Biphasic nature of propranolol's microelectrophysiologic effects.

The microelectrophysiologic effects of d,l-propranolol were observed to be biphasic when a wide range of concentrations was used in canine atrial tissue; therefore, the 2 isomers were used separately to test the hypothesis that the opposite effects of propranolol could be separated by isomeric isolation. Whole atria were detached from dog hearts and 3 by 0.5 cm strips were cut from the atria. Transmembrane action potentials were monitored from the atrial strips mounted in an isolation chamber containing Tyrode's solution. Concentrations of d-propranolol, l-propranolol, and the d,l-racemate ranging from 0.03 to 1.0 micrograms/ml were added to the bath and the effects observed for 60 minutes. In lower concentrations d,l-propranolol decreased the effective refractory period and activation time while increasing maximal rise velocity, action potential amplitude, and resting membrane potential. Higher concentrations produced the opposite effects. The l-isomer produced effects very similar to those of the lower concentrations of the racemate, whereas the d-form mimicked those effects of the higher concentrations of the racemic mixture. We conclude that d,l-propranolol is capable of exerting 2 opposite electropharmacologic actions, which may be demonstrated by varying the concentration of d,l-propranolol or by separating the isomers.

Vascular alpha-adrenergic blocking properties of quinidine.

The specificity of the alpha-adrenergic vascular blockade by quinidine was tested in the intact dog, in rabbit isolated aortic strips, and in rats under ganglionic blockade. Quinidine did not affect the pressor response of angiotensin II in dogs, the contractile response of histamine nor angiotensin II in aortic strips, nor the dose-pressor response curve of the alpha-agonist, B-HT 933 in the rat. However, the pressor effect of adrenaline and noradrenaline (NA) were significantly reduced in dogs, and the dose-response curves to NA in aortic strips and to the alpha-agonist, phenylephrine in rats was shifted to the right in a parallel manner by quinidine. In the rat, quinidine is at least 14 times more potent in antagonizing the vasopressor effect of an alpha 1-vs. and alpha 2-adrenoceptor agonist.

alpha-Adrenergic blocking properties of quinine HCl.

In the anesthetized dog, quinine HCl (50 mg/kg, i.v.) infused over a 20 min period produced 1 22% maximum decrease in diastolic blood pressure, a 53% increase in pulse pressure and a 52% increase in myocardial contractile force. The initial positive inotropic response was maximal in the first 5--15 min of the quinine infusion and decreased to near control levels 40 min following the quinine infusion. Quinine caused a marked reduction in the noradrenaline (NA) pressor response, blockade of the adrenaline (A) pressor response, partial blunting of the angiotensin II (AII) pressor effect but no change in the depressor effect of isoprenaline (I). The positive inotropic effects of CaCl2 were reduced and the duration of contractile action to both I and CaCl2 was significantly prolonged by quinine. In isolated rabbit thoracic aortic strips, quinine produced a parallel, dose-related shift of the concentration-response curve for NA to the right but did not affect the maximum responses. A pA2 of 4.91 was estimated by the method of Schild. The determined line had a slope of -0.84 which is similar to a theoretical slope of -1.0 and indicates a direct relationship between the number of receptors occupied and the contractile response. The responses to AII and histamine (H) were not altered by quinine. These results suggest that quinine HCl produces alpha-adrenergic blockade; additionally, quinine modifies catecholamine- and calcium-induced myocardial contractile force responses.

Pharmacological studies of a new 4-aminosugar cardiac glycoside (ASI-222).

ASI-222,3beta-O-(4-amino-4,6-dideoxy-beta-d-galactopyranosyl) digitoxigenin, is a semi-synthetic cardiac glycoside patterned after a natural glycoside obtained from Cambodia. Effects of ASI-222 on contractile force in the isolated rabbit atria, cardiac contractile force, cardiac rate, ventricular excitability and functional refractory period in dogs, and acute toxicity in mice have been compared to those effects of ouabain. Both electrically driven and spontaneously beating atria demonstrated more rapid onset and greater maximum increases in contractile force with ASI-222 than with ouabain in equal bath concentrations. In the dog, ASI-222 increased cardiac contractile force more rapidly and at a lower cumulative dose than ouabain. Moreover, the maximum increase in contractile force obtained with ASI-222 was greater than that obtained with ouabain. The occurrence of ventricular ectopic beats was observed at a higher cumulative dose of ASI-222 than for ouabain. Also, ASI-222 produced a decrease in ventricular excitability and an increase in functional refractory period ot the ventricle. Ouabain, in the same molar dose, produced either no change or a slight increase in these parameters. Our data indicate that ASI-222 has a greater therapeutic index than ouabain. This difference may be partially explained by effects of ASI-222 on electrical properties of the heart.

Ventricular electrophysiology of quinidine-propranolol combinations in the dog heart.

The effects of quinidine and propranolol, singly and in combination, on certain electrophysiological responses were studied in open-chest dogs. Parameters measured were the diastolic threshold, conduction time, excitability and functional refractory period of canine right ventricular muscle. Potentiation was judged to have occurred if the combination of a half-dose of quinidine plus a half-dose of propranolol produced a significantly greater response than the full dose of either quinidine or propranolol. Two combinations were employed in this study: combination I contained 2.5 mg/kg of quinidine plus 0.1 mg/kg of propranolol while combination II contained 5.0 mg/kg of quinidine plus 0.2 mg/kg of propranolol. Results of the study indicated that combination II potentiated the activity of quinidine and propranolol to depress excitability, increase conduction time and prolong refractoriness. However, combination I only potentiated the change in refractory state. Neither combination potentiated the individual drug action to increase the diastolic threshold. From the results of this study, it may be concluded that the combination was especially effective in prolonging ventricular refractoriness which may account to a significant degree for the enhanced therapeutic effectiveness of the combination in resistant ventricular arrhythmias.

Effect of an aminocardenolide and digoxin upon atrioventricular refractory period in the dog.

The ability of digoxin and a 4-aminocardenolide, ASI-222, to alter atrioventricular nodal refractory period (AVRP) was determined as a function of the maximum subarrhythmic dose (MSAD) in the dog anesthetized with morphine-pentobarbital. ASI-222, a highly polar and potent inhibitor of Na+, K+-adenosine triphosphatase produces a cardiotoxicity in dogs prominently involving atrioventricular nodal blockade rather than ventricular premature ectopic beats and tachycardia seen with digoxin. AVRP was assessed with trains of electrically isolated stimuli of decreasing pulse interval delivered to the right atria. Digoxin and ASI-222 were infused i.v. at rates which produced cardiac arrhythmias in about 100 min in dogs either: 1) with intact nerves, 2) pretreated with atropine, 3) without reflex receptors (without vagus and carotid sinus nerves, 4) without cardiac sympathetic nerves and adrenals or 5) pretreated with metoprolol. In dogs with intact nerves, ASI-222 produced greater increases in AVRP than digoxin at fractions of the MSAD; however, both glycoside produced a similar elevation at the MSAD (approximately equal to 30% increase). Atropine did not alter the AVRP response to ASI-222 but prevented the lengthening due to digoxin except for that which occurred near the MSAD. Removal of reflex receptor afferents (and vagi) had an effect similar to atropine on the AVRP response to digoxin, but completely prevented any response to ASI-222. Prior sympathectomy or beta adrenergic blockade abolished the AVRP response to ASI-222 but did not alter the responses to digoxin.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the combination of quinidine and propranolol upon atrial and ventricular automatically in dogs.

The effects of quinidine, propranolol and their combination on atrial and ventricular automaticity were studied in pentobarbital-anesthetized dogs with complete heart block produced by injection of 40% formalin. The indices of atrial and ventricular automaticity were the intrinsic rate and the asystole interval, 10-beat period, and beats per 30 seconds after cessation of a 2-minute overdrive. Potentiation was considered to be a response produced by the combination of a half-dose of quinidine plus a half-dose of propranolol significantly greater than that produced by the full dose of either drug. Two combinations were studied: combination I consisted of 1.0 mg/kg of quinidine and 0.04 mg/kg of propranolol while combination II consisted of 2.0 mg/kg of quinidine and 0.08 mg/kg of propranolol. Neither combination potentiated the action of the individual drugs on the ventricle. Both combinations produced a potentiation of the individual drug effects on atrial intrinsic rate, asystole interval, and 10-beat period while only combination II potentiated the individual drug effects on atrial beats per 30 seconds. These studies indicate that the enhanced effect of the quinidine-propranolol combination in conversion of atrial tachyarrhythmias to sinus rhythm may be a function of its potentiation of the ability of the individual drug to depress atrial automaticity.

A comparison of the effects of quinetholate, lidocaine and procainamide on ouabain-induced ventricular tachycardia and cardiac function.

The antiarrhythmic effects of the quinoline derivative, quinetholate, on ouabain-induced tachycardia were compared with those of lidocaine and procainamide in dogs. In addition, the effects of these three agents on cardiac function were compared. Quabain was injected intravenously until ventricular ectopic beats accounted for at least 60% of the heart rate. Then one of the above antiarrhythmic agents was infused until sinus rhythm was reestablished for a minimum of 3 min or until it became evident that successful reversion would not occur. All three agents effectively reversed ouabain-induced ventricular tachycardia in most instances. Quinetholate was especially consistent, causing 16 reversions out of a total of 17 experiments. The relative molar antiarrhythmic potencies of the three agents in responding animals were as follows: lidocaine = 1.0, procainamide = 1.5, and quinetholate = 3.1. Quinetholate caused the longest lasting reversions, i.e. usually lasting at least 30 min after infusion was stopped. The degree of cardiac depression caused by the three agents at their mean effective antiarrhythmic doses was determined using the left ventricular function curve method. Lidocaine produced a significant depression of the ventricular function curve at the antiarrhythmic dose while procainamide and quinetholate did not.

Comparison of the effects of aminosugar cardiac glycosides with ouabain and digoxin on Na+, K+ -adenosine triphosphatase and cardiac contractile force.

Two aminosugar cardiac glycosides, 3-beta-O-(4-amino-4,6-dideoxy-beta-D-galactopyranosyl) digitoxigenin (ASI-222) and its 4-aminoglucose analog (ASI-254) have been shown in our laboratory to have a greater therapeutic index than ouabain (O) or digoxin (D). We have now compared the ability of ASI-222, its nonamino galactose analog (ASI-253), ASI-254, ouabain and digoxin to inhibit swine brain Na+,K+-adenosine triphosphatase (Na+,K+-ATPase) and to increase contractile force of isolated, driven rabbit atria. As inhibitors of Na+,K+ -ATPase, both ASI-222 and ASI-254 were found to be about 10 times more potent than ASI-253, O or D (I50:ASI-222, 1.3 X 10(-7) M; ASI-254, 1.4 X 10(-7) M; ASI-253, 1.15 X 10(-6) M; D, 1.6 X 10(-6) M; O, 1.75 X 10(-6) 7). Moreover the potency of these glycosides in inhibiting Na+, K+ -ATPase correlates closely with the ability of these same glycosides to increase contractile force. The concentration needed to obtain 50% of the maximum increase in contractile force was 9.7 X 10(-8) M for ASI-254, 1.5 X 10(-7) M for ASI-222, 8.8 X 10(-7) M for ASI-253 8.4 X 10(-7) M for O and 1.2 X 10(-6) M for D. Since ASI-253, a nonaminogalactose analog of ASI-222, exhibits a potency in both of our test systems which is similar to the other neutral sugar cardenolides, our data also indicate that the presence of an aminosugar group at position 4 of a sugar in a cardiac glycoside confers greater potency.

Effect of cardiac beta-adrenergic blockade or denervation on cardiotoxicity of digoxin and an aminosugar cardenolide.

This study examined the role of cardiac beta-adrenergic receptors and cardiac sympathetic and vagal nerves in the cardiotoxicity of 3-beta-0-(4-amino-4,6-dideoxy-beta-D-glucopyranosyl) digitoxigenin hydrochloride (ASI-254). Vagally intact dogs received a constant rate intravenous infusion of either digoxin or ASI-254 in the presence and absence of practolol. Practolol pretreatment increased the dose of digoxin required to produce arrhythmias and markedly altered the pattern of toxicity, but did not alter the lethal dose. The terminal event was cardiac standstill rather than ventricular fibrillation as seen in digoxin control dogs. Practolol did not alter the toxic dose of ASI-254 and produced little change in the pattern of cardiotoxicity; both control and practolol-treated dogs died in cardiac standstill. Surgical sympathectomy did not alter the toxic dose of ASI-254, the character of toxicity, or the lethal dose compared to neurally intact dogs. However, vagal innervation may play a role in determining the type of cardiotoxicity produced by ASI-254. Vagotomy alone did not alter the toxic or the lethal dose of ASI-254; vagotomy did, however, alter the character of cardiotoxicity and terminal event. Our results indicate that ASI-254 infused intravenously does not interact with sites, central or peripheral, which activate the sympathetic nervous system. ASI-254 administered into the lateral ventricles produced signs of increased cardiac sympathetic nervous system activity. Tachycardia and arrhythmias produced by ICV ASI-254 appear to be neurally mediated since ganglionic blockade blunted these effects. These results suggest that ASI-254 is capable of interacting with central sympathetic nervous system structures, but in contrast to digoxin, access to these structures from intravenous administration is limited.

Changes in cardiac glycoside activity produced by aminosugar addition.

In anesthetized dogs, structure activity relationships among three cardiotonic compounds were determined by comparing the cardiovascular effects of digotoxigenin (the genin) to digitoxigenin-galactose (a genin-neutral sugar combination) and to digitoxigenin-aminogalactose (ASI-222, a genin-aminosugar combination) using either bolus i.v. injections or constant i.v. infusions. We recorded the effects of these drugs upon cardiac rate, mean blood pressure, left ventricular dP/dt, cardiac index, systolic time intervals, tension-time index, therapeutic index, ventricular excitability and the ventricular refractory period. The addition of an aminosugar group to digitoxigenin or an amine group to galactose-digitoxigenin results in an agent with greater ability to reduce heart rate and to increase cardiac contractility and cardiac index without affecting the tension-time index. Moreover, the addition of an amino group significantly increased the therapeutic index and ventricular refractory period but reduced the toxic index (lethal dose/toxic dose) when compared to the neutral-sugar cardenolide and genin. Our data indicate that such a substitution confers greater potency, prolongs the duration of activity and results in a compound with a greater therapeutic index.

Structure-activity relationships of an aminosugar cardiac glycoside, ASI-222 HCl, in the heart-lung preparation of the dog.

ASI-222 [3-beta-O-(4-amino-4,6-dideoxy-beta-D-galactopyranosyl) digitoxigenin HCl] is a semisynthetic aminosugar cardiac glycoside that has been shown to have a greater therapeutic index than ouabain or digoxin in dogs. We have compared the effects of ASI, digitoxigenin, digitoxigenin-beta-D-galactose, and digoxin in the dog heart-lung preparation. Minute work and stroke work were calculated. Controls were obtained before and after the hearts were failed with sodium pentobarbital. ASI-222 is about three times more potent than digitoxigenin and about twice as potent as digitoxigenin-beta-D-galactose in producing similar increases in the left ventricular stroke work. ASI-222 is about three times more potent than digoxin in creasing left ventricular stroke work. Our results indicate that the addition of an aminosugar group to the genin further increased potency over that observed with the addition of a single neutral sugar and prolonged the duration of activity in failing myocardial tissue.

Comparison of the cardiac effects of ASI-222 HCl, an aminosugar cardiac glycoside, and digoxin.

The effects of ASI-222 HCl and digoxin on cardiac contractile force, dP/dt, heart rate and mean blood pressure in the dog were evaluated following i.v. administration. Additional studies were performed on isolated, electrically driven rabbit atria with both ASI-222 and digoxin. Our data show that ASI-222 (40 microgram/kg) in non-vagotomized dogs produced a two-fold greater increase in cardiac contractile force and dP/dt than did an equimolar dose of digoxin. ASI-222 caused a peak increase in contractile force and dP/dt 10 min after administration, whereas the peak to digoxin occurred at 30 min. ASI-222 was more effective in non-vagotomized dogs in increasing contractile force and dP/dt than in vagatonized dogs. The inotropic responses to digoxin were not reduced by vagotomy. Studies on isolated electrically driven rabbit atria indicate ASI-222 to be 7-8 times more potent than digoxin in increasing contractile force. These data demonstrate that ASI-222 produces an earlier and greater increase in contractile force and dP/dt than digoxin in equimolar doses.

Interactions of pyridostigmine with cardiopulmonary systems and their relationships to plasma cholinesterase activity.

Three dose levels of pyridostigmine (0.5, 2, and 5 mg/kg) were given iv to dogs anesthetized with sodium pentobarbital. Hemodynamic measurements made were cardiac output, blood pressure, left ventricular dP/dT, and heart rate. Pulmonary function data obtained were airway resistance, respiratory rate, and tidal volume. Activity of blood cholinesterase was also measured. Increasing doses of pyridostigmine promptly and progressively lowered the acetylcholinesterase activity of blood to a minimum of 40% of control at the 5 mg/kg dose. Airway resistance was most sensitive to the drug. Resistance increased significantly with 2 mg/kg, and the 5 mg/kg dose resulted in more than a 10-fold increase, which persisted for more than 2 hr. Tidal volume was decreased and minute volume was increased due to an increase in respiratory rate. With the higher doses, heart rate decreased and stroke volume rose sufficiently to compensate so that cardiac output was unchanged. The lowest dose produced minimal effects on both cardiovascular and respiratory systems, and since this dose is greater than that proposed for organophosphate poisoning in humans, it seems likely that this drug would not cause important effects in normal humans when used as a protective agent.

Metabolism and cardiac actions of a polar aminocardenolide and digoxin in the conscious dog.

We studied the metabolism and cardiac actions of a polar aminocardenolide, 3-beta-O-(4-amino-4,6-dideoxy-beta-D-galactopyranosyl) digitoxigenin (ASI-222), in conscious, chronically instrumented dogs and compared the cardiac actions of this compound with those of digoxin. Chloroform-soluble metabolites and the excretion patterns of ASI-222 in urine and feces were identified and measured using thin-layer chromatography. The deaminated metabolite of ASI-222 appeared both in the urine and the feces together with the genin, digitoxigenin and the parent drug which constituted the majority of the radioactivity excreted. There was a secondary rise in the plasma concentration of ASI-222 starting 2 hr after the i.v. administration, which strongly suggests its enterohepatic recycling. The secondary increase in the plasma concentration was not seen in the dogs receiving digoxin. ASI-222 produced increases in cardiac contractility and systolic blood pressure which were more rapid in onset and shorter in duration than those produced by an equimolar dose of digoxin. Amplitudes of these physiologic responses to these two compounds in conscious dogs were approximately 2 times higher than the effects previously reported to similar doses in anesthetized dogs.

Cardiovascular effects of KM-13, a new, orally effective, cardiotonic sympathomimetic amine.

Dobutamine's chemical structure was modified to make it orally effective, while its pharmacological profile was preserved. Testing on anesthetized dogs showed that replacement of the para hydroxyl group with carboxyamide at the phenyl end of the molecule increased inotropic potency threefold, but it introduced pressor activity that spoiled the inotropic selective profile of dobutamine. However, shifting carboxyamide to the meta position avoided pressor activity and further enhanced inotropic potency to nine times that of dobutamine. When administered orally to conscious dogs, this compound, KM-13 (5 mg/kg), produced a sustained increase in left ventricular dP/dt with only immediate changes in heart rate; 10 mg/kg dobutamine was without cardiovascular effects. The (-) isomer of KM-13 contained twice the inotropic activity of the (+) isomer; in contrast, the (-) isomer had no effect on diastolic blood pressure, while the (+) isomer lowered blood pressure. The inotropic and chronotropic effects of dobutamine and KM-13 are both largely due to beta-adrenergic stimulation as shown by propranolol blockade. In contrast to dobutamine, KM-13 is an agent that is active by either oral or buccal administration and has greater inotropic potency.

Digoxin uptake into peripheral autonomic cardiac nerves: possible mechanism of digitalis-induced antiarrhythmic and toxic electrophysiologic actions.

It is generally accepted that certain cardiac rhythm disturbances are due to imbalances between the sympathetic and parasympathetic nervous systems. We have provided evidence that digoxin is concentrated in the peripheral nervous system of the heart as well as in the central nervous system. Previous findings have indicated that cardiac glycosides may directly or indirectly affect autonomic neurotransmitters. Therefore the uptake of digoxin into the peripheral cardiac nervous system may play an important role in both the antiarrhythmic and toxic electrophysiologic actions of digoxin.

Adrenergic and cholinergic mechanisms in digitalis inotropy.

In anesthetized dogs, the effects of peripheral cardiac nerves upon cardiotonic steroid-induced contractile force increases were determined by comparing the effects seen with cardiac nerves intact, cardiac denervation, stellate ganglia removed or vagi sectioned. Additionally, structure-activity relationships among four cardiotonic steroids were determined by comparing the contractile force effects of bolus i.v. injections of digitoxigenin (the genin), digitoxigenin-galactose (genin-neutral sugar combination), digitoxigenin-aminogalactose (ASI-222, genin-aminosugar combination) and digoxin. The effects of these drugs upon cardiac rate, mean blood pressure and cardiac contractile force were recorded. Cardiotonic steroids differ in their interaction with cardiac nerves. Digitoxigenin, in addition to its direct contractile force effect on the myocardium, modulates contractile force through adrenergic mechanisms. In contrast, both digoxin and ASI-222 influence their direct inotropic responses through cholinergic mechanisms. Neither adrenergic nor cholinergic mechanisms significantly affect the peak inotropic response of digitoxigenin-galactose. Our data indicate that alterations in both the aglycone and the sugar moieties can significantly alter the contribution of the autonomic nervous system to the contractile force response.