The cardiac pharmacology of tolbutamide.

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

Decreased myocardial adenyl cyclase activity in hypothyroidism

It has been suggested that hypothyroidism may alter the responsiveness of the heart to sympathetic stimulation. To define more precisely the interrelationship between hypothyroidism and catecholamine responsiveness we: (a) studied the effects of norepinephrine and fluoride on the activation of adenyl cyclase in the particulate fraction of heart homogenates from euthyroid and hypothyroid cats; and (b) assessed the contractile response of isolated right ventricular papillary muscles from the same cats to increasing concentrations of norepinephrine. It was found that maximal accumulation of cyclic 3',5'-adenosine monophosphate (3',5'-AMP) was significantly lower at peak norepinephrine concentrations in the hypothyroid (284 +/-5 pmoles) than in the euthyroid group (326 +/-10 pmoles) (P < 0.02). However, the K(m) for norepinephrine was similar in both groups (1-2 x 10(-5) moles/liter), and there was no apparent change in the threshold concentration. Fluoride-mediated increases in Cyclic 3',5'-AMP accumulation were also significantly lower in the hypothyroid (585 +/-25 pmoles) as compared to the euthyroid group (790 +/-20 pmoles) (P < 0.02). In contrast, norepinephrine produced a similar augmentation of contractility in isolated papillary muscles from the hypothyroid and euthyroid cats. It thus appears that although the hypothyroid state is associated with a decrease in the total amount of myocardial adenyl cyclase per milligram of tissue capable of being activated by norepinephrine or fluoride, there is no change in the sensitivity of the enzyme to norepinephrine stimulation. Moreover, the finding that the inotropic response to norepinephrine is unaltered in hypothyroidism is compatible with the hypothesis that only a fraction of the total intracellular cyclic 3',5'-AMP produced by norepinephrine activation of adenyl cyclase is required to elicit the inotropic response.     

Increased negative inotropic effect of calcium-channel blockers in hypertrophied and failing rabbit heart.

The effects on ventricular function of calcium channel blockers and isoproterenol were studied in isovolumically beating perfused control rabbit hearts and in hearts subjected to a double pressure plus volume overload studied at the early phase of heart failure. In control hearts, isoproterenol produced an increase of systolic ventricular function and relaxation that was maximal at 10(-7) M. In failing hearts, inotropic state increase in response to isoproterenol was significantly smaller (P less than .01) with no observed lusitropic effect. In control hearts, verapamil and diltiazem produced dose-dependent decreases of ventricular function which were larger with verapamil than with diltiazem (median drug concentration50 of developed pressure was, respectively, 1163 +/- 131 nM and 4524 +/- 451 nM, P less than .001). In failing hearts, contractility decrease was larger than in control hearts (median drug concentration50 of developed pressure was 604 +/- 69 nM and 2691 +/- 580 nM with verapamil and diltiazem, respectively). In contrast, Ro 40-5967, a new calcium-channel blocker, did not produce reductions of inotropic state with concentrations up to 10(-5) M. All three calcium-channel blockers produced a 2-fold increase of coronary flow at 10(-6) M. We conclude that the deleterious effect of verapamil and diltiazem in heart failure is due, at least in part, to a direct depressant effect of these drugs on contractility, which is larger than in control hearts. Additionally, the in vivo sympathetic compensation is probably reduced, as indicated by the decreased ventricular responsiveness to isoproterenol.     

Relationship among isoproterenol, cyclic AMP, cyclic AMP-dependent protein kinase and lipolysis in perfused fat cells

By using perfused fat cells the effect of isoproterenol on adenosine 3':5'-monophosphate (cAMP) levels, cAMP-dependent protein kinase activity and lipolysis was studied. An infusion of isoproterenol (10(-7) M) resulted in a time-dependent increase in cAMP levels and protein kinase activity in the fat cells. Both parameters reached maximum values after 5 min of drug infusion, then declined to steady-state values by 10 min. At 60 min, cAMP levels were still significantly (P less than .05) elevated over basal. Dose-response curves were determined for isoproterenol on cAMP levels, protein kinase activity and glycerol release. All three parameters were increased by isoproterenol over the same concentration range (10(-9)--10(-7) M). A plot of cAMP levels or protein kinase activity ratios vs. glycerol release resulted in linear relationship with high degrees of correlation (r = 0.98). The rates at which cAMP levels and glycerol release decline after termination of isoproterenol infusion were studied. Half-life values of 5.8 and 6.9 min were obtained for cAMP levels and glycerol release, respectively. These results support the hypotheses that cAMP, acting through protein kinase, is an intracellular mediator of the lipolytic response to isoproterenol. It is concluded that cAMP is not formed in great excess of that necessary to maximally increase lipolysis.     

Differential effects of somatostatin on atrial and ventricular contractile responses in guinea pig heart: influence of pretreatment with islet-activating protein

The effects of somatostatin on the contractile response of guinea pig cardiac preparations were investigated and compared with those of carbachol and adenosine. Somatostatin produced a concentration-dependent negative inotropic effect in the left atria, which was accompanied by a decrease in action potential duration. The maximum decrease in contractility which was obtained at 3 x 10(-6) M was around 40% of the predrug control values and far less than those produced by carbachol and adenosine. Somatostatin failed to produce inotropic effect on the papillary muscle and did not influence the spontaneously beating rate of the right atria. In the papillary muscles, however, somatostatin inhibited the positive inotropic effect of isoproterenol in a concentration-dependent manner as did carbachol and adenosine. In addition, somatostatin caused a significant inhibition of the isoproterenol-induced increase in cyclic AMP levels without affecting the basal level of cyclic AMP. In the papillary muscle, the inhibitory effect of somatostatin on the positive inotropic response to isoproterenol was significantly attenuated by pretreatment with islet-activating protein, and was significantly antagonized by the somatostatin antagonist cyclo[7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)]. These results suggest that somatostatin receptors in guinea pig ventricular muscles are coupled with adenylate cyclase via islet-activating protein-sensitive GTP-binding protein, whereas the negative inotropic effect of somatostatin in the left atria might be mediated by a subtype of somatostatin receptors which is different from that in the ventricle.     

Molecular basis for the cardiovascular activities of amrinone and AR-L57

It has been suggested that amrinone and AR-L57 enhance cardiac contractility either by inhibiting phosphodiesterase activity or altering Ca++ homeostasis. Because these novel agents are potentially useful in the management of heart failure, it was of interest to more clearly define their mechanism(s) of action. Amrinone and AR-L57 caused concentration-dependent increases in the contractile states of either perfused guinea-pig hearts or cultured rat cardiomyocytes. To determine whether these actions might result from an increase in sarcolemmal Ca++ movement, the effects of these agents on Ca++ accumulation were studied in a simple system, dog erythrocytes. Both agents promoted erythrocyte Ca++ accumulation in time and concentration-dependent manners, effects that resulted primarily from increased Ca++ entry. However, because these effects were not measurable at inotropic drug concentrations and were apparent only after a 30-min incubation, they did not provide an explanation for the inotropic effects of these agents. Amrinone and AR-L57 inhibited dog heart phosphodiesterase activity (isozyme III) with EC50 values of 23 and 420 microM, respectively; however, only the inotropic responses to amrinone were attenuated by the muscarinic agonist, carbachol, thereby implying a cAMP (cyclic AMP)-dependent mechanism. In cultured ventricular cells, concentrations of amrinone (2 X 10(-4) M) and AR-L57 (3 X 10(-5) M) that caused maximal inotropic responses were associated with the activation of glycogen phosphorylase, but neither drug significantly increased the activation state of cAMP-dependent protein kinase. To further probe the effects of these drugs on intracellular cAMP and Ca++ metabolism, their effects on protein phosphorylation were studied.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electrophysiological actions of the pimobendan metabolite, UD-CG 212 Cl, in guinea pig myocardium.

Pimobendan (UD-CG 115 BS), an inotropic agent and inhibitor of type III phosphodiesterase activity, is demethylated in vivo to form UD-CG 212 Cl, which is a more potent type III phosphodiesterase inhibitor. This study examined cyclic AMP (cAMP)-mediated actions of UD-CG 212 Cl. In guinea pig papillary muscles, UD-CG 212 Cl increased cAMP and stimulated Ca(++)-dependent slow action potentials (APs) in a dose-dependent manner. When compared to previous studies using pimobendan, UD-CG 212 Cl was approximately 100-fold more potent. UD-CG 212 Cl had no additional effects on slow APs in the presence of a maximal dose of isoproterenol (1 microM). Propranolol had little effect on UD-CG 212 Cl-induced slow APs. These results, along with previous studies, indicate that slow AP induction by UD-CG 212 Cl was cAMP-dependent, and the increase in cAMP levels was most likely due to phosphodiesterase inhibition and not beta receptor stimulation. Experiments with tetraethylammonium.Cl suggested that UD-CG 212 Cl probably did not induce slow APs by blocking K+ channels. In voltage-clamped ventricular myocytes UD-CG 212 Cl (100 microM) could stimulate Ca++ current (+21 +/- 5%) when basal cAMP levels were enhanced with a submaximal dose of isoproterenol (10(-9)-10(-8) M). Isoproterenol was not required to observe the stimulating effect of UD-CG 212 Cl on Ca++ current in intact, nondialyzed cells prepared using the nystatin-perforated patch method. Studies with the stereoisomers of UD-CG 212 Cl showed that the D-isomer was more potent than the L-isomer.(ABSTRACT TRUNCATED AT 250 WORDS)     

Desensitization to the inotropic effect of isoproterenol in cultured ventricular cells

To determine whether monoalyers of cultured chick embryo ventricular cells would show tachyphylaxis to chronotropic and inotropic effects of a beta adrenergic agonist, spontaneously contracting monolayers of primary cell cultures were studied using a phase-contrast microscope-video motion detector system that permitted quantitation of the chronotropic and inotropic state. The monoalyers were chronotropically unresponsive to isoproterenol between 10(-9) to 10(-6) M and chronotropically unresponsive to a 6-fold increase in perfusate calcium concentration. However, the cells were very inotropically responsive to calcium and to isoproterenol. Expressing the isoproterenol inotropic response as a percentage of response to 3.6 mM Ca, the response to 10(-6) M isoproterenol was 79 +/- 4% of the Ca response and the EC50 for isoproterenol was 3 x 10(-9) M. The monolayers rapidly developed dose-dependent desensitization to the inotropic effect of isoproterenol; after a 30-min exposure to 1 x 10(-6) M isoproterenol, the inotropic response was 40 +/- 5% of the initial response; desensitization was long-lasting and could be prevented by propranolol. Response to calcium remained unchanged after exposure to 10(-6) M isoproterenol. Thus, the cultured cell preparation shows rapid, sustained, beta receptor specific desensitization to the inotropic effects of a catecholamine.     

Norepinephrine- and isoproterenol- induced changes in cardiac contractility and cyclic adenosine 3':5' -monophosphate levels during early development of the embryonic chick.

Changes in contractility and cyclic adenosine 3':5'-monophosphate (cAMP) levels in response to norepinephrine and isoproterenol were monitored in isolated 4-day-old (noninnervated) and 7-day-old (innervated) embryonic hearts to determine whether a relationship between beta adrenergic receptor, adenylate cyclase and altered cardiac function is established at a very early stage in embryonic development before innervation takes place. Norepinephrine and isoproterenol promoted rapid time- and dose-dependent rises in cAMP levels which were greater in the 4-day-old hearts. These increases paralleled observed functional alterations within a specific range of drug concentrations and time. The elevation of cAMP levels and effect on cardiac function produced by isoproterenol (10(-7)M) were blocked by propranolol (10(-6)M). Dissociations between changes in tissue cAMP levels and cardiac function were also uncovered. Maximal increases in contractility were achieved with lower drug concentrations than were required to promote maximal accumulation of cAMP. Relatively high concentrations of norepinephrine or isoproterenol were less effective than lower concentrations in stimulating contractility but were more effective in promoting cAMP accumulation, The results indicate that both cardiac beta receptors adenylate cyclase are present and functionally related early in embryogenesis before sympathetic innervation occurs and that cAMP accumulation is associated with modulation of contractile activity whithin a certain concentration range and time course.     

Anti-shock actions of a new converting enzyme inhibitor, enalaprilic acid, in hemorrhagic shock in cats

A new angiotensin converting enzyme inhibitor, enalaprilic acid (MK-422), was given in a bolus of 0.5 mg/kg i.v., followed by an infusion of 0.25 mg/kg/hr to determine its effects in hemorrhagic shock. MK-422 produced no significant hemodynamic effects in sham shock controls, yet it effectively blocked the pressor effect of exogenously administered angiotensin I throughout the 260-min experimental period and reduced angiotensin converting enzyme activity by 90% as determined by radiochemical assay. In vitro studies on cat papillary muscles and pancreatic homogenates revealed no direct inotropic or antiproteolytic effect of enalaprilic acid. Nevertheless, converting enzyme inhibitor treatment maintained postreinfusion mean arterial blood pressure at a significantly higher value (P less than .01) than that of untreated hemorrhaged animals (66 +/- 5 vs. 27 +/- 10 mm Hg, respectively). Superior mesenteric artery flow for hemorrhaged cats was significantly higher (P less than .05) in the treated group both during the end of the oligemic period (6.1 +/- 0.4 vs. 3.8 +/- 0.8 ml/kg/min) and during the postreinfusion period (6.5 +/- 0.7 vs. 1.9 +/- 1.0 ml/kg/min). Moreover, enalaprilic acid blunted the marked rise in plasma cathepsin D (P less than .01) and myocardial depressant factor activities (P less than .01), and plasma amino-nitrogen concentrations (P less than .05) observed in the untreated hemorrhaged cats. These results indicate that enalaprilic acid improved the hemodynamic and metabolic status of cats in hemorrhagic shock.     

Age-related decrease in beta adrenergic receptor-mediated vascular smooth muscle relaxation

Beta adrenergic receptor-mediated vascular smooth muscle relaxation decreases with increasing age. We have examined the mechanism responsible for this phenomenon using rat mesenteric arteries from young (5-6 weeks) and older (10-12 months) rats. The beta adrenergic agonist isoproterenol produced a dose-dependent relaxation of serotonin-constricted mesenteric artery rings from young rats, whereas the maximal ability of isoproterenol to relax arterial rings from the older rats was found to be reduced markedly (92.7 vs. 27.6%, P less than .0001). The relaxation responses caused by acetylcholine and nitroglycerin, which appear to act independently of cyclic AMP (cAMP), are similar in the two groups. The loss in responsiveness of the mesenteric artery to isoproterenol was not explained by a change in beta receptor number in the vessels (29 +/- 4 in young rats vs. 31 +/- 7 fmol/mg of protein in the older rats). The maximal stimulation of cAMP accumulation by isoproterenol was lower in the older vessels; forskolin activated cAMP accumulation equally in the two groups. However, the vessels from the older rats were less sensitive to forskolin-induced vascular relaxation. Also, the ability of dibutyryl cAMP to promote vascular relaxation was diminished in the older vessels. These data suggest that the diminished cAMP accumulation in older vessels in response to isoproterenol might not necessarily in itself explain completely the reduced physiological response and that an additional defect in the beta adrenergic-mediated relaxation in the vascular smooth muscle of older rats may lie at the level of cAMP-dependent protein kinase activation or more distally.     

Comparison of beta adrenergic receptor binding characteristics and coupling to adenylate cyclase in rat pulmonary artery versus aorta

In an effort to define the mechanisms regulating pulmonary vasodilatation and explain the greater in vitro response to iso-proterenol in the pulmonary artery (PA) vs. aorta (AO), we compared beta adrenergic receptor binding characteristics and coupling to adenylate cyclase in PA and AO obtained from adult male rats. Beta adrenergic receptor binding characteristics and affinity for agonists were determined with [125I]-iodocyanopindolol. Agonist displacement studies were characteristic of a beta-2 adrenergic receptor subtype. Receptor density (44.7 +/- 7.3 vs. 39.6 +/- 0.8 fmol/mg of protein means +/- S.E.M., PA vs. AO) and the dissociation constant for the radioligand (10.3 +/- 2.6 vs. 13.4 +/- 3.5 pM) were similar in the two arteries. However, affinity for l-isoproterenol was greater (the inhibition constant was lower) in PA compared to AO (0.08 +/- 0.03 vs. 1.20 +/- 0.18 microM, P less than .05), as was affinity for l-epinephrine (0.89 +/- 0.20 vs. 3.87 +/- 0.62 microM, P less than .05). Affinity was similar for l-norepinephrine (18.93 +/- 3.63 vs. 13.49 +/- 3.12 microM). Base-line cyclic AMP (cAMP) content, basal adenylate cyclase activity and adenylate cyclase activity stimulated by GTP, isoproterenol plus GTP and forskolin were measured by radioimmunoassay for cAMP. Base-line cAMP content was greater in PA than in AO (513.5 +/- 46.9 vs. 125.5 +/- 19.1 pmol of cAMP per mg of protein, P less than .001), as was basal adenylate cyclase activity (10.8 +/- 1.2 vs. 5.7 +/- 1.3 pmol of cAMP per mg of protein per min, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Alteration in sensitivity to isoproterenol and acetylcholine in the rat heart after repeated administration of isoproterenol

Alteration of sensitivity to the inotropic responses to isoproterenol and acetylcholine (ACh) and of saturation constants for the specific binding of [3H]dihydroalprenolol and [3H]quinuclidinyl benzilate to particulate fractions were investigated in rat hearts chronically treated with isoproterenol. The EC50 value for the inotropic response to isoproterenol in the atria of rats injected i.p. with isoproterenol (0.01 or 0.1 mg/kg) twice a day for 10 days, was 9.0-fold higher than in controls. Isoproterenol (0.17 mg/kg i.p.)-induced stimulation of cardiac ornithine decarboxylase activity was significantly (P less than .05) attenuated in atria from animals of the treated groups. Scatchard analysis of specific binding of [3H]dihydroalprenolol revealed that repeated isoproterenol injection produced a significant decrease in the maximum number of binding sites (Bmax), but not in the dissociation constant (KD) for this antagonist. In comparison to the isoproterenol-induced changes in beta adrenergic positive inotropic response, repeated treatment with isoproterenol decreased 2-fold the EC50 value for the negative inotropic response to ACh in isolated atria. Scatchard analysis of specific [3H]quinuclidinyl benzilate binding, however, indicated a significant (P less than .01) decrease in Bmax. Repeated isoproterenol injections also elicited a significant increase in cardiac weight. It is suggested that repeated administration of isoproterenol induces a decrease in density of beta adrenergic receptors which may in part underlie a cardiac hyposensitivity to isoproterenol and an increase in the number of muscarinic ACh receptors and inotropic responsiveness to ACh.     

Influence of Calcium on the Inotropic Actions of Hyperosmotic Agents, Norepinephrine, Paired Electrical Stimulation, and Treppe

To analyze the interaction of calcium ion concentration with hypertonic agents and with other inotropic interventions, isolated right ventricular cat papillary muscles were studied under isometric conditions in Krebs-Ringer bicarbonate solution. Extracellular calcium concentrations were varied between 2.5 and 11.0 mM. Maximal inotropic effects occurred between 5 and 8.0 mM calcium and further elevation to 11.0 mM was without additional influence. The effect of hyperosmotic sucrose and mannitol on papillary muscle performance was compared with that of 10(-6) M norepinephrine at calcium concentrations of 2.5 and 10.0 mM and with paired electrical stimulation in 10.0 mM calcium. Both norepinephrine and the hyperosmotic agents produced significant increases in developed tension and in the maximal rate of tension rise (dT/dt) in Krebs-Ringer in 2.5 and 4.0 mM calcium. In 10 mM calcium norepinephrine increased developed tension and dT/dt, but sucrose and mannitol caused no change or small reductions in both. Paired electrical stimulation, like hyperosmolality, caused no increase in dT/dt in 10 mM calcium.The presence of a potent pharmacological inhibitor of systolic calcium transfer across the cell membrane (D600, 10(-6) M) reduced developed tension and dT/dt by 76+/-2.7 and 74+/-2.0%, respectively, and prevented and in fact reversed the expected increase in dT/dt associated with an increase in rate of stimulation (treppe). However, hypertonic mannitol and paired pacing persisted in causing marked increases in developed tension and dT/dt even in the presence of D600, suggesting that their inotropic effects are not dependent on increased intracellular transfer of calcium during systole through cell membrane channels in which D600 acts as a competitive inhibitor.The results of these studies suggest that apparent functional saturation of intracellular calcium receptor sites eliminates any additional inotropic effect of hyperosmolality or paired pacing. The data are compatible with the hypothesis that the inotropic effects of hyperosmolality and of paired pacing result from an increase in calcium concentration at the myofilaments during contraction. The increase induced by hyperosmolality might occur because of an increase in the total amount of calcium released into the cytosol with each action potential and/or as a passive consequence of cellular dehydration. Norepinephrine has the capacity to increase contractility even when intracellular calcium receptor sites appear to be functionally saturated, suggesting that it may act at least in part by a mechanism that is independent of changes in net intracellular calcium concentration.     

Inotropic and lusitropic effects of chlorpromazine on rat left ventricular papillary muscle

The in vitro effects of chlorpromazine on rat cardiac papillary muscle were tested at 10(-6), 10(-5) and 10(-4) M. Mechanical parameters were determined from the contraction and relaxation phases under isotonic and isometric conditions in order to assess contraction, relaxation, contraction-relaxation coupling and load sensitivity of relaxation. The peak power output Emax was determined from the force-velocity relationship. At 10(-6) M, a slight positive inotropic effect was observed, probably related to modifications in cross-bridges kinetics. Negative inotropic effects were observed with 10(-5) and 10(-4) M chlorpromazine. At 10(-5) M, shortening of the isometric relaxation and decrease in R2 = (+dF.dt-1max)/(-dF.dt-1max) suggest that chlorpromazine also diminishes myofilament Ca++ sensitivity. Emax was increased at 10(-6) M (19 +/- 5%, P less than .05), but decreased at 10(-5) M (-28 +/- 10%, P less than .05) and 10(-4) M (-82 +/- 2%, P less than .05). Modifications in the force-velocity relationship at 10(-4) M indicated that lowering myocardial performance by chlorpromazine was associated with a low muscle efficiency from a thermoenergetic point of view. At all concentrations, chlorpromazine impaired the isotonic relaxation and load sensitivity of relaxation. At 10(-4) M, muscle contracture and slowed isometric relaxation were probably due to "calcium overload." These results showed that chlorpromazine finely modulates intrinsic cardiac energetics and mechanics by acting on the sarcoplasmic reticulum, myofilament Ca++ sensitivity and cross-bridges kinetics, according to the level of load and chlorpromazine concentration used.     

Effects of levosimendan versus dobutamine on pressure load-induced right ventricular failure

OBJECTIVE: A transient increase in pulmonary arterial (PA) pressure can persistently depress right ventricular (RV) contractility. We investigated the effects of dobutamine and levosimendan on RV-PA coupling in this model of RV failure. DESIGN: Prospective, controlled, randomized animal study. SETTING: University research laboratory. SUBJECTS: Fifteen anesthetized dogs. INTERVENTIONS: Transient (90-min) PA constriction to induce persistent RV failure. Random assignment to dobutamine 5 and 10 microg/kg/min or levosimendan 12 microg/kg for 10 mins followed by 0.1 and 0.2 microg/kg/min. MEASUREMENTS AND MAIN RESULTS: We measured PA distal resistance and proximal elastance by pressure-flow relationships and vascular impedance. We measured RV contractility by the end-systolic pressure-volume relationship (Ees), PA effective elastance by the end-diastolic to end-systolic relationship (Ea), and RV-PA coupling efficiency by the Ees/Ea ratio. PA constriction persistently increased PA resistance and elastance, increased Ea from 0.95 +/- 0.07 to 3.01 +/- 0.28 mm Hg/mL, decreased Ees from 1.17 +/- 0.09 to 0.58 +/- 0.07 mm Hg/mL, and decreased Ees/Ea from 1.26 +/- 0.09 to 0.22 +/- 0.03 (p < .05). Dobutamine did not affect pulmonary hemodynamics, markedly increased RV contractility, and improved RV-PA coupling. Levosimendan decreased PA resistance and elastance, increased RV contractility, and restored RV-PA coupling. Compared with dobutamine, levosimendan decreased RV afterload and therefore better restored RV-PA coupling at similar inotropic state. CONCLUSIONS: A transient increase in PA pressure persistently worsens PA hemodynamics, RV contractility, RV-PA coupling, and cardiac output. Levosimendan restores RV-PA coupling better than dobutamine because of similar inotropic effects and additional pulmonary vasodilatory effects.     

Inhibition by 5-hydroxytryptamine of the beta adrenoceptor-mediated positive inotropic responses to catecholamines in rabbit papillary muscles: direct interaction with beta adrenoceptors.

The effects of 5-hydroxytryptamine (5-HT) on the positive inotropic responses to catecholamines were investigated in isolated rabbit papillary muscles. 5-HT produced a concentration-dependent positive inotropic effect, an effect which was antagonized by prazosin, but not by propranolol. The positive inotropic effect of 5-HT diminished greatly in muscles from rabbits pretreated with 6-hydroxydopamine. Thus, it is likely that 5-HT causes a release of norepinephrine and increases force of contraction indirectly through alpha-1 adrenoceptors. In the presence of prazosin, 5-HT exerted a concentration-dependent inhibition of the positive inotropic response to isoproterenol. The positive inotropic responses to tyramine and a beta-1 adrenoceptor agonist T-1583 were also inhibited by the addition of 5-HT. The inhibitory effect of 5-HT on the beta adrenoceptor-mediated responses was unaffected by methysergide, ketanserin, ICS 205-930 or atropine. Pretreatment with pertussis toxin did not block the inhibitory effect of 5-HT on the inotropic response to isoproterenol, while abolishing the cholinergic interaction against the isoproterenol response. In contrast to its antagonizing effect on the inotropic response to isoproterenol, 5-HT produced an additive effect on the positive inotropic response to norepinephrine. However, when neuronal amine uptake was blocked by cocaine, the positive intropic response to norepinephrine was suppressed by the addition of 5-HT. 5-HT inhibited (-)-[125I]iodocyanopindolol binding to the membranes from rabbit ventricles with a monophasic displacement curve.(ABSTRACT TRUNCATED AT 250 WORDS)     

Negative inotropic effect of rapamycin on isolated human cardiomyocytes

Rapamycin is an increasingly important immunosuppressive drug and reduces restenosis after coronary stenting, but its effects on cardiac contractility are largely unknown. We investigated the acute inotropic effects of rapamycin on isolated human cardiomyocytes. Cardiomyocytes were enzymatically isolated from right atrial appendages obtained during routine coronary artery bypass surgery. Cell morphology was examined by confocal microscopy. Cell contraction was recorded after electrical stimulation. Rapamycin elicited a concentration-dependent decrease in fractional cell shortening ranging from 14.3 +/- 2.6% at 10(-8) M rapamycin to 26.4 +/- 4.2% at 10(-5) M. Rapamycin also caused a concentration-dependent decrease in diastolic cell length. Contractile performance of isolated cardiomyocytes was well preserved, as evidenced by the profound positive inotropic effects of high extracellular calcium concentration and the beta-adrenoreceptor agonist isoproterenol. The acute negative inotropic effect of rapamycin on human cardiomyocytes might be due to altered calcium homeostasis through the binding of rapamycin to FKBP12.6 and its regulatory function on the ryanodine receptor, with increased calcium leakage from the sarcoplasmic reticulum.     

Effects of verapamil on myocardial contractility, cardiac adenosine 3,'5'-monophosphate and heart phosphorylase.

The effects of verapamil on myocardial isometric force on contraction, cardiac adenosine 3,'5'-monophosphate (cyclic AMP) and heart phosphorylase alpha activity were studied in the isolated perfused rat heart. When hearts were perfused with verapamil (5.98 times 10- minus 8 M), force of contraction was reduced approximately 50% within 4 to 5 minutes; at this point, the concentration of cyclic AMP was significantly lower than control but phosphorylase alpha activity was unchanged. In hearts perfused continuously for 60 minutes with verapamil, force of contraction and cyclic AMP levels returned to normal within 20 minutes after administration of verapamil was begun. Isoproterenol (0.355 nmol/min) reversed the depressant effect of verapamil on cardiac contractility and restored heart cyclic AMP levels to normal. Methoxamine (35.5 nmol/min) given to verapamil-depressed hearts, caused contractile force to return to normal, but cardiac cyclic AMP levels remained low. Mephentermine (23.0 nmol/min) had no effect on cardiac contraction, cyclic AMP or phosphorylase alpha activity in hearts depressed by verapamil. It was concluded that with the concentration of verapamil used in these experiments, the drug caused a transient decrease in force of contraction and myocardial cyclic AMP. Both the depression in myocardial contractility and in cardiac cyclic AMP caused by verapamil were reversed promptly by isoproterenol, whereas methoxamine overcame acutely only the negative inotropic effect of verapamil. Mephentermine had no effect on hearts depressed by verapamil.     

Beta adrenergic receptors in lymphocytes and granulocytes from patients with cystic fibrosis.

Intact lymphocytes from patients with cystic fibrosis (CF) produce significantly (P less than 0.001) less adenosine 3':5' cyclic monophosphate (cAMP) than normal lymphocytes in response to isoproterenol (10(-8)-10(-4) M), although the basal cAMP content and the response to prostaglandin E1 are normal. Obligate heterozygotes for CF have significantly (P less than 0.005) reduced cAMP response to isoproterenol as well, suggesting a genetic component in the beta adrenergic deficiency in CF. The number of beta adrenergic receptors, as determined by equilibrium binding of [3H]dihydroalprenolol to lymphocyte particulates, is the same in normal lymphocytes (969 +/- 165 receptors/cell) and lymphocytes from patients with CF (1,333 +/- 263 receptors/cell). Binding properties of the receptor for both antagonist and agonist, as assessed by KD for dihydroalprenolol and Ki for (-)-isoproterenol, are also normal in the CF lymphocytes. Similarly, in granulocytes from patients with CF, the cAMP response to isoproterenol (10(-8)-10(-4) M) is significantly reduced compared with healthy controls (P less than 0.03), as is the response of granulocytes from obligate heterozygotes (P less than 0.05). Again, the basal cAMP levels and the response to prostaglandin E1 are normal. The number of beta adrenergic receptors, as determined by equilibrium binding of [3H]dihydroalprenolol to granulocyte particulates, was the same in normal (1,462 +/- 249 receptors/cell) and CF (1,621 +/- 221 receptors/cell) preparations. Binding properties of the receptor for both agonist and antagonist, as assessed by KD for dihydroalprenolol and Ki for isoproterenol, are normal in CF granulocyte particulates. The lymphocyte and granulocyte beta adrenergic defect in CF cannot be explained by abnormalities of the beta adrenergic receptor or of adenylate cyclase itself. Receptor-cyclase coupling is the most likely site of the heritable beta adrenergic defect in CF.