Concentration-dependent Positive Inotropic Effect Research Articles

Positive inotropic and chronotropic effects of trypsin and some other proteolytic enzymes in the guinea-pig heart.

1 In atrial preparations of the young guinea-pig (body weight 150-250 g), five proteolytic enzymes (trypsin, chymotrypsin, bacterial-Al-proteinase (nagarse), bromelain and kallikrein) produced concentration-dependent positive inotropic and chronotropic effects, while they exerted only minimal effects on the papillary muscle preparations. 2 To characterize the effects, further experiments were conducted in atrial preparations using trypsin. There was a strong tendency for tachyphylaxis: a second exposure to the same concentration of trypsin resulted in considerably smaller positive inotropic and chronotropic effects. The positive inotropic and chronotropic effects of this substance were not affected by propranolol (5 X 10(-7)M). However, an accumulation of cyclic AMP was observed and the positive inotropic and chronotropic effects were potentiated by aminophylline (10(-4)M) in association with an augmentation of the accumulation of cyclic AMP. In preparations partially depolarized with high K+ (22mM) medium (contractions ceased under this condition) trypsin 100 micrograms ml-1 reinstated the contraction. Treatment of the preparation with aprotinin (200 u ml-1) resulted in a strong inhibition of the positive inotropic and chronotropic effects. 3 Islet activating protein (IAP), a specific inhibitor of the 'inhibition specific' guanine nucleotide binding regulatory protein of the adenylate cyclase system, did not produce significant inhibition of the positive inotropic and chronotropic effects of trypsin, whereas it produced a complete inhibition of the negative inotropic and chronotropic effects of carbachol. 4. These results suggest that the positive inotropic and chronotropic effects ofproteolytic enzymes are intimately connected with the proteolytic activities through which adenylate cyclase is activated to produce an accumulation of cyclic AMP within the myocardium. The destruction of the 'inhibition specific' guanine nucleotide regulatory protein of the adenylate cyclase was not substantiated as a mechanism of activation of the adenylate cyclase.

Muscarinic receptors mediate negative and positive inotropic effects in mammalian ventricular myocardium: differentiation by agonists

The concentration-dependence of the negative and positive inotropic effect of choline esters and of oxotremorine was studied in isometrically contracting papillary muscles of the guinea-pig. The preparations were obtained from reserpine-pretreated animals and were electrically driven at a frequency of 0.2 Hz. In the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methyl xanthine (IBMX, 100 mumol l-1), choline esters and oxotremorine produced concentration-dependent negative inotropic effects. Oxotremorine exhibited the highest negative inotropic potency (with a half-maximal effective concentration, EC50, of 20 nmol l-1) followed by carbachol (139 nmol l-1), methacholine (490 nmol l-1), acetylcholine in the presence of 10 mumol l-1 physostigmine (1.36 mumol l-1) and bethanechol (10 mumol l-1). Atropine was a competitive antagonist of the negative inotropic effects. Carbachol and oxotremorine decreased Vmax, overshoot and duration of slow Ca2+-dependent action potentials which had been elicited in the presence of 100 mumol l-1 IBMX. Choline esters produced a concentration-dependent positive inotropic effect. With an EC50 of 32 mumol l-1, carbachol was the most potent compound, followed by methacholine (35 mumol l-1), acetylcholine in the presence of 10 mumol l-1 physostigmine (46 mumol l-1) and bethanechol (142 mumol l-1). Compared to carbachol and methacholine which increased force by 100% of control, the increase induced by acetylcholine and bethanechol was only 64 and 58%, respectively. Atropine shifted the concentration-effect curves of all choline esters to higher concentrations. Choline esters caused intracellular Na+ activity to increase in the quiescent papillary muscle. This effect was reversed by atropine. Oxotremorine produced a small concentration-dependent positive inotropic effect (about 30% of the maximal effect of carbachol) which was resistant to atropine. Oxotremorine was a potent inhibitor of the positive inotropic effect of choline esters, and did not cause an increase in intracellular Na+ activity in the quiescent papillary muscle. The results show that muscarinic receptors of the ventricular myocardium mediate two inotropic effects, which are opposite in direction and differ in their concentration-dependence by a factor of 100. Although agonists differentiate between both inotropic effects, it is unknown whether the receptors involved represent receptor states or separate receptor subpopulations. The negative inotropic effect of choline esters and of oxotremorine can be best explained by adenylate cyclase inhibition. While stimulation of phosphoinositide hydrolysis might have been responsible for the positive inotropic effect of choline esters via modulation of cation-fluxes across the cell membrane, such a mechanism was not involved in the positive inotropic effect of oxotremorine.

Beta-2 adrenergic receptors are not only for circulating catecholamines in ventricular muscles of carp heart ( Cyprinus carpio)

1. Whether beta-2 adrenergic receptors are only for the circulating catecholamines in isolated ventricular muscles of carp heart was studied. 2. Isoproterenol, epinephrine and norepinephrine had concentration dependent positive inotropic effects. The ED 50 values for isoproterenol, epinephrine and norepinephrine were 6.23 ± 1.9, 87.3 ± 27.3 and 4500 ± 580 nM. respectively. Phentolamine did not alter the inotropic effects. 3. Tyramine increases the force of contraction and this action was completely blocked by propranolol and reserpine. but not by atenolol. 4. Norepinephrine levels, in comparison with those of epinephrine, were similar in plasma and higher in ventricular muscles. However, the norepinephrine levels in ventricular muscles of carp heart were markedly lower than those in ventricular muscles of rat heart. 5. These results suggest that ventricular muscles of carp heart contain adrenergic neurons, and that a catecholamine released from the nerve terminals, presumably epinephrine, may stimulate bela-2 adrenergic receptors.

Are beta-adrenergic receptors in ventricular muscles of carp heart ( Cyprinus carpio) mostly the beta-2 type?

I. The positive inotropic effect of isoproterenol was quantified in the presence of several beta-adrenergic blocking agents in ventricular strips of carp heart. 2. Isoproterenol had a concentration-dependent positive inotropic effect. The effect was markedly inhibited by propranolol and carteolol, but was extremely insensitive to atenolol. Practolol totally failed to alter the effect. 3. These results indicated that the positive inotropic effect of isoproterenol may be mediated by mostly beta-2 adrenergic receptors in ventricular strips of carp heart.

Studies on the Mechanism of the Positive Inotropic Effect of Piroximone in Cat Papillary Muscle

The mechanism of the inotropic effect of piroximone HCl [MDL 19205A, 4-ethyl-1,3-dihydro-5-(4-pyridinylcarbonyl)-2H-imidozol-2-on e HCl] was studied in the cat papillary muscle paced electrically in vitro. Piroximone produced a concentration-dependent positive inotropic effect accompanied by an increase in rate of contraction and rate of relaxation, but abbreviated time to peak tension and relaxation time. The positive inotropic effect produced by piroximone was antagonized by carbachol, 3 X 10(-6) M, whereas that produced by increasing calcium concentration was not affected by carbachol. In potassium chloride (22 mM) depolarized muscle, piroximone restored contractility, which was not affected by propranolol (10(-6) M) or by tetrodotoxin (2 X 10(-5) M), but was inhibited by nifedipine (10(-7) M). Piroximine also elevated tissue cyclic AMP (cAMP) content in the papillary muscle. Although nifedipine inhibited the restoration of contractility, it did so without altering the increase of cAMP produced by piroximone. These results suggest that piroximone causes an increase in calcium influx that is mediated by an increase in cAMP, and the results are consistent with the hypothesis that specific inhibition of the high affinity cAMP phosphodiesterase (PDE III) plays a role in the positive inotropic effect of piroximone.

DPI 201-106, a novel cardioactive agent. Combination of cAMP-independent positive inotropic, negative chronotropic, action potential prolonging and coronary dilatory properties.

The in vitro cardiac effects of DPI 201-106, a novel piperazinyl-indole, were investigated. DPI 201-106 produced concentration-dependent positive inotropic effects in guinea-pig and rat left atria, kitten, rabbit and guinea-pig papillary muscles and Langendorff perfused hearts of rabbits between 10(-7) and 3 X 10(-6) mol/l. During isometric twitches, contraction and relaxation phases were prolonged in guinea-pig left atria and right ventricular papillary muscles from kitten and guinea-pigs. Spontaneous sinus rate was decreased in right atria of guinea-pigs and rats. Coronary flow increased in rabbit isolated hearts. Functional refractory period was increased in left atria from guinea-pigs and rats with EC50 values of 1.7 and 0.24 mumol/l respectively. In electrophysiological measurements, DPI 201-106 prolonged the action potential duration (APD70) in guinea-pig papillary muscles up to 70% and in rabbit atria up to 120% at 3 mumol/l. Other action potential characteristics were not changed in guinea-pig papillary muscles but Vmax was decreased in rabbit left atria. The electrophysiological as well as the positive inotropic effects were stereoselective with the activity residing in the S-enantiomer. DPI 201-106 increased the Ca2+-sensitivity of skinned fibres from porcine trabecula septomarginalis with an EC50 of 0.2 nmol/l. DPI 201-106 dit not change cAMP levels in guinea-pig atria and rabbit papillary muscles. Slow action potentials were not induced by DPI 201-106 in partially depolarized guinea-pig papillary muscles. Phosphodiesterase activity of rat hearts was not inhibited by DPI 201-106 at pharmacologically relevant concentrations. The presence of propranolol did not influence the inotropic potency of DPI 201-106 in guinea-pig atria. In conclusion, DPI 201-106 represents a novel type of positive inotropic agents with a synergistic sarcolemmal and intracellular mechanism of action.

The positive inotropic effect of sodium arachidonate on auricles from diabetic rats

Contractile responses to exogenous sodium arachidonate (NaA) were studied in auricles from normal and acutely diabetic (streptozotocin-treated) rats. NaA induced a concentration-dependent positive inotropic effect in atria from normal and diabetic rats but the magnitude of the contractile influence of the fatty acid was different in both types of auricles, i.e. diabetic atria had a greater resonsiveness to NaA than normal preparations. Blockers of arachidonic acid (AA) metabolism via cyclo-oxygenase (indomethacin and acetylsalicylic acid), abolished the stimulatory influence of NaA in normal and in diabetic auricles. Moreover, incubation with tranylcipromine, a prostacyclin synthetase inhibitor, reduced the inotropic effect of NaA in normal atria, but failed to modify it in the diabetic atria. On the other hand, inhibitors of thromboxane (TX) synthetase (imidazole and L-8027) diminished the effect of NaA in diabetic preparations without altering the contractile responses in the controls. In the presence of lipoxygenase blocking agents (nordihydroguaiaretic acid and dithizone), the influence of NaA was reduced at normal values only in the atria from diabetic rats. Both atrial preparations (normal and diabetic) converted ([1- 14C]arachidonic acid (AA) into PGI 2 (assessed as 6-oxo-PGF 1α) and TXB 2. Moreover, normal atria generated more PGI 2 than TXB 2 whereas TXB 2 production was greater than that of PGI 2 in diabetic atria. The foregoing results suggest that in non-diabetic atria the effect of exogenous NaA may be attributed to metabolites generated by the sequential action of cyclo-oxygenase and prostacyclin synthetase. On the other hand, the contractile action of exogenous NaA in diabetic atria may be ascribed to thromboxanes and lipoxygenase(s) metabolites. A possible modulatory influence of TX on the action of lipoxygenase products is postulated.

Isoproterenol-induced desensitization to the positive inotropic effect of isoproterenol in ventricular strips isolated from carp heart ( Cyprinus carpio)

1. Whether the positive inotropic effect of isoproterenol in ventricular strips of carp heart is altered by previous exposure to the agonist was studied. 2. Isoproterenol produced a concentration-dependent positive inotropic effect in these preparations which was competitively antagonized by propranolol. 3. Isoproterenol dose-response curves were shifted significantly downward and to the right after previous treatment with and removal of isoproterenol. 4. The desensitization could be demonstrated after 5 min incubation and near maximal desensitization was observed after 30 min exposure to 1000 nM isoproterenol. The responsiveness was only partially recovered after 180min incubation in drug-free medium. 5. Isoproterenol produces a rapidly developing desensitization to inotropic effects of the beta-adrenergic agonist. This phenomenon may be mediated by alterations in beta-adrenergie receptors, adenylate cyclase and/or the beta receptor-adenylate cyclase coupling mechanism.

Effects of dihydropyridine calcium channel modulators in the heart: Pharmacological and radioligand binding correlations

Bay k 8644 produced a concentration-dependent positive inotropic effect followed by a negative inotropic effect in isolated and intact cardiac preparations. Nimodipine in low concentrations produced slight positive inotropy and in higher concentrations, the usual negative inotropic action. Radioligand binding experiments revealed equilibrium dissociation constants that, taken together with the pharmacological data, suggest that dihydropyridines bind to receptor subtypes and have varying intrinsic activities.

Competitive inhibition of phosphodiesterase activity by amrinone: Its implication in the cardiac effect of the drug

The effects of amrinone on partially purified bovine heart PDE and on resting tension of isolated guinea-pig left atria were investigated. Amrinone strongly inhibited the high affinity Mg2+ dependent PDE activity present in the 105000xg supernatant fraction of heart homogenate (IC50 = 60 microM), but only weakly blocked the Ca2+-CaM-dependent PDE (IC50 = 2 mM). The inhibition of high affinity PDE was competitive in nature. In guinea-pig left atria driven at 1 Hz, amrinone induced a concentration-dependent positive inotropic effect (EC50 = 200 microM). This action was not followed by a rise of the diastolic tension, even when high amrinone concentrations (1 mM) and/or high external Ca2+ concentrations (up to 12.8 mM) were used. It is suggested that the competitive inhibition of PDE by amrinone can only partially account for its positive inotropic effect. Moreover, because of the lack of effect on diastolic tension, amrinone differs from methylxanthines and digitalis glycosides in regard to its influence on intracellular Ca2+ movements.

Amiloride: Relationship between cardiac effects and inhibition of Na +/Ca 2+ exchange

The potassium sparing diuretic amiloride at concentrations ranging between 0.1–0.8 mM inhibited the Na +/Ca 2+ exchange in sarcolemmal vesicles isolated from beef heart. The rate of exhange activity was 50% reduced by 0.35 mM amiloride. In spontaneously beating atria isolated from normal and reserpinized guinea-pigs, amiloride produced a concentration-dependent positive inotropic effect and negative chronotropic effect (EC 50=0.7 mM). Amiloride protected spontaneously beating atria and left atria driven at 1 Hz from digitalis cardiotoxicity assessed in terms of a raised end-diastolic tension. It is suggested that the positive inotropic effect, negative chronotropic effect of amiloride and heart protection against digitalis toxicity are related to the observed inhibition of sarcolemmal Na +/Ca 2+ exchange activity.

Forskolin, a New Positive Inotropic Agent, and Its Influence on Myocardial Electrogenic Cation Movements

Forskolin exerts concentration-dependent positive inotropic effects in guinea pig isolated papillary muscles; resting potential, overshoot, and upstroke velocity remain unchanged. It markedly potentiates Ca2+-mediated action potentials elicited in partially depolarized myocardial fibers (increases in upstroke velocity, height, and action potential duration) and restores slow-channel-mediated transmembrane Ca2+ influx and contractile force under conditions of external Ca2+ withdrawal or in the presence of the specific Ca2+ antagonist D 600. In partially depolarized ventricular muscle, the potentiation of transmembrane inward Ca2+ current by forskolin is correlated with a reciprocal inhibition of Mg2+-induced electrogenic membrane effects. In this respect, forskolin resembles other Ca2+ influx promoters such as beta-adrenoceptor agonists, histamine, and dibutyryl-cyclic AMP (cAMP). All these agents favor sarcolemmal Ca2+ accumulation through increases in intracellular cAMP and the subsequent formation of Ca2+-binding phosphorylated membrane proteins; Mg2+ ions seem to be displaced by Ca2+. These findings support the concept that the effects of forskolin result from an increase in intracellular cAMP.

Effects of histamine on mechanical performance and biochemical and electrical activity in the heart of monkeys ( Macaca fuscata)

The properties of the cardiac effects of histamine on the isolated heart muscles of the Japanese monkey ( Macca fuscata) were investigated. Histamine had a concentration-dependent positive inotropic effect on left atria and papillary muscles and a positive chronotropic effect on right atria. Histamine increased the levels of cyclic AMP and shortened the duration of the action potential in Purkinje fibers, measures of its biochemical and electrophysiological effects. All of these effects of histamine were blocked by 10 −5 M cimetidine. These results indicate that histamine H 2-receptors mediate the cardiac effects of histamine on the monkey heart. Histamine (10 −5 M) also restored the action potentials and contractility of K +-depolarized preparations. These effects were inhibited by verapamil (10 −6-10 −5 M) but not by TTX (10 −5 M), suggesting that, in the monkey heart, histamine may act by increasing the slow inward current.

Sanguinarine: A positive inotropic alkaloid which inhibits cardiac Na +,K +-ATPase

In isolated, isometrically contracting left guinea pig atria, sanguinarine, a benzophenanthridine alkaloid from the papaveracea Sanguinaria canadensis, produced a concentration-dependent positive inotropic effect. Between 2.3 × 10 −6 M and 6.5 × 10 −5 M, sanguinarine increased contractility by 108% which was comparable to the maximal inotropic effect of ouabain. Within the same concentration range, sanguinarine caused inhibition of Na +,K +-ATPase isolated from guinea pig myocardium. 100% inhibition of Na +,K +-ATPase activity occurred at 1 × 10 −4 M sanguinarine. The I 50 for enzyme inhibition and the ED 50 for the inotropic action of sanguinarine were the same (6–6.5 × 10 −6 M) indicating that both effects may be causally related.

An intracellularly induced positive inotropic effect of manganese in guinea-pig ventricular myocardium.

1. Manganese ions (0.3–2.4 mmol/l, applied as MnCl2) produced a concentration-dependent rapid decrease in force of contraction (Fc) of guinea-pig papillary muscles; 0.6 mmol/l diminisched Fc by about 50%. The initial decrease in Fc was followed by a concentration-dependent positive inotropic effect which, within 60–90 min, led to a Fc value of 100% and more above the control (contraction frequency 0.625 Hz). 2. The positive inotropic effect of Mn2+ was associated with an increase in the rate of contraction and a marked, concentration-dependent prolongation of the relaxation time (by 80% in the presence of 1.2 mmol/l Mn2+). Concomitantly, the duration of the action potential was slightly shortened. 3. The development of the positive inotropic effect of Mn2+ depended on the stimulation frequency. The half-time values for the increase in Fc were 27, 11 and 5 min at the frequencies of 0.5, 0.625 and 1 Hz, respectively. 4. After washout of Mn2+ (0.6 mmol/l) the negative inotropic effect disappeared rapidly with a half-time of 0.5 min whereas the positive inotropic effect declined slowly (half-time 38 min). 5. It is concluded that, in addition to a negative inotropic effect due to an action on the sarcolemmal membrane, Mn2+, following its stimulation-induced uptake, exerts a positive inotropic effect by an intracellular action. Inhibition of sequestration of Ca2+ by intracellularly accumulated Mn2+ probably plays a role in the positive inotropic effect.

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.

Response of isolated human ventricular myocardium to cyclic AMP and its dibutyryl derivative.

The contractile responses to c-AMP and DB-c-AMP were studied in isolated electrically stimulated human papillary muscle strips. C-AMP (1×10−4 to 1×10−3 M) had no effect on contractile force in all of 6 human papillary muscle preparations studied. In contrast, DB-c-AMP (10−4 to 5×10−3 M) produced a concentration-dependent and reversible positive inotropic effect which was associated by a decrease in time to peak force and in relaxation time and which was not inhibited by 10−6 M propranolol. The possibility of a clinical applicability of DB-c-AMP is discussed.

Influence of cyclization and acyl substitution on the inotropic effects of adenine nucleotides.

This study was designed to further elucidate relevance and mechanism of the positive inotropic action of cyclic N6-2′-O-dibutyryl-AMP (DB-c-AMP). For this purpose the effects of cyclic N6-monobutyryl-AMP (N6-MB-c-AMP), noncyclic N6-2′-O-3′-O-tributyryl-5′-AMP (TB-AMP), c-AMP, adenosine and various adenine nucleotides (ATP, ADP, AMP) on myocardial contractile force (CF) were investigated and compared to that of DB-c-AMP. The experiments were performed on isolated, electrically driven (frequency 2 Hz) rat left auricles, i.e. on a preparation in which DB-c-AMP consistently produced positive inotropic effects. The following results were obtained: 1. N6-MB-c-AMP (2×10−4–5×10−3 M) produced a concentration-dependent positive inotropic effect (Fig. 2). This effect began 5.5 to 1.8 min after addition of the drug (Table 2), developed gradually (Fig.2), and was maximal within 44.5 to 9.5 min (Table 2). It was preceded by a transitory decrease in CF which was also concentration-dependent (Fig.2, Table 1). 2. The inotropic effects of cyclic N6-MB-AMP and cyclic DB-AMP were qualitatively similar. Quantitatively, the positive inotropic effect of N6-MB-c-AMP developed more slowly (Table 2) and was smaller in magnitude than that of DB-c-AMP (Fig.2). The initial negative inotropic effect, however, was more pronounced (Table 1) and longer in duration (Table 2) with N6-MB-c-AMP than with DB-c-AMP. 3. The time course of the reversal of the positive inotropic effects during washout in drug-free solution was practically not different with both drugs (Fig.3). Predrug levels were reached within 50–60 min. 4. No positive inotropic effects were found with the non-cyclic but butyryl substituted adenine nucleotide, TB-AMP. In contrast, this compound (10−7 to 10−3 M) produced a concentration-dependent decrease in CF (Fig.4). 5. c-AMP, ATP, ADP, AMP and adenosine depressed CF. No secondary positive inotropic effects could be detected within 60 min (Fig.5; Fig.6).

The positive ionotropic effect of dibutyryl-3',5'-AMP on isolated rat atria

1. In isolated electrically driven (frequency 2 Hz) rat left atria the N6-2′-O-dibutyryl derivative of cyclic 3′,5′-AMP (DB-AMP; monosodium salt) produced a concentration-dependent and reversible positive inotropic effect. In normal Tyrode solution (1.8 mM Ca++, the threshold concentration was 5×10−5 M; the peak concentration (10−3 M) increased contractile force to 203% of control values. Negative inotropic responses were not observed in these experiments even with supramaximal DB-AMP concentrations (Fig. 1). 2. The positive inotropic effect of DB-AMP developed gradualy (Fig. 2). It began 0.3–7.7 min after addition of the drug and was maximal within 3.5–28.3 min (Tab. 2). Once established, the effect was prolonged and decreased only slightly within 60 min (Fig. 2). 3. The positive inotropic effect of DB-AMP was due to an increase in the rate of tension development. The time to peak tension and the duration of contraction remained unchanged (Tab. 3). 4. At high concentrations (10−3–5×10−3M) DB-AMP caused contractures in some experiments (Tab. 1). 5. The time course and the magnitude of the inotropic effects of DB-AMP were influenced by the [Ca]e. The increment in myocardial contractility caused by 10−3M DB-AMP developed faster at high than at low [Ca]e (Tab. 4; Fig. 5). On the other hand, the positive inotropic effect of 10−3M DB-AMP was most evident at normal (1.8 mM) and reduced (0.45; 0.9 mM) [Ca]e, less pronounced at 3.6 mM Ca++ and was insignificant at 7.2 mM Ca++ (Fig.3). 6. Sodium butyrate (10−4–10−2M) did not alter the contractile behaviour of isolated rat atria. 7. In isolated electrically driven (frequency 3 Hz) left guinea-pig atria no positive inotropic effects could be demonstrated with DB-AMP at concentrations up to 10−3M.