Mechanisms of positive inotropic effects

Characterization of Stimulatory Action on Voltage-Gated Na+ Currents Caused by Omecamtiv Mecarbil, Known to Be a Myosin Activator

Omecamtiv mecarbil (OM, CK-1827452) is recognized as an activator of myosin and has been demonstrated to be beneficial for the treatment of systolic heart failure. However, the mechanisms by which this compound interacts with ionic currents in electrically excitable cells remain largely unknown. The objective of this study was to investigate the effects of OM on ionic currents in GH3 pituitary cells and Neuro-2a neuroblastoma cells. In GH₃ cells, whole-cell current recordings showed that the addition of OM had different potencies in stimulating the transient (I_Na(T)) and late components (I_Na(L)) of the voltage-gated Na⁺ current (I_Na) with different potencies in GH₃ cells. The EC₅₀ value required to observe the stimulatory effect of this compound on I_Na(T) or I_Na(L) in GH₃ cells was found to be 15.8 and 2.3 µM, respectively. Exposure to OM did not affect the current versus voltage relationship of I_Na(T). However, the steady-state inactivation curve of the current was observed to shift towards a depolarized potential of approximately 11 mV, with no changes in the slope factor of the curve. The addition of OM resulted in an increase in the decaying time constant during the cumulative inhibition of I_Na(T) in response to pulse-train depolarizing stimuli. Furthermore, the presence of OM led to a shortening of the recovery time constant in the slow inactivation of I_Na(T). Adding OM also resulted in an augmentation of the strength of the window Na⁺ current, which was evoked by a short ascending ramp voltage. However, the OM exposure had little to no effect on the magnitude of L-type Ca²⁺ currents in GH₃ cells. On the other hand, the delayed-rectifier K⁺ currents in GH₃ cells were observed to be mildly suppressed in its presence. Neuro-2a cells also showed a susceptibility to the differential stimulation of I_Na(T) or I_Na(L) upon the addition of OM. Molecular analysis revealed potential interactions between the OM molecule and hNa_V1.7 channels. Overall, the direct stimulation of I_Na(T) and I_Na(L) by OM is assumed to not be mediated by an interaction with myosin, and this has potential implications for its pharmacological or therapeutic actions occurring in vivo.

Alpha-adrenergic responsiveness in rat isolated perfused heart after abdominal aortic coarctation

Chronic isoproterenol pre-treatment, a well-known model of compensatory hypertrophy associated with cardiac beta-adrenoceptor desensitization, enhances the inotropic response to phenylephrine in rat isolated perfused hearts, supporting the hypothesis that myocardial alpha-adrenoceptor stimulation contributes to the maintenance of myocardial performance in situations in which cardiac beta-adrenoceptor function is compromised. To further corroborate this hypothesis, the effects of abdominal aortic coarctation on cardiac alpha-adrenergic responsiveness were investigated in Langendorff heart preparations. Abdominal aortic coarctation causes cardiac hypertrophy (21%) as shown by a significant increase in the ratio of ventricular dry weight to bodyweight. In preparations from hypertrophied rats, both maximum increases in left ventricular systolic pressure and heart rate elicited by isoproterenol (10(-12) to 10(-4) M) were significantly reduced (the isoproterenol concentration producing 50% of the maximum positive inotropic and chronotropic responses was enhanced almost 21- and 2-fold, respectively). However, the positive inotropic response to phenylephrine (10(-12) to 10(-4) M) remained unaffected following abdominal aortic coarctation, when compared with sham-operated rats. In preparations from both groups, phenylephrine infusion did not induce significant changes in heart rate. These results show that although abdominal aortic stenosis induced desensitization of cardiac beta-adrenoceptors, it did not enhance cardiac alpha-adrenoceptor responsiveness. This suggests that such an enhancement depends on the experimental model used to induce cardiac hypertrophy associated with desensitization of cardiac beta-adrenoceptors.

Effects of long-term pretreatment with isoproterenol on inotropic responsiveness to alpha-adrenoceptor stimulation: study in isolated perfused rat hearts

The effects of chronic pretreatment with isoproterenol (5 mg kg(-1)) daily for 10 days on cardiac alpha-adrenergic responsiveness in Langendorff heart preparations were investigated. Isoproterenol pretreatment caused cardiac hypertrophy (29%) as shown by a significant increase in the ratio of ventricular dry weight to body weight. In preparations from isoproterenol-pretreated rats, both maximum increases in left ventricular systolic pressure and heart rate elicited by isoproterenol (10(-12) to 10(-4) M) were significantly reduced (the isoproterenol concentration producing 50% of the maximum positive inotropic and chronotropic responses was enhanced almost 32- and 4-fold, respectively), while the positive inotropic response to phenylephrine (10(-12) to 10(-4) M) was significantly enhanced (the phenylephrine concentration producing 50% of the maximum positive inotropic effect was reduced almost 100-fold), compared with saline-pretreated rats. In preparations from both groups, phenylephrine infusion induced non-significant changes in heart rate and its positive inotropic response was reduced in the presence of propranolol (10(-7) M) in the perfusion medium. Even under beta-adrenoceptor blockade, the curve for the phenylephrine-induced positive inotropic effect remained shifted upward after isoproterenol pretreatment. Chronic isoproterenol pretreatment induces the expected cardiac beta-adrenoceptor desensitization while simultaneously enhancing the positive inotropic responsiveness to phenylephrine in Langendorff heart preparations. These findings support the hypothesis that cardiac alpha1-adrenoceptor stimulation may contribute to the maintenance of myocardial function under conditions in which beta-adrenoceptor function is compromised.

Adrenergic-mediated effects of cocaine on the myocardial force-frequency relationship

We studied the role of sarcolemmal alpha- and beta-adrenoceptors activation in the effects of cocaine on the positive force staircase in isolated guinea pig atria. The preparations were superfused with Tyrode's solution at 31 degrees C while attached to a force transducer to measure peak tension developed (PTD), maximum velocity of development of tension (Vmax T) and time to peak tension (TPT). The positive force staircase was not affected by propranolol or phentolamine, but it was abolished by nifedipine. Cocaine 1 mg/l (2.9 microM) enhanced PTD and Vmax T, while TPT remained unchanged. On the other hand, cocaine did not modify the increase in PTD induced by the increase in frequency of stimulation, but significantly reduced the magnitude of the increase in Vmax T. The cocaine-induced attenuation of the increase in Vmax T in response to changes in the frequency of stimulation was abolished by both propranolol and phentolamine. It is concluded that the effect of cocaine on the force-frequency relationship required background activation of alpha- and beta-adrenergic receptors.

Mechanism of the positive inotropic action of cocaine in the guinea pig atrium

We studied the mechanism of the positive inotropic action of cocaine in isolated guinea pig atria superfused with Tyrode's solution at 31 degrees C while attached to a force transducer to measure peak tension developed, maximum velocity of development of tension, and time to peak tension. Cocaine 2.9 microM enhanced peak tension developed and velocity of development of tension, and prolonged time to peak tension. The increase in peak tension developed produced by cocaine was not affected by propranolol. On the other hand, the cocaine-induced increase in velocity of development of tension was reduced, but not abolished. In the presence of propranolol and phentolamine combined, the cocaine-induced prolongation of time to peak tension was abolished and the increases of both peak tension developed and velocity of development of tension were significantly smaller than those observed in the absence of the two adrenergic blockers. For all practical purposes, nifedipine completely abolished the increase in peak tension developed induced by cocaine. It is concluded that the positive inotropic effect of cocaine in the guinea pig atrial muscle is predominantly the result of adrenergic-dependent, both alpha- and beta- receptor mediated, as well as adrenergic-independent increases in calcium influx through the L-type calcium channels in the sarcolemma.

Reduced muscarinic cholinoceptor density and sensitivity in various models of experimental cardiac hypertrophy

1. In the present study we investigated functional and binding characteristics of muscarinic receptors in experimental cardiac hypertrophy. 2. As models of cardiac hypertrophy we used hearts of spontaneously hypertensive rats (SHR) and Wistar rats with surgically induced abdominal aorta stenosis (ASR). Wistar Kyoto rats (WKY) and sham operated Wistar rats were used as respective controls. 3. The hypertrophy was more pronounced in hearts of ASR compared to SHR, although the mean arterial pressure was found to be lower. 4. Isolated, perfused Langendorff heart preparations (paced with 5 Hz) from both groups of hypertrophied hearts were less sensitive to the muscarinic agonists oxotremorin and methacholine (P < 0.05, all n = 6) when compared with control organs. The maximal reduction in contractile force induced by methacholine was 59.3 +/- 4.5% in SHR and 41.6 +/- 3.4% in ASR hearts versus 26.4 +/- 4.1% and 25.0 +/- 2.6% in control organs, respectively. 5. The density (fmol/mg protein-1) of muscarinic receptors in membrane homogenates of hearts from SHR (127.6 +/- 11.5) was unchanged, whereas in hearts from ASR (221.0 +/- 8.9) it was found to be reduced (P < 0.05) when compared to the respective controls (142.5 +/- 14.7 and 308.8 +/- 16.1, respectively, all n = 6). 6. From the present data we conclude that cardiac hypertrophy results in a loss of sensitivity towards muscarinic receptor stimulation. A corresponding reduction of left ventricular receptor density could only be demonstrated in massively hypertrophied hearts of ASR.

Sensitivity of cardiac tissues with moderate and advanced hypertrophy to calcium ions

1. Prolonged existence of hypertension is known to induce a compensatory increase in cardiac weight, later followed by a loss of functional responsiveness to biological stimuli. 2. It was the aim of the present study to investigate the functional responses of hypertrophied hearts to rising levels of intracellular calcium. The experiments were performed using two different degrees of cardiac hypertrophy, the first as obtained in spontaneously hypertensive rats (SHR) of 18-20 weeks old, the second by using rats, 32-34 weeks old, with a surgically induced stenosis of the thoracic aorta (ASR). The ASR, which showed signs of overt heart failure, may be presented as a model for hypertension-induced end-stage cardiac hypertrophy. Age-matched normotensive Wistar-Kyoto rats (WKY) and sham-operated Wistar rats served as respective controls. 3. Different methods were employed such as increasing the extracellular Ca2+ concentration, stimulation of calcium influx by means of the calcium entry promoter Bay K 8644, or altering the sodium-calcium exchange by means of the sodium ionophore monensin. 4. The inotropic responses induced by increasing the extracellular Ca2+ concentration or provoked by the calcium entry promoter Bay K 8644 proved more pronounced in hearts taken from SHR of 18 weeks old than in those from normotensive control rats, whereas the response to monensin was found to be the same in both types of hearts. In the hearts of ASR, however, the inotropic responses to Ca2+, Bay K 8644 and monensin were strongly impaired. 5. These data demonstrate that in functional experiments the sensitivity to Ca2+, which represents the main pathway in establishing a contraction, is strongly reduced in advanced cardiac hypertrophy.

Differential response of hypertrophied rat hearts to various alpha 1-adrenoceptor agonists

With respect to the heart, the prolonged existence of hypertension, both in man and in experimental animals is predominantly characterized by an increase in left ventricular myocardial mass. In this process, the autonomic nervous system plays an important role. Although endogenous catecholamine stimulation of the heart is mainly exerted via the beta-adrenoceptors, in several mammalian species, the stimulation of cardiac alpha 1-adrenoceptors also mediates positive inotropic actions. We investigated the functional responses of isolated hypertrophied hearts taken from spontaneously hypertensive rats (SHR) and rats with an induced aortic stenosis (ASR) to various alpha 1-adrenoceptor agonists and compared them with those from age matched Wistar Kyoto (WKY) and "sham" operated controls. Accordingly, we studied the functional response to: methoxamine (alpha 1), cirazoline (alpha 1) and phenylephrine (alpha 1 > beta 1). The inotropic response to the alpha 1-adrenoceptor agonists cirazoline and methoxamine proved to be significantly weaker in hypertrophied hearts from SHR and ASR than in non-hypertrophied hearts from WHY and "sham" operated controls (p < 0.05). The inotropic response to phenylephrine remained intact in hypertrophied myocardial tissue. However, it was significantly reduced when the hearts were pre-treated with the intracellular Ca(2+)-antagonists ryanodine and TMB-8. These findings show that the mechanism of sarcolemmal Ca2+ release, activated by phenylephrine, is still intact in the hypertrophied myocardial cell. In conclusion, these data show that cardiac hypertrophy, be it of genetical or mechanical origin, leads to a reduced response of the isolated heart to alpha 1-adrenoceptor stimulation.

Mechanisms for the positive inotropic effect of alpha 1-adrenoceptor stimulation in rat cardiac myocytes

alpha 1-Adrenoceptor activation can enhance myocardial contractility, and two possible inotropic mechanisms are an increase in myofilament Ca2+ sensitivity and action potential prolongation, which can increase net Ca2+ entry into cells. In adult rat ventricular myocytes (bath Ca2+, 1 mM; stimulated at 0.2-0.5 Hz), the drug 4-aminopyridine and the whole-cell voltage clamp have been used to control Ca2+ entry and differentiate between the two mechanisms. At 22-23 degrees C the specific alpha 1-adrenoceptor agonist methoxamine (100 microM) prolonged action potential duration at 50% repolarization from 55 +/- 2 to 81 +/- 5 msec, delayed time to peak contraction, and increased shortening amplitude from 5.3 +/- 0.6 to 7.8 +/- 1 microns (n = 18). Reduction of the transient outward current and other K+ currents by methoxamine was the major cause of action potential prolongation in rat myocytes with little change in the L-type calcium current. Block of the transient outward current with 2 mM 4-aminopyridine prolonged action potential duration from 52 +/- 6 to 98 +/- 12 msec and increased unloaded cell shortening from 2.9 +/- 0.4 to 6.6 +/- 0.6 microns (n = 4). Subsequently, methoxamine no longer increased cell shortening, although significant potentiation of twitch amplitude was still seen after a brief rest interval. In voltage-clamp experiments, with 70-500-msec pulses, although membrane currents were reduced, methoxamine had no positive inotropic effect and reduced cell shortening from 5.3 +/- 0.7 to 4.97 +/- 0.8 microns at pulse potentials positive to -40 mV. Similar alpha 1-adrenoceptor responses were observed at 35 degrees C during action potential and voltage-clamp experiments, which could be blocked by 10 microM prazosin. In myocytes loaded with the Ca2+ indicator indo-1, alpha 1-adrenoceptor stimulation or 4-aminopyridine both increased cell contraction and intracellular Ca2+ transients by similar amounts. As in unloaded cells, prior exposure to 4-aminopyridine prevented any inotropic effect of methoxamine without changing the systolic intracellular Ca2+ transient. The results indicated that under our experimental conditions positive inotropy in rat cardiomyocytes on exposure to alpha 1-adrenoceptor agonists was strongly correlated with the action potential prolongation that accompanied K+ current reduction. In addition, modulation of K+ channels could occur independent of changes in contractility and/or [Ca2+]i.