Effects of isomazole on force of contraction and phosphodiesterase isoenzymes I-IV in nonfailing and failing human hearts.

The phosphodiesterase (PDE) inhibitor isomazole increased the force of contraction to 278.3 +/- 89.1% (n = 7) of the predrug value in ventricular trabeculae carneae isolated from nonfailing human hearts. This effect can be attributed mainly to a PDE III or a combined PDE III/IV inhibition since at the concentration of the maximal positive inotropic effect of isomazole, PDE III and PDE IV were completely inhibited. In explanted failing human hearts (end-stage myocardial failure, NYHA IV), isomazole increased the force of contraction only marginally to 110.1 +/- 10.7% of the predrug value. The lack of a distinct positive inotropic efficacy of isomazole in failing human hearts could not be explained by an impairment of PDE inhibition since the properties of the PDE I-IV isoenzymes separated by DEAE-Sepharose chromatography and the inhibitory effects of isomazole did not differ in both preparations. The positive inotropic effect of the beta-adrenoceptor agonist isoprenaline was also reduced in failing hearts. However, in the presence of isomazole, the diminished positive inotropic effect of isoprenaline was restored to values obtained with isoprenaline alone in nonfailing hearts. Thus, the decreased effect of inotropic drugs like isoprenaline or isomazole in preparations from failing human heart might be explained mainly by a diminished cAMP formation due to a defect in receptor-adenylate cyclase coupling.     

Effects of cantharidin on force of contraction and phosphatase activity in nonfailing and failing human hearts.

1. The effect of the phosphatase inhibitor, cantharidin (3-300 microM) on force of contraction was studied in isolated electrically driven right ventricular trabeculae carneae from human myocardium. 2. The positive inotropic effect of cantharidin started at a concentration of 100 microM with a positive inotropic effect to 199% and to 276% of the predrug value in nonfailing and failing human hearts, respectively. 3. Under basal conditions the contraction time parameters were prolonged in human heart failure vs. nonfailing preparations. However, the positive inotropic effect of cantharidin did not affect contraction time parameters. Thus, time to peak tension, time of relaxation and total contraction time were not shortened by cantharidin in nonfailing and failing preparations. 4. The phosphatase activity was unchanged in preparations from failing hearts compared to nonfailing hearts. 5. Cantharidin inhibited phosphatase activity in a concentration-dependent manner. The IC50 value of cantharidin was about 3 microM in both nonfailing and failing human myocardium. 6. The positive inotropic effect of cantharidin was similar in nonfailing and failing human hearts, accompanied by a similar inhibitory effect of cantharidin on the phosphatase activity. The positive inotropic effect of cantharidin in failing hearts was as strong as the effect of isoprenaline in nonfailing hearts. 7. It is concluded that the treatment with a phosphatase inhibitor may offer a new positive inotropic modality for the treatment of human heart failure.     

Effect of trapidil and papaverine on thrombocyte aggregation and phosphodiesterase activity

Trapidil effect on platelet aggregation in vitro and in vivo as well as on phosphodiesterase (PDE) activity was studied. Both ADP- and collagen-induced aggregation of human and rabbit platelets was inhibited by trapidil at concentrations of 60 mumol/l. The PDE activity of human platelet lysates was blocked at concentrations that were necessary to inhibit aggregation. Papaverine exerted 5 times higher inhibitory effects on aggregation and PDE activity as compared to trapidil. Infusion of trapidil to rabbits caused aggregation inhibition in vivo. Papaverine at equimolar doses was more effective than trapidil. At therapeutic doses trapidil is unlikely to produce adequate plasma levels for inhibition of platelet aggregation.     

Effects of the benzimidazole derivatives pimobendan and 2-(4-hydroxy-phenyl)-5-(5-methyl-3-oxo-4,5-dihydro-2H-6- pyridazinyl) benzimidazole . HCl on phosphodiesterase activity and force of contraction in guinea-pig hearts

The effects of pimobendan (UD-CG 115 BS) and UD-CG 212 Cl (2-(4-hydroxy-phenyl)-5-(5-methyl-3-oxo-4,5-dihydro-2H-6- pyridazinyl)benzimidazole X HCl) on force of contraction, beating frequency, and on adenylate cyclase and phosphodiesterase activity were investigated in isolated preparations from guinea-pig hearts. Both benzimidazole derivatives exerted a concentration-dependent positive inotropic effect in guinea-pig papillary muscles. The efficacies were similar to that of dihydroouabain. The positive inotropic effect of both benzimidazoles was accompanied by an enhancement of the rate of force development and a prolongation of the contraction. Both benzimidazole derivatives inhibited phosphodiesterase (PDE) activity in a crude preparation from guinea-pig ventricles. However, at the concentrations producing maximal positive inotropic effects in papillary muscles, pimobendan and UD-CG 212 Cl diminished PDE activity only by about 20-30%. Since both benzimidazoles did not affect adenylate cyclase in a particulate membrane preparation a stimulation of the cAMP synthesis can be ruled out. As recently reported for pimobendan, this study provides functional evidence that the positive inotropic effect of UD-CG 212 Cl is also at least partially mediated by cAMP. Firstly, the positive inotropic effect of UD-CG 212 Cl was inhibited by carbachol, adenosine and (-)-N6-phenyl-isopropyladenosine. Secondly, UD-CG 212 Cl potentiated the inotropic effects of isoprenaline and histamine. UD-CG 212 Cl had no positive chronotropic effect and pimobendan increased the beating frequency only slightly.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of phosphodiesterase isoenzyme inhibitors on cutaneous inflammation in the guinea-pig.

1. Inflammation is central to the pathophysiology of asthma. The recent findings that different inflammatory cells may express different phosphodiesterase (PDE) isoenzymes have centered attention on inhibitors of these isoenzymes as new drugs for the treatment of asthma. In this study, we investigated the effect of different PDE isoenzyme inhibitors on the accumulation of 111In-labelled eosinophils and local oedema formation at sites of allergic- and mediator-induced inflammation in guinea-pig skin. 2. Systemic treatment with SK&F 94120, a type III PDE inhibitor, or zaprinast, a type V PDE inhibitor, had no effect on the 111In-eosinophil accumulation and oedema formation induced by i.d. injection of zymosan-activated plasma (ZAP), PAF, histamine or in a passive cutaneous anaphylaxis (PCA) reaction. 3. Systemic treatment with rolipram, a type IV PDE inhibitor, effectively inhibited 111In-eosinophil accumulation induced by ZAP, PAF, histamine and in a PCA reaction. However, oedema formation measured in the same sites was not affected. Systemic administration of higher doses of theophylline produced similar results. In contrast, 111In-neutrophil accumulation induced by ZAP or in a PCA reaction was not altered by systemic treatment with rolipram. 4. Locally-injected rolipram had little effect on 111In-eosinophil accumulation and oedema formation induced by histamine, PAF and in a PCA reaction. 5. These data show that systemic, but not local, treatment with rolipram effectively inhibits allergic- and mediator-induced 111In-eosinophil accumulation but not oedema formation or 111In-neutrophil accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Anti-spasmogenic activity of isoenzyme-selective phosphodiesterase inhibitors in guinea-pig trachealis.

1 The anti-spasmogenic potential of SK&F 94120 (PDE3-selective), rolipram (PDE4-selective), zaprinast (PDE5-selective), zardaverine (dual PDE3/4 inhibitor) and theophylline (non-selective) was evaluated in guinea-pig trachealis. 2 SK&F 94120 or rolipram (10 and 100 microM) antagonized histamine-induced tension generation in a concentration-dependent and non-competitive manner whereas ACh-induced contractions were unaffected. Similarly, SK&F 94120 and rolipram in combination were anti-spasmogenic with respect to both contractile agonists to an extent that was greater than the effect of either drug alone. Identical results were obtained with zardaverine (1, 10 and 100 microM) and theophylline (100 microM and 1 mM). 3 Zaprinast protected guinea-pig trachealis against histamine-, but not ACh-induced contractile responses in a manner that was indistinguishable from the results obtained with SK&F 94120. However, in contrast to the interaction between SK&F 94120 and rolipram, no further antagonism was seen when zaprinast and rolipram were used in combination. 4 Pre-treatment of tissues with SNP (10 and 100 microM) antagonized histamine-induced tension generation in a concentration-dependent and non-competitive manner. However, no further antagonism was produced when SNP and rolipram were used concurrently. Likewise, the protection afforded by a combination SNP and SK&F 94120 was no greater than that produced by SNP alone. 5 These results demonstrate that an inhibitor of PDE3 enhances the anti-spasmogenic activity of rolipram but not drugs that elevate cyclic GMP mass. Moreover, the ability of SNP and zaprinast to protect guinea-pig trachealis against histamine-induced contractions apparently is not due to the inhibition of PDE3.     

Effects of midodrine, ST 1059, methoxamine and glycine on spontaneously beating guinea-pig atria.

The chronotropic effects of midodrine, glycine (10(-8) to 3.10(-3) M), ST1059 and methoxamine (10(-8) to 10(-3) M) were investigated in the spontaneously beating guinea-pig right atrial preparation. Midodrine and glycine produced a slight, but significant rise in atrial rate over a wide concentration range. The midodrine-induced rise in atrial rate was not influenced by the beta-adrenergic receptor blocking drug propranolol (10(-6) M). The histamine (H2)-receptor blocking drug metiamide (3.10(-5) M) abolished the positive chronotropic actions of both midodrine and glycine. No positive chronotropic effect was seen after the administration of ST 1059 or methoxamine. A decrease in atrial rate was elicited by high concentrations (above 10(-4) to 10(-3) M) of the sympathomimetic agents midodrine, ST 1059, and methoxamine, but not by the amino acid glycine.     

Prevention by phosphodiesterase inhibitors of antigen-induced contraction of guinea-pig colonic smooth muscle.

1. The ability of various phosphodiesterase (PDE) inhibitors to reduce the initial and/or late response to ovalbumin (OVA) in isolated strips of guinea-pig colonic smooth muscle from sensitized animals was examined. 2. Both the initial and late responses to OVA (0.05 mg ml-1) were inhibited by the non-selective PDE inhibitor, 3-isobutyl-1-methyl-xanthine (IBMX, EC50 = 26.0 and 6.1 microM, respectively), and the selective inhibitors of low Km cyclic AMP specific PDE (PDE IV), (R)- and (S)-rolipram. The (S)-isomer (EC50 = approximately 1.0 microM for both responses) was about 10 fold less potent than the (R)-isomer (EC50 = approximately 0.1 microM for both responses). 3. Zaprinast, a selective inhibitor of the cyclic GMP-specific PDE (PDE V) inhibited only the late response (EC50 = 1.4 microM). 4. SK&F 94120, a selective inhibitor of the cyclic GMP-inhibited low Km cyclic AMP PDE (PDE III), inhibited the initial response (45.9 +/- 11.9%, P < 0.05) at the highest concentration tested (100 microM), and had no effect on the late response. 5. The results suggest that PDE inhibitors, especially PDE IV inhibitors, can attenuate the contractile response of guinea-pig colon to OVA.     

Regulation by phosphodiesterase isoenzymes of non-adrenergic non-cholinergic contraction in guinea-pig isolated main bronchus.

1. We have investigated the role of phosphodiesterase isoenzymes in modulating electric field stimulation (EFS), substance P and capsaicin-induced contraction of the guinea-pig isolated main bronchus. 2. Non-adrenergic non-cholinergic contractile responses were elicited by EFS (3 Hz, 20 s) in the guinea-pig isolated main bronchus in the presence of the non-selective muscarinic antagonist, atropine (0.1 microM), the non-selective beta-adrenoceptor antagonist, propranolol (1 microM), the neutral endopeptidase inhibitor, thiorphan (10 microM) and the cyclo-oxygenase inhibitor, indomethacin (5 microM). The type III, type III/IV, type IV and type V phosphodiesterase isoenzyme inhibitor, SKF 94836, benzafentrine, Ro-20-1724 and zaprinast respectively, significantly attenuated the contractile response to EFS. The IC50 (95% confidence limits) value for SKF 94836, benzafentrine, Ro-20-1724 and zaprinast was 8.3 microM (0.89-78); 0.7 microM (0.1-4.5); 0.5 microM (0.2-1.2) and 13 microM (2-87) respectively. 3. The phosphodiesterase isoenzyme inhibitors, SKF 94836, Ro-20-1724 and zaprinast, partially attenuated the contractile response to substance P (10 nM). Benzafentrine significantly inhibited the contractile response to substance P, yielding an IC50 value of 1.9 microM (0.9-3.8). 4. The phosphodiesterase isoenzyme inhibitor, Ro-20-1724 (0.1-100 microM) failed to reduce significantly the contractile potency of capsaicin (P > 0.05). In contrast, SKF 94836 (1 microM), benzafentrine (10 microM) and zaprinast (100 microM) significantly reduced the contractile potency of capsaicin (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of D-600 and its optical isomers on force of contraction in cat papillary muscles and guinea-pig auricles.

1 (--)-D-600 and (+)-D-600 exerted concentration-dependent negative inotropic effects in papillary muscles from cats, the potency of (--)-D-600 being about 100 times greater than that of (+)-D-600. The action was more pronounced at high than at low frequencies of stimulation. Time to peak force, relaxation time and therefore also total duration of contraction were not significantly affected by the drugs. 2 The negative inotropic effects of both drugs were not reversible after washing in drug-free solution for 60 min at all concentrations tested. 3 The same negative inotropic effects were seen in guinea-pig left auricles with the racemic mixture of both isomers. 4 Uptake measurements of D-600-[nitrile-14C] in isolated left auricles of guinea-pigs showed the drug to be accumulated in the myocardial tissue; tissue-medium ratios from 1.25 to 6.0 were found at exposure times from 1 to 100 minutes. Preparations which were loaded first with D-600-[nitrile-14C] for 100 min and then washed in drug-free solution for different times lost up to about 80% of the initial radioactivity, whereas the depression of force of contraction was not reversible. 5 The results show that there are marked quantitative differences between the actions of (--)-D-600 and (+)-D-600 on isometric force of contraction in cat papillary muscles. Although the time course of the uptake of D-600-[nitrile-14C] and the development of the negative inotropy in guinea-pit left auricles were reasonably well related, the magnitude of the negative inotropy does not seem to be dependent on the total tissue concentration once the negative inotropic effect has been fully developed.     

Mechanism of action and cardiotonic activity of a new phosphodiesterase inhibitor,the benzimidazole derivative adibendan (BM 14.478), in guinea-pig hearts

1. The effects of adibendan (BM 14.478; 7,7-dimethyl-2-(4-pyridyl)-6,7-dihydro-3H,5H-pyrrolo[2,3-f] benzimidazole-6-one) on force of contraction and beating frequency were analysed in guinea-pig electrically driven papillary muscles and spontaneously beating right auricles, respectively. The effects of 3-isobutyl-1-methylxanthine (IBMX) and milrinone were studied for comparison. 2. Adibendan exerted a concentration-dependent (0.03-300 mumol/l) positive inotropic effect in papillary muscles (EC50 = 1.3 mumol/l) which was only partially reversible. The efficiency of adibendan was less than that of milrinone, but adibendan was about two orders of magnitude more potent and had only slight positive chronotropic effects (113% of pre-drug values) at most. Milrinone increased the frequency of beating maximally to 140% of pre-drug values. The positive inotropic effect of adibendan is probably at least partially mediated by cAMP since carbachol reduced the increase in force of contraction by about 75%. 3. To elucidate the mechanism of action of adibendan we investigated its effects on phosphodiesterase I-III and adenylate cyclase activity in isolated preparations from guinea-pig hearts. 4. Adibendan selectively inhibited phosphodiesterase III (PDE III) activity concentration-dependently (IC50 = 2.0 mumol/l). The IC50 values for the inhibition of PDE I or II were more than 60-fold higher. Since adibendan did not affect adenylate cyclase activity a stimulation of the cAMP synthesis as a mechanism of action can be ruled out. 5. The results provide evidence that the positive inotropic action of adibendan is at least in part due to an inhibition of cAMP-PDE III.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of N-ethylmaleimide on action potential and force of contraction of guinea-pig papillary muscles.

1. Standard microelectrode techniques were used to investigate the influence of N-ethylmaleimide on the action potential, slow response action potential and force of contraction of guinea-pig papillary muscles. 2. N-ethylmaleimide, 3 x 10(-5) to 10(-4) mol l-1, concentration-dependently increased the force of contraction. The positive inotropic effect developed quickly and, with the largest drug concentration, was followed by a progressive decline of the contractile force. The action potential duration was progressively shortened by N-ethylmaleimide. 3. The effects of N-ethylmaleimide were not prevented in the presence of tetrodotoxin 3 x 10(-8) mol l-1 and propranolol 4 x 10(-6) mol l-1 or by a reduction of the Na(+)-concentration to 70 mmol l-1. 4. Verapamil, 10(-6) mol l-1, reduced the positive inotropic, but not the action potential shortening effect of N-ethylmaleimide. 5. In K(+)-depolarized muscles in the presence of propranolol and tetrodotoxin, N-ethylmaleimide 10(-4) mol l-1 increased the maximum depolarization velocity and the duration of the slow response action potential. The latter effect was transient and was followed by a progressive reduction of the action potential duration. 6. The most likely explanation for the action potential shortening effect of N-ethylmaleimide seems to be an increase of an outward potassium current while the transient inotropic effect of the drug may be caused, at least in part, by an increase of the slow inward calcium current.     

Effects of aminoglycoside antibiotics on bound calcium and contraction in guinea-pig atria.

We studied the effect of three aminoglycoside antibiotics which have been shown to replace Ca from lipid monolayers on the superficially bound Ca of isolated beating left atria from guinea-pigs. The cellular Ca content was determined by means of 45Ca after having attained complete exchange. The antibiotics dibekacin, sisomicin, and gentamicin, all reduced the cellular Ca content by 10-20% in a dose-dependent manner. The loss of superficially bound Ca was accompanied by a decline of the contractile force by 40-90%. It is concluded that in isolated atrial muscle it is the amount of Ca bound to the outer surface of the cardiac plasmalemma, rather than the extracellular Ca2+ concentration, that determines contraction height.     

Leukotriene D4 increases both the force of contraction and polyphosphoinositide formation in guinea-pig papillary muscle.

Leukotriene D4 (LTD4) increased the force of contraction in guinea-pig papillary muscle. A rapid (less than 1 min), transient (less than 5 min) response to LTD4 (1 microM) reached 19.3 +/- 5.4% of isoproterenol maximum. A single exposure to LTD4 resulted in complete and homologous desensitization which was not influenced by indomethacin. LTD4 (0.1-3.0 microM) increased total inositol phosphates released from [3H]inositol-labeled tissue. ICI 198,615, a selective LT receptor antagonist, blocked both the increase in force of contraction and the increase in inositol phosphates by LTD4, but had no effect on the inotropic response to isoproterenol. These data support the existence of specific functional LTD4 receptors in myocardial tissue of guinea-pigs.     

Effects of type-selective phosphodiesterase inhibitors on glucose-induced insulin secretion and islet phosphodiesterase activity.

1. We examined various type-selective phosphodiesterase (PDE) inhibitors on glucose-induced insulin secretion from rat isolated islets, on islet PDE activity and on islet cyclic AMP accumulation in order to assess the relationship between type-selective PDE inhibition and modification of insulin release. 2. The non-selective PDE inhibitor, 3-isobutyl-1-methylxanthine (IBMX, 10(-5)-10(-3) M), as well as the type III selective PDE inhibitors SK&F 94836 (10(-5)-10(-3) M), Org 9935 (10(-7)-10(-4) M), SK&F 94120 (10(-5)-10(-4) M) and ICI 118233 (10(-6)-10(-4) M) each caused concentration-dependent augmentation (up to 40% increase) of insulin release in the presence of a stimulatory glucose concentration (10 mM), but not in the presence of 3 mM glucose. 3. Neither the type IV PDE inhibitor rolipram (10(-4) M) nor the type I and type V PDE inhibitor, zaprinast (10(-4)-10(-3) M) modified glucose-induced insulin release when incubated with islets, although a higher concentration of rolipram (10(-3) M) inhibited secretion by 55%. However, when islets were preincubated with these drugs followed by incubation in their continued presence, zaprinast (10(-6)-10(-4) M) produced a concentration-dependent inhibition (up to 45% at 10(-4) M). Under these conditions, rolipram inhibited insulin secretion at a lower concentration (10(-4) M) than when simply incubated with islets. 4. A combination of SK&F 94836 (10(-5) M) and forskolin (5 x 10(-8) M) significantly augmented glucose-induced insulin secretion (30% increase), although neither drug alone, in these concentrations, produced any significant effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of captopril on membrane current and contraction in single ventricular myocytes from guinea-pig.

1. Single guinea-pig ventricular myocytes were voltage-clamped and cell length was measured with a photodiode array. 2. Captopril (1 x 10(-5) M) reduced both peak early current and active shortening in response to a depolarizing clamp pulse along a similar time course. 3. From a holding potential of around -45 mV peak early inward current was reduced by 37 +/- 9% (P less than 0.001) on exposure to captopril. The early current-voltage relationship was shifted outwards by captopril indicating a reduction in membrane conductance through the L-type calcium channel (ICa). 4. The amplitude of cell shortening in response to depolarizing voltage steps was reduced but the voltage-dependence of contraction after captopril was unchanged. 5. A small negative shift of the potential at which ICa was half-activated was observed after captopril. There was no change in the voltage-dependence of the inactivation variable or in the time-dependence of repriming for ICa. 6. The actions of captopril on ICa and developed shortening were dose-dependent and took place in the same proportion when Ica was increased by isoprenaline. 7. These results are discussed in relation to the effects of captopril on Ica and contraction and to its clinical usage.     

Effects of ageing on nicotine-induced contraction and substance P-like materials release in guinea-pig bronchus.

1. Effects of ageing on nicotine-induced contraction and release of substance P-like materials in the bronchial preparations from guinea-pigs of different ages were studied. 2. The pD2 value (potency) of nicotine decreased with age from 10 to 100 weeks. The pD2 value of substance P did not change with age suggesting that substance P receptor mechanisms do not alter with age. 3. The amount of substance P-like materials released by nicotine (10(-4) M) decreased with age from 10 to 100 weeks, supporting our previous findings that nicotine contracts the guinea-pig bronchus through the release of substance P-like materials. 4. These results suggest that the age-related decrease in the pD2 value (potency) of nicotine is due to the reduction in the amount of substance P-like materials released by nicotine.     

Effects of phosphodiesterase inhibitors on vasoactive intestinal peptide-induced relaxation of isolated guinea-pig trachea.

The relaxant effect of vasoactive intestinal peptide (VIP) was investigated in isolated guinea-pig trachea in the presence of the phosphodiesterase (PDE) inhibitors, papaverine and 3-isobutyl-1-methylxanthine (IBMX), and the results were compared to those obtained with the cyclic AMP-dependent bronchodilators, isoproterenol and prostaglandin E2 (PGE2). The relaxant effect of VIP was greater when the magnitude of the leukotriene D4 (LTD4)-induced contraction was smaller. A similar effect was also observed for the relaxation induced by isoproterenol but not by PGE2. In the presence of papaverine (1 microM) and IBMX (3 microM), which reduced the 30 nM LTD4-induced contraction to the same extent, the relaxant effect of VIP was not changed, whereas the relaxant effects of isoproterenol and PGE2 were significantly potentiated. The potentiating effect of PDE inhibitors was also observed for the relaxation induced by the adenylate cyclase activator, forskolin, but not for the relaxation induced by the guanylate cyclase activator, sodium nitroprusside. These results suggest that the relaxation induced by VIP is different from that induced by cyclic AMP-dependent bronchodilator in the guinea-pig trachea.