Drug-free Solution Research Articles

Permeability of latex and polyvinyl chloride gloves to fluorouracil and methotrexate

The permeability of latex surgical and polyvinyl chloride (PVC) examination gloves to fluorouracil and methotrexate was studied using a radiotracer method. Drug-tracer solutions for both drugs were prepared by combining tritiated fluorouracil or methotrexate with fluorouracil or methotrexate injection, respectively. Drug-free solutions were also prepared to be as similar as possible to the vehicles of the commercial injections. Each half of a total of 40 equilibrium-dialysis cells (20 for each glove type) was filled with either the drug-tracer solution or the drug-free solution. The cell halves were then sealed with glove membranes cut from each type of glove and fastened together so that the drug-tracer and drug-free solutions of each respective drug were opposite one another. Samples were collected after 5 to 45 minutes from each dialysis-cell well and analyzed for drug using liquid scintillation counting techniques. Microgram levels of both drugs were detected in the drug-free dialysis wells for both glove types at each sampling time. Except for the methotrexate drug-labeled well sealed with the PVC glove membrane, the amount of drug in the dialysis wells was significantly different at different sampling times. The variability in results using gloves from different lots was also significantly different for both types of gloves. Overall, latex gloves were significantly less permeable than PVC gloves to fluorouracil, and PVC gloves were significantly less permeable to methotrexate than latex gloves. The latex and PVC gloves used in this study both exhibit time-dependent permeability to fluorouracil and methotrexate.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium Channel Block by a Potent, New Antiarrhythmic Agent, Transcainide, in Guinea Pig Ventricular Myocytes

Transcainide is a new lidocaine analog that has been shown to suppress a range of cardiac arrhythmias in an initial clinical trial. We have evaluated the effects of transcainide on sodium channel current in guinea pig ventricular myocytes using the whole-cell patch-clamp technique. Reduction of sodium current by transcainide was concentration dependent, with an ED50 of approximately 0.5 microM (n = 9). This reduction of the sodium current exhibited little use dependence, and block did not accumulate even after 10-Hz pulse trains. Moreover, little or no recovery from block was observed even when cells were hyperpolarized to -160 mV for 1 min. We observed no reversal of sodium channel block after superfusing cells up to 1 h in drug-free solution. Thus, transcainide, unlike many other clinically useful antiarrhythmic agents, blocked sodium channels with very little time dependence or voltage dependence. These novel electrophysiological properties of transcainide may endow the agent with a unique spectrum of efficacy against certain arrhythmias. At the same time, the great potency of transcainide and its lack of reversibility, at least within an hour, mandate that the drug should be used with great caution.

Tetraethylammonium blockade of calcium-activated potassium channels in clonal anterior pituitary cells

The effects of extracellular and intracellular tetraethylammonium (TEA) ions on single Ca2+-activated K+ channels were studied in excised membrane patches from the anterior pituitary clone AtT-20/D16-16 with the patch-clamp technique. The presence of TEA on either surface of the membrane resulted in a decrease in the single-channel current. Dissociation constants at zero voltage for the TEA-receptor complex were calculated to be 52.2 mM and 0.08 mM for external and internal TEA, respectively. The high sensitivity of AtT-20/D16-16 cells to internal TEA is of considerable interest, since in other preparations, the greater TEA sensitivity for Ca2+-activated K+ channels has thus far been found to occur on the external membrane surface. Hill plot analysis of the dose-response data yielded a slope of 0.92, indicating a one-to-one stoichiometry for TEA-receptor binding. The blockade by TEA showed little voltage or current sensitivity over the membrane potential range studied, and could be fully reversed by washout in drug-free solution. The results suggest the presence of TEA receptors on both the external and internal membrane surfaces but with different binding affinities. Occupancy of either site by TEA leads to an apparent decrease in the single-channel conductance of Ca2+-activated K+ channels.

Electrophysiological effects of alpha-adrenoceptor antagonists in rabbit sino-atrial node, cardiac Purkinje cells and papillary muscles.

The effects of prazosin, labetalol, and medroxalol were studied in the rabbit sino-atrial node, Purkinje cells and papillary muscles. At concentrations producing similar bradycardia, labetalol and medroxalol reduced the maximum rate of depolarization (Vmax) and overshoot potential in the sinus node. Prazosin had no such effects. These large and highly significant reductions in Vmax and overshoot in sinus node cells were observed at concentrations of medroxalol and labetalol which had no negative inotropic effect. If depolarization in the sinus node was due to the second inward current, this would imply that such currents in the sinus node and contracting cardiac muscle are pharmacologically distinct. All three drugs prolonged action potential duration in the sinus node, Purkinje cells and papillary muscles in a dose-related manner. Recovery after 1 h in drug-free solution from the effects of medroxalol and labetalol was only partial in the sinus node, but almost complete in Purkinje cells and papillary muscle. Recovery from prazosin was complete in all tissues. All three drugs depressed Vmax in Purkinje cells and papillary muscles in a dose-related manner, and recovery was complete. It is concluded that all three drugs had class 1 and class 3 antiarrhythmic actions, which could contribute to their protective effects in ischaemia and reperfusion independently of blockade of myocardial alpha-adrenoceptors.

Effect of Baralgin on isolated preparations of the upper urinary tract in man

The effects of the constituents of Baralgin ( metamizole , fenpiverinium , and pitofenone ) on mechanical activity were studied in isolated human preparations of the upper urinary tract. Metamizole in concentrations up to 10(-3) mol/l did influence neither spontaneous phasic activity nor activation by high external potassium, norepinephrine, or acetylcholine. Fenpiverinium blocked the increase in frequency of phasic-rhythmic contractions and the tonic tension development induced by acetylcholine. Fenpiverinium did influence neither spontaneous phasic activity nor activation by high potassium or norepinephrine. Pitofenone (10(-3)mol/l) antagonized completely spontaneous phasic-rhythmic as well as norepinephrine- or acetylcholine-induced phasic activity. The tonic activation induced by norepinephrine and acetylcholine was nearly completely inhibited. Pitofenone (10(-6) to 10(-3) mol/l) antagonized the high potassium-induced activation in a concentration-dependent way. The EC50 of pitofenone amounted to 2 X 10(-4) mol/l. Uptake measurements of 14C- pitofenone (2 X 10(-4) mol/l) in isolated ureteral segments showed the drug to be accumulated in the ureteral tissue; a tissue/medium ratio of about 3 was found at an exposure time of 60 min. The accumulation of the drug was reversible after washing in drug-free solution, indicated by the loss of radioactivity increasing with longer washing times. The results show that pitofenone has a direct relaxant effect on smooth muscle of the upper urinary tract, whereas fenpiverinium has exclusively anticholinergic properties. Metamizole had no direct effect on smooth muscle activity, and its clinical effects may be rather due to its analgesic action.

Electrophysiological studies of some semisynthetic cardiac glycoside derivatives in isolated papillary muscle of the guinea-pig.

The effects of digitoxin, 3 alpha-methyl-digitoxigenin-3 beta-monoglucoside (3 alpha-MDM), 3 alpha-methyl-digitoxigenin (3 alpha-MD), proscillaridin, 4, 5-methylene-procillaridin (4, 5-MP), and 3 beta-hydroxy-4, 5-methylene-A, B-trans-scillarenin (3 beta-HMTS) on force of contraction and on the transmembrane action potentials were examined in isolated papillary muscles of guinea-pigs. All derivatives exhibited the typical cardiac glycoside effects: i.e. they increased the force of contraction and shortened the action potential duration at 20% (plateau phase) and 90% of repolarization. With digitoxin, 3 beta-HMTS and 4, 5-MP a transient prolongation in action potential duration was observed at the lower concentrations. The action potential amplitude and the resting membrane potential were reduced consistently only with the higher concentrations used. The onset of the positive inotropic effects of 3 alpha-MDM, 3 alpha-MD and 3 beta-HMTS was more rapid than that of digitoxin and proscillaridin. The increment in contractile force reached a maximum well before the full shortening effect on the action potential duration had developed. The shortening of the action potential is thought to be responsible for the biphasic nature of the positive inotropic effect. With 3 alpha-MD and 3 alpha-MDM even toxic effects, e.g. increase in baseline tension, were completely reversible after washing in drug-free solution. The dose-response curves for the positive inotropism can only be compared reliably once the equilibrium of drug action has been established. This steady state is probably reflected by the development of the full shortening in action potential duration.

Inotropic and electrophysiological effects of dantrolene on guinea-pig papillary muscle

1. The mechanism of the inotropic effects of dantrolene, an inhibitor of intracellular Ca release in skeletal muscle, was analysed in isolated guinea-pig papillary muscles contracting isometrically at a frequency of 1 Hz. 2. In muscles from reserpine-pretreated animals and in muscles exposed to 30 nmol/l TTX, but taken from nonreserpinized animals, dantrolene produced a monophasic negative inotropic effect if applied at 1, 10, or 100 μmol/l. Within 24 min this effect amounted to ca. 30% of control force of contraction and was reached faster at the higher concentrations. It was not reversed during a subsequent 75-min superfusion with drug-free solution. 3. In muscles from non-reserpinized guinea pigs, the effects of 1 or 10 μmol/l dantrolene were identical to those described above, but 100 μmol/l induced a slight positive inotropic effect in 5 of 6 preparations. 4. The reduction of peak force of contraction in catecholamine-depleted muscles was accompanied by reductions in the rate of force development, time to peak force, and relaxation time. Resting potential, \(\dot V_{\max } \), and overshoot of the action potential were unaffected, and action potential duration (90% repol.) was prolonged slightly. Slow action potentials elicited at 24 mmol/l [K]0 at a frequency of 0.5 Hz were likewise unaffected with respect to \(\dot V_{\max } \) and overshoot and also showed a prolonged duration. Equilibration with 50 μmol/l dihydroouabain at 0.6 mmol/l [Ca]0 did not alter the extent of the negative inotropic dantrolene effect. The negative inotropic effect persisted in the presence of and after equilibration with caffeine (20 mmol/l) or ryanodine (2 μmol/l). 5. We conclude that the direct negative inotropic effect of dantrolene cannot be attributed to overt effects on the sarcolemma such as an inhibition of fast Na or slow Ca channels. The experiments with dihydroouabain argue against an impairment of the Nai-dependent regulator of contractility (Na−Ca exchange). While this evidence is compatible with an intracellular site of action of dantrolene, such as the sarcoplasmic reticulum, the drug acts differently from two other presumed inhibitors of this organelle, caffeine and ryanodine.

The nature of the membrane potential of glial cells associated with the medial giant axon of the crayfish

Electrophysiological, pharmacological and electron microscopic methods were used to characterize the satellite glial (Schwann-like) cells associated with the medial giant axon of the crayfish, Procambarus clarkii. The satellite glial cell layer surrounding the axon is formed by 15–20 cells deeply interdigitated into each other, forming a vast system of intercellular channels which communicate the axo-glial space with the external medium. The satellite cell layer varies from 0.2–3 μm in thickness. Membrane potentials of the giant axon and the satellite glial cells were monitored before, during and after treatment with ouabain and a variety of cholinergic agonists and antagonists. The membrane potentials of 63 control satellite cells averaged−42.6 ± 0.6 mV. The intracellular localization of the glial cell potential difference was corroborated by lithium carminate marking of the microelectrode tip recording site. Superfusion of satellite cells with 10 −7 m carbachol, nicotine or acetylcholine caused a 15 to 20 mV hyperpolarization from resting level. Muscarine (10 −6 m) had no effect on the glial cell potential. The effect of nicotine was prevented or reversed by-tubocurarine (10 −9 m). The effects of cholinomimetics were reversed by washing the cells in drug-free solution. None of these agents had an obvious effect on axon membrane potential or action potential generation at the concentration used in this study. Ouabain (10 −3 m) also caused a rapid hyperpolarization of the glial cells. The effect lasted 15–18 min after which the membrane rapidly depolarized. The results suggest that (1) the satellite glial cell of the crayfish giant axon system may be studied simultaneously with the axon using electrophysiological techniques; (2) the satellite glial cell membrane appears to have typical acetylcholine receptors of the nicotinic type; (3) the membrane potential of the glial cell is sensitive to ouabain and (4) properties of the glial cell associated with the giant axon of the crayfish are similar to those of the Schwann cell of the tropical squid Sepioteuthis sepioidea.

Effects of loperamide on the guinea pig taenia coli

The effects of an antidiarrhoel agent, loperamide, were studied using isolated guinea pig taeni coli. Loperamide in concentrations ranging from 10 −7 M to 10 −6 M inhibited the cholinergic, contractile responses induced by electrical transmural stimulation, nicotine and serotonin, and the nonadrenergic, relaxing responses induced by electrical transmural stimualtion and nicotine. However,t he adrenergic responses to perivascular nerve stimulation was not affectesd by these concentrations. The inhibitory effects of loperamide were not reversed by washing with a drug-free solution. Morphine (10 −6 M to 10 −5 M) also inhibited both cholinergic and nonadrenergic responses, but the effect was reversible. Naloxone (10 −6 M) attenuated the inhibitory effects of both drugs. Unlike morphine, loperamide in concentrations higher than 5 × 10 −6 M relaxed the strips and reduced the contractile response to acetylcholine noncempetitively, and these effects were not blocked by naloxone. These results suggest that loperamide at low concentration acts selectively ont he opiate receptors located in both cholinergic and of the intestinal tone.

Avermectin B1a irreversibly blocks postsynaptic potentials at the lobster neuromuscular junction by reducing muscle membrane resistance.

Avermectin B1a, a macrocyclic lactone with broad spectrum anthelmintic activity, affects neuromuscular transmission in the lobster stretcher muscle. Perfusion of the muscle with 1-10 microgram of the drug per ml eliminates inhibitory postsynaptic potentials within a few minutes. Intracellularly recorded excitatory postsynaptic potentials are gradually reduced in amplitude over 20-30 min, and their falling phases become faster; there is no effect, however, on extracellularly recorded excitatory potentials. Avermectin B1a reduced the input resistance of the muscle fibers with a time course similar to that of the reduction of excitatory potentials. Washing for up to 2 hr with drug-free solution fails to reverse the drug's effects. However, perfusion with 20 microgram of picrotoxin per ml results in recovery of the excitatory potentials and input resistance. Avermectin B1a also blocks the firing of the crayfish stretch receptor neuron, and this block is also reversed by picrotoxin. We hypothesize that the reduction in excitatory postsynaptic potentials after avermectin B1a treatment is caused solely by reduction in membrane resistance; additional experiments suggest that the reduction in membrane resistance is due to the opening of membrane Cl- channels, perhaps including those regulated by gamma-aminobutyric acid at the inhibitory synapse.

The effects of adenosine-und guanosine 3?,5?-phosphoric acid benzyl esters on guinea-pig ventricular myocardium

1. The inotropic effects of benzyl-and substituted benzyl triesters of cyclic AMP and cyclic GMP were studied in isometrically contracting guineapig papillary muscles driven at a rate of 0.2 Hz. 2. The triesters of cyclic AMP produced marked concentration-dependent positive inotropic effects. The maximum increase of contraction force (ΔFc) was 2.2 g (n=5) for p-MeBncAMP and 1.8 g for BncAMP and o-NO2BncAMP, respectively (n=5 for each ester). Total contraction time (t1+t2) was thereby shortened by 44 ms (n=15). Threshold and maximally effective concentrations for the positive inotropic effects were 1×10−4 M and 6×10−4M (BncAMP), 3×10−4 M and 3×10−3 M (p-MeBncAMP) and 1×10−5 M and 2×10−4 M (o-NO2BncAMP), respectively. 3. The phosphodiesterase inhibitor 1-methyl,3-isobutyxanthine (IBMX) (2×10−5 M) shifted the concentration-effect curves of all 3 triesters to the left, decreasing the concentrations required to produce a half-maximal increase of the force of contraction by a factor of 6. 4. The time necessary to produce a half-maximum inotropic effect was 2 min for p-MeBncAMP, 10 min for BncAMP and 66 min for o-NO2BncAMP. The inotropic effects could not be reversed upon washing the preparations for 1 h with a drug-free solution. 5. The concentration-effect curves of the triesters of cyclic AMP were not affected by previous blockade of the β-adrenoceptors with 5×10−6 M (±)-propranolol. 6. BncGMP at concentrations ranging from 1×10−5 to 2×10−3 M, produced no inotropic effects on 3 guinea-pig papillary muscles, while a decrease of the contraction force was detectable 30 min after the addition of 2×10−3 M p-MeBncGMP, reaching 75% of the control value within 1 h (n=3). At 3×10−3 M, both esters induced a positive inotropic effect (ΔFc) of 0.2 g which was completed in 3–4 min. The negative inotropic effect of 2×10−3 M p-MeBncAMP was immediately reversed upon washing the preparation while the positive inotropic effect was not affected. 7. Spontaneous hydrolysis of the benzyl triesters of cyclic AMP or cyclic GMP in Krebs-Henseleit buffer gave rise to cyclic AMP or cyclic GMP and to the respective benzyl alcohols. The electron donating substituent p-CH3 of the p-MeBncAMP or the p-MeBncGMP increased the hydrolytic reactivity by a factor of 14 when compared to the unsubstituted benzyl esters which in turn were 5 times more reactive than the triesters which contained the electron withdrawing substituent o-NO2. 8. Triesters of cyclic AMP and cyclic GMP (3×10−3 M) inhibited beef heart phosphodiesterase (PDE) by 17–29% and by 58–65%, respectively (n=4 for each ester). The specific PDE-inhibitor IBMX (2×10−5 M) blocked PDE by 52% (n=4). 9. The data presented in this study are in agreement with the hypothesis that benzyl triesters of cyclic AMP are simply a transport form of cyclic AMP across the cell membrane and, once inside, they are hydrolyzed with a characteristic time course to cyclic AMP which is responsible for the positive inotropic effects. Assuming that the benzyl triesters of cyclic GMP are also intracellularly converted to cyclic GMP, its postulated mediator role of a negative inotropic effect is not in accordance with the present results.

Electrophysiologic effects of encainide (MJ 9067) on canine purkinje fibres

Microelectrodes were used to characterize the actions of a new antiarrhythmic compound, encainide, 4-methoxy-2′-[2-(1-methyl-2-piperidyl)-ethyl] benzanilide, on transmembrane potentials generated by isolated canine Purkinje fibres in tissue bath. Encainide, 1×10 −7 M, had no effect on resting potential (RP) and action potential (AP) of stimulated fibres or automaticity of spontaneously fibres. Encainide (1×10 −6 M) shortened phase 2 and action potential duration (APD) at 50, 75 and 100% of repolarization; AP rate of rise (V̇max) decreased only ∼15% at 3 h exposure, while RP was unchanged. Higher concentrations also shortened phase 2 and APD, markedly decreased V̇mac, depressed the membrane responsiveness curve and increased conduction time. Encainide, 1×10 −6 M, increased ERP/APD 100. These effects were rapidly reversed by washing with drug-free solution. Encainide (1×10 −5 M) decreased the automatic rate of spontaneously beating untreated fibres and stretch-depolarized fibres, but had little effect on catecholamine-enhanced automaticity. Encainide, 1×10 −5 M, delayed the onset of ouabain toxicity. These findings suggesting that the antiarrhythmic effect of encainide may arise from its depression of V̇max and memrane responsiveness, increase in ERP/APD 100 and inhibition of automaticity, are discussed in light of the similarity of the actions of this new drug to the known properties of lidocaine and quinidone.

Sites of action and active forms of lidocaine and some derivatives on cardiac Purkinje fibers.

We have compared the effects of lidocaine (L) to those of several derivatives in an attempt to determine the sites of action and active forms of the molecules. We studied cardiac Purkinje fibers with intracellular microelectrodes, and drugs were administered by superfusion or by iontophoretic intracellular injection. We compared to L the effects of QX-314 and QX-572, two quaternary derivatives, and also compound 6603, a tertiary analogue with a pKa of 9.81. When administered by superfusion in concentrations of 10(-5) and 10(-4) M, all four agents exerted qualitatively similar effects on the transmembrane action potential. The rate of onset of action of the quaternary derivatives was considerably slower than that of L and their actions were not reversed by superfusion for 1 hour with drug-free solution. Comparison of effects of L and QX-314 after intracellular injection showed that attenuation of Vmax, and thus of the fast inward current, results from interaction of the charged form acting form acting from the inner surface of the sarcolemma. After intracellular injection, QX-314 diffused readily along the long axis of fiber bundles, and it was thus possible to expose the inner surface of a large number of contiguous cells to drug. Comparison of results of extracellular and intracellular application suggests that effects on the voltate during phase 2 result from the charged form acting from the inner surface, but effects on total action potential duration result only from drug acting from the outer surface of the sarcolemma. Unlike L, the three derivatives did not decrease the slope of normal phase 4 depolarization. The results may help relate antiarrhythmic action to specific effects on transmembrane potentials.

Effect of phentolamine, alprenolol and prenylamine on maximum rate of rise of action potential in guinea-pig papillary muscles

Effects of phentolamine (13.3, 26.5 and 53.0 micron), alprenolol (3.5, 7.0 and 17.5 micron) and prenylamine (2.4, 4.8 and 11.9 micron) on the transmembrane potential were studied in isolated guinea-pig papillary muscles, superfused with Tyrode's solution. 1. Phentolamine, alprenolol and prenylamine reduced the maximum rate of rise of action potential (.Vmax) dose-dependently. Higher concentrations of phentolamine and prenylamine caused a loss of plateau in a majority of the preparations. Resting potential was not altered by any of the drugs. Readmittance of drug-free Tyrode's solution reversed these changes induced by 13.3 micron of phentolamine and all conconcentrations of alprenolol almost completely but those induced by higher concentrations of phentolamine and all concentrations of prenylamine only slightly. 2. .Vmax at steady state was increased with decreasing driving frequencies (0.5 and 0.25 Hz) and was decreased with increasing ones (2--5 Hz) in comparison with that at 1 Hz. Such changes were all exaggerated by the above drugs, particularly by prenylamine. 3. Prenylamine and, to a lesser degree, phentolamine and alprenolol delayed dose-dependently the recovery process of .Vmax in premature responses. 4. .Vmax in the first response after interruption of stimulation recovered toward the predrug value in the presence of the above three drugs. The time constants of recovery process ranged between 10.5 and 15.0s for phentolamine, between 4.5 and 15.5s for alprenolol. The time constant of the main component was estimated to be approximately 2s for the recovery process with prenylamine. 5. On the basis of the model recently proposed by Hondeghem and Katzung (1977), it is suggested that the drug molecules associate with the open sodium channels and dissociated slowly from the closed channels and that the inactivation parameter in the drug-associated channels is shifted in the hyperpolarizing direction.

Effects of antipsychotic drugs on action potential production in skeletal muscle. II. Haloperidol: nonspecific and opiate drug receptor mediated effects

The effects of haloperidol, an antipsychotic butyrophenone, on excitability and action potential production in frog's sartorius muscle fibers were studied. This drug produced a local-anestheticlike effect which developed slowly over 1 to 5 h with lower concentrations (2.7 to 5.3 X 10(-6 M) but was completely reversed by exposing the muscles to a drug-free solution. In studies with intracellular microelectrodes, evidence was obtained showing that haloperidol decreased excitability and depressed action potential production by inhibiting the specific increase in sodium conductance (gNa) which normally follows an adequate stimulus. Evidence also was obtained showing an inhibition of the secondary increase in potassium conductance (gK). Haloperidol is structurally related to meperidine and it was found that the inhibition of gNa produced by haloperidol is partially antagonized by low concentrations of naloxone (2.8 X 10(-8) and 2.8 X 10(-7) M); as was previously shown for meperidine. Thus haloperidol, like meperidine, suppresses action potential production by two mechanisms of action: one, a nonspecific local-anaestheticlike effect; and the other, a specific inhibition of gNa mediated by means of an opiate drug receptor associated with the muscle fiber membrane. Naloxone did not antagonize the effects of chlorpromazine on gNa.

Effects of antipsychotic drugs on action potential production in skeletal muscle. I. Chlorpromazine and promethazine

The effects of chlorpromazine, an antipsychotic phenothiazine, and promethazine, an antihistaminic phenothiazine, on excitability and action potential production in frog's sartorius muscle fibers were studied and compared. Both drugs produced a local anaesthetic effect which developed slowly over 3 to 5 h with lower concentrations (1 to 15 × 10−6 M) and was only partially reversed by exposing the muscles to a drug-free solution for 3 to 4 h. The resting potential and the input resistance of the muscle fibers were unaffected by drug concentrations which reduced the action potential maximum rate of rise, the threshold current of 2-ms injected pulses and the intracellularly measured threshold depolarization. The effects on the action potential were antagonized in an apparently competitive manner by sodium ions. Thus both drugs depressed excitability and the rising phase of the action potential by inhibiting the specific increase in sodium conductance (gNa) which normally follows an adequate stimulus. It was shown that both drugs also inhibited the secondary rise in potassium conductance (gK) which normally occurs during an action potential. Although quantitatively similar, lower concentrations of chlorpromazine (> 15 × 10−6 M) were more potent and higher concentrations (> 15 × 10−6 M) were less potent than promethazine. The qualitatively identical and the quantitatively similar effects of these two drugs would suggest that the antipsychotic effect produced by some of the phenothiazines is unrelated to their effects on action potential production.

Effects of D-600 and its optical isomers on force of contraction in cat papillary muscles and guinea-pig auricles.

1 (--)-D-600 and (+)-D-600 exerted concentration-dependent negative inotropic effects in papillary muscles from cats, the potency of (--)-D-600 being about 100 times greater than that of (+)-D-600. The action was more pronounced at high than at low frequencies of stimulation. Time to peak force, relaxation time and therefore also total duration of contraction were not significantly affected by the drugs. 2 The negative inotropic effects of both drugs were not reversible after washing in drug-free solution for 60 min at all concentrations tested. 3 The same negative inotropic effects were seen in guinea-pig left auricles with the racemic mixture of both isomers. 4 Uptake measurements of D-600-[nitrile-14C] in isolated left auricles of guinea-pigs showed the drug to be accumulated in the myocardial tissue; tissue-medium ratios from 1.25 to 6.0 were found at exposure times from 1 to 100 minutes. Preparations which were loaded first with D-600-[nitrile-14C] for 100 min and then washed in drug-free solution for different times lost up to about 80% of the initial radioactivity, whereas the depression of force of contraction was not reversible. 5 The results show that there are marked quantitative differences between the actions of (--)-D-600 and (+)-D-600 on isometric force of contraction in cat papillary muscles. Although the time course of the uptake of D-600-[nitrile-14C] and the development of the negative inotropy in guinea-pit left auricles were reasonably well related, the magnitude of the negative inotropy does not seem to be dependent on the total tissue concentration once the negative inotropic effect has been fully developed.

EFFECT OF PROSTAGLANDIN E1 ON NEUROMUSCULAR TRANSMISSION IN THE RAT

1 The effect of prostaglandin E1 on neuromuscular transmission in the phrenic nerve-diaphragm muscle preparation of the rat was studied with intra- and extracellular recording techniques. 2 Prostaglandin E1, in concentrations from 10 nM, induced intermittent failures in the generation of the end-plate potential in response to repeated indirect stimulation. 3 Failures appeared abruptly, the end-plate potential behaving in an all-or-nothing fashion. The effect occurred only at 36-38 degrees C when the nerve was stimulated at 30-80 Hz and was reversible upon washing with drug-free solution. 4 Since miniature end-plate potentials were not affected, such failures must be attributed to a presynaptic action of prostaglandin E1. 5 Extracellular recording suggested that prostaglandin E1 prevented the action potential from reaching the nerve terminal.

Intracellular electrophysiological alterations in canine cardiac conducting tissue induced by aprindine and lignocaine

The effects of aprindine and lignocaine (lidocaine USP) were compared on intracellularly recorded potentials from normal canine conducting tisues. Only aprindine produced marked depression of the maximum rate of rise (Vmax) of action potentials elicited at normal cycle lengths (1000 ms). Both compounds, however, produced enhanced depression of Vmax at short cycle lengths or at short S1-S2 coupling intervals at normal membrane resting potential. Both lignocaine and aprindine shorten the action potential duration (APD) and effective refractory period (ERP) but increase the EPR/APD ratio. These effects are easily reversible upon perfusion with drug-free solution only in the case of lignocaine.

Protection by phentolamine against the effects of phenoxybenzamine on transmitter release elicited by nerve stimulation in the perfused cat heart.

1 The effects of cocaine, phentolamine and phenoxybenzamine on neuronal uptake of [(3)H]-noradrenaline and on (3)H-transmitter and noradrenaline overflow elicited by nerve stimulation were determined in the perfused heart of the cat.2 During perfusion with cocaine 3.4 x 10(-7)M, there was a 2-fold increase in transmitter overflow while neuronal uptake of [(3)H]-noradrenaline was inhibited by 31.3 +/- 2.1%.3 After exposure to phenoxybenzamine 8.7 x 10(-7)M for 20 min and washing with drug-free solution for 165 min there was an 8-fold increase in transmitter overflow during nerve stimulation. Under these conditions neuronal uptake of [(3)H]-noradrenaline was inhibited by only 17.5 +/- 5.4%.4 There was no significant change in transmitter overflow or in neuronal uptake of [(3)H]-noradrenaline, 155 min after a 30 min exposure to phentolamine (3.2 x 10(-5)M).5 Perfusion with phentolamine (3.2 x 10(-5)M) before and during exposure to phenoxybenzamine (8.7 x 10(-7)M), prevented the increase in transmitter overflow observed after perfusion with phenoxybenzamine alone.6 Protection by phentolamine against the effects of phenoxybenzamine supports the view that the effects on transmitter release obtained after perfusion with phenoxybenzamine are due to the blockade of presynaptic alpha-adrenoceptors which regulate transmitter release through a negative feed-back mechanism.