Isolated Guinea Pig Left Atria Research Articles

The surmountable effect of FSCPX, an irreversible A1 adenosine receptor antagonist, on the negative inotropic action of A1 adenosine receptor full agonists in isolated guinea pig left atria

A1 adenosine receptors (A1 receptors) are widely expressed in mammalian tissues; therefore attaining proper tissue selectivity is a cornerstone of drug development. The fact that partial agonists chiefly act on tissues with great receptor reserve can be exploited to achieve an appropriate degree of tissue selectivity. To the best of our knowledge, the A1 receptor reserve has not been yet quantified for the atrial contractility. A1 receptor reserve was determined for the direct negative inotropic effect of three A1 receptor full agonists (NECA, CPA and CHA) in isolated, paced guinea pig left atria, with the use of FSCPX, an irreversible A1 receptor antagonist. FSCPX caused an apparently pure dextral displacement of the concentration-response curves of A1 receptor agonists. Accordingly, the atrial A1 receptor function converging to inotropy showed a considerably great, approximately 80-92 % of receptor reserve for a near maximal (about 91-96 %) effect, which is greater than historical atrial A1 receptor reserve data for any effects other than inotropy. Consequently, the guinea pig atrial contractility is very sensitive to A1 receptor stimulation. Thus, it is worthwhile considering that even partial A1 receptor agonists, given in any indication, might decrease the atrial contractile force, as an undesirable side effect, in humans.

Effects of GBE50 on physiological characteristics and intracellular free calcium of myocardium

To investigate effects of GBE50 on the physiological characteristics and intracellular free calcium of myocardium. The preparations of isolated atria of guinea pigs were used for recording the myocardial physiological characteristics and the technique of Fluo-3/AM was used for measuring the concentration of free calcium in myocytes. GBE50 (10, 20, 40, 80, 160 mg x L(-1)) prominently inhibited the contractive force and decreased spontaneous beat of right atrium in concentration-dependent manner. GBE50 (10, 20, 40, 80, 160 mg x L(-1)) inhibited the contractive force of left atrium, and markedly decreased the post rest potentiation of myocardial contractility in left atrium in concentration-dependent manner. GBE50 (10, 20, 40, 80, 160 mg x L(-1)) prolonged the function refractory period of isolated guinea pig left atrium. 50 mg x L(-1) GBE50 decreased [Ca2+]i of guinea pig ventricular myocytes in normal Tyrode's solution by (46.53 +/- 8.44)% in 12 minutes. GBE50 induces inhibitory effects of the spontaneous beat, contractility, post-rest potentiation and prolonged the function refractory period. GBE50 may decrease [Ca2+]i of guinea pig ventricular myocytes.

Effect of extracellular calcium on the additive effect of theophylline on the cardiac response to catecholamine

At diff erent extracellular calcium concentrations, the positive inotropic eff ect of isoproterenol and isoproterenol in combination with theophylline, a phosphodiesterase inhibitor have been evaluated in the isolated frog heart and the isolated guinea pig left atria to investigate whether extracellular calcium produces any eff ect on the additive eff ect of the theophylline on the cardiac response to catecholamine. Cumulative dose response study of isoproterenol and isoproterenol in presence of theophylline at diff erent extracellular calcium concentration was performed. Th e study revealed an increase in additive eff ect of theophylline on the cardiac response to catecholamine with increase in extracellular calcium concentration, but increase in extracellular calcium concentration decreased the myocardial responsiveness to additive eff ect of theophylline and isoproterenol combination. Th e mechanism of positive inotropic eff ect of isoproterenol and in combination with theophylline involves increase in intracellular CAMP by diff erent pathways and extracellular calcium produces positive inotropic eff ect by initiating the interaction between the contractile proteins actin and myosin.The study revealed that an increase in the concentration of extracellular calcium increased the additive eff ect of theophylline and isoproterenol combination, but a decrease in the myocardial responsiveness was observed. Key words: Additive eff ect, cAMP, isoproterenol, positive inotropic eff ect, theophylline and calcium

A unique xanthine derivative KMCP-98 with activation of adenosine receptor subtypes

KMCP-98 is a newly synthesized adenosine receptor agonist by alkylation at the 7-position of the xanthines nucleus. We first investigated the pharmacological activities of KMCP-98 under in vivo and in vitro conditions. Acute intravenous injection of KMCP-98 (1.0, 2.0 and 3.0 mg/kg) produced a temporary fall in blood pressure and heart rate, followed by a sustained fall in heart rate in pentobarbital-anesthetized Wistar rats. The hypotensive and bradycardiac responses were inhibited by pretreatment with an A 1 adenosine receptor antagonist 8-phenyltheophylline (8-PT, 0.5 mg/kg). Both KMCP-98 and adenosine (0.3–100 μM) produced negative inotropic activitity in isolated guinea pig left atria. The negative inotropic activity of KMCP-98 was significantly blocked by pretreatment with A 1 receptor antagonists 8-PT (10 μM) and xanthine amine congener (XAC, 10 μM), a nonselective adenosine antagonist theophylline (10 μM), a K + channel blocker tetraethylammonium (TEA, 10 mM) and a K ATP channel blocker glibenclamide (1 μM). KMCP-98 (0.03–30 μM) produced concentration-dependent relaxations in carbachol (1 μM) precontracted guinea pig tracheal smooth muscle. The trachea relaxant response of KMCP-98 was markedly inhibited by A 2, A 2a and A 2b adenosine receptor antagonists 3,7-dimethyl-1-propargylxanthine (DMPX, 10 μM), 8-(3-chlorostyryl)caffeine (CSC, 10 μM) and alloxazine (10 μM), respectively, the nitric oxide synthase (NOS) inhibitor l-NAME (100 μM) and also by TEA and glibenclamide. In addition, KMCP-98 (0.03–30 μM) elicited relaxant response in norepinephrine (3 μM) precontracted rat thoracic aorta in a concentration-dependent manner. The thoracic aorta relaxant response of KMCP-98 was also significantly inhibited by DMPX, CSC, alloxazine, l-NAME, TEA and glibenclamide. Furthermore, the binding characteristics of KMCP-98, adenosine and 5′- N-ethylcarboxaminoadenosine (NECA) were evaluated in [ 3H]DPCPX and [ 3H]CGS 21680 binding to rat cortex and striatum, respectively. The K i values of KMCP-98 for predominate A 1 and A 2 adenosine receptor sites were 3908±952 and 158±10 nM, respectively. In conclusion, KMCP-98 was found to be a xanthine-based adenosine receptor agonist associated cardiac depression, tracheal and aortic smooth muscle relaxations.

17beta-O-Aminoalkyloximes of 5beta-androstane-3beta,14beta-diol with digitalis-like activity: synthesis, cardiotonic activity, structure-activity relationships, and molecular modeling of the Na(+),K(+)-ATPase receptor.

A series of digitalis-like compounds with a 17-aminoalkoxyiminoalkyl or -alkenyl substituent was synthesized and evaluated for inhibition of Na(+),K(+)-ATPase and for inotropic activity. The highest inhibition was found with compounds having the substituent in configuration 17beta and the amino group at a distance of 6 or 7 bonds from C(17) of the digitoxigenin skeleton. The presence of the oxime function strengthens the interaction with the receptor, more if alpha,beta-unsaturated, thus mimicking the electronic situation of the unsaturated lactone in natural digitalis compounds. The most active compounds showed Na(+),K(+)-ATPase inhibitory potencies (IC(50)) 17-25 times higher than the standards digitoxigenin and digoxin and 3-11 times higher inotropic potencies (EC(50)) in isolated guinea pig left atria. These features are supported by a molecular model suggesting the possible interactions of the groups described above with particular amino acid residues in the H1-H2 domains of Na(+),K(+)-ATPase. Some interactions are the classical ones already described in the literature; a new, very strong interaction of the basic group with the Cys138 was found and adds new possibilities to design compounds interacting with this region of the receptor. The most interesting compounds were also studied in vivo in the anesthetized guinea pig for evaluating their inotropic effect versus the lethal dose. Compounds 9 and 12 showed a slightly higher safety ratio than digoxin and deserve further evaluation.

Universal template approach to drug design: polyamines as selective muscarinic receptor antagonists.

The concept that polyamines may represent a universal template in the receptor recognition process is embodied in the design of new selective muscarinic ligands. Tetraamines 4-7 and 16-20 and diamine diamides 8-15 were synthesized, and their pharmacological profiles at muscarinic receptor subtypes were assessed by functional experiments in isolated guinea pig left atrium (M2) and ileum (M3) and by binding assays in rat cortex (M1), heart (M2), submaxillary gland (M3), and NG 108-15 cells (M4). It has been confirmed that appropriate substituents on the terminal nitrogens of a tetraamine template can tune both affinity and selectivity for muscarinic receptors. The novel tetraamine C-tripitramine (17) was able to discriminate significantly M1 and M2 receptors versus the other subtypes, and in addition it was 100-fold more lipophilic than the lead compound tripitramine. Compound 14 (tripinamide), in which the tetraamine backbone was transformed into a diamine diamide one, retained high affinity for muscarinic subtypes, displaying a binding affinity profile (M2 > M1 > M4 > M3) qualitatively similar to that of tripitramine. Both these ligands, owing to their improved lipophilicity relative to tripitramine and methoctramine, could serve as tools in investigating cholinergic functions in the central nervous system. Furthermore, notwithstanding the fact that the highest affinity was always associated with muscarinic M2 receptors, for the first time polyamines were shown to display high pA2 values also toward muscarinic M3 receptors.

A mathematical model for analysis of pharmacologically induced changes in the kinetics of cardiac muscle

A mathematical model of the isometric contraction of cardiac muscle is developed and utilized to characterize the inotropic and lusitropic effects of cardioactive compounds in isolated guinea pig left atria. In contrast to metrics that are based on minima and maxima of an isometric twitch and its derivative function, the entire time course of the twitch is used to quantify the kinetics of the contraction-relaxation cycle. The model relates observed tension to a time-dependent activation function that describes generation of internal force and a coupling function that determines mechanical response to the activation function. The model is structured so that it is suitable for nonlinear curve fitting to observed data. Results obtained using the model for fitting experimental data from tissues treated with different classes of cardioactive compounds agree with more qualitative results presented by other authors. Experiments using the model to fit data over an extended (90 min) time course revealed differences in the kinetic profiles of milrinone and forskolin. Computer simulations that demonstrate the effect of each model parameter on twitch kinetics are presented, and the relationships between the model and other theoretical and empirical models of cardiac muscle are discussed. The mathematical model is useful to enable a more quantitative understanding of the kinetics of cardiac muscle contraction and relaxation and identify compounds that may be selective for inotropic or lusitropic effects.

Pyrrolo[2,1-c][1,4]benzothiazines: synthesis, structure-activity relationships, molecular modeling studies, and cardiovascular activity.

The synthesis and pharmacological evaluation of a series of pyrrolo[1,4]benzothiazine derivatives are described. These compounds, related to diltiazem, have been shown to be representative of a novel series of calcium channel antagonists. The IC50S for inhibition of [3H]nitrendipine binding calculated by radioreceptor assay on rat cortex and rat heart homogenates showed that some of the described compounds possess an affinity equal to or higher than those of the reference calcium antagonists verapamil and cis-(+)-diltiazem. Furthermore, the alteration of the benzothiazepinone system of diltiazem to the pyrrolo[1,4]benzothiazine system of the title compounds resulted in a clear-cut selectivity for cardiac over vascular tissue, as shown in functional studies. In fact comparison of calcium antagonist activity on guinea pig aorta strips with the negative inotropic activity, determined by using an isolated guinea pig left atrium, revealed that the compounds examined displayed higher selectivity than the reference standard, within a wide variation of data. A number of structure-activity relationship trends have been identified, and possible explanation is advanced in order to account for the observed differences in selectivity. Prerequisite for in vitro calcium channel-blocking activity is the presence of two pharmacophores, namely, the substitution at C-4 and the substitution on the pyrrole ring. Two of the tested compounds, 8b and 28a, were identified as potent calcium antagonists selective for cardiac over vascular tissue.

Cooliatoxin, the first toxin from Coolia monotis (Dinophyceae).

Coolia monotis is a benthic dinoflagellate previously thought to be non-toxic. We describe a new toxin, named cooliatoxin, purified from cultures of a strain of C. monotis isolated from Australia. Cooliatoxin is likely a mono-sulphated, polyether toxin (M = 1,062; i.p. LD50 = 1 mg/kg in mice) that induces hypothermia and respiratory failure in mice after a pronounced delay period during which there are no obvious signs of intoxication. These signs in mice are similar to those reported for the shellfish toxin named yessotoxin and the molecular weight of cooliatoxin corresponds to the mono-sulphated form of yessotoxin, suggesting that cooliatoxin may be an analogue of yessotoxin. Cooliatoxin has no effect on the mouse phrenic nerve or diaphragm musculature in vitro but causes initial stimulation and subsequent block of unmylenated nerves in vitro. In isolated guinea pig left atria, cooliatoxin (above 20 nm) induced a slow developing concentration dependent sustained inotropic response. Propranolol or tetrodotoxin reversed the positive inotropic effects, indicating that the majority of the cooliatoxin induced response was mediated by stimulation of nerves associated with the atrial musculature, resulting in the release of noradrenaline. Cooliatoxin induced transient contractions in isolated guinea pig vas deferens preparations. Atria and vas deferens preparations were tachyphylactic to a second equivalent dose of cooliatoxin applied after the effects of the first dose had diminished. The observed in vitro effects of cooliatoxin on peripheral nerves are unlikely to account for the lethal effects in mice and a central action of this toxin is suspected.

Enhancement of adenosine A1 receptor functions by benzoylthiophenes in guinea pig tissues in vitro.

Previous reports on a series of benzoylthiophenes, including PD 81,723 [2-amino-4,5-dimethyl-3-(3-trifluoromethyl-benzoyl) thiophene], have shown specific enhancement of agonist binding at the adenosine A1 receptor. We have studied the effects of two substituted benzoylthiophenes, PD 78,416 (thieno[2,3-c]pyridine-6(5H)-carboxylic acid, 2-amino-3-benzoyl-4,7-dihydro-ethyl ester) and RS-74513-000 [2-amino-4-ethyl-5-methyl-3-(3-trifluoro-methyl-benzoyl) thiophene] on response elicited by adenosine A1 receptors in isolated guinea pig left atrium and ileum. In the electrically paced left atrium, PD 78,416 antagonized negative inotropic effect elicited by the agonist CPA [N6-cyclopentyladenosine] with a pKB value of 6.2 +/- 0.2 (n = 4). At a low concentration which had no antagonistic effect (0.1 microM), PD 78,416 enhanced the effect of CPA. The concentration-response curve to CPA was shifted leftward by 5.1 fold (95% confidence limits 2.4-11.2). In field stimulated isolated ileum, PD 78,416 (0.1, 0.3, 1 microM) did not enhance or antagonize effects of CPA. At concentrations above 1 microM, PD 78,416 decreased electrically induced contraction. This effect was not sensitive to adenosine deaminase and was not antagonized by the A1 antagonist CPX [8-cyclopentyl-1,3-dipropyl-xanthine] (1 microM). Unlike PD 78,416, RS-74513-000 (0.01, 0.1, 1, 3, 10 microM) did not antagonize or enhance effects of CPA in the left atrium. However, effects of CPA in ileum were enhanced by RS-74513-000 (1 and 3 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of inotropic effect of cyclosporin A in isolated guinea pig left atria

Cyclosporin A is being widely used to prevent graft rejection in organ transplantation and to treat autoimmune diseases. Since various toxic side effects have been observed, the aim of this study was to look for even a subtle deleterious effect of cyclosporin A on cardiac inotropy in electrically stimulated guinea pig left atria. The left atrial muscles of guinea pigs, in Tyrode's solution containing 2.7 or 5.4 mM potassium, were electrically stimulated by one of two methods: (1) continuously at 3 Hz, during which cyclosporin A was applied cumulatively (from 10(-9) to 10(-5) M); or (2) stimulated intermittently at 2.5 Hz in 5 mM cyclosporin A, with rest periods of 4 s duration interposed every 4 min. The effects of cyclosporin A on contractile force were observed for 150 min in the first stimulation method, and the effects on the steady state contractile force and amplitude of post-rest contraction were observed for 240 min in the second method. The steady state contractile force of the atria declined within the 4 h period at 2.7 mM potassium in Tyrode's solution both in the cyclosporin A group (n = 10) and in the control group (n = 5) to 68(SD 11)% and to 63(4)%, respectively. After 4 h the amplitudes of the post-rest contraction were 101(16)% and 101(4)% in cyclosporin A and control groups, respectively. At 5.4 mM potassium, the following values were obtained (cyclosporin A v control): steady state force 70(8)% (n = 11) v 69(8)% (n = 5); post-rest force 105(9)% v 102(7)%. Cyclosporin A does not influence the steady state contractile force or the amplitude of the post-rest contraction, suggesting the absence of inotropic effects on isolated guinea pig left atria.

Effects of S-312, a New Calcium Antagonist, on the Mechanical and Electrophysiological Responses of Isolated Cardiovascular Preparations

S-312, a new calcium antagonist with a bicyclic dihydrothienopyridine structure, potently relaxed the helical strips of various isolated rabbit arteries precontracted with high K+-depolarization. serotonin (5-HT) and U46619 (thromboxane A2 analogue), and it competitively inhibited Ca++-induced contractions in depolarized basilar and femoral arteries. These effects of S-312 were more potent than nifedipine and almost comparable to or slightly more potent than those of nicardipine. In comparison with nifedipine and nicardipine, the calcium antagonistic effect and the relaxant effect on 5-HT-induced contractions of S-312 were most prominent in the basilar artery. The potent vasodilating action of S-31 2in the high K+-depolarized basilar artery was not easily reversed by washing S-312 did not affect Ca++-induced contraction in the skinned fiber of guinea pig taenia caecum. The negative inotropic effect of S-312 in isolated guinea pig left atria was much less potent than those of nifedipine and nicardipine. S-312 above 10-7 M preferentially increased AV nodal conduction time in Langendorff-perfused isolated rabbit hearts, and above 3×10-8 M. it mainly decreased the maximum upstroke velocity of the action potential in isolated rabbit sinus node preparations. In summary, the present results indicate that S-312 is a potent new calcium antagonist possessing vasculoselectivity. especially for cerebral vessels.

Effects of S-312, a new calcium antagonist, on the mechanical and electrophysiological responses of isolated cardiovascular preparations.

S-312, a new calcium antagonist with a bicyclic dihydrothienopyridine structure, potently relaxed the helical strips of various isolated rabbit arteries precontracted with high K+-depolarization, serotonin (5-HT) and U46619 (thromboxane A2 analogue), and it competitively inhibited Ca++-induced contractions in depolarized basilar and femoral arteries. These effects of S-312 were more potent than nifedipine and almost comparable to or slightly more potent than those of nicardipine. In comparison with nifedipine and nicardipine, the calcium antagonistic effect and the relaxant effect on 5-HT-induced contractions of S-312 were most prominent in the basilar artery. The potent vasodilating action of S-312 in the high K+-depolarized basilar artery was not easily reversed by washing. S-312 did not affect Ca++-induced contraction in the skinned fiber of guinea pig taenia caecum. The negative inotropic effect of S-312 in isolated guinea pig left atria was much less potent than those of nifedipine and nicardipine. S-312 above 10(-7) M preferentially increased AV nodal conduction time in Langendorff-perfused isolated rabbit hearts; and above 3 x 10(-8) M, it mainly decreased the maximum upstroke velocity of the action potential in isolated rabbit sinus node preparations. In summary, the present results indicate that S-312 is a potent new calcium antagonist possessing vasculoselectivity, especially for cerebral vessels.

Studies on the calcium antagonistic action of tetrandrine: XVI. Effects of tetrandrine on positive staircase phenomenon and post-rest potentiation of contraction of isolated guinea pig left atrium.

Tetrandrine (Tet) 0.32 mmol/1 and verapamil (Ver) 30 μmol/1 inversed positive staircase phenomenon of contraction in left alrium of guinea pig. After the preparation was partially depolarized with K+ 20 mmol/1, such effect of Tet and Ver became much more pronounced. Propranolol could not inverse the positive staircase phenomenon. In addition, after treatment with Tet 0.32 mmol/1 for 20 min the post -rest potentiation of myocardial contraction in the left atrium was depressed by 2.8±0.3 g (P<0.01). The results suggest that the negative inotropism of Tet is related not only to the inhibition of Ca++ influx to the cells, but also to the decrease of the intracellular Ca++ release.

Cardiovascular Profile of Ro 13–6438, a Novel Positive Inotropic Agent with Vasodilating Properties

The cardiovascular properties of Ro 13-6438 (R-6-chloro-1,5-dihydro-3- methylimidazo -[2,1-b] quinazolin -2[ 3H]-one), a novel nonglycoside , noncatechol cardiotonic agent, were investigated in vitro and in vivo by both intravenous and oral administration. Ro 13-6438 increased tension development of isolated guinea pig left atria in a concentration-dependent manner with an EC50 of 30 microM, but had no stimulant effect on the spontaneous rate of right atria. The positive inotropic effect of Ro 13-6438 was additive to that of ouabain (0.1 microM); Ro 13-6438 suppressed the arrhythmogenic activity of high concentrations of ouabain (10 microM). IN anesthetized open-chest dogs 10-300 micrograms/kg Ro 13-6438 i.v. produced a significant and dose-dependent increase in myocardial force, with a duration of action exceeding 60 min following the highest dose. It also slightly increased heart rate, cardiac output, and blood flow. Ro 13-6438 decreased systolic and diastolic blood pressure, left ventricular end-diastolic pressure, and total peripheral resistance. Thus, the direct positive inotropic effects of Ro 13-6438 were supported by a decrease in preload and afterload. In chronically instrumented, conscious dogs Ro 13-6438 increased myocardial contractility after administration of 0.03-0.3 mg/kg i.v. or 3-10 mg/kg p.o. The effects persisted for greater than 8 h after oral administration of 10 mg/kg. The inotropic effects were accompanied by a modest increase in heart rate, which, however, had a clearly shorter duration of action than the former.(ABSTRACT TRUNCATED AT 250 WORDS)

Two ouabain binding sites in guinea pig heart Na+-K+-ATPase. Differentiation by sodium and erythrosin B.

3H-ouabain binding to guinea pig heart NaK-ATPase resulted in two different cardiac glycoside binding sites: A high affinity, low capacity binding was obtained at a KD = 2.6 X 10(-7) mol/l (about 40% of the binding sites) whereas a low affinity, high capacity binding was established at a KD = 1.3 X 10(-6) mol/l (about 60% of the binding sites). Similar results were obtained when studying the effect of increasing Na+-concentrations on ouabain binding: again two distinct processes involved in the Na+/ouabain interaction could be demonstrated. Whereas one ouabain binding process was activated at rather low Na+-concentrations (K0.5 = 4.5 mmol/l), the other ouabain binding process was predominant at high Na+-concentrations only (K0.5 = 69 mmol/l). Comparing these data with the ouabain action on contractile force and with NaK-ATPase activity in guinea pig heart, the high affinity binding of ouabain seems to correlate with the inotropic action, whereas the low affinity ouabain binding is more related to NaK-ATPase inhibition. To further discriminate the two cardiac glycoside binding sites operative in guinea pig heart muscle, erythrosin B, shown to be an inhibitor of the high affinity ouabain binding in this preparation, was applied to isolated guinea pig left atria in the presence of increasing ouabain concentrations. There was no change in the ouabain induced inotropy or toxicity by erythrosin B. These results could indicate that the low affinity ouabain binding is responsible for both inotropic and toxic actions in guinea pig heart. However, the functional significance of the high affinity ouabain binding would then remain obscure.(ABSTRACT TRUNCATED AT 250 WORDS)

Active transport and inotropic state in guinea pig left atrium.

Although the positive inotropic effect of cardiac glycosides correlates well with inhibition of Na+ pump activity in many preparations, digitalis at low concentrations (10(-9) to 10(-8) M) may produce an apparent stimulation of monovalent cation transport in isolated intact myocardium or produce an inotropic effect that does not correlate with pump inhibition. Digitalis is known to modify tissue metabolism of endogenous neurotransmitters that may affect inotropic state, Na,K-ATPase activity, and K+ permeability. We examined the interactions of low concentrations of ouabain with adrenergic and cholinergic influences in isolated guinea pig left atria stimulated at 3.3 Hz in which inotropic state and monovalent cation transport (measured as 86Rb+ uptake) were assessed simultaneously. Ouabain (10(-9) M) stimulated Rb+ transport (+25%) without an inotropic response; the stimulatory effect on transport was abolished by propranolol or atropine pretreatment. In atria pretreated with atropine, 10(-8) M ouabain produced a small positive inotropic effect (+10%) without measurable associated Na+-K+ pump inhibition. This inotropic response was abolished in catecholamine-depleted atria. Ouabain (10(-7) M) always produced a positive inotropic response (about +25%) independent of catecholamine depletion, beta-adrenergic blockade, or muscarinic blockade, but Rb+ uptake inhibition was observed only in beta-adrenergically-blocked atria. In all preparations, ouabain concentrations greater than 10(-7) M caused an inotropic response associated with pump inhibition. At concentrations 3 X 10(-7) M and higher, mechanical toxicity was observed in all preparations except those pretreated with propranolol. Incubation with low concentrations of ouabain did not modify the inotropic response to isoproterenol. At concentrations of isoproterenol sufficient to stimulate Rb+ transport by 25%, there was a large (+80%) inotropic response. We conclude first, that, in guinea pig atria exposed to ouabain, the mechanism as well as the extent of inotropic response and of monovalent cation transport modification is concentration dependent, second, that at low concentrations (1-10 X 10(-9) M), in vitro inotropic and monovalent cation transport responses are in part mediated by an effect of ouabain on endogenous neurotransmitters; and third, that in this preparation at concentrations between 10(-9) and 10(-7) M ouabain, monovalent cation transport as measured by tissue 86Rb+ uptake does not correlate with inotropic response.

Stimulation of monovalent cation active transport by low concentrations of cardiac glycosides. Role of catecholamines.

The stimulatory effect of low concentrations of ouabain on the Na-K pump in isolated guinea pig left atria was studied in vitro by assessing active transport of the K(+) analog Rb(+). Active transport of Rb(+) was stimulated 20+/-8% (SEM, P < 0.05) above control values by 3 nM ouabain, but was inhibited by concentrations >10 nM. Preincubation with the beta-adrenergic antagonist propranolol (1 muM) completely blocked stimulation of active transport of Rb(+) by 3 nM ouabain. Norepinephrine, 10 nM, increased Rb(+) active transport 29+/-10% (P < 0.02) above control values. The beta-adrenergic agonist l-isoproterenol, 10 nM, increased active transport of Rb(+) by 33+/-10% (P < 0.01) above control levels. This stimulatory effect was abolished if tissues were first exposed to propranolol. Tyramine (0.1 muM), a stimulator of endogenous catecholamine release, increased active transport of Rb(+) 26+/-12% (P < 0.05) above control values. Rb(+) active transport was not significantly changed when left atrial tissues were incubated with alpha-adrenergic agonists or antagonists. Ouabain stimulation of Rb(+) active transport was prevented by in vivo depletion of myocardial endogenous catecholamines by either reserpine or 6-hydroxydopamine. These findings indicated that in myocardial tissue, Na-K pump stimulation by low concentrations of ouabain is mediated at least in part through beta-adrenergic effects of endogenous catecholamines.

Effect of dantrolene sodium on the transmembrane potentials and contractility of guinea pig atrial myocardium

Dantrolene sodium (3 × 10 −4 M) exerted a biphasic effect on the mechanical and electrical activity of the isolated guinea pig left atrium. In the first phase, the drug increased the contractile force by about 200%, and prolonged the action potential duration. This transient positive inotropic effect was antagonized by D-600, a slow Ca 2+ channel inhibitor. In the later phase, the drug gradually decreased the amplitude of contractions, shortened the action potential duration and induced contracture. Dantrolene sodium increased the rate of depolarization and overshoot of Ca 2+ -mediated slow action potentials. The results suggest that dantrolene sodium increases the slow inward Ca 2+ current, causing a positive inotropic effect in the atrial myocardium. The late negative inotropic effect of the drug could be due to a secondary inhibition of the slow inward Ca 2+ current due to an increased intracellular free Ca 2+ concentration as a consequence of other actions of the drug.

A Comparison of the Myocardial Depressant Effects of Several Aminoglycoside Antibiotics

The myocardial depressant action of aminoglycosides has been known for a number of years but there was no available data on newer derivatives. We have thus studied the contractile responses of isolated guinea pig left atria to increasing concentrations of gentamicin, tobramycin, lividomycin and amikacin (0.02, 0.04, 0.08, 0.13, 0.17 and 0.22 mM). Our results are consistent with those of previous workers and show that newer aminoglycosides do exert a similar depressant action. As gentamicin and tobramycin are more depressant than lividomycin, then amikacin at similar concentrations, no clear-cut difference was obtained when respecting a relative range of concentrations mimicking that of clinical situations. However, these depressant effects were attained using concentrations far exceeding those reached in man.