Masao Endoh

Relationship between relaxation and cyclic GMP formation caused by nicorandil in canine mesenteric artery

SummaryIn the isolated canine mesenteric artery relaxation caused by nicorandil [N-(2-hydroxyethyl)nicotinamide nitrate ester; SG-75] in the presence of noradrenaline was accompanied by a concomitant elevation of cGMP level, whilst the cAMP level was not changed by the drug. The relaxation and cGMP level after the administration of nicorandil showed a good correlation. In this respect nicorandil is very similar to nitrogen oxide-containing vasodilators.

CHARACTERIZATION OF ADRENOCEPTORS MEDIATING POSITIVE INOTROPIC RESPONSES IN THE VENTRICULAR MYOCARDIUM OF THE DOG

1 The pharmacological characteristics of adrenoceptors mediating the positive inotropic action in the dog heart were assessed by the use of blood‐perfused papillary muscles and isolated strips of ventricular myocardium. 2 On the blood‐perfused papillary muscle driven at 2 Hz and in sinus node preparations, phenylephrine induced positive inotropic and chronotropic responses in the same dose range and was much less potent than isoprenaline. The dose‐response curve for the chronotropic action of phenylephrine was parallel to that of isoprenaline, whilst the dose‐response curve for the inotropic action of phenylephrine was less steep than that of isoprenaline. 3 The infusion of pindolol, a β‐adrenoceptor blocking agent, at a rate of 1 μg/min, shifted the isoprenaline dose‐response curves to the right, and to the same extent, in both papillary muscle and sinus node preparations. In contrast to isoprenaline, the antagonism of phenylephrine by pindolol was noncompetitive. Phentolamine did not affect the positive inotropic and chronotropic actions of phenylephrine. 4 On isolated ventricular strips α‐adrenoceptor blockade by 10−6m phentolamine did not affect dose‐response curves to phenylephrine or dopamine. Pindolol shifted the dopamine dose‐response curves to the right in a competitive manner and those of phenylephrine in a noncompetitive manner. 5 On ventricular strips from reserpine‐pretreated dogs phenylephrine and tyramine dose‐response curves were shifted markedly to the right and downwards. Desipramine (10−5m) which enhanced the action of noradrenaline considerably reduced the myocardial responses of phenylephrine. 6 Papaverine (10−5m) decreased the threshold concentration of phenylephrine required to stimulate the myocardium and shifted phenylephrine dose‐response curves to the left. 7 Raising the temperature from 32°C to 37°C shifted phenylephrine dose‐response curves to the right; when the temperature was raised from 37°C to 42°C the affinity of the drug was not changed. 8 Other α‐adrenoceptor stimulants, methoxamine and clonidine, decreased the active tension of ventricular strips. The responses to noradrenaline and adrenaline (in the presence of pindolol; 3 × 10−8m) were not affected by phentolamine (10−6m). 9 The results indicate that adrenoceptors mediating positive inotropic responses in the dog ventricle are of the /J‐type and that post‐synaptic α‐adrenoceptors are not involved. Phenylephrine acts mainly by releasing noradrenaline from adrenergic nerve endings and partly by a weak direct action on β‐adrenoceptors.

The time course of changes in cyclic nucleotide levels during cholinergic inhibition of positive inotropic actions of isoprenaline and theophylline in the isolated canine ventricular myocardium

SummaryThe time course of changes in cyclic AMP and cyclic GMP levels during the cholinergic inhibition of the positive inotropic actions of isoprenaline and theophylline was assessed in the isolated canine right ventricular myocardium.1.Carbachol (3×10−6 mol/l) caused biphasic changes in force of contraction and cyclic AMP levels when administered during the positive inotropic response to isoprenaline (10−6 mol/l): a positive inotropic action superimposed on the slowly developing negative inotropic action which was preceded by a reduction of cyclic AMP levels. The positive and negative inotropic actions of, and the reduction of cyclic AMP levels induced by carbachol in the presence of isoprenaline were inhibited by atropine (3×10−7 mol/l). The cyclic GMP levels were increased after the administration of carbachol in the presence of isoprenaline in a monophasic manner and reached a maximal level 120 s after the administration; the elevation was also abolished by atropine. The half-time (T1/2) for cyclic GMP elevation after the administration of carbachol was about 50 s.2.Carbachol (3×10−6 mol/l) administered during the positive inotropic response to theophylline (2×10−3 mol/l) abolished the inotropic action of theophylline in a monophasic manner:T1/2, 40 s; a reduction of cyclic AMP levels (T1/2, 10 s) and an elevation of cyclic GMP levels (T1/2, 20 s) preceded the changes in force of contraction caused by carbachol.3.The present results indicate that the reduction of cyclic AMP levels occurred immediately after the administration of carbachol during cholinergic inhibition of the positive inotropic actions of isoprenaline and theophylline; the elevation of cyclic GMP levels and reduction of force of contraction occurred following the changes in cyclic AMP levels. Considering the mediator role of cyclic AMP in cardiac contraction, it appears that the reduction of cyclic AMP levels is responsible for the cholinergic inhibition of the actions of isoprenaline and theophylline in the canine ventricular myocardium.

Differentiation by cholinergic stimulation of positive inotropic actions mediated via α- and β-adrenoceptors in the rabbit heart

Abstract In the isolated rabbit papillary muscle, experiments were carried out in order to elucidate whether or not cholinergic stimulation produces a differential antagonistic action on the positive inotropic effects mediated via β- and α-adrenoceptor stimulation. Carbachol (0.1–30 μM) alone scarcely affected the basal tension developed. The postive inotropic effects of phenylephrine (30 μM) in the presence of phentolamine and of isoprenaline, which were mediated via β-adrenoceptors, were markedly inhibited by carbachol. Carbachol (3 μM) shifted the dose-response curve for isoprenaline in a parallel manner, and that for phenylephrine with phentolamine to the right and downwards. Carbachol administered during induction of the positive inotropic effects via α-adrenoceptors by phenylephrine (30 μM) with pindolol or by methoxamine failed to inhibit these effects and increased further the tension developed. The dose-response curve for phenylephrine determined in the presence of pindolol was not affected by carbachol. The present results indicate that the cholinergic antagonism of the adrenergic action on the contractility of the mammalian ventricular myocardium is exerted specifically to the β-adrenoceptor-mediated action, but not to the α-adrenoceptor-mediated one.

Comparison of the mechanisms underlying the positive inotropic actions of dopamine adrenaline and isoprenaline on the isolated rabbit papillary muscle

SummaryIn the isolated rabbit papillary muscle the effects of dopamine on the contractile force and on the level of 3′,5′-cyclic adenosine monophosphate (cAMP) at different frequencies of stimulation were studied and compared with those of isoprenaline and adrenaline.1.When the frequency of stimulation was increased from 0.5–2.5 Hz the dose-response curves for the positive inotropic effect of dopamine as well as of isoprenaline were shifted to the left, whereas the maximum of the developed tension reached for both drugs remained unchanged.2.At a frequency of stimulation of 0.5 Hz pindolol (3×10−8 M) and phentolamine (10−6 M), respectively, did not affect the dose-response curve for dopamine; only the simultaneous administration of pindolol plus phentolamine shifted the dose-response curve to the right. In the presence of cocaine (3×10−5 M) as well as in that of cocaine plus corticosterone (4×10−5 M) the dose-response curve for dopamine was shifted to the right. On the other hand, the upper part of the dose-response curve for adrenaline was shifted to the right by pindolol (3×10−8 M), the lower part by phentolamine (10−6 M) and the whole curve by the application of both antagonists.3.At a frequency of stimulation of 2.5 Hz neither pindolol (3×10−8 M) nor phetolamine (10−6 M) influenced the dose-response curve for dopamine, whereas the simultaneous administration of both drugs shifted the whole curve to the right.4.Dopamine (10−4 M) increased significantly the content of the cAMP after 60 s by about 40% (at 0.5 Hz) and 50% (at 1.0 Hz), respectively, but this increase was by far less compared with that obtained by isoprenaline (3×10−7 M).5.Pindolol (3×10−8 M) completely abolished the increase of the cAMP-content evoked by dopamine (10−4 M), while phentolamine (10−6 M) enhanced the elevation of the cAMP-level to nearly the same extent as isoprenaline (3×10−7 M) did.6.The increase of the cAMP level induced by adrenaline (10−5 M) was comparable with that caused by isoprenaline (3×10−7 M). While phentolamine (10−6 M) did not influence the adrenaline induced increase of the cAMP content, pindolol completely abolished it.7.The present results are compatible with the view, that the positive inotropic effect via stimulation of β-adrenoceptors is mediated by cAMP, while that of α-adrenoceptors is not. Furthermore it is concluded, that dopamine produces its positive inotropic effect by a cAMP-dependent direct and/or indirect β-adrenoceptor stimulation as well as by a cAMP-independent direct α-adrenoceptor stimulation to about the same degree.

Effects of papaverine and its interaction with isoprenaline and carbachol on the contractile force and cyclic nucleotide levels of the canine ventricular myocardium.

SummaryIn the isolated ventricular myocardium of the dog the effects of papaverine on the contractile force and on the cyclic nucleotide levels were studied. Furthermore the interaction between papaverine and the adrenergic β-or cholinergic stimulation was investigated.1.Papaverine (3×10−5 M) induced a positive inotropic action and increased the cyclic AMP level in the majority of preparations. Ventricular muscles isolated from certain dogs showed only a negative inotropic response to papaverine. As a whole, a significant correlation was found between the tension developed and the cyclic AMP level after the administration of papaverine. The cyclic GMP level was not changed or decreased by papaverine.2.The positive inotropic action of papaverine and elevation of the cyclic AMP level in response to papaverine were not inhibited by a β-adrenoceptor blocking drug, pindolol (3×10−8 M), indicating that these effects are not caused by catecholamine release.3.In muscles, in which papaverine failed to cause the positive inotropic action, contractile as well as cyclic AMP responses to isoprenaline were significantly enhanced by papaverine.4.Carbachol (3×10−6M) diminished the positive inotropic actions of isoprenaline and papaverine, abolished the accumulation of cyclic AMP produced by these agents, and increased significantly the cyclic GMP level. The elevation of cyclic GMP level by carbachol in the presence of papaverine was especially marked and amounted to 4-fold the corresponding control value. These results indicate that papaverine inhibits the break down of the intracellular cyclic AMP and GMP in the intact myocardial cells and may thereby interact functionally with the autonomic nervous system.

DIFFERENTIAL RESPONSES TO CARBACHOL, SODIUM NITRO‐PRUSSIDE AND 8‐BROMO‐GUANOSINE 3‘,5’‐MONOPHOSPHATE OF CANINE ATRIAL AND VENTRICULAR MUSCLE

1 The relation between force of contraction and cyclic nucleotide levels during muscarinic receptor stimulation, and after administration of sodium nitroprusside was assessed in canine isolated atrial and ventricular muscle. 2 The pD2 value (negative logarithm of ED50) for carbachol to decrease force of atrial contraction was similar to that required to inhibit adenosine 3′,5′‐monophosphate (cyclic AMP)‐mediated positive inotropic responses in ventricular muscle. 3 The cyclic AMP level of atrial muscle did not significantly change during carbachol‐induced negative inotropic action, whilst the guanosine 3′,5′‐monophosphate (cyclic GMP) level was elevated immediately after administration. 4 Sodium nitroprusside elevated cyclic GMP levels (without changing cyclic AMP levels) both in atrial and ventricular muscle. The force of atrial contraction was significantly reduced by the drug, whilst ventricular contractile force was unaffected. 5 8‐Bromo‐cyclic GMP markedly decreased contractile force in atrial muscle. In contrast, similar concentrations of 8‐bromo‐cyclic GMP had no effect on ventricular contractile force. 6 The positive inotropic action of phenylephrine on canine cardiac muscle, which is mediated through β‐adrenoceptors, was unaffected either by sodium nitroprusside or by 8‐bromo‐cyclic GMP. 7 The present results suggest that the effect of muscarinic receptor stimulation in canine atrial and ventricular muscle is related to different changes in intracellular cyclic nucleotide metabolism. The direct myocardial depressant action on atrial muscle seems to be related to an elevation of cyclic GMP level, whilst a reduction of cyclic AMP may be responsible for the indirect action (‘accentuated antagonism’) in both atrial and ventricular muscle.

Effects of calcium-antagonistic coronary vasodilators, nifedipine and verapamil, on ventricular automaticity of the dog

SummaryIn isolated, blood-perfused canine papillary muscles intra-arterial injection of calcium-antagonistic coronary vasodilators, nifedipine and verapamil, produced a dose-related decrease in force of contraction. The ventricular rate of about 40 beats/min was not significantly changed by nifedipine even in doses which profoundly decreased the force of contraction. Verapamil changed the ventricular rate in a biphasic manner, but the changes remained as small as about 10% of the basal rate in doses which markedly suppressed the force of contration. Calcium chloride elicited an increase in force of contraction but depressed automaticity. The present results show that in response to nifedipine, verapamil and calcium ions, ventricular automaticity has characteristics different from those of the sinus node.

adrenoceptors mediating the positive inotropic effect of phenylephrine in the right ventricular muscle of the monkey (Macaca fuscata)

SummaryThe property of adrenoceptors mediating the positive inotropic effect (PIE) in the ventricular muscle of the Japanese monkey (Macaca fuscata) was investigated by the use of phenylephrine (PE) and adrenoceptor antagonists. The intrinsic activity (0.6) and the pD2-value (5.41) for PE were comparable to those in other mammalian species. The β-adrenoceptor antagonist, pindolol (3×10−8 mol/l) shifted only the upper part of the concentration-response curve (CRC) for PE to the right; the pD2-value for PE was not significantly affected by pindolol. On the other hand, phentolamine (10−6 mol/l) shifted the lower part of the CRC for PE more than the upper part. In the presence of both pindolol and phentolamine the curve was shifted to the right in a parallel manner. The time required for twitch relaxation was negatively correlated to the degree of PIE of PE in the presence of phentolamine but not pindolol. These results indicate that both β- and α-adrenoceptors mediate the positive inotropic action in the ventricular muscle of the Japanese monkey and that in contrast to the action via β-adrenoceptors the action via α-adrenoceptors is not accompanied by the “relaxant effect”.

Inhibition of cyclic AMP phosphodiesterase activity and myocardial contractility: Effects of cilostamide, a novel PDE inhibitor, and methylisobutylxanthine on rabbit and canine ventricular muscle

Abstract The effects of cilostamide (N-cyclohexyl-N-methyl-4-[6-carbostyriloxy]butyramide; OPC-3689), a novel cyclic AMP phosphodiesterase (PDE) inhibitor were compared with those of 1-methyl-3-isobutylxanthine (IBMX) on the rabbit and canine heart preparations. Cilostamide was about three times less potent than IBMX in inhibiting the crude PDE activity of rabbit and canine heart in the cell-free system, while it was 10 times more potent than IBMX in enhancing the positive inotropic action of isoprenaline in the rabbit and canine ventricular myocardium: 10−6 M cilostamide shifted the concentration-response curve for isoprenaline to the left in a parellel manner to the same extent as did 10−5 M IBMX. Thus, cilostamide enhanced β-adrenoceptor stimulation more potently than did IBMX and the substances examined previously. Accumulation of intracellular cyclic AMP caused by 10−6 M isoprenaline in the isolated canine ventricular myocardium was significantly enhanced by 10−6 M cilostamide and 10−5 M IBMX; isoprenaline (10−6 M) induced cyclic AMP accumulation was greater with IBMX (10−5 M) than with cilostamide (10−6 M). The threshold concentration for cilostamide itself to induce positive chronotropic and inotropic actions in the rabbit heart was lower than that for IBMX, while the intrinsic activity of IBMX was greater than that of cilostamide. In the canine ventricular myocardium, the positive inotropic actions of cilostamide were comparable to those of IBMX; the action of cilostamide in concentrations of 10−5 M and higher was partly inhibited by a β-adrenoceptor blocking agent, pindolol (3 × 10−8 M). During the washout period of the drugs after the maximal response to the drugs had been reached, the positive inotropic action of cilostamide disappeared more rapidly than that of IBMX. The present results suggest that cilostamide is able to permeate the myocardial cell membrane more easily than IBMX and reach the PDE in the functionally important cyclic AMP compartment. The difference in turnover rate of cyclic AMP even in the same tissue in the physiological condition may also affect the direct action of the PDE inhibitors thereon.