The effect of quinidine and drugs with quinidine-like action (propranolol, verapamil and tetracaine) on the calcium transport system in isolated sarcoplasmic reticulum vesicles of rabbit skeletal muscle

Hexanol and lidocaine affect the oligomeric state of the Ca-ATPase of sarcoplasmic reticulum

Hexanol at 7 degrees C stimulates the activity of the Ca-ATPase of sarcoplasmic reticulum (SR). Time-resolved phosphorescence spectroscopy studies of SR whose Ca-ATPase is covalently labeled with erythrosin isothiocyanate (ERITC) indicate that at 7 degrees C hexanol (1) cause a concentration-dependent increase in the rate of decay of phosphorescence anisotropy, (2) causes larger oligomers of Ca-ATPase to dissociate into smaller oligomers, and (3) increases the rotational mobility of Ca-ATPase in all its oligomeric states. Electron paramagnetic resonance (EPR) spectroscopy of spin-labeled stearic acid (SASL) in SR suggests that at 7 degrees C hexanol diminishes the fraction of SR lipids in the boundary lipid domain and disorders and fluidizes both the boundary lipid and the unrestricted lipid domain. In protein-free liposomes of extracted SR lipids hexanol increases fluidity and decreases order to a greater extent near the center of the lipid bilayer than near the polar head groups. At 25 degrees C hexanol has biphasic effects on Ca-ATPase activity: at 10 and 20 mM hexanol increases activity, but at 30 mM and especially at 40 mM there is inhibition of Ca-ATPase activity. The influence of hexanol at 25 degrees C on the oligomeric state of Ca-ATPase is also biphasic. At 10 and 20 mM, hexanol promotes the dissociation of larger oligomers into smaller ones, whereas at higher concentrations, 30 and 40 mM, hexanol causes larger oligomers to be formed from smaller ones. Lidocaine at 25 degrees C inhibits Ca-ATPase activity and causes dramatic slowing of the decay of phosphorescence anisotropy of ERITC-labeled SR by causing the formation of larger oligomers of Ca-ATPase from smaller ones. In protein-free liposomes of SR lipids at 25 degrees C, lidocaine disorders and fluidizes the acyl chains near the center of the bilayer (as did hexanol), but has opposite effects near the polar head groups. The opposite effects of hexanol and lidocaine on the oligomeric state of the SR Ca-ATPase provide a new molecular explanation for the opposite effects of hexanol and lidocaine on the activity of the Ca-ATPase. We conclude that the biphasic effects of hexanol on the activity of Ca-ATPase can be accounted for by biphasic effects of hexanol on the oligomeric state of the Ca-ATPase. This study supports the view that anesthetics can alter interactions between membrane proteins.

The effect of lidocaine on myocardial ischemia with asanguinous reperfusion. An in vitro study

The effect of lidocaine on the ischemic nor-mothermic rat heart was studied in a Langendorff preparation. Ventricular fibrillation, total retrograde coronary flow and effluent lactate concentration were monitored in preischemia (control), ischemia (20 min) and reperfusion (20 min). Myocardial metabolites were determined in specimens excised at termination of reperfusion. Six hearts were infused with lidocaine in Ringer solution at onset of ischemia (group A) and six with only Ringer solution (group B). Sinus rhythm proceeded directly to diastolic arrest after 17 sec in group A, while all group B hearts showed ventricular fibrillation before arrest at 174 sec. Effluent lactate concentration was reduced in group A during the first 10 min of ischemia, but not subsequently. After 10 min of reperfusion, coronary flow was reduced by 12% in group A and 20% in group B. ATP was higher and ADP, AMP and IMP were lower in group A than in group B after 20 min of reperfusion. Creatine phosphate showed no intergroup difference, but creatine was higher in group B. Cardiac arrest with lidocaine thus reduced lactate formation during ischemia and lessened high-energy phosphate depletion after reperfusion.

Direct binding of verapamil to the ryanodine receptor channel of sarcoplasmic reticulum

Radioligand binding experiments and single channel recordings demonstrate that verapamil interacts with the ryanodine receptor Ca2+ release channel of the sarcoplasmic reticulum of rabbit skeletal muscle. In isolated triads, verapamil decreased binding of [3H]Ryanodine with an IC50 of approximately 8 microM at an optimal pH 8.5 and pCa 4.3. Nitrendipine and d-cis-diltiazem did not interfere with binding of [3H]Ryanodine to triads, suggesting that the action of verapamil does not involve the dihydropyridine receptor. Single channel recordings showed that verapamil blocked Ca2+ release channels by decreasing open probability, duration of open events, and number of events per unit time. A direct interaction of verapamil with the ryanodine receptor peptide was demonstrated after purification of the approximately 400 kDa receptor protein from Chaps-solubilized triads. The purified receptor displayed high affinity for [3H]Ryanodine with a Kd of approximately 5 nM and a Bmax of approximately 400 pmol/mg. Verapamil and D600 decreased [3H]Ryanodine binding noncompetitively by reducing the Bmax. Thus the presence of binding sites for phenylalkylamines in the Ca2+ release channel was confirmed. Verapamil blockade of Ca2+ release channels may explain some of the paralyzing effects of phenylalkylamines observed during excitation-contraction coupling of skeletal muscle.

Drug-induced calcium release from heavy sarcoplasmic reticulum of skeletal muscle

Calcium release from isolated heavy sarcoplasmic reticulum of rabbit skeletal muscle by several calmodulin antagonistic drugs was measured spectrophotometrically with arsenazo III and compared with the properties of the caffeine-induced calcium release. Trifluoperazine and W7 (about 500 microM) released all actively accumulated calcium (half-maximum release at 129 microM and 98 microM, respectively) in the presence 0.5 mM MgCl2 and 1 mg/ml sarcoplasmic reticulum protein; calmidazolium (100 microM) and compound 48/80 (70 micrograms/ml) released maximally 30-40% calcium, whilst bepridil (100 microM) and felodipin (50 microM) with calmodulin antagonistic strength similar to trifluoperazine (determined by inhibition of the calcium, calmodulin-dependent protein kinase of cardiac sarcoplasmic reticulum) did not cause a detectable calcium release, indicating that this drug-induced calcium release is not due to the calmodulin antagonistic properties of the tested drugs. Calcium release of trifluoperazine, W7 and compound 48/80 and that of caffeine was inhibited by similar concentrations of magnesium (half-inhibition 1.4-4.2 mM compared with 0.97 mM for caffeine) and ruthenium red (half-inhibition for trifluoperazine, W7 and compound 48/80 was 0.22 microM, 0.08 microM and 0.63 micrograms/ml, respectively, compared with 0.13 microM for caffeine), suggesting that this drug-induced calcium release occurs via the calcium-gated calcium channel of sarcoplasmic reticulum stimulated by caffeine or channels with similar properties.

The effects of quinine on the calcium and magnesium content of the sarcoplasmic reticulum and the temperature-dependence of quinine contractures

A significant decrease in the Ca2+ and increase in the Mg2+ content of the terminal cisternae (TC) of the sarcoplasmic reticulum (SR) during quinine contraction was demonstrated by electron probe analysis of rapidly frozen frog muscles. The extent of Ca2+ release (71% of total) from the TC and the absence of an increase in total cell Ca2+ support the conclusion that quinine contractures are caused by passive efflux of Ca2+ from the SR when the latter is uncompensated due to inhibition of the SR Ca2+ pump by quinine. A rapid warming contraction (RWC) was observed, in the presence of quinine, when the temperature of intact and skinned muscles was increased from about 5 degrees C to 18-23 degrees C. The duration of the latency of quinine contracture, in intact muscle bundles, was approximately 31 s at 3 degrees C and 2 s at 23 degrees C. The results suggest a significant temperature sensitivity of the passive Ca2+ channels of the SR membrane, although an effect of temperature on the lipid partition coefficient of quinine into the SR has not been ruled out.

Ba2+-inhibitable 86Rb+ fluxes across membranes of vesicles from toad urinary bladder

86Rb+ fluxes have been measured in suspensions of vesicles prepared from the epithelium of toad urinary bladder. A readily measurable barium-sensitive, ouabain-insensitive component has been identified; the concentration of external Ba2+ required for half-maximal inhibition was 0.6 mM. The effects of externally added cations on 86Rb+ influx and efflux have established that this pathway is conductive, with a selectivity for K+, Rb+ and Cs+ over Na+ and Li+. The Rb+ uptake is inversely dependent on external pH, but not significantly affected by internal Ca2+ or external amiloride, quinine, quinidine or lidocaine. It is likely, albeit not yet certain, that the conductive Rb+ pathway is incorporated in basolateral vesicles oriented right-side-out. It is also not yet clear whether this pathway comprises the principle basolateral K+ channel in vivo, and that its properties have been unchanged during the preparative procedures. Subject to these caveats, the data suggest that the inhibition by quinidine of Na+ transport across toad bladder does not arise primarily from membrane depolarization produced by a direct blockage of the basolateral channels. It now seems more likely that the quinidine-induced elevation of intracellular Ca2+ activity directly blocks apical Na+ entry.

Effects of (+)-propranolol on intracellular mechanisms of contraction in striated muscle of the rabbit

In functionally skinned muscle fibers from the rabbit, we studied the effect of propranolol on calcium activation of the contractile proteins and, in separate experiments, on calcium uptake and release from the sarcoplasmic reticulum (SR) while measuring physiological tension. Pieces from isolated papillary muscle (PM), soleus (SL) (slow-twitch skeletal muscle), and adductor magnus (AM) (fast-twitch skeletal muscle) were homogenized (sarcolemma disrupted). A fiber bundle from PM and single fibers from SL and AM were dissected from the homogenate and mounted on a photodiode tension transducer. To study Ca2+-activated tension development of the contractile proteins, we used high EGTA (7 mmol/l) to control the free calcium concentration. To study SR function, we used five different solutions to load the calcium into the SR and to release it from the SR with 25 mmol/l caffeine, thus producing a tension transient. In general, propranolol has similar mechanisms of action in the three muscle types. Propranolol (0.1-1.0 mmol/l) increased the submaximal calcium-activated tension development in all muscles but with PM = SL greater than AM, and this increase was correlated with increases in calcium binding to isolated troponin C. Propranolol increased the maximal calcium-activated tension development in PM and SL, but decreased that in AM. Propranolol at concentrations of 0.3-1.0 mmol/l decreased calcium uptake by the SR but did not change calcium release in any of the three muscles. In PM, however, propranolol at a concentration of 0.1 mmol/l increased calcium uptake by the SR. We conclude that propranolol induces decreases in muscle contraction mainly by decreasing calcium uptake by the SR.

Fasciola hepatica: the effects of neuropharmacological agents upon in vitro motility

The effects of a wide range of neuropharmacological agents on the motility in vitro of Fasciola hepatica have been determined using an isometric transducer system. The neuromuscular blocking agents tubocurarine and decamethonium cause a long-term stimulation of the basal activity of the fluke. Acetylcholine causes an inhibition of activity. This effect is mimicked by the cholinergic agonists carbachol and nicotine, antagonised by the cholinergic blocking agents atropine and mecamylamine, and potentiated by eserine, a cholinesterase inhibitor. With nicotine and atropine the effects are accompanied by an increase in muscle tone at a concentration of 1 X 10(-2) M. Noradrenaline and adrenaline also cause some inhibition of activity, an effect antagonised by guanethidine, which blocks the release of noradrenaline. In contrast, dopamine stimulates fluke motility, whilst its antagonist dihydroergotamine causes an inhibition of activity. The monoamine oxidase inhibitors iproniazid and p-chloromercuribenzoic acid induce a stimulation of activity; with the latter there is an increase in muscle tone at a concentration of 1 X 10(-3) M. The amine depleting agents chloroamphetamine and reserpine, and the monoamine uptake inhibitors desipramine and nortriptyline produce an inhibition of fluke activity, as does the serotonin uptake inhibitor fluoxetine. High concentrations of chloroamphetamine (1 X 10(-2) M) and the uptake inhibitors (1 X 10(-3) M and above) also induce an increase in muscle tone. Serotonin causes a marked stimulation of motility. The pharmacological evidence is consistent with a neurotransmitter role of acetylcholine (inhibitory), dopamine (excitatory), and noradrenaline (inhibitory). The status of serotonin is discussed.

Intracellular mechanism of quinidine action on muscle contraction. A comparison between rabbit cardiac and skeletal muscle

The mechanism of quinidine action on rabbit cardiac and skeletal muscle was examined with "functionally skinned" muscle-fiber preparations. By using these preparations we could correlate measurements of muscle tension with the effect of quinidine on the Ca2+ activation of the contractile proteins and on the Ca2+ uptake and release from the sarcoplasmic reticulum (SR).

Dysfunction of the sarcoplasmic reticulum in polymyositis

The transport of calcium in vesicles of sarcoplasmic reticulum isolated from muscle specimens from 6 patients with early, active polymyositis and from 11 controls was examined. The time courses of calcium uptake and calcium-dependent ATPase activity were measured simultaneously. Calcium uptake by sarcoplasmic reticulum vesicles from patients with polymyositis was 50% less than that by vesicles from controls (P less than 0.001). In contrast, no difference in calcium-dependent ATPase activity was noted between vesicles from patients with polymyositis and controls. The demonstrated defect may be important in the pathogenesis of muscle weakness in polymyositis.

Inhibition of calcium-dependent ATPase from sarcoplasmic reticulum by a new class of indolizidine alkaloids, pumiliotoxins A, B, and 251D

Pumiliotoxins (PTX) A, B, and 251D, members of a new class of indolizidine alkaloids isolated from the skin of poison frogs of the family Dendrobatidae, inhibit Ca2+-ATPase activity in sarcoplasmic reticulum vesicles from frog and rat hind-limb muscles. PTX-B and PTX-A appear to be relatively specific inhibitors of Ca2+-ATPase; PTX-A is much less potent than PTX-B. PTX-251D is a potent inhibitor of Ca2+-ATPase, and was also found to inhibit Na+, K+, and Mg2+-ATPases in rat brain synaptosomes. Caffeine and verapamil, two drugs known to affect calcium translocation, are very weak inhibitors of the Ca2+-ATPase. The Ki values for inhibition of the Ca2+-ATPase of rat and frog sarcoplasmic reticulum by PTX-B were comparable and ranged between 22 and 36 microM. Inhibition of calcium-dependent ATPase in sarcoplasmic reticulum by pumiliotoxin-B is noncompetitive with calcium and is not readily reversible. Based on structure-activity profiles, it is concluded that inhibition of Ca2+-ATPase by the indolizidine alkaloids is responsible for the alkaloid-elicited prolongation of twitch in intact muscle.

Calcium uptake in isolated brush-border vesicles from rat small intestine

Ca2+ uptake in brush-border vesicles isolated from rat duodena was studied by a rapid-filtration technique. Ca2+ uptake showed saturation kinetics, was dependent on the pH and ionic strength of the medium and was independent of metabolic energy. Uptake activity was readily inhibited by Ruthenium Red, La3+, tetracaine, EGTA, choline chloride and Na+ or K+. The effect of variations in medium osmolarity on Ca2+ uptake and the ionophore A23187-induced efflux of the cation from preloaded vesicles indicated that the Ca2+-uptake process involved binding to membrane components, as well as transport into an osmotically active space. Scatchard-plot analyses of the binding data suggested at least two classes of Ca2+-binding sites. The high-affinity sites, Ka = (2.7 +/- 1.1) x 10(4) M-1 (mean +/- S.D.) bound 3.2 +/- 0.8 nmol of Ca2+/mg of protein, whereas the low-affinity sites (Ka = 60 +/- 6 M-1) bound 110 +/- 17 nmol of Ca2+/mg of protein. In the presence of 100 mM-NaCl, 1.7 and 53 nmol of Ca2+/mg of protein were bound to the high- and low-affinity sites respectively. Decreased Ca2+-uptake activity was observed in vesicles isolated from vitamin D-deficient as compared with vitamin D-replete animals and intraperitoneal administration of 1,25-dihydroxycholecalciferol to vitamin D-deficient rats 16 h before membrane isolation stimulated the initial rate of Ca2+ uptake significantly. The data indicated that Ca2+ entry and/or binding was passive and may involve a carrier-mediated Ca2+-uptake component that is associated with the brush-border membrane. Altering the electrochemical potential difference across the membrane by using anions of various permeability and selected ionophores appeared to increase primarily binding to the membrane rather than transport into the intravesicular space. Since there is considerable binding of Ca2+ to the vesicle interior, a comprehensive analysis of the transport properties of the brush-border membrane remains difficult at present.

Negative inotropic effect of beta-blockers in the guinea-pig atrium after preincubation with isoprenaline

In the isolated guinea-pig atrium, pretreated with a high concentration of isoprenaline (8 x 10(-7) M), beta-blockers produced a marked negative inotropic effect in a concentration which by itself was too low to have a direct depressing action. No negative inotropic effect was caused by isoprenaline when beta-blockers were applied first. The depressing action of beta-blockers was unaffected by phentolamine or atropine. During the depression, the electrical parameters of the action potential were not significantly different from those of the control, though the contractions were much smaller than control ones. The negative inotropic effect of beta-blockers was weaker in excess Ca (7.5--10 mM). The observations indicate that isoprenaline possesses a dual action but that the negative inotropic effect is usually masked by the positive inotropic effect and that beta-blockers, by quickly abolishing the positive inotropic effect, reveal the underlying negative inotropic effect.

Stimulatory and inhibitory effects of dimethylsulfoxide, propranolol and chlorpromazine on the partial reactions of ATPase of sarcoplasmic reticulum

The effects of dimethylsulfoxide, propranolol and chlorpromazine on the partial reactions of the ATPase of sarcoplasmic reticulum were investigated. When analyzed according to a reaction scheme in which the ADP-sensitive (E1P) and ADP-insensitive (E2P) phosphoenzymes occur sequentially and P1 is derived from the latter, dimethylsulfoxide decreased the rate of E2P hydrolysis whereas it stimulated the rate of the E1P to E2P conversion. Propranolol increased the rate of E2P hydrolysis while it decreased the rate of the E1P to E2P conversion. Propranolol exerted an additional effect, presumably inhibition of the phosphoenzyme formation. These effects of dimethylsulfoxide and propranolol can account for both the stimulatory and inhibitory effects of these drugs on the overall rate of ATP hydrolysis observed in the presence and absence of added alkali metal salts. Chlorpromazine accelerated E2P hydrolysis whereas it appeared to inhibit the E1P to E2P conversion. These effects of chlorpromazine appear able to account for its stimulatory and inhibitory effects on the overall rate of ATP hydrolysis in the presence and absence of alkali metal salts. In the presence of chlorpromazine, however, the rate of Pi liberation during the steady state ATP hydrolysis was found to be greater than the hydrolysis rate of E2P. This finding suggests that under these conditions Pi is derived not only from E2P but also from source(s) other than E2P.

The influence of (+)-propranolol on the inotropic effect of dihydro-ouabain in relation to stimulation frequency

1. The influence of (+)-propranolol, 10(-6) to 4 x 10(-5) mol/l, upon the positive inotropic effect of dihydro-ouabain was studied on guinea-pig papillary muscle. At 1 Hz, negative inotropically effective concentrations of (+)-propranolol decreased the inotropic effectiveness of the glycoside. 2. For equivalent diminution of the basal force of contraction, either by reduction of stimulation frequency or by increasing concentrations of (+)-propranolol, a similar decrease in the inotropic effectiveness of dihydro-ouabain was observed. In contract, reduction of [Ca2+]o had almost no effect on the effectiveness of the glycoside. 3. The inhibitory effect of (+)-propranolol on the inotropic effectiveness of dihydro-ouabain declined with reduction of stimulation frequency. At any stimulation frequency tested, the inhibitory effect of (+)-propranolol was proportional to the effect of a reduction of that particular frequency to the rested-state condition in the absence of (+)-propranolol. 4. The results are consistent with the hypothesis that the inhibition of the excitation-coupled sodium influx by (+)-propranolol is the cause of its inhibiting effect on the effectiveness of dihydro-ouabain.

Effect of propranolol and related drugs on transmembraneous pH differences in liposomes

Propranolol (1-isopropylamino-3-(1-naphtoloxy)-propan-2-ol) a beta-adrenergic receptor blocking agent was found to cause changes of transmembraneous pH in liposomes prepared from Soy-lecithin and cardiolipin. When the external pH was neutral and the internum of the liposomes acidic, the drug decreased the pH gradient. When the externum was acidic and the internum neutral, the gradient was increased by the drug. The effect of butacaine was similar to that of propranolol, while procaine, timolol and practolol were ineffective. It is suggested that the charged form of propranolol is bound to the membrane and dislocates protons from binding sites in the membrane and that the uncharged form of propranolol penetrates the membrane. After penetration it could associate with protons in the intraliposomal compartment and hence increase the pH of the interior. Depending on the direction of the pre-existing proton gradient propranolol would thus be able to increase or decrease the pH difference across the liposomal membrane.

Roles of prostaglandins suggested by the prostaglandin agonist/antagonist actions of local anaesthetic, anti-arrhythmic, anti-malarial, tricyclic anti-depressant and methyl xanthine compounds. Effects on membranes and on nucleic acid function

We have recently found that local anaesthetic, anti-malarial, anti-arrhythmic, tricyclic antidepressant and methyl xanthine compounds behave as prostaglandin antagonists at drug concentrations readily attainable in human body fluids. We have also found that various hormones, including prolactin, vasopressin and angiotensin when present in physiological concentrations can be potent stimulators of prostaglandin synthesis. Cortisol has no effect on basal prostaglandin systhesis but in physiological concentrations is able to reverse the effects of other hormones. The drugs are thought to act mainly by interfering with either calcium interactions with membranes, or with cyclic nucleotide synthesis or degradation or with nucleic acid function. We propose that prostaglandins play crucial roles at all three points. The hypothesis leads to new approaches to many aspects of cell regulation, to the functions of many systems, especially the neuromuscular, cardiovascular, renal and respiratory systems. It also has profound implications for drug design and evaluation and for the understanding of many clinical disorders.

Chloroquine, quinine, procaine, quinidine, tricyclic antidepressants, and methylxanthines as prostaglandin agonists and antagonists

Chloroquine, quanine, procaine, quinidine, clomipramine, theophylline, and caffeine have been shown to be strong prostaglandin antagonists and weak agonists. The antagonist effect is clearly demonstrable at concentrations reached in human plasma when the drugs are used therapeutically. This suggests that prostaglandins are important in several situations in which their role has hitherto been unsuspected. New approaches to the development of prostaglandin antagonists and new uses for established drugs are indicated. In a preliminary study chloroquine has been successfully used to close patent ductus arteriosus in three infants.

Aspects of the mechanism of action of local anesthetics on the sarcoplasmic reticulum of skeletal muscle

1. The effect was studied of local anesthetics (tetracaine, dibucaine, procaine and xylocaine) on the forward and the backward reactions of the calcium pump of skeletal muscle sarcoplasmic reticulum. 2. The inhibition of the rate of calcium uptake, the rate of calcium-dependent ATP splitting and the rate of calcium-dependent ATP-ADP phosphate exchange by sarcoplasmic reticulum in the presence of the above drugs is at least partially due to the inhibition of the phosphoprotein formation from ATP. 3. The rate of the ADP-induced calcium release from sarcoplasmic reticulum and the rate of ATP synthesis driven by the calcium efflux are inhibited on account of a reduction of the phosphoprotein formation by orthophosphate. 4. The phosphorylation of calcium transport ATPase by either ATP or orthophosphate is diminished by the local anesthetics owing to a reduction in the apparent calcium affinity of sarcoplasmic reticulum emmbranes on the outside and on the inside, respectively. 5. The drug-induced calcium efflux from calcium-preloaded sarcoplasmic reticulum vesicles, a reaction not requiring ADP, is probably not mediated by calcium transport ATPase.

Cell shape changes and transmembrane receptor uncoupling induced by tertiary amine local anesthetic

Tertiary amine local anesthetics (dibucaine, tetracaine, procaine, etc.) modify cell morphology, concanavalin A (Con A)-mediated agglutinability and redistribution of Con A receptors. Con A agglutination of untransformed mouse 3T3 cells was enhanced at low concentrations of local anesthetics, and the dynamics of fluorescent-Con A indicated that ligand-induced clustering was increased in the presence of the drugs. In contast, these drugs inhibited Con A-induced receptor capping on mouse spleen cells. These effects can be duplicated by combinations of vinblastine (or colchicine) and cytochalasin B suggesting that local anesthetics act on microtubule and microfilament assemblies which are involved in the trans-membrane control of cell surface receptor mobility and distribution. It is proposed that tertiary amine local anesthetics displace plasma membrane-bound Ca2+, resulting in disengagement of microfilament systems from the plasma membrane and increased cellular Ca2+ concentration to levels which disrupt microtubular organization. The possible involvement of cellular Ca2+ in cytoskeletal destruction by local anesthetics was investigated utilizing Ca2+-specific ionophores A23187 and X537A. In media containing Ca2+ and cytochalasin B these ionophores caused effects similar to tertiary amine local anesthetics.

Comparative study of the effects of propranolol and tetracaine on cation movements in resealed human red cell ghosts

1. The effects of two positively charged local anaesthetic amines, tetracaine and propranolol, on cation permeability were studied in resealed human red cell ghosts prepared from metabolically depleted erythrocytes. 2. The K permeability was reduced by tetracaine but increased by propranolol. The effect of tetracaine was independent of extracellular Ca concentration but was raised to 2-5 x 10(-7) M. The effect of propranolol, which was enhanced when the external Ca concentration was raised, could be completely inhibited by lowering the internal free Ca to less than 10(-7) M. 3. Propranolol, but not tetracaine, increased the intracellular Ca ion concentration by releasing up to 20% of the membrane-bound Ca to the cell interior. This increase in intracellular Ca was sufficient to mediate the observed change in K permeability. 4. Tetracaine and propranolol reduced the Ca binding capacity of the ghost membrane by about 20 and 40% respectively. The Ca permeability was increased by propranolol and was slightly reduced by tetracaine. 5. In high concentrations (2-7 mM) propranolol by itself moderately increased K and Na permeability, but supressed completely the Ca-induced increase in K permeability. Tetracaine in concentrations up to 4 mM enhanced the Ca-induced increase in K permeability. Higher concentrations of the drug caused lysis of the cells. 6. Maximally effective concentrations of tetracaine and propranolol inhibited the ATP-dependent Ca outward transport by 30 and 70% respectively. 7. The effects of tetracaine on K permeability were shared by the local anaesthetics prilocaine and lidocaine, those of propranolol were shared by practolol, a beta-adrenergic antagonist and tetraethylammonium, a ganglionic blocking agent. 8. It is suggested that the differences in the effects of tetracaine and propranolol on cation permeability reflect qualitatively different interactions of the two drugs with Ca binding sites on the inner surface of the membrane.