Role of alpha1-adrenoceptor subtypes in production of the positive inotropic effects in mammalian myocardium: implications for the alpha1-adrenoceptor subtype distribution

Action and therapeutic targets of myosin light chain kinase, an important cardiovascular signaling mechanism

The global incidence of cardiac diseases is increasing, imposing a substantial socioeconomic burden on healthcare systems. The pathogenesis of cardiovascular disease is complex and not fully understood, and the physiological function of the heart is inextricably linked to well-regulated cardiac muscle movement. Myosin light chain kinase (MLCK) is essential for myocardial contraction and diastole, cardiac electrophysiological homeostasis, vasoconstriction of vascular nerves and blood pressure regulation. In this sense, MLCK appears to be an attractive therapeutic target for cardiac diseases. MLCK participates in myocardial cell movement and migration through diverse pathways, including regulation of calcium homeostasis, activation of myosin light chain phosphorylation, and stimulation of vascular smooth muscle cell contraction or relaxation. Recently, phosphorylation of myosin light chains has been shown to be closely associated with the activation of myocardial exercise signaling, and MLCK mediates systolic and diastolic functions of the heart through the interaction of myosin thick filaments and actin thin filaments. It works by upholding the integrity of the cytoskeleton, modifying the conformation of the myosin head, and modulating innervation. MLCK governs vasoconstriction and diastolic function and is associated with the activation of adrenergic and sympathetic nervous systems, extracellular transport, endothelial permeability, and the regulation of nitric oxide and angiotensin II. Additionally, MLCK plays a crucial role in the process of cardiac aging. Multiple natural products/phytochemicals and chemical compounds, such as quercetin, cyclosporin, and ML-7 hydrochloride, have been shown to regulate cardiomyocyte MLCK. The MLCK-modifying capacity of these compounds should be considered in designing novel therapeutic agents. This review summarizes the mechanism of action of MLCK in the cardiovascular system and the therapeutic potential of reported chemical compounds in cardiac diseases by modifying MLCK processes.

The Antiarrhythmic and Hypotensive Effects of S-61 and S-73, the Pyrrolidin-2-one Derivatives with α1-Adrenolytic Properties

Heart rhythm abnormalities are a cause of many deaths worldwide. Unfortunately, the available antiarrhythmic drugs show limited efficacy and proarrhythmic potential. Thus, efforts should be made to search for new, more effective, and safer pharmacotherapies. Several studies suggested that blocking the α₁-adrenoceptors could restore normal heart rhythm in arrhythmia. In this study, we aimed to assess the antiarrhythmic potential of S-61 and S-73, two novel pyrrolidin-2-one derivatives with high affinity for α₁-adrenergic receptors. First, using radioligand binding studies, we demonstrated that S-61 and S-73 did not bind with β₁-adrenoceptors. Next, we assessed whether S-61 and S-73 could protect rats against arrhythmia in adrenaline-, calcium chloride- and aconitine-induced arrhythmia models. Both compounds showed potent prophylactic antiarrhythmic properties in the adrenaline-induced arrhythmia model, but the effect of S-61 was more pronounced. None of the compounds displayed antiarrhythmic effects in calcium chloride- or aconitine-induced arrhythmia models. Interestingly, both derivatives revealed therapeutic antiarrhythmic activity in the adrenaline-induced arrhythmia, diminishing heart rhythm irregularities. Neither S-61 nor S-73 showed proarrhythmic potential in rats. Finally, the compounds decreased blood pressure in rodents. The hypotensive effects were not observed after coadministration with methoxamine, which suggests the α₁-adrenolytic properties of both compounds. Our results confirm that pyrrolidin-2-one derivatives possess potent antiarrhythmic properties. Given the promising results of our experiments, further studies on pyrrolidin-2-one derivatives might result in the development of a new class of antiarrhythmic drugs.

Chiral recognition of doxazosin enantiomers in 3 targets for therapy as well as adverse drug reactions in animal experiments

Doxazosin used in benign prostatic hyperplasia has the side effects of causing hypotension and the risk of heart failure. The 3 targets of α1A-adrenoceptors (in the prostate), α1D-adrenoceptors (in the aorta), and an unknown mechanism (in the heart) are involved, respectively. We hypothesized that there is a chiral recognition of doxazosin enantiomers in the 3 targets. Using isolated rat aorta (α1D-adrenoceptors) and rabbit prostate (α1A-adrenoceptors), we examined pA2 and pKB values of doxazosin enantiomers. We observed chronotropic and inotropic effects of doxazosin enantiomers in isolated rat and rabbit heart tissues. (–)Doxazosin and (+)doxazosin produced a shift to the right of concentration–contraction curves for noradrenalin (aorta) and phenylephrine (prostate smooth muscle). The pA2 value of (–)doxazosin (8.625 ± 0.053) was smaller than (+)doxazosin (9.503 ± 0.051) in rat aorta, but their pKB values in rabbit prostate were the same. In rat and rabbit heart tissues, (+)doxazosin (3–30 µmol·L−1) significantly decreased atrial rate, and produced negative inotropic effects; however, (–)doxazosin did not affect the atrial rate, and produced positive inotropic effects in the atria. Thus, the chiral carbon atom of doxazosin does not affect its activity at the therapeutic target of α1A-adrenoceptors in the prostate, but significantly changes its blocking activity against α1D-adrenoceptors in the aorta, and produces opposite inotropic effects in the atria via an α1-adrenoceptor-independent mechanism.

Selective attenuation of norepinephrine release and stress-induced heart rate increase by partial adenosine A1 agonism

The release of the neurotransmitter norepinephrine (NE) is modulated by presynaptic adenosine receptors. In the present study we investigated the effect of a partial activation of this feedback mechanism. We hypothesized that partial agonism would have differential effects on NE release in isolated hearts as well as on heart rate in vivo depending on the genetic background and baseline sympathetic activity. In isolated perfused hearts of Wistar and Spontaneously Hypertensive Rats (SHR), NE release was induced by electrical stimulation under control conditions (S1), and with capadenoson 6 · 10(-8) M (30 µg/l), 6 · 10(-7) M (300 µg/l) or 2-chloro-N(6)-cyclopentyladenosine (CCPA) 10(-6) M (S2). Under control conditions (S1), NE release was significantly higher in SHR hearts compared to Wistar (766+/-87 pmol/g vs. 173+/-18 pmol/g, p<0.01). Capadenoson led to a concentration-dependent decrease of the stimulation-induced NE release in SHR (S2/S1  =  0.90 ± 0.08 with capadenoson 6 · 10(-8) M, 0.54 ± 0.02 with 6 · 10(-7) M), but not in Wistar hearts (S2/S1  =  1.05 ± 0.12 with 6 · 10(-8) M, 1.03 ± 0.09 with 6 · 10(-7) M). CCPA reduced NE release to a similar degree in hearts from both strains. In vivo capadenoson did not alter resting heart rate in Wistar rats or SHR. Restraint stress induced a significantly greater increase of heart rate in SHR than in Wistar rats. Capadenoson blunted this stress-induced tachycardia by 45% in SHR, but not in Wistar rats. Using a [(35)S]GTPγS assay we demonstrated that capadenoson is a partial agonist compared to the full agonist CCPA (74+/-2% A(1)-receptor stimulation). These results suggest that partial adenosine A(1)-agonism dampens stress-induced tachycardia selectively in rats susceptible to strong increases in sympathetic activity, most likely due to a presynaptic attenuation of NE release.

Enhancement of methoxamine-induced contractile responses of rat ventricular muscle in streptozotocin-induced diabetes is associated with alpha1A adrenoceptor upregulation

AIM

To clarify the time-related changes in cardiac function and the mechanism underlying the cardiac dysfunction present in diabetes mellitus, we studied mechanical responses induced by alpha(1)- and beta-adrenoceptors, the Ca(2+)-entry promoter Bay K 8644- and ryanodine (an agent known to inhibit Ca(2+) release from the sarcoplasmic reticulum) in papillary muscles from streptozotocin (STZ)-induced diabetic and age-matched control rats.

METHODS

Male Wistar rats received a single injection of STZ (60 mg kg(-1)) via the tail vein to induce diabetes. For the mechanical studies, papillary muscle preparations were suspended in an organ bath and isometric contractions were measured in 1-, 4-, and 10-week STZ-induced diabetic and age-matched control rats.

RESULTS

In 1-week diabetic rats, the contractions induced by isoproterenol, methoxamine and Bay K 8644 were unchanged (vs. age-matched controls). In 4-week diabetic rats, (a) the isoproterenol- and Bay K 8644-induced contractions were impaired, (b) sensitivity to ryanodine was reduced, whereas (c) the methoxamine-induced contraction was unchanged. In 10-week diabetic rats, the isoproterenol- and Bay K 8644-induced contractile responses were impaired and the sensitivity to ryanodine was reduced, but in sharp contrast the methoxamine-induced contraction was enhanced. Both the mRNA level for the alpha(1A) adrenoceptor (but not the alpha(1B) or alpha(1D) mRNAs) and alpha(1A) adrenoceptor protein were increased in 10-week diabetic rats (vs. age-matched controls).

CONCLUSION

These results suggest that impairments of beta-adrenergic and Ca(2+)-handling mechanisms occur early in the development of cardiomyopathy in STZ-induced diabetic rats, and that this is followed by augmentation of alpha(1A) adrenoceptor-mediated inotropy due to alpha(1A) adrenoceptor upregulation.

Modulation of intracellular Ca2+ via L-type calcium channels in heart cells by the autoantibody directed against the second extracellular loop of the alpha1-adrenoceptors

The effects of methoxamine, a selective alpha1-adrenergic receptor agonist, and the autoantibody directed against the second extracellular loop of alpha1-adrenoceptors were studied on intracellular free Ca2+ levels using confocal microscopy and ionic currents using the whole-cell patch clamp technique in single cells of 10-day-old embryonic chick and 20-week-old fetal human hearts. We observed that like methoxamine, the autoantibody directed against the second extracellular loop of alpha1-adrenoreceptors significantly increased the L-type calcium current (I(Ca(L))) but had no effect on the T-type calcium current (I(Ca(T))), the delayed outward potassium current, or the fast sodium current. This effect of the autoantibody was prevented by a prestimulation of the receptors with methoxamine and vice versa. Moreover, treating the cells with prazosin, a selective alpha1-adrenergic receptor antagonist blocked the methoxamine and the autoantibody-induced increase in I(Ca(L)), respectively. In absence of prazosin, both methoxamine and the autoantibody showed a substantial enhancement in the frequency of cell contraction and that of the concomitant cytosolic and nuclear free Ca2+ variations. The subsequent addition of nifedipine, a specific L-type Ca2+ channel blocker, reversed not only the methoxamine or the autoantibody-induced effect but also completely abolished cell contraction. These results demonstrated that functional alpha1-adrenoceptors exist in both 10-day-old embryonic chick and 20-week-old human fetal hearts and that the autoantibody directed against the second extracellular loop of this type of receptors plays an important role in stimulating their activity via activation of L-type calcium channels. This loop seems to have a functional significance by being the target of alpha1-receptor agonists like methoxamine.

Alpha-1 adrenergic signaling in a cardiac murine atrial myocyte (HL-1) cell line

Activation of alpha-1 adrenergic receptors in the heart has been shown to result in increased contractile activity, cardiac fetal gene re-expression, and myocyte hypertrophy. Three alpha-1 adrenergic receptors have been identified through molecular cloning. Due to the limited selectivities of the currently available alpha-1 adrenergic receptor antagonists, the signaling pathways activated by specific subtypes in the heart remain unresolved. To resolve this dilemma, we have used a molecular approach to identify the signaling pathways and downstream genes that are engaged in response to activation of individual alpha-1 adrenergic subtypes in cardiac cells. We have transfected constitutively active alpha-1 adrenergic receptors (alpha1a-S290/293-AR [1] or the alpha1b-S288/294-AR [2]) subtypes into the cardiac murine myocyte cell line (HL-1) and studied the signal transduction pathway(s) and cardiac gene(s) activated by them. In this study, we demonstrate that the alpha1a-S290/293 -AR [1] subtype preferentially couples to cardiac-specific atrial natriuretic factor (ANF) gene expression, while the alpha1b-S288/294-AR preferentially couples to activation of mitogen-activated protein kinase (MAPK), Ets-like transcription factor-1 (Elk1) and serum response element (SRE) signaling pathways. Endogenous alpha-1 adrenergic receptors are expressed, and stimulate phosphatidylinositol-hydrolysis upon activation with the alpha-1 agonist, phenylephrine.

Penile erection and cardiac risk: pathophysiologic and pharmacologic mechanisms

Erection is initiated through the parasympathetic nervous system, activation of which overrides the sympathetic tone that maintains the penis in a nonerectile (flaccid) state. This state is maintained mainly through the release of norepinephrine from penile adrenergic nerves. Norepinephrine contracts the vasculature and cavernosal smooth muscle. Arousal/erection is associated with a decrease of norepinephrine release in the penis, with a release of nitric oxide, and with a reduction in penile smooth muscle tone. It is also associated with minor cardiovascular changes. Heart rate increases by 4-8 beats per minute, on average, and the rate-pressure product and oxygen consumption increase by approximately 25%. There may be no changes in systemic venous norepinephrine concentrations; systemic venous epinephrine concentrations increase by about 60%. Drugs initiating or enhancing erection act by inhibiting norepinephrine-induced contraction (e.g., phentolamine) or by enhancing or directly inducing relaxation of the corpora cavernosa and the penile vasculature (e.g., sildenafil). Despite potentially negative hemodynamic actions when given parenterally, oral phentolamine-in doses required for enhancing erection-appears to produce few cardiovascular adverse effects. The hemodynamic effects of sildenafil are small, even in patients with coronary artery disease. However, the effects of the drug on human myocardium have not been conclusively established, and should be further investigated. As judged by available information, the cardiac risk associated with erection, with or without enhancement of drugs currently used for treatment of erectile dysfunction, is low.