Comparative species studies on the effect of monovalent cations and ouabain on cardiac Na+, K+-adenosine triphosphatase and contractile force.

The effects of ouabain, Rb+ and Tl+ on Na+, K+-adenosine triphosphatase (Na+,K+-ATPase; Mg++-dependent, Na+,K+-activated ATP phosphohydrolase, EC 3.6.1.3) and contractile force were compared in guinea-pig and rat hearts. Although ouabain produced a dose-dependent positive inotropic effect in rat as well as in guinea-pig atrial preparations, concentrations of ouabain needed to produce comparable positive inotropic effects were more than an order of magnitude higher in rats than in guinea pigs. Additionally, the time to reach the plateau of the inotropic response was significantly shorter in rat than in guinea-pig atrial preparations. Concentrations of ouabain needed to produce comparable inhibition of cardiac Na+, K+-ATPase in vitro observed with partially purified cardiac enzyme preparations were also more than an order to magnitude higher in rats than in guinea pigs.     

The influence of ketamine on inotropic and chronotropic responsiveness of heart muscle.

The influence of ketamine on the inotropic and chronotropic responsiveness of heart muscle was examined in spontaneously beating right atrial preparations and in electrically driven left atrial preparations of guinea pigs. Ketamine (2.63 X 10(-5) to 4.2 X 10(-4) M) decreased heart rate of right atria and decreased contractile tension and its maximum rate of increase in both right and left atrial preparations (right atria greater than left atria). Ketamine did not prevent the heart rate increase produced by norepinephrine (NE; 1 X 10(-8) to 1 X 10(-4) M) in right atria; however, the maximum heart rate was consistently lower in ketamine-treated than in control muscles even after exposure to NE. Although contractile tension was decreased by ketamine, the maximum inotropic response to NE was consistently greater in ketamine-treated atria than in control atria. An inhibitor of the slow Ca++ current in heart muscle, D600, depressed the contractile effects of NE but did not prevent the positive inotropic interaction of ketamine and NE. Ketamine similarly enhanced the inotropic responses to norepinephrine (1 X 10(-6) M), epinephrine (1 X 10(-6) M), isoproterenol (1 X 10(-7) M) and dibutyryl cyclic adenosine 3':5'-monophosphate (AMP; 4 X 10(-3) M) in left atria electrically paced at a constant frequency of contraction of 1 Hz; however, ketamine inhibited the positive inotropic response to increased frequency of stimulation (0.1-3.0 Hz) and to ouabain (3 X 10(-7) M). These findings demonstrate that ketamine can exert a selective positive inotropic influence in heart muscle independent of heart rate or direct or reflexogenic autonomic nervous system changes, and suggest that this activity could in some way be associated with an alteration of the intracellular disposition of cyclic AMP.     

Digitalis-induced mechanical toxicity: protection by slow Ca++ channel blockers

In the in vitro perfusion of the isolated heart, toxic doses of cardiac glycosides produce an inotropic response which is followed by a decline in contractile force and an increase in the resting tension. Several reports in the literature indicate that the subsequent decline in contractile force may be related to cardiac cellular Ca++ overload. The purpose of the present study was to determine if the slow Ca++ channel blockers such as verapamil and nifedipine, which block Ca++ influx through voltage-dependent gated channels, can reduce or prevent the digitalis-induced decline in contractile force (mechanical toxicity). Langendorff preparations of isolated perfused guinea pig heart were used for the present study. The data obtained demonstrate that 1 to 2 microM ouabain in the perfusion medium produced mechanical toxicity in the hearts after an initial inotropic response. Verapamil or nifedipine, when combined with ouabain in the perfusion medium, increased the magnitude of the inotropic response and delayed or abolished the mechanical toxicity in a dose-dependent manner. No changes in the sarcolemmal Na+,K+-adenosine triphosphatase or ouabain binding were observed in the presence of verapamil or nifedipine. The data suggest that simultaneous use of verapamil or nifedipine may protect against digitalis-induced mechanical toxicity.     

Cardiotonic action of [8]-gingerol, an activator of the Ca++-pumping adenosine triphosphatase of sarcoplasmic reticulum, in guinea pig atrial muscle

[8]-Gingerol (gingerol), a component of ginger, produced a concentration-dependent positive inotropic effect on guinea pig isolated left atria at concentrations of 1 X 10(-6) to 3 X 10(-5) M. Gingerol also exhibited positive inotropic and chronotropic effects on guinea pig right atria. The gingerol-induced inotropic effect was abolished by ryanodine, but was little affected by propranolol, chlorpheniramine, cimetidine, tetrodotoxin, diltiazem or reserpine. The time to peak tension and relaxation time within a single contraction were shortened by gingerol (1 X 10(-5) M) as well as isoproterenol, whereas they were prolonged by BAY K 8644. In guinea pig isolated atrial cells, gingerol (3 X 10(-6) M) caused an increase in the degree and the rate of longitudinal contractions. In guinea pig left atria, gingerol (1 X 10(-6) to 3 X 10(-5) M) gave little influence on the action potential, although it increased the contractile force of the atria. The whole-cell patch-clamp experiments showed that the slow inward current was little affected by gingerol (1 X 10(-6) to 3 X 10(-5) M) in voltage-clamped guinea pig cardiac myocytes. The measurement of extravesicular Ca++ concentration using a Ca++ electrode indicated that gingerol (3 X 10(-6) to 3 X 10(-5) M) accelerated the Ca++ uptake of fragmented sarcoplasmic reticulum (SR) prepared from canine cardiac muscle in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of cytochalasin-B and phloretin on digitalis inotropy

Phloretin and cytochalasin-B are known to inhibit sugar transport across the cell membrane of many tissues. Both of these agents at concentrations of 100 and 20 microM, respectively, blocked the inotropic effects of ouabain and acetylstrophanthidin (AS) in isolated rabbit atria and papillary muscle preparations. Neither of these agents had any effect of its own on contractile force. Addition of phloretin or cytochalasin-B after the inotropic response to ouabain was fully established did not reverse the effect. The potency of cytochalasin-B was greater in atria than in papillary muscles, 1 microM of cytochalasin caused significant inhibition of the inotropic effect of ouabain in atria without significant effect in papillary muscles. Phloretin but not cytochalasin-B decreased the binding of [3H]ouabain to a semipurified sarcolemmal preparation isolated from canine left ventricular muscle. Neither ouabain nor AS had a substantial positive inotropic effect in atria suspended in substrate-free medium. Substitution of pyruvate (5 mM) for glucose did not fully support their inotropic effect in atria. Papillary muscles behaved differently, in that substrate-free as well as pyruvate media almost fully supported the inotropic effects of ouabain and of low concentrations of AS. Higher concentrations (greater than 250 ng/ml) of As produced a negative inotropic response in substrate-free medium. The possibility that an "active" sugar transport system is required for digitalis inotropy is ruled out by the observation that 2-deoxyglucose also prevents the inotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Negative chronotropic and positive inotropic actions of phencyclidine on isolated atrial muscle in guinea pigs and rats

Chronotropic and inotropic actions of phencyclidine were studied in spontaneously beating right atrial muscle and electrically paced left atrial muscle preparations isolated from guinea-pig or rat hearts. In right atrial muscle preparations, phencyclidine (10-100 microM) decreased the frequency of spontaneous beating. Guinea-pig and rat heart preparations had similar sensitivities to this action of phencyclidine. The negative chronotropic effect was not altered by atropine. A high concentration of naloxone failed to affect the chronotropic effect of phencyclidine in guinea-pig muscle, but significantly reduced the effect in rat heart muscle preparations. Phencyclidine (1-100 microM) caused positive inotropic effects in both guinea-pig and rat heart left atrial muscle electrically stimulated at 1.5 Hz; rat heart preparations had a higher sensitivity to the positive inotropic action of phencyclidine. The positive inotropic effect was reduced by verapamil, nifedipine and relatively high concentrations of diltiazem, but was not affected by propranolol, phentolamine, tripelennamine, atropine or ryanodine, indicating that the effect is not mediated by adrenergic, histaminergic or cholinergic systems or does not involve ryanodine-sensitive calcium pools. Inactivation of the fast sodium channels by partial membrane depolarization, and subsequent restoration of the contraction by raising the extracellular Ca++ concentration, did not abolish the positive inotropic action of phencyclidine. These results suggest that the negative chronotropic effect of phencyclidine is not mediated by a stimulation of the muscarinic receptor. The positive inotropic effects of phencyclidine seem to result from an increase in Ca++ influx through the slow channels of the cardiac cell membrane.     

Leukotrienes C4, D4 and E4: effects on human and guinea-pig cardiac preparations in vitro

The effects of leukotrienes C4, D4 and E4 (LT C4, D4 and E4) were studied in isolated preparations of guinea-pig and human myocardium in order to assess their contribution to cardiac dysfunction associated with systemic anaphylaxis. LT C4, D4 and E4 all caused long-lasting and dose-related decreases in the contractile force and coronary flow rate of the isolated guinea-pig heart. The rank order of potency was LT D4 greater than C4 greater than E4. The effects of LT C4 and D4 were antagonized by the anti-slow-reacting-substance compound FPL 55712. The negative inotropic effect of LT is unlikely to be secondary to the concomitant reduction in coronary flow because: 1) the same reduction in coronary flow by angiotensin II resulted in a negligible decrease in contractility and 2) the negative inotropic effect of LT also occurred in the electrically paced, noncoronary perfused left atrium and right ventricular papillary muscle of the guinea pig and in pectinate muscles obtained from surgical specimens of human right atrial appendage. LT D4 potentiated the positive chronotropic effect of histamine, supporting the concept that functional interactions occur between the various mediators of immediate hypersensitivity. The cardiac effects of pure synthetic LT are similar to those previously obtained with crude slow-reacting substance of anaphylaxis indicating that the prolonged contractile failure associated with systemic anaphylaxis largely could be due to the negative inotropic effect of LT. Because LT are released in a variety of immunological and inflammatory reactions, their potent myocardial depressant effects may play a role in cardiac dysfunction associated with these reactions.     

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.     

Ouabain effects on intracellular potassium activity and contractile force in cat papillary muscle.

The relationship between the positive inotropic and toxic effects of cardiac glycosides and their effects on intracellular ionic composition is incompletely defined. We measured intracellular potassium activity (alpha ik), extracellular potassium activity (alpha ek), resting potential, action potential duration, and contractile force at 32 degrees C in paired papillary muscles from feline right ventricles exposed to ouabain. Muscles used for electrophysiological measurements were quiescent except for isolated stimulation to confirm impalement and record action potential duration. Muscles used for contractile force measurements were quiescent except for 4-min periods when force was measured at a cycle length of 1,400 ms. Muscle length was adjusted to achieve 50% of maximal tension at this cycle length before each experiment. In four experiments, alpha ik and contractile force were measured in the same muscle. Alpha iK was measured with single and double-barrel K-sensitive electrodes. At 10 nM ouabain, action potential duration is prolonged. Among the concentrations tested, the threshold for a clear positive inotropic effect is 0.1 microM ouabain. The threshold for decrease in alpha iK, increase in alpha eK, and decrease in membrane potential is 1 microM, at which concentration toxic signs develop, including arrhythmias, aftercontractions, and alteration in the staircase response of contractile force to repetitive stimulation. Ouabain need not change alpha iK to effect positive inotropy in ventricular muscle, a relationship different from that reported between [K]i (intracellular potassium concentration) and positive inotropy. Higher ouabain concentrations, which others have shown to clearly inhibit active Na and K transport, are shown to upset intracellular potassium activity homeostasis and to consistently produce toxicity.     

Effect of PGD2 on cardiac contractility: a negative inotropism secondary to coronary vasoconstriction conceals a primary positive inotropic action

The purpose of this study was to characterize by pharmacological means the inotropic action of prostaglandin D2 (PGD2) in the guinea-pig heart. In the whole heart perfused at constant pressure, PGD2 (0.01-10 micrograms) reduced coronary flow rate and decreased left ventricular contractile force in a dose-dependent manner. When the coronary vasoconstricting effect of PGD2 was antagonized by the PG antagonist sodium p-benzyl-4[1-oxo-2-(4-chlorobenzyl)-3-phenyl propyl]phenyl phosphonate (N-0164), by the cyclooxygenase inhibitor indomethacin or by the thromboxane synthetase inhibitor sodium (E)-3-[4-(1-imidazolyl-methyl)phenyl]-2-propanoate (OKY 046), the negative inotropic response to PGD2 was attenuated or completely abolished and a positive inotropic effect was unmasked. In the isolated left atrium or right ventricular papillary muscle preparations, PGD2 induced only a positive inotropic response. The atrial response to PGD2 was unaffected by N-0164, indomethacin or propranolol but was markedly decreased by carbachol or adenosine. Conversely, the response of the papillary muscle to PGD2 was potentiated by papaverine. Thus, these data indicate that PGD2 has a primary positive inotropic effect, which may involve cyclic AMP metabolism. On the other hand, because PGD2 is also a potent coronary vasoconstrictor, the secondary negative inotropic effect of PGD2 predominates.     

Enhancement of cardiac actions of ouabain and its binding to Na+, K+-adenosine triphosphatase by increased sodium influx in isolated guinea-pig heart

The rate of development of the positive inotropic action of ouabain is enhanced when the heart is stimulated at higher frequencies. A hypothesis that this enhancement is due to a stimulation of the glycoside binding to sarcolemmal Na+,K+-adenosine triphosphatase (ATPase) caused by an increase in intracellular Na+ available to the sodium pump was tested in isolated left atrial muscle preparations of guinea-pig heart, incubated at 30 degrees C and electrically stimulated at 0.5, 1 or 2 Hz. The rate of development of the positive inotropic action of ouabain was dependent on the frequency of stimulation. Each preparation was homogenized at a predetermined time and the fractional occupancy of Na+,K+-ATPase by ouabain was estimated from the decrease in the initial velocity of ATP-dependent [3H]ouabain binding reaction. A parallel relationship was observed between effects of stimulation frequency of the positive inotropic action and those on the occupancy of Na+,K+-ATPase by ouabain. In quiescent preparations, a sodium ionophore, monensin, enhanced the development of contracture caused by a toxic concentration of ouabain and also the glycoside binding to Na+,K+-ATPase. Similar effects on the ouabain-induced contracture and on the glycoside binding were observed with either grayanotoxin I or batrachotoxin, agents known to increase sodium influx, when muscle preparations were exposed to these agents under 1.5 Hz stimulation and were subsequently tested for the actions of ouabain in quiescence. When the exposure to ouabain and either grayanotoxin I or batrachotoxin was restricted to quiescent period, the development of ouabain-induced contracture and glycoside binding to Na+,K+-ATPase were not significantly altered. Monensin, grayanotoxin I or batrachotoxin failed to significantly affect [3H]ouabain binding to muscle homogenates when added to the medium for the labeled glycoside binding assay. These results indicate that intracellular sodium ions promote the ouabain binding to Na+,K+-ATPase and thereby enhance the development of glycoside actions in the isolated atrial muscle of guinea-pig heart. The "beat-dependent" onset of the glycoside action is at least partially explained from the effect of membrane depolarization to increase Na+ available to the sodium pump and to enhance the glycoside binding.     

The cardiac pharmacology of tolbutamide.

Recent clinical studies have suggested an association of tolbutamide therapy with an increased incidence of cardiovascular deaths. Due to the paucity of information concerning the acute cardiac actions of tolbutamide, the effects of this agent upon cardiac contractility and automaticity were examined under in vivo and in vitro conditions in rabbit, cat and dog heart muscle preparations. Tolbutamide (10(-6) to 3 x 10(-3) M) produced a biphasic inotropic response with a peak positive inotropic response at 3 X 10(-3) M which was 13.7 +/- 5.1% of the maximal obtainable increase in tension. Similar studies in cat papillary muscle resulted in a response that averaged 19% of the maximal increase in contractile force. In contrast, canine papillary muscles as well as the intact canine heart failed to develop a positive inotropic response to tolbutamide. Responses of rabbit atrial strips to isoproterenol were not potentiated by previous exposure to tolbutamide. Exposure of rabbit atria to theophylline, 2.5 X 10(-4) M, did not potentiate the inotropic effects of tolbutamide. Stidies in spontaneously beating rabbit right atria and cat papillary muscle-Purkinje fiber preparations demonstrated that tolbutamide does not have the potential to augment automaticity in these tissues. In intact dog heart, the intracoronary administration of tolbutamide did not lead to disturbances in cardiac rhythm, providing additional evidence that tolbutamide does not increase ventricular automaticity. It is concluded that tolbutamide possess a species-specific positive inotropic effect in rabbit and cat but not in the dog. The inotropic effect is small when compared to the maximum inotropic response and is observed only in vitro. Tolbutamide lacks the ability to enhance cardiac pacemaker activity. These data do not support the conclusions of previous investigatirs concerning the possible deleterious cardiac effects of tolbutamide.     

Positive inotropic effects in isolated ventricular myocardium from non-failing and terminally failing human hearts

The positive inotropic responses to isoprenaline, dobutamine, histamine, forskolin, isobutyl-methylxanthine (IBMX), dibutyryl-cyclic adenosine monophosphate (db-cAMP), ouabain and calcium were studied in isolated, electrically driven papillary muscle strips from either terminally failing human hearts or non-failing donor myocardium. The positive inotropic effect of calcium has been taken to evaluate the maximal force of contraction of each individual muscle strip ('contractile reserve'). In the non-failing heart, the maximal positive inotropic effect of isoprenaline, dobutamine, IBMX, ouabain and calcium were not significantly different, but were significantly greater than histamine. In terminally failing hearts, the positive inotropic effects of agents stimulating the adenylate cyclase by a receptor-dependent mechanism (isoprenaline, dobutamine and histamine) and the phosphodiesterase inhibitor IBMX are less than in the normal heart. Furthermore, these compounds gave a markedly reduced inotropic effect compared with forskolin, db-cAMP and ouabain, which gave maximal responses similar to calcium in the failing hearts. The data did not differ when the increase of force of contraction was related to the diameter of each preparation. These results indicate that a defect in adenylate cyclase occurs in the failing human heart, presumably located at the regulatory stimulatory subunit (Gs) of the adenylate cyclase since effects through stimulatory receptors were reduced. Responses from activation of the catalytic subunit or through cAMP-dependent protein kinases were less affected. Since the positive inotropic effect of IBMX is also impaired, it is suggested that the basal rate of cAMP production is also reduced in heart failure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of ouabain on the nerve stimulation-evoked release of norepinephrine from the isolated perfused guinea-pig heart

The isolated perfused guinea-pig heart prelabeled with (-)-[7-3H]-norepinephrine was used to examine the effects of increasing concentrations of ouabain on the sympathetic nerve stimulation-evoked release of endogenous norepinephrine and [3H]norepinephrine in the presence and absence of physostigmine or atropine. The overflow of norepinephrine and [3H]norepinephrine from guinea-pig hearts was measured during postganglionic stimulation of the cardiac accelerator fibers (5 Hz for 60 sec, 2 msec duration, for 300 pulses). Perfusion with 10(-7) M ouabain for 20 min had no effect on the release of norepinephrine or [3H]norepinephrine after nerve stimulation. However, perfusion with either 10(-6) or 3 X 10(-6) M ouabain for 15 min resulted in a significant decrease in the nerve stimulation-evoked release of norepinephrine (44.6 +/- 2.24 and 44.0 +/- 2.17%, respectively) and [3H]norepinephrine (43.8 +/- 1.62 and 44.9 +/- 2.16%, respectively) compared with previous control outputs. Perfusion of hearts with physostigmine (10(-6) M), an acetylcholinesterase inhibitor, or atropine (3 X 10(-6) M), a muscarinic blocking agent, did not alter the release of norepinephrine or [3H]norepinephrine after nerve stimulation. Perfusion with physostigmine during perfusion with 10(-6) M ouabain resulted in a decrease in the release of norepinephrine and [3H]norepinephrine only slightly greater than 10(-6) M ouabain alone, which was not significant, but the release of norepinephrine during stimulations performed after a 45-min washout of 10(-6) M ouabain was decreased significantly when 10(-6) M physostigmine was present. Perfusion of hearts with atropine during perfusion with either 10(-6) or 3 X 10(-6) M ouabain reversed the inhibitory effect of ouabain on the release of norepinephrine and [3H]norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dual effect of ouabain on the palytoxin-induced contraction and norepinephrine release in the guinea-pig vas deferens

Palytoxin (PTX), C129H223N3O54, isolated from marine coelenterates of Palythoa tuberculosa, caused a first rapid contraction followed by the slow phasic contraction of guinea-pig vas deferens. In the presence of ouabain (10(-5) M), PTX (10(-8) M) failed to cause the first contraction; however, the second contraction was potentiated. In the presence of phentolamine (10(-6) M), the second contraction was inhibited selectively. When ouabain was applied to the muscle in the presence of phentolamine, both first and second contractile responses to PTX were abolished. When the muscle was exposed to the potassium-depleted solution, the first contractile response to PTX was rather potentiated. PTX caused the release of norepinephrine from the muscle. Exposure of the muscle to ouabain (10(-5) M) markedly increased the PTX-induced release. It is indicated that the first and second contractile responses to PTX have entirely different properties. The second response is due to a release of norepinephrine from nerves and was potentiated by ouabain through the increase in the norepinephrine release, whereas the first response was not due to the norepinephrine release but presumably to a direct action on smooth muscle cell and was inhibited by ouabain. The mechanism of the action of PTX was discussed in the relation with Na,K-ATPase.     

Pharmacological evaluation of new calcium antagonists: 2-substituted 3-dimethylamino-5,6-methylenedioxyindenes. Effects on isolated hearts and coronary vessels.

The 2-n-propyl and 2-n-butyl aminoindene hydrochlorides were previously reported to interfere with excitation-contraction coupling in uterine and intestinal smooth muscle, and with stimulus-secretion coupling in the adrenal medulla, by interfering with the action of calcium at an intracellular site. The present investigation was designed to apply this knowledge to the potential coronary and cardiac effects of such a pharmacological action. The 2-n-propyl and 2-n-butyl aminoindenes (5 X 10(-6)-10(-4) M) produced a concentration-dependent relaxation of potassium-contracted strips of bovine extramural coronary vessels, which was reversible upon elevation of the calcium concentration in the medium. In the nonstimulated isolated perfused rabbit heart, the 2-n-propyl aminoindene (3 X 10(-5) M) and the 2-n-butyl aminoindene (3 X 10(-5) and 10(-4) M) increased coronary flow without affecting cardiac chronotropic activity. However, the 2-n-propyl aminoindene (3 X 10(-5) and 10(-4)M) and the 2-n-butyl aminoindene (10(-4) M) produced a negative inotropic action in this preparation. The combination of coronary dilation and decreased force of cardiac contraction may represent desirable properties for drugs useful in the treatment of ischemic heart disease.     

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.     

Chronotropic and inotropic effects of terikalant on isolated, blood-perfused atrial and ventricular preparations of dogs

Summary— We investigated the effects of terikalant, which blocks inward rectifier K⁺ current, on the sinus rate, atrial and ventricular contractile force in the isolated, blood-perfused right atrial and left ventricular preparations of dogs, and the effects of terikalant on the negative cardiac responses to acetylcholine, adenosine or pinacidil (an ATP-sensitive K⁺ channel opener) and on the positive cardiac responses to norepinephrine. Terikalant (1–100 nmol) decreased sinus rate and briefly and slightly increased atrial contractile force in isolated atria. However, terikalant did not increase ventricular contractile force in isolated ventricles. Neither propranolol nor atropine inhibited the positive inotropic and negative chronotropic responses to terikalant, respectively. Terikalant (10 or 30 nmol) did not significantly affect the negative cardiac responses to acetylcholine, adenosine nor pinacidil and the positive responses to norepinephrine. These results suggest that terikalant decreases sinus rate with a small changes in myocardial contractile force and does not affect the cardiac responses to muscarinic and adenosine receptor agonists, ATP-sensitive K⁺ channel openers nor β-adrenoceptor agonists in the dog heart.     

Characterization of the beta adrenoceptor subtype(s) mediating the positive inotropic effects of epinine, dopamine, dobutamine, denopamine and xamoterol in isolated human right atrium.

In patients with chronic heart failure cardiac beta-1 adrenoceptors are reduced, whereas beta-2 adrenoceptor changes vary depending on the etiology of the disease. Beta Adrenoceptor agonists can be used for short-term inotropic support in chronic heart failure; their clinical efficacy might depend on which beta adrenoceptor subtype(s) mediates their positive inotropic effect. Thus, the beta adrenoceptor subtype(s) involved in the positive inotropic effects of clinically used beta adrenoceptor agonists was characterized on isolated electrically driven human right atria by the use of the selective beta-1 adrenoceptor antagonist CGP 20712 A (300 nmol/l) and/or the selective beta-2 adrenoceptor antagonist ICI 118,551 (30 nmol/l). Epinine evoked positive inotropic effects through stimulation of beta-1 and beta-2 adrenoceptors to about the same degree, whereas dobutamine acted mainly at beta-1 adrenoceptors but had a significant beta-2 adrenoceptor component. Both agonists were full agonists causing the same maximum increase in contractile force (Emax) as did isoprenaline or Ca++ (Emax = 1.0). In contrast, denopamine was a partial selective beta-1 adrenoceptor agonist (Emax = 0.75-0.85). Dopamine was in the presence of uptake1-blockade (by 5 mumol/l phenoxybenzamine) a partial agonist (Emax = 0.60-0.70) acting selectively at beta-1 adrenoceptors; in the absence of uptake1-blockade, however, dopamine was a full agonist, indicating that part of its positive inotropic effect is indirect via the release of endogenous noradrenaline. Xamoterol did not exert positive inotropic effects, but concentration-dependently slightly decreased basal force of contraction.     

Kinetics of ouabain binding and changes in cellular sodium content, 42K+ transport and contractile state during ouabain exposure in cultured chick heart cells

To define further the mechanism of positive inotropic action of cardiac glycosides, the temporal relationships among ouabain binding, sodium pump inhibition and positive inotropy were examined using cultured chick embryo ventricular cells. In K+-free medium, specific [3H]ouabain binding to intact cells followed pseudo first-order kinetics with saturation of binding sites occurring at 1 microM ouabain. The KD values calculated from the association and dissociation rate constants were 1.4 and 4.9 X 10(-7) M, respectively, in K+-free and 4 mM K+ medium. The Scatchard plot of binding in K+-free medium was linear, consistent with the presence of a single class of binding sites (KD = 1.3 X 10(-7) M). In 4 mM K+, 0.1 microM ouabain occupied 10% of the total binding sites and failed to produce an inotropic effect, inhibit 42K+ uptake or alter [Na+]i. Exposure of cells to 1 microM ouabain caused a significant increase in contractile state after 30 sec, reaching a plateau after 7 min with 50 +/- 6% augmentation of the amplitude of cell motion; the 42K+ uptake rate was concurrently inhibited by 36% accompanied by a 35% increase in [Na+]i and occupation of 38% of total ouabain binding sites. The initial rate of 42K+ uptake in cells loaded with Na+ by incubation in K+-free medium was 4 times greater than that observed without Na+ loading. These results indicate that more than 10% of sodium pump sites must be inhibited to produce an appreciable change in the rate of monovalent cation transport, [Na+]i or contractile state, due to the reserve capacity of uninhibited sodium pumps.(ABSTRACT TRUNCATED AT 250 WORDS)