Inotropic and electrophysiological effects of BDF 9148, a congener of DPI 201-106, in guinea-pig atria and papillary muscles

Linked biaromatic compounds as cardioprotective agents

Cardiovascular diseases (CVDs) are widespread in the modern world, and their number is constantly growing. For a long time, CVDs have been the leading cause of morbidity and mortality worldwide. Drugs for the treatment of CVD have been developed almost since the beginning of the 20th century, and a large number of effective cardioprotective agents of various classes have been created. Nevertheless, the need for the design and development of new safe drugs for the treatment of CVD remains. Literature data indicate that a huge number of cardioprotective agents of various generations and mechanisms correspond to a single generalized pharmacophore model containing two aromatic nuclei linked by a linear linker. In this regard, we put forward a concept for the design of a new generation of cardioprotective agents with a multitarget mechanism of action within the indicated pharmacophore model. This review is devoted to a generalization of the currently known compounds with cardioprotective properties and corresponding to the pharmacophore model of biaromatic compounds linked by a linear linker. Particular attention is paid to the history of the creation of these drugs, approaches to their design, and analysis of the structure-action relationship within each class.

Effective and steady differentiation of a clonal derivative of P19CL6 embryonal carcinoma cell line into beating cardiomyocytes

The P19CL6 cell line is a useful model to study cardiac differentiation in vitro. However, large variations were noticed in the differentiation rates among previous reports as well as our individual experiments. To overcome the unstable differentiation, we established P19CL6-A1, a new clonal derivative of P19CL6 that could differentiate into cardiomyocytes more efficiently and stably than the parent using the double stimulation with 5-Aza and DMSO based on the previous report. We also introduced a new software, Visorhythm, that can analyze the temporal variations in the beating rhythms and can chart correlograms displaying the oscillated rhythms. Using P19CL6-A1-derived cardiomyocytes and the software, we demonstrated that the correlograms could clearly display the enhancement of beating rates by cardiotonic reagents. These indicate that a combination of P19CL6-A1 and Visorhythm is a useful tool that can provide invaluable assistance in inotropic drug discovery, drug screening, and toxicity testing.

Sodium channel enhancer restores baroreflex sensitivity in conscious dogs with heart failure

We compared the cardiac inotropic, lusitropic, and chronotropic responses to the Na+ channel enhancer LY-368052 in conscious dogs before and after development of congestive heart failure (CHF). We also examined the effect of LY-368052 on baroreflex sensitivity and the efferent neural mechanisms of the bradycardic response in heart failure. Dogs were chronically instrumented, and heart failure was induced by right ventricular pacing at 240 beats/min for 3–4 wk. LY-368052 dose-dependently increased left ventricular contractile performance before and after the development of CHF to a similar extent. The inotropic effect of LY-368052 in heart failure was not altered by either ganglionic or β-adrenergic receptor blockade. LY-368052 improved cardiac relaxation and induced bradycardia in dogs with heart failure but not in normal dogs. The negative chronotropic effect of LY-368052 was eliminated by ganglionic blockade but not β-adrenergic blockade, suggesting that the bradycardia was mediated by the autonomic nervous system via enhanced parasympathetic tone. Baroreflex sensitivity was assessed as the pulse interval-mean arterial pressure slope in response to temporary pharmacological (nitroglycerin or phenylephrine) and mechanical (brief occlusion of inferior vena cava) alterations of arterial pressure in conscious dogs before and after development of heart failure. Baroreflex sensitivity was significantly depressed in heart failure and restored completely by acute treatment with LY-368052. Thus the Na+ channel enhancer LY-368052 maintains its β-receptor-independent inotropic effect in chronic CHF and specifically improves ventricular relaxation and depressed baroreflex function.

Combined inotropic and bradycardic effects of a sodium channel enhancer in conscious dogs with heart failure: a mechanism for improved myocardial efficiency compared with dobutamine

We compared the cardiac inotropic, chronotropic, and myocardial O(2) consumption (MVO(2)) responses to the sodium (Na(+)) channel enhancer, LY341311 [(S)-4-[3-[[1-(diphenyl-methyl)-3-azetidinyl]oxy]-2-hydroxypropoxy]-1H-indole-2-carbonitrile monohydrate], with the beta-receptor agonist dobutamine in conscious dogs with heart failure. Heart failure was induced in chronically instrumented dogs by right ventricular pacing at 240 beats per minute for 3 to 4 weeks. LY341311 (10-100 microg/kg/min i.v.) dose dependently increased cardiac contractile function as reflected, at the highest dose, by increases in left ventricular dP/dt(max) (55 +/- 7%), and fractional shortening (62 +/- 9%), accompanied by increases in cardiac stroke work (111 +/- 18%) and minute work (34 +/- 10%) and decreases in heart rate (33 +/- 4%). Dobutamine (2-15 microg/kg/min i.v.) increased contractile responses to a similar degree but also increased heart rate (15 +/- 5%) at the highest dose. Complete ganglionic blockade with hexamethonium and atropine or with hexamethonium alone abolished the bradycardic effect but not the inotropic response to LY341311. At similar levels of inotropic response, dobutamine (10 microg/kg/min) increased MVO(2) by 23 +/- 7% (P < 0.05), whereas LY341311 (100 microg/kg/min) had no effect. In the presence of left atrial pacing at a constant heart rate and at matched contractile work, MVO(2) was increased by LY341311 to the same extent as dobutamine. These data indicate that autonomically mediated bradycardia produced by LY341311 contributes to a favorable net metabolic effect on myocardial O(2) utilization in the failing heart while providing inotropic support comparable to a beta-receptor-mediated agonist.

Identification of a cardiac sodium channel insensitive to synthetic modulators

BACKGROUND

DPI 201-106 (DPI) was the first synthetic compound showing cardioselective modulation of voltage-gated sodium channels (VGSCs) resulting in a positive inotropic effect. Currently, the exact mode of action for this class of compounds is not known.

METHODS

Effects of different natural and synthetic sodium channel modulators were investigated in cardiac tissue of several species with conventional electrophysiologic methods.

RESULTS

In electrically driven cardiac tissues, all compounds investigated increased force of contraction (FC) and action potential duration (APD) with increasing concentrations except for DPI in cattle trabecular muscle, which demonstrated no effect. Interestingly, calculation of EC50 levels at 30% repolarization demonstrates that natural VGSC-ligands were highly potent in prolonging the APD in cattle whereas no positive trends could be obtained for DPI and SDZ 211-939 (SDZ) in cattle.

CONCLUSIONS

These results demonstrate that the binding site for DPI and SDZ is distinct from sites 2 or 3 of the VGSC alpha-subunit. Moreover, this is the first time that these compounds show no effect or even shortening of APD. This finding will enable the characterization of the mode of action and probably the binding site for synthetic VGSC-modulators.

Effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes

BDF 9198 (a congener of DPI 201 - 106 and BDF 9148) was found to be a positive inotrope on guinea-pig isolated ventricular muscle strips. The effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes were studied using the whole cell patch clamp method. In normal external solution, at 37 degrees C, action potential duration at 50% repolarization (APD(50)) was 167.4+/-8.36 ms (n=37). BDF 9198 produced a concentration-dependent increase in APD(50) (no significant increase at 1x10(-10) M; and APD(50) values of 273.03+/-35.8 ms at 1x10(-9) M; n=6, P<0.01 and 694.7+/-86.3 ms at 1x10(-7) M; P<0.001, n=7). At higher concentrations in the range tested, BDF 9198 also induced early and delayed and after-depolarizations. Qualitative measurements of I(Na) with physiological [Na](o) showed prolongation of the current by BDF 9198, and the appearance of transient oscillatory inward currents at high concentrations. Quantitative recording conditions for I(Na) were established using low external [Na] and by making measurements at room temperature. The current - voltage relation, activation parameters and time-course of I(Na) were similar before and after a partial blocking dose of Tetrodotoxin (TTX, 1 microM), despite a 2 fold difference in current amplitude. This suggests that voltage-clamp during flow of I(Na) was adequately maintained under our conditions. Selective measurements of I(Na) at room temperature showed that BDF 9198 induced a concentration-dependent, sustained component of I(Na) (I(Late)) and caused a slight left-ward shift in the current - voltage relation for peak current. The drug-induced I(Late) showed a similar voltage dependence to peak current in the presence of BDF 9198. Both peak current and I(Late) were abolished by 30 microM TTX and were sensitive to external [Na]. Inactivation of control I(Na) during a 200 ms test pulse to -30 mV followed a bi-exponential time-course. In addition to inducing a sustained current component, BDF 9198 left the magnitude of the fast inactivation time-constant unchanged, but increased the magnitude of the slow inactivation time-constant. Additional experiments with a longer pulse (1 s) raised the possibility that in the presence of BDF 9198, I(Na) inactivation may be comprised of more than two phases. No significant effects of 1x10(-6) M BDF 9198 were observed on the L-type calcium current, or delayed and inward rectifying potassium currents measured at 37 degrees C. It is concluded that the prolongation of APD(50) by BDF 9198 resulted from selective modulation of I(Na). Reduced current inactivation induced a persistent I(Na), increasing the net depolarizing current during the action potential. This action of the drug indicates a potential for 'QT prolongation' of the ECG. The observation of after-depolarizations suggests a potential for proarrhythmia at some drug concentrations.

Effects of BDF 9148 on the action potentials and contractions of left ventricles from normo- and hypertensive rats

1. The aim of the present study was to test the hypothesis that responses to BDF 9148, which prolongs the opening of sodium channels, are reduced in the spontaneously hypertensive rat (SHR) left ventricle in the presence of hypertrophy and failure. 2. We studied the effects of BDF 9148 on the action potentials and contractions of left ventricles from 5-week-old prehypertensive, 14-week-old hypertensive, 6- and 12-month-old hypertension-associated hypertrophy and 18-month-old hypertension-induced heart failure SHR and age-matched Wistar-Kyoto normotensive (WKY) rats. 3. Action potentials and left ventricular contractions did not alter in the early stages of hypertension (14-week-old SHR). The diastolic membrane potential did not change with hypertension-associated hypertrophy, but there was a reduction in amplitude and a prolongation of action potentials in the left ventricles of 6-18-month-old SHR. Cardiac stimulation responses and maximum contractions to 10(-6) mol/L isoprenaline were reduced at 6 months, whereas the maximum contractions to 10(-2) mol/L CaCl2 were only reduced in left ventricles of 18-month-old SHR. 4. At concentrations ranging from 10(-7) to 3 x 10(-6) mol/L, BDF 9148 increased the amplitude and prolonged the duration of action potentials and augmented the force in WKY rat left ventricles. The augmenting effects of BDF 9148 at 3 x 10(-6) mol/L were smaller than at 10(-6) mol/L, possibly because the high concentration of BDF 9148 was also blocking calcium channels. Similar effects were observed with BDF 9148 in the early stages of hypertension (14-week-old SHR). 5. In the presence of persistent hypertension-associated hypertrophy of the SHR left ventricle at > or = 6 months, the effects of BDF 9148 on action potentials and contractions were significantly reduced to a small extent. This impairment of the response to BDF 9148 may reflect the reduced contractility of the SHR left ventricle and/or it may indicate that the response to the opening of sodium channels is altered from 6 months of age. 6. In summary, most of the response of BDF 9148 is maintained in the presence of hypertrophy and failure. Thus, BDF 9148 may have some potential for the treatment of heart failure.

Positive inotropic effects of the novel Na+-channel modulator BDF 9198 in human nonfailing and failing myocardium

The aim of this study was to investigate the inotropic properties of the novel Na+-channel modulator BDF 9198 in human nonfailing and failing myocardium. For comparison the Na+-channel modulator BDF 9148, the beta-adrenoceptor-agonist isoprenaline, and calcium were studied. Concentration-response curves for BDF 9198 (0.01-30 microM), BDF 9148 (0.01-30 microM), isoprenaline (0.001-1 microM), and calcium (1.8-15 mM) were obtained in electrically driven left ventricular human papillary muscle strips (1 Hz, 37 degrees C; dilated cardiomyopathy, NYHA IV, heart transplantation; nonfailing, donor hearts). Whereas isoprenaline was significantly less effective and less potent in increasing the force of contraction in failing human myocardium than in nonfailing myocardium (p < 0.01), BDF 9198 and BDF 9148 were (in NYHA IV) as effective as in nonfailing human tissue. In both tissues, BDF 9198 and BDF 9148 exerted similar positive inotropic effects as calcium, with the novel Na+-channel modulator BDF 9198 being more potent in increasing force of contraction than was the preceding agent BDF 9148. The potencies of both Na+-channel modulators, BDF 9198 and BDF 9148, were enhanced in human failing myocardium when compared with nonfailing myocardium. In summary, the novel Na+-channel modulator BDF 9198 increases force of contraction to the same extent as calcium and with a higher potency than BDF 9148. The sensitivity of failing human myocardium to Na+-channel modulators is increased when compared with nonfailing myocardium, which might be the result of an altered Na+ homeostasis in human heart failure.

Investigations of the mechanism of the positive inotropic action of BDF 9148: comparison with DPI 201-106 and the enantiomers

The electromechanical and biochemical activities of the positive inotropic compounds BDF 9148 and DPI 201-106 were compared in guinea-pig myocardic preparations. Additionally, the properties of the BDF 9148 enantiomers were studied to compare their positive inotropic effects. In guinea pig papillary muscles, BDF 9148 exerted a concentration-dependent increase of force of contraction with a 50% effective concentration (EC50) value of 0.6 microM, compared with 1.3 microM for DPI 201-106. Like that of DPI, the inotropic effect of BDF 9148 was abolished by treatment with tetrodotoxin (TTX) but not affected by treatment with carbachol. Likewise, pretreatment of the papillary muscles with propranolol, cimetidine, and histamine did not affect the contractile effects of BDF 9148. In the left atria, both agents had a positive inotropic effect with an EC50 of 0.2 microM for BDF and 0.8 microM for DPI. Incubation of single concentrations of the respective drugs for a period of 90 min with guinea pig papillary muscles resulted in slightly differing parameters of isometric contraction. In contrast to DPI, BDF 9148 prolonged the contraction time transiently. Time to peak force was not markedly influenced by either drug. The functional refractory period was prolonged by both drugs to a similar extent. At 10 microM, BDF 9148 showed a biphasic effect on the action potential duration (APD) most evident at APD90, whereas DPI prolonged APD90 progressively until the 90 min. The positive inotropic effect of BDF 9148 could be demonstrated by the (S-), whereas the (R+)-enantiomer was without effect. Neither DPI nor BDF 9148 increased myocardial cyclic adenosine monophosphate (cAMP) in isolated rat cardiomyocytes and guinea pig papillary muscles. Additionally, neither BDF 9148 nor DPI showed an inhibitory effect on the guinea pig myocardic Na+/K(+)-ATP'ase activity in the concentration range with a positive inotropic effect in the guinea-pig papillary muscle.

Enhanced sensitivity of the failing human myocardium to cardiac glycosides and Na(+)-channel activators

Cardiac glycosides and Na+ -channel activators increase intracellular Na+ and thereby enhance the transport rate of the sarcolemmal Na+/Ca2+ exchanger. We tested the hypothesis of whether increased expression of the Na+/Ca2+ exchanger in failing human myocardium is accompanied by enhanced sensitivity of the failing human myocardium toward cardiac glycosides and Na+ -channel activators. We studied the positive inotropic effects of the new Na+ -channel activator BDF and the cardiac glycoside ouabain in human failing (New York Heart Association [NYHA] functional class IV, heart transplants for dilated cardiomyopathy, n = 11) and nonfailing (donor hearts, n = 5) myocardium on electrically driven left ventricular papillary muscle strips (1 Hz, 37 degrees C). The effectiveness of ouabain and BDF to increase force of contraction was similar in human nonfailing and failing myocardium. BDF was more potent to increase force of contraction in failing than in nonfailing tissue (p < 0.05). The time until maximal inotropic effect developed after ouabain was significantly shorter in NYHA IV (mean 150 +/- 16 min) than in nonfailing myocardium (mean 240 +/- 20 min). These results suggest that human failing myocardium exerts and enhanced sensitivity to cardiac glycosides and Na+ -channel activators, possibly because of enhanced expression of the Na+/Ca2+ exchanger or because of an altered intracellular Na+ -homeostasis.

Activators of sodium, calcium and potassium channels modulate the cardiac effects of quinidine in vitro

Quinidine (25.5 mumol/l) reduced the beating frequency of isolated right guinea-pig atria, caused a negative inotropic effect in papillary muscles and slightly raised the contractile force of left atria. The functional refractory period of both tissues was prolonged. A 20% increase of the extracellular sodium concentration did not reverse the effects of quinidine. The Na-channel activator BDF 9148 (1 mumol/l) and the Ca-channel agonist Bay-K-8644 (0.5 mumol/l) further increased the contractile force and caused an additional prolongation of the functional refractory period in quinidine-pretreated atria. Only Bay-K-8644 was able to reverse the negative inotropic effect of quinidine in papillary muscles. The influence of Bay-K-8644 on the contractile force in quinidine-pretreated muscles was not attenuated by lemacalim (3 mumol/l), an activator of ATP-dependent potassium channels, but the duration of the functional refractory period was significantly reduced. These results suggest that a combination of a calcium channel activator and a potassium channel opener might be able to improve the treatment of quinidine intoxications.

Positive inotropic responses of the sodium channel modulator BDF 9148 in diseased rat myocardium

1. This study has defined the positive inotropic responses to the sodium channel modulator BDF 9148 in rats with hypertension, thyroid dysfunction, diabetes or dwarfism. Concentration-response curves to BDF 9148 and calcium chloride were determined in isolated left atria and left ventricular papillary muscles. 2. BDF 9148 increased force of contraction in left ventricular papillary muscles in all disease states with maximal responses comparable to calcium chloride. BDF 9148 potency was significantly decreased in muscles from diabetic rats only. 3. BDF 9148 produced similar responses in left atria except from hyperthyroid rats where negative inotropic responses only were measured. This exception confirms that the left atria is an imperfect model for ventricular responsiveness. 4. Thus, the increase in force of contraction in the ventricles as a consequence of sodium channel modulation by BDF 9148 is maintained in these disease states unlike responses to alpha-or beta-adrenoceptor agonists.

The effects of veratridine and BDF 9148 on the action potentials and contractility of the rat right ventricle

1. The effects of veratridine, BDF 9148 and lignocaine on the action potentials and contractile force of the electrically-driven rat right ventricle have been determined. 2. Veratridine at 10(-7)-10(-6) M and BDF 9148 at 10(-7)-10(-5) M had no effect on the threshold potential or amplitude but prolonged the ventricular action potentials. 3. In contractility studies, veratridine at 10(-7)-10(-6) M augmented the cardiac stimulation responses and the augmenting effects with 3 x 10(-7) and 10(-6) M were greater at 2 than 4 Hz. In the presence of veratridine at 3 x 10(-6) M, the ventricle would not pace. 4. BDF 9148 at 10(-7)-10(-5) M augmented the cardiac stimulation responses and the augmenting effects with 10(-7) and 3 x 10(-7) M were greater at 2 than 4 Hz and the effect was maximal at 3 x 10(-7) M and submaximal at 10(-5) M. The effects of BDF 9148 at 10(-5) were not readily reversible. 5. Lignocaine at 10(-4) M had no effect on the ventricular action potential duration but decreased the threshold potential and amplitude and also reduced the cardiac stimulation force responses. In the presence of lignocaine, the augmenting effects of veratridine and BDF 9148 on ventricular force were reduced. 6. In summary this study has shown that BDF 9148 prolongs the action potential and augments the contractile force responses of the rat right ventricle by a lignocaine-sensitive mechanism. BDF 9148 or similar drugs may have potential as positive inotropes in the treatment of heart failure.

The inotropic agents DPI 201-106 and BDF 9148 differentially affect potassium currents of guinea-pig ventricular myocytes

The inotropic agents DPI 201-106 and BDF 9148 increase action potential duration (APD) of heart muscle. This effect can be explained by inhibition of inactivation of sodium current, which is affected by both agents to a similar extent (Ravens et al. 1991, Br J Pharmacol 104:1019-1023). However, as DPI 201-106 prolongs APD of guinea-pig ventricle to a larger extent than BDF 9148, other currents may also be involved. The aim of the present study was to measure the effects of DPI 201-106 and BDF 9148 on the inward rectifier IK1, and the two components of the delayed rectifier, IKs and IKr. The methyl-for-carbonitrile-substituted derivative BDF 8784 was included to study structure-activity relationships. Single-electrode whole-cell voltage-clamp technique was used to measure membrane currents of guinea-pig ventricular myocytes. Only DPI 201-106 reduced IK1 at potentials both negative and positive to the reversal potential. Three microM of DPI 201-106 reduced IKs, whereas 1 microM of BDF 9148 had no effect on this current. These concentrations were equieffective with respect to positive inotropic action (Ravens et al. 1991, Br J Pharmacol 104:1019-1023). BDF 9148 did however block IKs at higher concentrations, as did BDF 8784. It is concluded that block of outward current by DPI 201-106, but insignificant effects of BDF 9148, are responsible for the differential effects of these compounds on APD at equieffective concentrations with respect to inotropy.

Pharmacological modulation of human cardiac Na+ channels

Pharmacological modulation of human sodium current was examined in Xenopus oocytes expressing human heart Na+ channels. Na+ currents activated near -50 mV with maximum current amplitudes observed at -20 mV. Steady-state inactivation was characterized by a V1/2 value of -57 +/- 0.5 mV and a slope factor (k) of 7.3 +/- 0.3 mV. Sodium currents were blocked by tetrodotoxin with an IC50 value of 1.8 microM. These properties are consistent with those of Na+ channels expressed in mammalian myocardial cells. We have investigated the effects of several pharmacological agents which, with the exception of lidocaine, have not been characterized against cRNA-derived Na+ channels expressed in Xenopus oocytes. Lidocaine, quinidine and flecainide blocked resting Na+ channels with IC50 values of 521 microM, 198 microM, and 41 microM, respectively. Use-dependent block was also observed for all three agents, but concentrations necessary to induce block were higher than expected for quinidine and flecainide. This may reflect differences arising due to expression in the Xenopus oocyte system or could be a true difference in the interaction between human cardiac Na+ channels and these drugs compared to other mammalian Na+ channels. Importantly, however, this result would not have been predicted based upon previous studies of mammalian cardiac Na+ channels. The effects of DPI 201-106, RWJ 24517, and BDF 9148 were also tested and all three agents slowed and/or removed Na+ current inactivation, reduced peak current amplitudes, and induced use-dependent block. These data suggest that the alpha-subunit is the site of interaction between cardiac Na+ channels and Class I antiarrhythmic drugs as well as inactivation modifiers such as DPI 201-106.

Cardiotonic actions of selective phosphodiesterase inhibitors in rat isolated ventricular cardiomyocytes

1. The contractile effects of the novel cardiotonic agent HN-10200 (2-[3-methoxy-5-methylsulphinyl-2-thienyl]-1H-imidazo-[4,5-c]-p yri dine hydrochloride), were examined and comparisons made with the responses obtained to a structurally similar compound, sulmazole, and to a number of other compounds which are known to inhibit phosphodiesterase (PDE) isoenzymes with differing selectivities; namely, enoximone (PDE III inhibitor), Ro 20-1724 (PDE IV inhibitor) and 3-isobutyl-1-methylxanthine (non-selective PDE inhibitor). 2. Contractile function, as measured by mechanical shortening, and biochemical systems involving cyclic AMP were investigated in ventricular cardiomyocytes isolated from adult Sprague-Dawley rats (200-250 g). 3. HN-10200 exerted a concentration-dependent (10(-8) M-10(-4) M) positive contractile effect, which was independent of alpha- or beta-adrenoceptor, or histamine receptor stimulation. 4. The efficacies of the contractile responses to the PDE inhibitors were of the order: HN-10200 > IBMX > sulmazole > enoximone and maximum stimulations, which were obtained at concentrations of 10(-4) M, were 54 +/- 4%, 41 +/- 7%, 38 +/- 7% and 26 +/- 5% (mean +/- s.e.) greater than basal levels, respectively (n = 6); the basal value of contractile amplitude (dL), in the absence of PDE inhibitors was 7.39 +/- 0.18% (mean +/- s.e.). Ro 20-1724 did not have any effect on contractile activity. 5. Due to low basal levels of cyclic nucleotides in isolated cells, accumulation of cyclic AMP due to the presence of the PDE inhibitors was detected only when the levels of cyclic nucleotide were enhanced with forskolin (10 microM). 6. The PDE inhibitors increased levels of cyclic AMP only at concentrations> 10-4 M. HN-10200 and sulmazole had similar concentration-dependent profiles for the accumulation of cyclic AMP; their potencies were lower than that of IBMX (concentrations of forskolin required to increase cyclic AMP by 4 pmol mg-1 protein, in the presence of maximum concentrations of the PDE inhibitors, were 13 +/- 311M, 14 +/- 3 JAM and 3 +/- 0.6 JAM [mean +/- s.e.], respectively).7. These results indicate that a similar mechanism, probably through a weak inhibition of the cyclic AMP-specific PDE isoenzymes, is responsible for the increase in levels of cyclic AMP by HN-10200 and sulmazole. However, cyclic AMP is only partially responsible for the positive contractile effect of HN-10200 and, similarly, sulmazole and IBMX. The lack of apparent increase in levels of cyclic AMP by enoximone, highlights its degree of selectivity for the PDE III isoenzyme, such that the PDE IV isoform is still present in sufficient quantity to degrade cyclic AMP within the cell. On the other hand,the potent action of Ro 20-1724 on accumulation of cyclic AMP, in addition to the lack of effect on contractile function, is in agreement with the selectivity of this compound for the PDE IV isoenzyme and compartmentalization of cyclic AMP in rat isolated ventricular cardiomyocytes.