?

Physiologically-based pharmacokinetic (PBPK) models explicitly incorporate tissue-specific blood flows, partition coefficients, and metabolic processes. Since PBPK models are derived using physiologic parameters and interactions of the compound with tissue components, these models are considered to be "bottom up" as opposed to "top down". Modeling approaches can be characterized as either a posteriori (observational) or a priori (based solely on theory). Furthermore, approaches can be mechanistic (structure and components based on mechanisms) or empirical (based on observations alone). Both "bottom up" and "top down" approaches can incorporate either empirical or mechanistic components. In this perspective, we discuss some of the methods and assumptions of current PBPK modeling approaches. Specifically, we discuss drug partitioning into phospholipids and neutral lipids, use of blood-plasma ratios to estimate basic drug tissue partitioning, and clearance of neutral and acidic drugs. Based on these discussions, we believe that current PBPK models are mechanistic but a posteriori and semi-empirical.

Induction of Apoptosis by Cationic Amphiphilic Drugs Amiodarone and Imipramine

Phospholipidosis is the excessive accumulation of intracellular phospholipids in cell lysosomes. Drugs that induce this disease often share common physiochemical properties and are collectively classified as cationic amphiphilic drugs (CADs). Although the cause of phospholipidosis and morphologic appearance of affected lysosomes have been studied extensively, less is known about the physiologic effects of the condition. In the current study, U-937 cells were incubated with the CADs amiodarone (2.5-10 microg/mL) and imipramine (2.5-20 microg/mL). Treatment of U-937 cells with these compounds for 96 h resulted in concentration-related increases in phospholipids, as assessed by flow cytometry using the fluorophore nile red. These results were verified by measuring the concentrations of choline-derived phospholipids, which were significantly increased in drug-treated cells. Cell number in amiodarone (10 microg/mL) and imipramine (20 microg/mL) cultures following the 96-h incubation period were markedly reduced compared to control cultures. These observations suggested that accumulation of cellular phospholipids could inhibit cell proliferation. Flow cytometric analysis revealed a decrease in the percentage of cells in the S-phase of the cell cycle following drug treatment, yet DNA replication still occurred in a significant portion of cells. Interestingly, amiodarone and imipramine induced apoptosis in U-937 cells as shown by annexin V-FITC staining and DNA fragmentation. Enzymatic assays demonstrated that amiodarone and imipramine induced the activity of caspases 2 and 3. These results suggest that disruption of cell lysosomes in U-937 cells following accumulation of phospholipids does not cause a cell cycle arrest but instead induces apoptosis by activation of caspase pathways.

Evaluation of the interaction of propranolol with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes: the partitioning model

The sorption behavior of the amine containing beta-receptor blocking agent propranolol (Ppn) in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles was investigated. Both protonated and unprotonated Ppn were measured in the continuous phase after removal of the vesicles containing sorbed Ppn by centrifugation. In contrast, by analyzing the surface charge density, deduced from electrophoretic mobility measurements, only the sorbed protonated Ppn was determined. A partitioning model was used to describe the sorption behavior. Sensitivity analysis revealed that sufficiently reliable and independent parameters were obtained. The partition coefficient of the unprotonated Ppn was about 22 times higher than that of the protonated analogue. Statistical analysis revealed a significant increase in the intrinsic partition coefficients of both Ppn analogues with an increase in the salt concentration.

Role of phospholipids in drug-LDL bindings as studied by high-performance frontal analysis/capillary electrophoresis

The binding study between basic drugs ((S)-verapamil (VER) and (S)-propranolol (PRO)) and phospholipid liposomes was performed by using high-performance frontal analysis/capillary electrophoresis (HPFA/CE) in order to investigate the effect of oxidative modification of low-density lipoprotein (LDL) upon drug-binding affinity from molecule-based viewpoint. 1-Palmitoyl-2-oleoyl-phosphatidylcholine (POPC, 16:0, 18:1), 1-palmitoyl-2-linoleoyl-phosphatidylcholine (PLPC, 16:0, 18:2), dilauloyl-phosphatidylcholine (DLaPC, 12:0, 12:0), 1-palmitoyl-2-oleoyl-phosphatidyl-glycerol (POPG, 16:0, 18:1), and 1-palmitoyl-sn-glycero-3-phosphocholine (monoPPC, 16:0) were used to prepare the model liposomes. At physiological pH (pH 7.4), the model liposome prepared from POPG+POPC had negative net charges, while the total net charge of the other model liposomes (POPC liposome, PLPC liposome, DLaPC liposome, and monoPPC+POPC liposome) was zero. The drug and the model liposome mixed solutions were subjected to HPFA/CE, and the total binding affinities (nK) were calculated. The nK values of VER and PRO to POPG+POPC liposome were more than six and 10 times higher than those of other liposomes, respectively. On the other hand, the nK values of the model drugs to POPC liposome, PLPC liposome, DLaPC liposome and monoPPC+POPC liposome showed small differences less than twice. These results indicate that the electrostatic interaction plays an important effect on drug-liposome binding, and suggest that the increase in the negative charge of LDL phospholipids gives more significant effect on the drug-binding affinity of the basic drugs than the acyl-chain structure.

Intracellular delivery and antibacterial activity of gentamicin encapsulated in pH-sensitive liposomes

Cell membranes are relatively impermeable to the antibiotic gentamicin, a factor that, along with the toxicity of gentamicin, precludes its use against many important intracellular bacterial infections. Liposomal encapsulation of this drug was used in order to achieve intracellular antibiotic delivery and therefore increase the drug's therapeutic activity against intracellular pathogens. Gentamicin encapsulation in several dipalmitoylphosphatidylcholine (DPPC) and pH-sensitive dioleoylphosphatidylethanolamine (DOPE)-based carrier systems was characterized. To systematically test the antibacterial efficacies of these formulations, a tissue culture assay system was developed wherein murine macrophage-like J774A.1 cells were infected with bacteria and were then treated with encapsulated drug. Of these formulations, DOPE-N-succinyl-DOPE and DOPE-N-glutaryl-DOPE (70:30;mol:mol) containing small amounts of polyethyleneglycol-ceramide showed appreciable antibacterial activities, killing greater than 75% of intracellular vacuole-resident wild-type Salmonella typhimurium compared to the level of killing of the control formulations. These formulations also efficiently eliminated intracellular infections caused by a recombinant hemolysin-expressing S. typhimurium strain and a Listeria monocytogenes strain, both of which escape the vacuole and reside in the cytoplasm. Control non-pH-sensitive liposomal formulations of gentamicin had poor antibacterial activities. A fluorescence resonance energy transfer assay indicated that the efficacious formulations undergo a pH-dependent lipid mixing and fusion event. Intracellular delivery of the fluorescent molecules encapsulated in these formulations was confirmed by confocal fluorescence microscopy and was shown to be dependent on endosomal acidification. This work shows that encapsulation of membrane-impermeative antibiotics in appropriately designed lipid-based delivery systems can enable their use in treating intracellular infections and details the development of a general assay for testing the intracellular delivery of encapsulated drug formulations.

Effect of propranolol on rat brain synaptosomal Na(+)-K(+)-ATPase, Mg(2+)-ATPase and Ca(2+)-ATPase

The beta blocker drug propranolol (PPL) significantly inhibited Na(+)-K(+)-ATPase, Mg(2+)-ATPase and Ca(2+)-ATPase activities in a concentration dependent manner in rat brain synaptosomes. The concentrations required for 50% inhibition (IC50) in the activity of these enzymes were 1.5-1.8 mM. The double-reciprocal plot of ATP-stimulated Na(+)-K(+)-ATPase activity in the presence of PPL showed apparent decrease in K(m) and Vmax and the inhibition was of an uncompetitive type with respect to ATP. The nature of inhibition by PPL of Na(+)-activated Na(+)-K(+)-ATPase activity was of a mixed type showing an increase in Km and decrease in Vmax. Potassium activation kinetics of Na(+)-K(+)-ATPase displayed uncompetitive type of inhibition with PPL since Km and Vmax were decreased. Magnesium activation of Mg(2+)-ATPase showed decrease in Vmax with no apparent change in Km in the presence of PPL. The drug inhibited synaptosomal Ca(2+)-ATPase in an uncompetitive manner. The observed inhibition of synaptosomal ATPases indicates possible alterations in the synaptic transmission by the beta blocker drug PPL.

New systematically active antimycotics from the beta-blocker category

Candida albicans secretes phospholipases, which are considered to be one of the mediators of cell penetration. It is known that other phospholipases from mammalian cells can be inhibited by lipophilic beta-blocking structures. As the result of a synthesis programme of several years' duration, structures deriving from beta-hydroxyethylamines were introduced. In vitro and in vivo results with these compounds are presented in comparison with standard antimycotics. In combination with fluconazole, several of the compounds can prevent death in mice infected with lethal inocula of C. albicans. Histological examinations confirm the inhibitory effect of the beta-blocker-like structures on tissue penetration. The structures therefore constitute new antimycotics that are endowed with extensive in vitro effectiveness against fungi and also definite in vivo effects in the animal model.

Molecular parameters involved in aminoglycoside nephrotoxicity

Aminoglycoside antibiotics are hydrophilic molecules consisting of an animated cyclitol associated with amino sugar. They bind in vivo as well as in vitro to negatively charged membranes. Their use as chemotherapeutic agents is unfortunately accompanied by oto- and nephrotoxic reactions, and the purpose of this review is to examine the role of the molecular interactions between aminoglycosides and membranes in the development of nephrotoxicity. 31P Nuclear magnetic resonance (NMR) and fluorescence depolarization have been used to characterize the effect of aminoglycosides on phosphate heads and fatty acyl chains of phospholipids. 15N NMR has been used to obtain interesting information on regioselective interactions of amino groups of antibiotics with phospholipids. The binding of aminoglycosides with negatively charged membranes is associated with impairment of phospholipid catabolism, change in membrane permeability, and membrane aggregation. Biochemical analysis and 1H NMR spectroscopy have brought information on the molecular mechanism involved in the impairment of phospholipid catabolism. Nephrotoxic aminoglycosides could induce sequestration of phosphatidylinositol and therefore reduce the amount of negative charge available for optimal lysosomal phospholipase activity toward phosphatidylcholine included in liposomes that also contain cholesterol and sphingomyelin. Conformational analysis shows that aminoglycosides, which have a high potency to inhibit lysosomal phospholipase activity, adopt an orientation parallel to the lipid/water interface. This orientation of the aminoglycoside molecule at the interface is also critical to explain the marked increase of membrane permeability induced by less nephrotoxic aminoglycosides such as isepamicin and amikacin. This effect is indeed only observed with aminoglycosides oriented perpendicular to this interface, probably related to the creation of a local condition of disorder. The impairment of phospholipid catabolism, which is considered to be an early and significant step in the development of aminoglycoside toxicity, is therefore not related to the change in membrane permeability. However, the role of this latter phenomenon and of membrane aggregation for aminoglycoside nephrotoxicity could be further investigated.

Concentration-dependent binding of the chiral beta-blocker oxprenolol to isoelectric or negatively charged unilamellar vesicles

Large unilamellar vesicles (LUVs) of different lipid compositions were used to study the type of binding of the beta-blocking cationic agent oxprenolol to the lipid matrix of biological membranes at a physiologic pH value of 7.4. When isoelectric membranes of pure egg lecithin or egg lecithin/cholesterol (7:3 mol/mol) were used, a linear relationship between membrane-bound and free oxprenolol indicated a constant molar partition coefficient of 54 or 44 between the liposomal and the aqueous phase over a wide concentration range of the drug up to 25 mM. This pointed to deep insertion of the drug molecules into the hydrophobic membrane interior. Drug binding to membranes of negatively charged phosphatidylserine from bovine brain was cooperative with a Hill coefficient h of 3.4 at concentrations below 0.5 mM and a molar ratio Re of bound drug per lipid of 1:10. Above drug concentrations of 2.5 mM and Re = 1:5, a constant molar partition coefficient of 33 could be estimated. R-oxprenolol or S-oxprenolol, as well as the racemic drug, showed no differences in membrane binding, even with egg lecithin LUVs containing 20 mol% of the negatively charged (2S, 4R)-N-(hexadecanoyl)-4-hydroxyproline, which has a pronounced chiral headgroup. Our results suggest that enantioselective interactions of the chiral oxprenolol with the chiral lipids of biological membranes can be excluded. Furthermore, surface adsorption of the drug is probable only on the negatively charged cytosolic side of biological plasma membranes, whereas on the isoelectric exterior the cationic drug is inserted deeply into the membrane.

The transport mechanisms of organic cations and their zwitterionic derivatives across rat intestinal brush-border membrane. 1. Binding characteristics to the bio- and lipid-membranes

The uptake mechanisms of organic cations such as tryptamine, tyramine, 5-benzyloxytryptamine (BOTA) and their zwitterionic derivatives (tyrosine, tryptophan, 5-benzyloxytryptophan (BOTP)) by rat intestinal brush-border membrane vesicles and liposome containing phosphatidylserine were studied and compared. As compared to their zwitterionic derivatives, uptake rates by rat intestinal brush-border membrane of these three cations were far superior. The binding of cationic compounds to the brush-border membrane was also higher than those of their zwitterionic derivatives. Furthermore, the binding behaviour of BOTA and tryptamine to phospholipid liposome clearly amplified with increasing amounts of phosphatidylserine. In contrast, the contents of phosphatidylserine, a negatively charged phospholipid, exhibited no effects on the binding of zwitterionic derivatives (tryptophan and BOTP). The double-reciprocal plot of tryptamine binding with BOTA to liposome showed competitive inhibition. These results suggest that the binding of organic cations to the membrane lipid has a relatively high specificity despite the absence of membrane protein such as a transport-carrier in the liposome, and that the binding of cationic compounds play an important role in the uptake to the cell membrane systems.

Effect of acidic phospholipids on the activity of lysosomal phospholipases and on their inhibition by aminoglycoside antibiotics--I. Biochemical analysis

Aminoglycoside antibiotics accumulate in lysosomes of kidney and cultured cells and cause an impairment of phospholipid catabolism which is considered to be an early and significant step in the development of their toxicity. Using liposomes, wer previously demonstrated that the activity of lysosomal phospholipases A1 and A2 towards phosphatidylcholine was markedly enhanced by the inclusion of phosphatidylinositol in the bilayer, and that gentamicin impaired this activity by binding to phosphatidylinositol. Since gentamicin-induced inhibition was inversely related to the amount of phosphatidylinositol included in the liposomes, we proposed that gentamicin impairs activity of phospholipases by decreasing the quantity of available negative charges carried by the bilayer surface (Mingeot-Leclercq et al., Biochem Pharmacol 37: 591-599, 1988). We now extend these observations to phosphatidylserine and phosphatidic acid, and compare the inhibition caused by gentamicin, amikacin and streptomycin towards lysosomal phospholipases on the hydrolysis of phosphatidylcholine in the presence of each of these acidic phospholipids. Inclusion of phosphatidic acid in liposomes, and, to a lesser extent, phosphatidylserine, caused a larger increase in phospholipases activity than phosphatidylinositol. In parallel, the three aminoglycosides tested were found less inhibitory towards phospholipases activity measured on phosphatidic acid-or phosphatidylserine-containing liposomes than was previously observed with phosphatidylinositol, even though equilibrium dialysis experiments failed to demonstrate significant difference in binding parameters of the drug towards each of these liposomes populations. Yet, as for phosphatidylinositol-containing liposomes, the inhibition was inversely related to the amount of phosphatidic acid or phosphatidylserine included in the bilayer and the inhibitory potency of the three drugs was consistently gentamicin greater than amikacin greater than streptomycin with the three types of negatively-charged liposomes used. We conclude that impairment of lysosomal phospholipases activity towards phosphatidylcholine included in negatively-charged membranes by aminoglycoside antibiotics is dependent upon drug binding to the bilayer, but that it is modulated by the nature of the acidic phospholipid that binds the drug as well as by that of the drug itself. A companion paper (Mingeot-Leclercq et al., Biochem Pharmacol 40: 499-506, 1990) will examine by computer-aided conformational analysis the parameters (drug-phospholipid energy of interaction, position of the drug in a monolayer and its accessibility to the aqueous phase) which may be important for these effects.

Correlation between structures of organic cations and their binding behaviours to brush border membrane isolated from rat small intestine

We have investigated the correlation between the molecular structures of various organic cations and their binding to rat small intestinal brush border membrane. The binding of small quaternary ammonium compounds such as tetramethylammonium and choline to brush border membrane was not sufficient to inhibit methylchlorpromazine binding. However, lauryltrimethylammonium and cetyltrimethylammonium, both quaternary amines with a long carbon chain, inhibited binding significantly. The inhibition was competitive. When the unbranched hydrocarbon chain of the quaternary amines was extended in steps from C1 (methyl) to C16 (cetyl), the inhibitory effect increased sharply with length from C7 (heptyl) to C16. These results suggest that the size of the hydrophobic part of the molecule is an important factor in binding of quaternary ammonium compounds to the brush border membrane. The structure of the hydrophilic part was another factor. In imipramine-related compounds, the order of binding was N-didesmethylimipramine (primary amine) greater than desipramine (secondary) greater than imipramine (tertiary) greater than methylimipramine (quaternary). However, with the small molecular ethylamine-related compounds, binding properties did not reflect differences in the hydrophilic component. Therefore, the effect of the hydrophilic part may be secondary and may depend on the size of the hydrophobic part. We suggest that organic cations which are amphiphilic can bind to a common binding site on brush border membrane through hydrophobic and/or hydrophilic interactions.

Model of gentamicin-induced nephrotoxicity and its amelioration by calcium and thyroxine

The exact mechanism of gentamicin-induced acute renal failure is presently unknown; various mechanisms have been proposed but there is no proposed commonality between them. In animals, dietary calcium loading and L-thyroxine administration have been shown to ameliorate toxicity, with again no common process. A mechanism of competitive displacement of calcium and other cations from anionic phospholipids at the plasma and organelle membrane level, resulting in a decrease in Na+ -K+ ATPase, adenylate cyclase, mitochondrial function and ATP production, protein synthesis, solute reabsorption and overall cellular function is proposed. A further proposal is dietary calcium loading and thyroxine (which increases intracellular calcium) reverse gentamicin-induced acute renal failure by increasing the calcium and solute flux, thereby competitively inhibiting the primary lesion: anionic phospholipid binding.

Involvement of sialic acid in high-affinity binding of quaternary ammonium compounds by brush border membrane from rat intestine

As one approach to clarify the absorption mechanisms of quaternary ammonium compounds (QACs), their binding characteristics have been studied using brush border membrane vesicles isolated from rat small intestine and liposomes composed of phospholipid and GM3 ganglioside. The binding of propantheline was significantly decreased when the vesicles were pretreated with neuraminidase. Propantheline and methochlorpromazine bound to the liposomes, the binding for the latter drug being significantly greater than that for propantheline. When GM3, isolated from rat small intestine, was incorporated into the liposomes their binding capacity for both drugs increased significantly. It is suggested that the binding of QACs to the lipid layer and sialic acid play a role in the high binding of drugs to the intestinal brush border membrane. Furthermore, a sensitive and reproducible high-performance liquid chromatographic method for sialic acid has been developed.

Inhibition of purified human postheparin lipoprotein lipase by beta-adrenergic blockers in vitro

We examined the effects of five beta-adrenergic blockers on the hydrolysis of phosphatidylcholine-stabilized triolein particles by purified human postheparin lipoprotein lipase (PHLpL) in order to evaluate the possible role of direct inhibition as a mechanism of drug-induced hypertriglyceridemia. The relative inhibitory potencies were observed in the following order: propranolol much greater than pindolol greater than metoprolol greater than atenolol greater than nadolol. There was a positive correlation between the octanol/water partition coefficients of these agents and their inhibition of lipoprotein lipase, suggesting that hydrophobicity may be one of the major determinants for PHLpL inhibition. The amount of the beta-adrenergic blockers required to produce 50% inhibition of human PHLpL was much greater than that required to inhibit purified bovine lipoprotein lipase.

Binding of organic cations to brush border membrane from rat small intestine

The binding of six organic cations (chlorpromazine, promethazine, imipramine, diphenhydramine, methochlorpromazine and propantheline) to the brush border membrane isolated from rat small intestine has been investigated. The cations were bound to the membrane to varying extents, the order of binding being chlorpromazine greater than promethazine greater than methochlorpromazine greater than imipramine greater than propantheline greater than diphenhydramine. There was no relation between binding and the chloroform-water partition coefficient. Chlorpromazine binding was significantly decreased in the presence of imipramine, methochlorpromazine and propantheline. Anionic compounds (indomethacin and xanthene-9-carboxylic acid) did not affect chlorpromazine binding. High and low affinity binding of the cations to the intestinal brush border membrane was demonstrated with Scatchard plots and Hill plots. Imipramine and methochlorpromazine inhibited chlorpromazine binding at both binding sites. From the results, it was suggested that the organic cations tested were specifically bound to common binding sites on the brush border membrane.

Gentamicin causes the fast depression of action potential and contraction in cultured cardiocytes

The short-term cardiotoxic effects of gentamicin were studied on electrical and mechanical activities in cultures of ventricular confluent cardiocytes of rats. Application of gentamicin to the bath lowered the spontaneous action potential firing rate which was restored when the external calcium was elevated. Fast local application and withdrawal of drugs to non-pacemaker cells allowed effect/dose curves to be made. Such application of 200 nmol gentamicin (approximated concentration: 0.8 x 10(-3)-8.0 x 10(-3) M) lowered the contraction amplitude by 50% and the action potential overshoot by 20% while the action potential duration and maximum diastolic potential remained unchanged. Gentamicin withdrawal was followed by a rapid (half time = 5 s) and complete restoration of contraction. Verapamil, Mn2+, EDTA and La3+ also diminished contraction and overshoot but their effects were reversed 28, 25, 6 and 1.5 times more slowly, respectively. Gentamicin, like La3+, could act extracellularly, probably by competing with calcium ions for some external membrane site.

Propranolol inhibition of the neutral phospholipases A of rat heart mitochondria, sarcoplasmic reticulum and cytosol

Membrane damage caused by phospholipase A action is thought to be an important factor in ischemic myocardial injury. Propranolol has been shown previously to have beneficial effects in both animal experiments and clinical trials, and it has membrane-stabilizing properties in vitro. To investigate the possibility that these effects might be due, in part, to effects on phospholipases, we determined the effects of propranolol on rat heart phospholipases A at physiological pH using small unilamellar liposomes of di[1-14C]oleoylphosphatidylcholine as substrate. Propranolol inhibited heart phospholipases A in vitro. The concentration required to give 50% inhibition was 0.2 mM for the mitochondrial and cytosolic phospholipases A and 2.9 mM for sarcoplasmic reticulum phospholipase A. The binding of [4-3H]propranolol to fresh membrane preparations was studied using an ultracentrifugation method. Propranolol bound readily to both membrane fractions in vitro with no significant difference in the saturation number (0.20 to 0.28 mol drug per mol phospholipid) but the association constant, KA, was higher for mitochondrial membranes (3760 +/- 350) than for the sarcoplasmic reticulum (2190 +/- 390). Our results show that propranolol inhibited heart phospholipases A in vitro at physiological pH. The mitochondrial and cytosolic phospholipases A were more susceptible to inhibition than the phospholipase A of sarcoplasmic reticulum. Propranolol bound to mitochondria and sarcoplasmic reticulum in vitro, suggesting the possibility that propranolol binding to heart membranes in vivo could result in drug concentrations in these membranes high enough to inhibit phospholipase A. This could represent an additional mechanism by which propranolol exerts beneficial effects in myocardial ischemia.

Inhibition of purified bovine milk lipoprotein lipase by propranolol and other beta-adrenergic blockers in vitro

Numerous clinical studies have shown that propranolol administration causes hypertriglyceridemia and a decrease in high-density lipoprotein in man. Although these findings have been attributed to diminution of triacylglycerol-rich lipoprotein catabolism by lipoprotein lipase, biochemical studies of the effects of propranolol on lipoprotein lipase activity in vitro have not been previously reported. We purified lipoprotein lipase from raw bovine skimmed milk and examined the effect of propranolol using as substrate phospholipid-stabilized, triolein emulsions containing purified human apolipoprotein C-II. These studies demonstrate that propranolol inhibits lipoprotein lipase activity. The inhibition was found to be noncompetitive with a Ki for propranolol of 0.55 mM. In addition, propranolol was shown to bind to phospholipid-stabilized triolein emulsions reaching local concentrations at the particle surface many times higher than its bulk concentration. Metoprolol, timolol and practolol, which are less hydrophobic than propranolol, were less inhibitory. Atenolol was the weakest inhibitor of purified bovine lipoprotein lipase in vitro.