Myocardial alpha1-adrenoceptor: inotropic effect and physiologic and pathologic implications

Cardiac and Vascular α1-Adrenoceptors in Congestive Heart Failure: A Systematic Review

As heart failure (HF) is a devastating health problem worldwide, a better understanding and the development of more effective therapeutic approaches are required. HF is characterized by sympathetic system activation which stimulates α- and β-adrenoceptors (ARs). The exposure of the cardiovascular system to the increased locally released and circulating levels of catecholamines leads to a well-described downregulation and desensitization of β-ARs. However, information on the role of α-AR is limited. We have performed a systematic literature review examining the role of both cardiac and vascular α1-ARs in HF using 5 databases for our search. All three α1-AR subtypes (α1A, α1B and α1D) are expressed in human and animal hearts and blood vessels in a tissue-dependent manner. We summarize the changes observed in HF regarding the density, signaling and responses of α1-ARs. Conflicting findings arise from different studies concerning the influence that HF has on α1-AR expression and function; in contrast to β-ARs there is no consistent evidence for down-regulation or desensitization of cardiac or vascular α1-ARs. Whether α1-ARs are a therapeutic target in HF remains a matter of debate.

Effect of acute alpha adrenergic stimulation on cardiac function

Background. Phenylephrine (PE) is a α-adrenergic agent commonly administered by perfusion and anesthesia. It is important to identify the effect of PE, especially on cardiac function. This study was intended to show the effects of PE on cardiac function in the murine model via pressure-volume loops. Methods. Six C57BL/6J twelve-week-old female mice were studied prior to and following PE administration at 50 μg/kg IV. In vivo pressure-volume loops were recorded at both time points. Results. There was an expected increase in maximum arterial pressure by 30% (p < 0.001) and end-systolic pressure by 34% (p < 0.001). However, there was a decrease in cardiac output by 30% (p = 0.0006), ejection fraction by 36% (p = 0.0003) and stroke volume by 25% (p < 0.004). Conclusions. This study demonstrates that PE has an effect on cardiac function beyond increasing vascular resistance. The data suggest the negative effects of PE on cardiac function may be related to stimulation of cardiac α-adrenergic receptors. Perfusion (2007) 22, 289—292.

Alpha-1-adrenergic receptors: targets for agonist drugs to treat heart failure

Evidence from cell, animal, and human studies demonstrates that α1-adrenergic receptors mediate adaptive and protective effects in the heart. These effects may be particularly important in chronic heart failure, when catecholamine levels are elevated and β-adrenergic receptors are down-regulated and dysfunctional. This review summarizes these data and proposes that selectively activating α1-adrenergic receptors in the heart might represent a novel and effective way to treat heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Testosterone-augmented contractile responses to α1- and β1-adrenoceptor stimulation are associated with increased activities of RyR, SERCA, and NCX in the heart

We hypothesized that testosterone at physiological levels enhances cardiac contractile responses to stimulation of both α1- and β1-adrenoceptors by increasing Ca2+ release from the sarcoplasmic reticulum (SR) and speedier removal of Ca2+ from cytosol via Ca2+-regulatory proteins. We first determined the left ventricular developed pressure, velocity of contraction and relaxation, and heart rate in perfused hearts isolated from control rats, orchiectomized rats, and orchiectomized rats without and with testosterone replacement (200 μg/100 g body wt) in the presence of norepinephrine (10−7 M), the α1-adrenoceptor agonist phenylephrine (10−6 M), or the nonselective β-adrenoceptor agonist isoprenaline (10−7 M) in the presence of 5 × 10−7 M ICI-118,551, a β2-adrenoceptor antagonist. Next, we determined the amplitudes of intracellular Ca2+ concentration transients induced by electrical stimulation or caffeine, which represent, respectively, Ca2+ release via the ryanodine receptor (RyR) or releasable Ca2+ in the SR, in ventricular myocytes isolated from the three groups of rats. We also measured 45Ca2+ release via the RyR. We then determined the time to 50% decay of both transients, which represents, respectively, Ca2+ reuptake by sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) and removal via the sarcolemmal Na+/Ca2+ exchanger (NCX). We correlated Ca2+ removal from the cytosol with activities of SERCA and its regulator phospholamban as well as NCX. The results showed that testosterone at physiological levels enhanced positive inotropic and lusitropic responses to stimulation of α1- and β1-adrenoceptors via the androgen receptor. The increased contractility and speedier relaxation were associated with increased Ca2+ release via the RyR and faster Ca2+ removal out of the cytosol via SERCA and NCX.

Increased diastolic time fraction as beneficial adjunct of α1-adrenergic receptor blockade after percutaneous coronary intervention

The effect of α1-receptor blockade with urapidil on coronary blood flow and left ventricular function has been attributed to relief of diffuse coronary vasoconstriction following percutaneous coronary intervention (PCI). We hypothesized that an increase in diastolic time fraction (DTF) contributes to the beneficial action of urapidil. In eleven patients with a 63% (SD 13) diameter stenosis, ECG, aortic pressure (Pa) and distal intracoronary pressure (Pd), and blood flow velocity were recorded at baseline and throughout adenosine-induced hyperemia. Measurements were obtained before and after PCI and after subsequent α1-receptor blockade with urapidil (10 mg ic). DTF was determined from the ECG and the Pa waveform. Functional parameters such as coronary flow velocity reserve, fractional flow reserve, and an index of hyperemic microvascular resistance (HMR) were assessed. Urapidil administration after PCI induced an upward shift in the DTF-heart rate relationship, resulting in a 3.1% (SD 2.7) increase in hyperemic DTF at a constant heart rate ( P < 0.005) due to a shorter duration of systole. Hyperemic Pa and Pd decreased, respectively, by 6.1% (SD 6.6; P < 0.05) and 5.7% (SD 5.8; P < 0.01) after α1-blockade. Although epicardially measured functional parameters were on average not altered by α1-blockade due to concurrent changes in pressure and heart rate, HMR decreased by urapidil in those patients where coronary pressure remained constant. In conclusion, α1-receptor blockade after PCI produced a modest but significant prolongation of DTF at a given heart rate, thereby providing an adjunctive beneficial mechanism for improving subendocardial perfusion, which critically depends on DTF.

Receptor subtype involved in alpha 1-adrenergic receptor-mediated Ca2+ signaling in cardiomyocytes

AIM

The enhancement of intracellular Ca2+ signaling in response to alpha 1-adrenergic receptor (alpha 1-AR) stimulation is an essential signal transduction event in the regulation of cardiac functions, such as cardiac growth, cardiac contraction, and cardiac adaptation to various situations. The present study was intended to determine the role(s) of the alpha 1-AR subtype(s) in mediating this response.

METHODS

We evaluated the effects of subtype-specific agonists and antagonists of the alpha 1- AR on the intracellular Ca2+ signaling of neonatal rat ventricular myocytes using a confocal microscope.

RESULTS

After being cultured for 48 h, the myocytes exhibited spontaneous local Ca2+ release, sparks, and global Ca2+ transients. The activation of the alpha 1-AR with phenylephrine, a selective agonist of the alpha 1-AR, dose-dependently increased the frequency of Ca2+ transients with an EC50 value of 2.3 micromol/L. Blocking the alpha 1A-AR subtype with 5-methylurapidil (5-Mu) inhibited the stimulatory effect of phenylephrine with an IC(50) value of 6.7 nmol/L. In contrast, blockade of the alpha 1B-AR and alpha 1D-AR subtypes with chloroethylclonidine and BMY 7378, respectively, did not affect the phenylephrine effect. Similarly, the local Ca2+ spark numbers were also increased by the activation of the alpha 1-AR, and this effect could be abolished selectively by 5-Mu. More importantly, A61603, a novel selective alpha 1A-AR agonist, mimicked the effects of phenylephrine, but with more potency (EC(50) value =6.9 nmol/L) in the potentiation of Ca2+ transients, and blockade of the alpha 1A-AR by 5-Mu caused abolishment of its effects.

CONCLUSION

These results indicate that alpha 1-adrenergic stimulation of intracellular Ca2+ activity is mediated selectively by the alpha 1A-AR.

Subtype-specific alpha1- and beta-adrenoceptor signaling in the heart

Recent studies of adrenoceptors have revealed subtype-specific signaling, promiscuous G-protein coupling, time-dependent switching of intracellular signaling pathways, intermolecular interactions within or between adrenoceptor subfamilies, and G-protein-independent signaling pathways. These findings have extended the classical linear paradigm of G-protein-coupled receptor signaling to a complex "signalome" in which an individual adrenoceptor initiates multiple signaling pathways in a temporally and spatially regulated manner. In particular, persistent stimulation of beta-adrenoceptor subtypes causes a time-dependent switch of signaling pathways and elicits different, even opposing, functional roles of these receptors in regulating cardiac structure and function. Recent progress in the understanding of subtype-specific functions and signaling mechanisms of cardiac adrenoceptor subtypes, particularly beta(1)-adrenoceptors, beta(2)-adrenoceptors, alpha(1A)-adrenoceptors and alpha(1B)-adrenoceptors, might have important pathogenic and therapeutic implications for heart disease.

Cardiac adrenoceptors: physiological and pathophysiological relevance

At present, nine adrenoceptor (AR) subtypes have been identified: alpha(1A)-, alpha(1B)-, alpha(1D)-, alpha(2A)-, alpha(2B)-, alpha(2C)-, beta(1)-, beta(2)-, and beta(3)AR. In the human heart, beta(1)- and beta(2)AR are the most powerful physiologic mechanism to acutely increase cardiac performance. Changes in betaAR play an important role in chronic heart failure (CHF). Thus, due to increased sympathetic activity in CHF, betaAR are chronically (over)stimulated, and that results in beta(1)AR desensitization and alterations of down-stream mechanisms. However, several questions remain open: What is the role of beta(2)AR in CHF? What is the role of increases in cardiac G(i)-protein in CHF? Do increases in G-protein-coupled receptor kinase (GRK)s play a role in CHF? Does betaAR-blocker treatment cause its beneficial effects in CHF, at least partly, by reducing GRK-activity? In this review these aspects of cardiac AR pharmacology in CHF are discussed. In addition, new insights into the functional importance of beta(1)- and beta(2)AR gene polymorphisms are discussed. At present it seems that for cardiovascular diseases, betaAR polymorphisms do not play a role as disease-causing genes; however, they might be risk factors, might modify disease, and/or might influence progression of disease. Furthermore, betaAR polymorphisms might influence drug responses. Thus, evidence has accumulated that a beta(1)AR polymorphism (the Arg389Gly beta(1)AR) may affect the response to betaAR-blocker treatment.

Upregulation of the α1-adrenoceptor-induced phosphoinositide and inotropic response in hypothyroid rat heart

In this study, we examined changes in the biochemical and inotropic events of the alpha(1)-adrenoceptor signaling pathway in hypothyroid rat hearts. Hypothyroidism was induced by treating experimental animals with 0.05% 6-n-propyl-2-thiouracil (PTU) in drinking water for 7 weeks. A significant decrease of beta- and an increase in alpha(1)-adrenoceptor density as well as an increase in the basal activity of the phosphoinositide (4,5) bisphosphate hydrolyzing phospholipase C was observed in sarcolemmal membranes purified from hypothyroid hearts as compared to age-matched euthyroid controls. Following stimulation with 10 microM phenylephrine (in the presence of 10 microM atenolol), the increase of contractile parameters over baseline values was significantly higher in hypo- than euthyroid hearts, while the opposite occurred under beta-stimulation with 0.1 microM isoproterenol. Interestingly, the increase in phenylephrine-mediated positive inotropy was accompanied by a significant increase in the sarcolemmal phospholipase C activity and in the inositol 1,4,5-trisphosphate content in hypothyroid as compared to euthyroid controls. Our results suggest that cardiac alpha(1)-adrenoceptor and its associated phosphoinositide signaling pathway may act as a reserve for catecholamine inotropic response in hypothyroidism, where the beta-adrenoceptors are compromised.

Dehydroepiandrosterone sulfate and estrone sulfate reduce GABA-recurrent inhibition in the hippocampus via muscarinic acetylcholine receptors

Several recent studies have established a role for estrogens in ameliorating specific neurodegenerative disorders, mainly those associated with the cholinergic neurons of the basal forebrain and their targets in the cortex and hippocampus. We have previously demonstrated that endogenous and exogenous application of the neurosteroid dehydroepiandrosterone sulfate (DHEAS) markedly reduces GABA-mediated recurrent inhibition and synchronizes hippocampal unit activity to theta rhythm (Steffensen (1995) Hippocampus 5:320-328). In this study, we evaluated the role of muscarinic receptors in mediating the effects of DHEAS and estrone sulfate (ES), the principal circulating estrogen in humans, on short-latency-evoked potential responses, paired-pulse inhibition (PPI), paired-pulse facilitation, and GABA interneuron activity in the dentate gyrus and CA1 subfields of the rat hippocampus. In situ microelectrophoretic application of the muscarinic M2 subtype cholinergic receptor agonist cis-dioxolane, DHEAS, and ES markedly reduced PPI in the dentate and CA1 that was blocked by the M2 receptor antagonist gallamine. Similar to DHEAS, microelectrophoretic administration of ES increased population spike amplitudes, without increasing excitatory transmission, but this effect was not blocked by gallamine. Microelectrophoretic application of cis-dioxolane and ES markedly increased the firing rate of dentate hilar interneurons and CA1 oriens/alveus interneurons and enhanced their synchrony to hippocampal theta rhythm. These findings suggest that select GABA-modulating neurosteroids and neuroactive estrogen sulfates alter septohippocampal cholinergic modulation of hippocampal GABAergic interneurons mediating recurrent, but not feedforward, inhibition of hippocampal principal cell activity.

alpha1-Adrenoceptor-mediated Negative Inotropic Effect Caused by Intracellular Ionic Activities in Guinea-pig Papillary Muscle

In guinea-pig papillary muscle, phenylephrine (PE), an agonist of alpha1-adrenoceptor (alpha1-AR), led to a transient negative inotropic effect (NIE) and a subsequent sustained positive inotropic effect (PIE). To clarify the ionic mechanisms underlying the NIE, we measured the [Na+]i or [pH]i by ion-selective microelectrodes. PE produced a decrease in the intracellular Na+ concentration ([Na+]i) and an increase in intracellular pH ([pH]i). During the phase of NIE, PE produced only a (-) change of [Na+]i (Delta[Na+]i). With a decrease in extracellular Na+ or an increase in extracellular Ca2+, the PE-induced NIE was attenuated and PE produced (+)Delta[Na+]i. The PE-induced NIE and (-)Delta[Na+]i were definitely strengthened by lowering the bath temperature or increasing the stimulation frequency. 2-(2,6-di-methoxyphenoxyethyl)amino-methyl-1,4-benzidioxane HCl, an antagonist of alpha1A-AR, completely abolished the PE-induced NIE and (-)Delta[Na+]i. Phorbol 12,13-dibutyrate, an activator of protein kinase C (PKC), decreased the baseline [Na+]i and twitch force and increased the baseline [pH]i in mimicry of PE. Pretreatment with 1-5(isoquinolinesulphonyl)-2-methylpiperazine, an inhibitor of PKC, abolished the PE-induced NIE and (-)Delta[Na+]i. During pretreatment with benzamil, an inhibitor of Na+/Ca2+ exchange, we found that the PE-induced NIE and (-)Delta[Na+]i were reversibly abolished. Our results indicate the PE-induced NIE may be elicited upon the activation of Na+/Ca2+ exchange which can be attributed to the (-)Delta[Na+]i. (-)Delta[Na+]i is mediated through a PKC-dependent pathway via an activation of alpha1A-AR subtype and its effect could be strengthened remarkably at high [Na+]i and [Ca2+]i values.

Phenylephrine acts via IP3-dependent intracellular NO release to stimulate L-type Ca2+ current in cat atrial myocytes

This study determined the effects of alpha1-adrenergic receptor (alpha1-AR) stimulation by phenylephrine (PE) on L-type Ca2+ current (I(Ca,L)) in cat atrial myocytes. PE (10 microm) reversibly increased I(Ca,L) (51.3%; n = 40) and shifted peak I(Ca,L) activation voltage by -10 mV. PE-induced stimulation of I(Ca,L) was blocked by each of 1 microm prazocin, 10 microm L-NIO, 10 microm W-7, 10 microm ODQ, 2 microm H-89 or 10 microm LY294002, and was unaffected by 10 microm chelerythrine or incubating cells in pertussis toxin (PTX). PE-induced stimulation of I(Ca,L) also was inhibited by each of 10 microm ryanodine or 5 microm thapsigargin, by blocking IP3 receptors with 2 microm 2-APB or 10 microm xestospongin C or by intracellular dialysis of heparin. In field-stimulated cells, PE increased intracellular NO (NOi) production. PE-induced NOi release was inhibited by each of 1 microm prazocin, 10 microm L-NIO, 10 microm W-7, 10 microm LY294002, 2 microm H-89, 10 microm ryanodine, 5 microm thapsigargin, 2 microm 2-APB or 10 microm xestospongin C, and unchanged by PTX. PE (10 microm) increased phosphorylation of Akt, which was inhibited by LY294002. Confocal microscopy showed that PE stimulated NOi release from subsarcolemmal sites and this was prevented by 2 mm methyl-beta-cyclodextrin, an agent that disrupts caveolae formation. PE also increased local, subsarcolemmal SR Ca2+ release via IP3-dependent signalling. Electron micrographs of atrial myocytes show peripheral SR cisternae in close proximity to clusters of caveolae. We conclude that in cat atrial myocytes PE acts via alpha1-ARs coupled to PTX-insensitive G-protein to release NOi, which in turn stimulates I(Ca,L). PE-induced NOi release requires stimulation of both PI-3K/Akt and IP3-dependent Ca2+ signalling. NO stimulates I(Ca,L) via cGMP-mediated cAMP-dependent PKA signalling. IP3-dependent Ca2+ signalling may enhance local SR Ca2+ release required to activate Ca2+-dependent eNOS/NOi production from subsarcolemmal caveolae sites.

Chronic Exercise Improves Myocardial Inotropic Reserve Capacity Through α1-Adrenergic and Protein Kinase C-Dependent Effects in Senescent Rats

We have previously demonstrated that α1-adrenergic (AR)-mediated contraction is diminished in the senescent rat heart, in part due to alterations in protein kinase C (PKC) signaling. Since chronic exercise training (EX) can exert independent effects on increasing α1-AR contraction in the adult rat heart, we sought to determine whether age-related defects in α1-AR contraction could be reversed by chronic EX. We further hypothesized that improved α1-AR contraction by EX may be PKC dependent. Adult (4 months; Y) and aged (24 months; O) male F344 rats were treadmill-trained (n = 12–13/group; TR) at ∼70% of VO2max for 12 weeks or remained sedentary (YSED, YTR, OSED, OTR). Training status was verified by plantaris citrate synthase activity and left ventricular (LV) contractile responses (dP/dt) to α1-AR stimulation were assessed in Langendorff-perfused hearts using the α1-AR agonist phenylephrine (PE; 10−5 M) with and without the PKC inhibitor chelerythrine (CE; 10−6 M). α1-AR stimulation elicited greater increases in LV dP/dt in hearts isolated from OTR (4525.4 ± 224.1 mmHg/s) versus OSED (3658.9 ± 291.0 mmHg/s), while CE abolished PE-induced effects (OTR, 4069.2 ± 341.2) versus (OSED, 3608.9 ± 321.2) (p &lt;.01). Upon western blotting, phosphospecific antibodies directed at PKCε (pSer729) revealed greater levels in LV isolated from YTR versus YSED, and EX ameliorated aged-related reductions in OSED (p &lt;.001). Basal PKCε mRNA levels were also greater in YTR and OTR versus YSED (p &lt;.01). PE-induced increases in phosphor-PKCδ (pThr507) levels observed in OSED were attenuated in OTR (p &lt;.03). Chronic EX was also associated with significant reductions in PKCα (pSer657) levels following PE in OTR (p &lt;.002). The results indicate that age-related reductions in α1-AR contraction can be partially reversed by EX in the rat heart. These results further suggest that alterations in PKC levels underlie, at least in part, EX-induced improvements in α1-AR contraction.

Autonomic receptor systems in the failing and aging human heart: similarities and differences

Changes in autonomic receptor systems (alpha- and beta-adrenoceptors and muscarinic receptors) were compared in the aging and failing human heart. In both settings responsiveness of beta-adrenoceptors and all other receptor systems that evoke their effects via cyclic AMP accumulation was diminished. Muscarinic receptor function, on the other hand, was decreased in the aging, but unchanged in the failing heart; in contrast, G protein-coupled receptor kinase activity was increased in the failing, but unchanged in the aging heart. alpha-Adrenoceptor function was unchanged or slightly decreased in the failing heart. However, nothing is known on alpha-adrenoceptor changes in the aging heart. These results indicate that in the failing human heart all autonomic receptor systems appear to be altered in the direction to attenuate beta-adrenoceptor responses to sympathetic (over)stimulation while in the aging human heart autonomic receptor systems appear to be altered in a direction that protects the heart against too pronounced reduction in beta-adrenoceptor responsiveness.

Alpha 1-adrenergic receptor responses in alpha 1AB-AR knockout mouse hearts suggest the presence of alpha 1D-AR

Two functional alpha(1)-adrenergic receptor (AR) subtypes (alpha(1A) and alpha(1B)) have been identified in the mouse heart. However, it is unclear whether the third known subtype, alpha(1D)-AR, is also present. To investigate this, we determined whether there were alpha(1)-AR responses in hearts from a novel mouse model lacking alpha(1A)- and alpha(1B)-ARs (double knockout) (ABKO). In Langendorff-perfused hearts, alpha(1)-ARs were stimulated with phenylephrine. For ABKO hearts, phenylephrine reduced left ventricular pressure and coronary flow (to 87 +/- 2% and 86 +/- 4% of initial, respectively, n = 11, P < 0.01). These effects were blocked by prazosin and 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-8-azaspirol[4,5]decane-7,9-dione] dihydrochloride, suggesting that alpha(1D)-AR-mediated responses were present. In contrast, right ventricular trabeculae from ABKO hearts did not respond to phenylephrine, suggesting that in ABKO perfused hearts, the effects of phenylephrine were not mediated by direct actions on cardiomyocytes. A novel finding was that alpha(1)-AR stimulation caused positive inotropy in the wild-type mouse heart, in contrast to negative inotropy observed in mouse cardiac muscle strips. We conclude that mouse hearts lacking alpha(1A)- and alpha(1B)-ARs retain functional alpha(1)-AR responses involving decreases of coronary flow and ventricular pressure that reflect alpha(1D)-AR-mediated vasoconstriction. Furthermore, alpha(1)-AR inotropic responses depend critically on the experimental conditions.

Prevention of heart failure in rats by trimetazidine treatment: a consequence of accelerated phospholipid turnover?

Heart failure is known for alteration of cardiac catecholamine responsiveness involving adrenergic receptor (AR) down-regulation. Trimetazidine, a metabolically active anti-ischemic drug, accelerates the turnover of phospholipids. The present study evaluated the consequences of trimetazidine treatment (supposed to increase phospholipid synthesis) on AR in heart failure in rats. In control rats, trimetazidine (7.5 mg/day supplied in the diet) induced after 8 weeks a significant increase in both beta- (+54%) and alpha-AR (+30%) density, although after 12 weeks, the receptor density was normalized. Heart failure was obtained by ascending aortic banding. These heart failure rats developed a severe cardiac hypertrophy, mainly affecting the left ventricle, which was significantly reduced in the trimetazidine-treated group. The plasma level of brain natriuretic peptide (BNP), a marker of heart failure severity, was significantly increased in the heart failure group as compared with the sham group (900 and 1200% after 8 and 12 weeks, respectively). In the trimetazidine-treated group, the plasma BNP increase was significantly lower. The development of heart failure was associated with a decrease in beta- and alpha-AR sites (-23 and -36% versus sham, respectively) after 8 weeks and continued to decrease after 12 weeks (-37 and -48% versus sham, respectively). This down-regulation was prevented by trimetazidine without alteration in affinity. These results suggest that trimetazidine prevents AR desensitization and cardiac hypertrophy, in a pressure-overload model of heart failure. This cytoprotection suggests that membrane homeostasis preservation may be considered as a therapeutic target in the treatment of heart failure.

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.

Trimetazidine effect on phospholipid synthesis in ventricular myocytes: consequences in alpha-adrenergic signaling

The anti-anginal drug trimetazidine (TMZ) has been shown to increase the synthesis of phospholipids in ventricular myocytes, including phosphatidyl-inositol (PI). This study focused on the consequences of increasing PI metabolism on alpha-adrenergic signaling pathway in cultured rat cardiomyocytes. In the cells treated with TMZ, the synthesis of PI from inositol was largely increased as compared with the control (+55% in 60 min). The stimulation of alpha-adrenergic receptors by phenylephrine (PE) induced a dose-dependent production of inositide phosphates (IPs) by phospholipase C (PLC) activation. However, the amount of available IPs was significantly lower in TMZ-treated cells, in a dose-dependent manner. This effect was observed in the presence and absence of the IP1-phosphatase inhibitor LiCl. The in vitro determination of PLC activity revealed that this effect could not be attributed to the direct inhibition of the enzyme by TMZ. The TMZ-induced reduction of IPs in the PE-stimulated cardiomyocytes should be attributed to the increase of inositol recycling and incorporation in membrane structures, elicited by increased phospholipid synthesis. The consequences of this reduction in IPs availability were investigated on the cardiomyocyte hypertrophy induced by alpha-adrenergic chronic stimulation. Acute stimulation with PE increased protein synthesis (+50%), but this increase was largely prevented by TMZ. In conclusion, TMZ reduces cell available IPs, by accelerating their recycling in membranes as PI. This effect results in a cytoprotection in the pathological process of hypertrophy elicited by chronic alpha-adrenergic stimulation.

Calreticulin inhibits the MEK1,2-ERK1,2 pathway in alpha 1-adrenergic receptor/Gh-stimulated hypertrophy of neonatal rat cardiomyocytes

In cardiac myocytes, stimulation of alpha(1)-adrenoceptor (AR) leads to a hypertrophic phenotype. The G(h) protein (transglutaminase II, TGII) is tissue type transglutaminase and transmits the alpha(1B)-adrenoceptor signal with GTPase activity. Recently, it has been shown that the calreticulin (CRT) down-regulates both GTP binding and transglutaminase activities of TGII. To elucidate whether G(h) mediates norepinephrine-stimulated intracellular signal transductions leading to activation of extracellular signal-regulated kinases (ERKs) and neonatal rat cardiomyocyte hypertrophy, we examined the effects of G(h) on the activation of ERKs and inhibitory effects of CRT on alpha(1)-adrenoceptor/G(h) signaling. In neonatal rat cardiomyocytes, norepinephrine-induced ERKs activation was inhibited by an alpha(1)-adrenoceptor blocker (prazosin), but not by an beta-adrenoceptor blocker (propranolol). Overexpression of the G(h) protein stimulated norepinephrine-induced ERKs activation, which was inhibited by alpha-adrenoceptor blocker (prazosin). Co-overexpression of G(h) and CRT abolished norepinephrine-induced ERKs activation. Taken together, norepinephrine induces hypertrophy in neonatal rat cardiomyocytes through alpha(1)-AR stimulation and G(h) is partly involved in norepinephrine-induced MEK1,2/ERKs activation. Activation of G(h)-mediated MEK1,2/ERKs was completely inhibited by CRT.

Alpha(1)-AR-induced positive inotropic response in heart is dependent on myosin light chain phosphorylation

The possible involvement of different kinases in the alpha(1)-adrenoreceptor (AR)-mediated positive inotropic effect (PIE) was investigated in rat papillary muscle and compared with beta-AR-, endothelin receptor- and phorbol ester-induced changes in contractility. The alpha(1)-AR-induced PIE was not reduced by the inhibitors of protein kinase C (PKC), MAPK (ERK and p38), phosphatidyl inositol 3-kinase, or calmodulin kinase II. However, PKC inhibition attenuated the effect of phorbol 12-myristate 13-acetate (PMA) on contractility. alpha(1)-AR-induced PIE was reduced by approximately 90% during inhibition of myosin light chain kinase (MLCK) by 1-(5-chloronaphthalene-1-sulfonyl)1H-hexahydro-1,4-diazepine (ML-9). Endothelin-induced PIE was also reduced by ML-9, but ML-9 had no effect on beta-AR-induced PIE. The Rho kinase inhibitor Y-27632 also reduced the alpha(1)-AR-induced PIE. The alpha(1)-AR-induced PIE in muscle strips from explanted failing human hearts was also sensitive to MLCK inhibition. alpha(1)-AR induced a modest increase in (32)P incorporation into myosin light chain in isolated rat cardiomyocytes. This effect was eliminated by ML-9. The PIE of alpha(1)-AR stimulation seems to be dependent on MLCK phosphorylation.

α1- and β-adrenoceptor stimulation differentially activate p38-MAPK and atrial natriuretic peptide production in the perfused amphibian heart

We investigated the activation of p38-MAPK by various adrenergic agents in the perfused Rana ridibunda heart. Phenylephrine (50 μmol l-1) rapidly induced the differential activation of all three mitogen-activated protein kinase (MAPK) subfamilies (ERK, JNKs and p38-MAPK)in this experimental system. Focusing on p38-MAPK response to phenylephrine,we found that the kinase phosphorylation reached maximal values at 30 s,declining thereafter to basal values at 15 min. p38-MAPK activation by phenylephrine was verified as exclusively α1-AR-mediated. Furthermore, SB203580 (1 μmol l-1) abolished the kinase phosphorylation by phenylephrine. Isoproterenol (50 μmol l-1)was also shown to activate p38-MAPK in a time- and temperature-dependent manner. A marked, sustained p38-MAPK activation profile was observed at 25°C, while at 18°C the kinase response to isoproterenol was modest. Isoproterenol effect on p38-MAPK stimulation was β-AR-mediated. Immunohistochemical studies revealed the enhanced presence of phosphorylated p38-MAPK and atrial natriuretic peptide (ANP) in both phenylephrine- and isoproterenol-stimulated hearts, a reaction completely blocked by the respective specific antagonists, or the specific p38-MAPK inhibitor SB203580. These findings indicate a functional correlation between p38-MAPK activation and ANP accumulation in the perfused amphibian heart.

Identification of alpha1-adrenergic receptors and their involvement in phosphoinositide hydrolysis in the frog heart

The aim of this study was to characterize alpha(1)-adrenergic receptors in frog heart and to examine their related signal transduction pathway. alpha(1)-Adrenergic binding sites were studied in purified heart membranes using the specific alpha(1)-adrenergic antagonist [(3)H]prazosin. Analysis of the binding data indicated one class of binding sites displaying a K(d) of 4.19 +/- 0.56 nM and a B(max) of 14.66 +/- 1.61 fmol/mg original wet weight. Adrenaline, noradrenaline, or phenylephrine, in the presence of propranolol, competed with [(3)H]prazosin binding with a similar potency and a K(i) value of about 10 microM. The kinetics of adrenaline binding was closely related to its biological effect. Adrenaline concentration dependently increased the production of inositol phosphates in the heart in the presence or absence of propranolol. Maximal stimulation was about 8.5-fold, and the half-maximum effective concentration was 30 and 21 microM in the absence and presence of propranolol, respectively. These data clearly show that alpha(1)-adrenergic receptors are coupled to the phosphoinositide hydrolysis in frog heart. To our knowledge, this is the first direct evidence supporting the presence of functional alpha(1)-adrenergic receptors in the frog heart.

alpha(1)-adrenergic stimulation mediates Ca(2+)-dependent inositol phosphate formation through the alpha(1B)-like adrenoceptor subtype in adult rat cardiac myocytes

We studied the effects of increased Ca(2+) influx on alpha(1)-adrenoceptor-stimulated InsP formation in adult rat cardiac myocytes. We further examined if such effects could be mediated through a specific alpha(1)-adrenoceptor subtype. [(3)H]InsP responses to adrenaline were dependent on extracellular Ca(2+) concentration, from 0.1 microM to 2 mM, and were completely blocked by Ca(2+) removal. However, in cardiac myocytes preloaded with BAPTA, a highly selective calcium chelating agent, Ca(2+) concentrations higher than 1 microM had no effect on adrenaline-stimulated [(3)H]InsP formation. Taken together these results suggest that [(3)H]InsP formation induced by alpha(1)-adrenergic stimulation is in part mediated by increased Ca(2+) influx. Consistent with this, ionomycin, a calcium ionophore, stimulated [(3)H]InsP formation. This response was additive with the response to adrenaline stimulation implying that different signaling mechanisms may be involved. In cardiac myocytes treated with the alpha(1B)-adrenoceptor alkylating agent, CEC, [(3)H]InsP formation remained unaffected by increased Ca(2+) concentrations, a pattern similar to that observed when intracellular Ca(2+) was chelated with BAPTA. In contrast, addition of the alpha(1A)-subtype antagonist, 5'-methyl urapidil, did not affect the Ca(2+) dependence of [(3)H]InsP formation. Neither nifedipine, a voltage-dependent Ca(2+) channel blocker nor the inorganic Ca(2+) channel blockers, Ni(2+) and Co(2+), had any effect on adrenaline stimulated [(3)H]InsP, at concentrations that inhibit Ca(2+) channels. The results suggest that in adult rat cardiac myocytes, in addition to G protein-mediated response, alpha(1)-adrenergic-stimulated [(3)H]InsP formation is activated by increased Ca(2+) influx mediated by the alpha(1B)-subtype.

Metabolism of DHEA by cytochromes P450 in rat and human liver microsomal fractions

Administration of dehydroepiandrosterone (DHEA) to rodents produces many unique biological responses, some of which may be due to metabolism of DHEA to more biologically active products. In the current study, DHEA metabolism was studied using human and rat liver microsomal fractions. In both species, DHEA was extensively metabolized to multiple products; formation of these products was potently inhibited in both species by miconazole, demonstrating a principal role for cytochrome P450. In the rat, use of P450 form-selective inhibitors suggested the participation of P4501A and 3A forms in DHEA metabolism. Human liver samples displayed interindividual differences in that one of five subjects metabolized DHEA to a much greater extent than the others. This difference correlated with the level of P4503A activity present in the human liver samples. For one subject, troleandomycin inhibited hepatic microsomal metabolism of DHEA by 78%, compared to 81% inhibition by miconazole, suggesting the importance of P4503A in these reactions. Form-selective inhibitors of P4502D6 and P4502E1 had a modest inhibitory effect, suggesting that these forms may also contribute to metabolism of DHEA in humans. Metabolites identified by LC-MS in both species included 16alpha-hydroxy-DHEA, 7alpha-hydroxy-DHEA, and 7-oxo-DHEA. While 16alpha-hydroxy-DHEA appeared to be the major metabolite produced in rat, the major metabolite produced in humans was a mono-hydroxylated DHEA species, whose position of hydroxylation is unknown.

Protecting the myocardium from ischemic injury: a critical role for alpha(1)-adrenoreceptors?

Ischemic preconditioning (IPC) refers to the ability of short periods of ischemia to make the myocardium more resistant to a subsequent ischemic insult. It is the most powerful form of endogenous protection against myocardial infarction and has been demonstrated in all species evaluated to date. However, the cellular mechanisms that drive IPC remain poorly understood. This hypothesis describes an important role for alpha(1)-adrenoreceptors in mediating IPC and discusses the underlying mechanisms by which this is likely achieved. alpha(1)-Adrenoreceptors are present in the myocardium of all mammalian species, and several lines of evidence suggest that they play an important role in mediating IPC. During periods of myocardial hypoxia/ischemia, cardiomyocytes have to rely solely on anaerobic glycolysis for energy production; for this, the cells have to depend on increased glucose entry inside the cell as well as increased glycolysis. Stimulation of alpha(1)-adrenoreceptors increases glucose transport inside the cardiomyocytes by translocating glucose transporter (GLUT)-1 and GLUT-4 from the cytoplasm to the plasma membrane, enhances glycogenolysis by activating phosphorylase kinase, increases the rate of glycolysis by activating the enzyme phosphofructokinase, reduces intracellular acidity produced during excessive glycolysis by activating the Na(+)/H(+) exchanger, and inhibits apoptosis by increasing the levels of the antiapoptotic protein Bcl-2. Myocardial ischemia produces an increase in the expression of alpha(1)-adrenoreceptors in cardiomyocytes, as well as increases the levels of its agonist norepinephrine by several fold. During ischemic states, upregulation of alpha(1)-adrenoreceptors and increase in norepinephrine release could be a powerful adaptive mechanism that drives IPC. An understanding into the role of alpha(1)-adrenoreceptors in mediating IPC could not only point to newer treatments for limiting myocardial damage during myocardial infarction or heart surgery, but could also help in avoiding the use of alpha(1)-antagonists in patients with ischemic heart disease.

Alpha 1-adrenergic receptor regulation: basic science and clinical implications

Adrenergic receptors (ARs) are members of the G-protein-coupled receptor family, which includes alpha 1ARs, alpha 2ARs, beta 1ARs, beta 2ARs, beta 3ARs, adenosine, muscarinic, angiotensin, endothelin receptors, and many others that are responsible for a large variety of physiologic effects through G-protein coupling. This review focuses on alpha 1ARs and their regulation at both the mRNA and protein levels. Currently, three alpha 1AR subtypes have been characterized both pharmacologically and at the gene level: alpha 1aAR, alpha 1bAR, and alpha 1dAR. These are expressed in a species- and tissue-dependent manner. Mutagenesis approaches have been extremely valuable in the identification of key residues that govern alpha 1AR ligand binding and signaling. These studies reveal that alpha 1ARs have evolved an exquisitely sensitive regulation of their activity in which any disruption of the native structure has profound effects on subsequent function and effector coupling. Significant advances have also been made in the elucidation of signaling pathway components, resulting in the identification of novel pathways that can lead to pathologic conditions. Specific topics include mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and G-protein-coupled receptor cross-talk pathways. Within this context, recent studies identifying underlying transcriptional mechanisms involved in the regulation of the alpha 1AR subtypes are also discussed. Finally, given the potentially important role of alpha 1ARs in the vasculature, as well as in the pathology of many diseases, such as myocardial hypertrophy and benign prostatic hyperplasia, the clinical relevance of alpha 1AR distribution, pharmacology, and therapeutic intervention is reviewed.

Properties of the ventricular adrenergic signal transduction system during ontogeny of spontaneous hypertension in rats

The purpose of this study was to characterize adrenergic receptors and associated G proteins in ventricles of spontaneously hypertensive rats (SHRs) at different stages of development. The beta- and alpha1-adrenoceptor densities and subtype distribution, and beta-adrenoceptor-G protein coupling were studied by radioligand binding, and levels of G(Salpha), G(ialpha), and G(q/11alpha) protein species were determined by Western blotting in SHRs and Wistar-Kyoto (WKY) control rats aged 3.5 weeks, 3 months, and 8 months. In 3.5-week-old SHRs, both the beta-adrenoceptor density and the percentage of agonist high-affinity binding sites were higher than in age-matched WKY rats. The beta1/beta2-subtype distribution, the alpha1-adrenoceptor density, and the alpha1B/alpha1A-subtype distribution were similar in rats of both strains at all ages. Although essentially no differences in G(salpha) levels between SHRs and WKY rats were detected, higher G(ialpha) and lower Gq/1alpha concentrations were found in 3.5-week-old SHRs. In 3-month-old SHRs, increased levels of Gq/11alpha proteins were observed. In 8-month-old SHRs, none of the parameters was different from those of controls. We conclude that the differences in properties of the adrenergic signal transduction system between SHRs and WKY rats are exclusively observable before and at the onset of the overt hypertension. Moreover, the hypertensive genotype apparently affects G proteins more readily than adrenoceptors.

Cardiovascular α1-adrenoceptor subtypes: functions and signaling

α1-Adrenoceptors (α1AR) are G protein-coupled receptors and include α1A, α1B, and α1D subtypes corresponding to cloned α1a, α1b, and α1d, respectively. α1AR mediate several cardiovascular actions of sympathomimetic amines such as vasoconstriction and cardiac inotropy, hypertrophy, metabolism, and remodeling. α1AR subtypes are products of separate genes and differ in structure, G protein-coupling, tissue distribution, signaling, regulation, and functions. Both α1AAR and α1BAR mediate positive inotropic responses. On the other hand, cardiac hypertrophy is primarily mediated by α1AAR. The only demonstrated major function of α1DAR is vasoconstriction. α1AR are coupled to phospholipase C, phospholipase D, and phospholipase A2; they increase intracellular Ca2+ and myofibrillar sensitivity to Ca2+ and cause translocation of specific phosphokinase C isoforms to the particulate fraction. Cardiac hypertrophic responses to α1AR agonists might involve activation of phosphokinase C and mitogen-activated protein kinase via Gq. α1AR subtypes might interact with each other and with other receptors and signaling mechanisms.Key words: cardiac hypertrophy, inotropic responses, central α1-adrenoreceptors, arrythmias.

Methoxamine inhibits transient outward potassium current through alpha1A-adrenoceptors in rat ventricular myocytes

alpha1-Adrenoceptor agonists are known to reduce transient outward potassium current (I(to)) in the heart. The aim of this study was to analyze the effect of methoxamine (mtx) on I(to) and to elucidate which adrenoceptor subtype was involved in this effect. We used the whole-cell configuration of the patch-clamp technique to record I(to). Our experiments confirm that mtx induces a dose-dependent decrease of I(to) that is characterized by an acceleration of time to peak (3.5 +/- 0.2 and 2.3 +/- 0.3 ms for control and mtx, respectively), and a decrease in both inactivation time constants (T(fast) was reduced from 20.8 +/-2.6 to 14.9 +/- 1.1 ms, and tau(slow) was reduced from 138 +/- 32.1 to 114 +/- 28.7 ms; n = 7). All these effects were antagonized by prazosin and the alpha1A-antagonist 5-methylurapidil but not by the irreversible alpha1B-antagonist chloroethylclonidine. These data indicate that stimulation of alpha1A-adrenoceptor subtype is involved in the methoxamine-induced reduction of I(to) in rat ventricular myocytes.

Modulation of adrenergic receptors during left ventricular hypertrophy development and after regression by captopril

The objective of this study was to analyze adrenergic receptors during cardiac hypertrophy development, after establishment of cardiac hypertrophy and after regression of cardiac hypertrophy by an angiotensin-converting enzyme inhibitor. Left ventricular hypertrophy (LVH) was induced by abdominal aortic stenosis. After surgery, plasma norepinephrine concentrations (PNE) and left ventricular adrenergic receptors from rat hearts subjected to aortic stenosis were assessed during cardiac hypertrophy development (at 3, 7, 15, and 30 days of aortic stenosis), once cardiac hypertrophy had been established (7 and 14 weeks after the stenosis) and after regression of cardiac hypertrophy by an antihypertensive dose (200 mg/kg/day) of captopril. The presence of LVH was observed from day 7 after stenosis. PNE had significantly increased after 15 days but returned to control values 30 days after surgery. The density of alpha1-adrenoceptors was found to decrease with development of hypertrophy. Once hypertrophy had been established, 7 weeks from stenosis, PNE was not different from control; however, the density of alpha1-adrenoceptors continued to diminish, whereas PNE and the density of beta-adrenoceptors were no different from control values. Fourteen weeks after stenosis, a significant decrease in PNE was recorded, and no change in alpha1- but an increase in beta-adrenoceptors was observed. LVH was reversed by treatment with captopril; PNE was similar in control and stenosed treated animals. The density of alpha1-adrenoceptors was decreased when compared with control animals, and no change in the density of beta-adrenoceptors was observed with treatment. In conclusion, a decrease of alpha1-adrenoceptors was associated with LVH development and earlier stages of established cardiac hypertrophy. Later stages of established cardiac hypertrophy were characterized by no change in alpha1- and an increase in beta-adrenoceptors. Treatment with captopril induced LVH regression and decreased the number of alpha1-adrenoceptors without any change in beta-adrenoceptors.

Experimental hyperlipidemia prevents the protective effect of ischemic preconditioning on the contractility and responsiveness to phenylephrine of rat-isolated stunned papillary muscle

This study was designed to establish a hyperlipidemic diet (significant increase in the cholesterol and triglycerides blood levels, but without atherogenic changes in heart muscle and coronary vessels) and to investigate the influence of experimental hyperlipidemia on the effects of ischemic preconditioning (PC) of rat-isolated papillary muscle on the time course of contractility during simulated ischemia and reperfusion and responsiveness to phenylephrine under such a condition. The animals were divided in four experimental groups: standard diet-fed control group (SD), SD underwent ischemic preconditioning (SD-PC), hyperlipidemic diet-fed group (HLD) and HLD underOFFt PC (HLD-PC). Force of contraction (Fc), velocity of contraction (+dF/dt), and velocity of relaxation (-dF/dt) were measured. HLD preparations were more sensitive to ischemia then SD ones. PC, performed by 5-min perfusion with no-substrate solution gassing with 95% N2/5% CO2 in the presence of fast electrical stimulation, and 10 min of reperfusion with normal solution and rate of stimulation, significantly increased the resistance of isolated cardiac tissues to simulated ischemia in SD-PC group, but not in HLD-PC group. Negative inotropic action of phenylephrine occured in SD group of preparations after simulated-ischemia/reperfusion period was also prevented by PC. Therefore, we conclude that experimental hyperlipidemia significantly influenced the function of rat heart muscle including the higher sensitivity to ischemia and different reaction to the same PC procedure.

Vascular alpha 1D-adrenoceptor function is maintained during congestive heart failure after myocardial infarction in the rat

BACKGROUND

During congestive heart failure, desensitization of beta-adrenoceptors is related to a lower adrenergic responsiveness in the heart; little is known about alpha 1-adrenoceptors in the vasculature under this condition. We evaluated alpha 1D-adrenoceptor response in aorta and carotid arteries in a model of congestive heart failure (CHF) post-myocardial infarction.

METHODS

Noradrenaline-elicited contraction was determined in endothelium-denuded arterial rings from young (10-week-old) Wistar rats in the absence and presence of the alpha 1D-adrenoceptor antagonist BMY 7378 (8-(2-(4-(2-methoxyphenyl)-1-piperazinyl) ethyl)-8-azaspiro(4,5)decane-7,9-dione dihydrochloride) in sham-operated rats and in rats that developed CHF 4 weeks or 7 months after myocardial infarction.

RESULTS

In the thoracic aorta, BMY 7378 displaced noradrenaline effect to the right with pA2 values of: sham, 8.58 +/- 0.12; CHF, 8.36 +/- 0.13, and sham, 8.56 +/- 0.10; CHF, 7.99 +/- 0.13 at 4 weeks and 7 months after myocardial infarction, respectively. While in carotid arteries, the pA2 values were: sham, 8.43 +/- 0.19; CHF, 8.81 +/- 0.19, and sham, 8.35 +/- 0.18; CHF, 8.29 +/- 0.08 at 4 weeks and 7 months after myocardial infarction, respectively. When adult (7-month-old) rats were subjected to myocardial infarction, CHF was not installed and pA2 values were similar and high in both sham and infarcted rats.

CONCLUSIONS

These results indicate that alpha 1D-adrenoceptors remained as the main receptors involved in contraction in aorta and carotid arteries, irrespective of CHF duration.

Preconditioning prevents the negative inotropic action of phenylephrine in rat isolated stunned papillary muscle

The aim of the present study was to establish a model of ischemic preconditioning of rat isolated papillary muscle and to investigate its effect on the simulated ischemia-induced disturbances in contractility and responsiveness to isoproterenol and phenylephrine. Experiments were performed in rat left ventricle papillary muscle. The following parameters were measured: force of contraction (Fc), velocity of contraction (+dF/dt), velocity of relaxation (-dF/dt), time to peak contraction (ttp), and relaxation time at the level of 10% of total amplitude (tt10). After 60 min of simulated ischemia induced by the perfusion of isolated tissues with no-substrate solution aerated by 95% N2/5% CO2, all of the measured parameters were markedly decreased. There was not complete recovery of Fc, +dF/dt and -dF/dt after 60 min of reperfusion. Positive inotropic action of isoproterenol does not differ before and after simulated ischemia. In contrast, phenylephrine induces a positive inotropic action in non-treated, but a significant negative one in simulated ischaemia/reperfusion treated preparations. The latter effect of phenylephrine was reversed by chloroethylclonidine (CEC), a selective blocker of alpha1b-adrenoceptor, but not by WB-4101, a selective blocker of alpha1a-adrenoceptor. Ischemic preconditioning of rat isolated cardiac tissue induced by the 5 min perfusion with no-substrate solution, aerated by 95% N2/ 5% CO2, in the presence of fast electrical pacing (BCL shortened from 2000 ms to 700 ms) and 10 min reperfusion, significantly improves a recovery of the contractility and prevents phenylephrine negative inotropic action.

Change of 7alpha-hydroxy-dehydroepiandrosterone levels in serum of mice treated by cytochrome P450-modifying agents

Dehydroepiandrosterone (DHEA) is 7alpha-hydroxylated in liver, brain and other organs of murine and in other species. Several works suggest that the 7alpha-hydroxy-DHEA produced may be one of the native antiglucocorticoids, and compounds modifying its production may prove useful in investigation of 7alpha-hydroxy-DHEA production and effects. After treatment of mice with dexamethasone, phenobarbital, trilostane, melatonin or metyrapone, we have used gas chromatography-mass spectrometry with negative ion detection for measurement of 7alpha-hydroxy-DHEA levels in serum of control and treated animals. The 7alpha-hydroxylating rates of liver and brain microsomes from the same animals were also measured. Results showed that serum levels of 7alpha-hydroxy-DHEA were significantly increased after treatment by all compounds except metyrapone. Significantly increased 7alpha-hydroxy-DHEA levels were directly related with significantly increased 7alpha-hydroxylation yields in liver and not in brain. In contrast, metyrapone decreased 7alpha-hydroxylation in liver and brain. These findings indicate that in brain and in liver, different enzyme systems may be responsible for production of 7alpha-hydroxy-DHEA and that treatment-induced modifications of circulating 7alpha-hydroxy-DHEA levels are mainly due to change of 7alpha-hydroxylating rates in liver.

alpha1-Adrenergic activation of L-type Ca current in rat ventricular myocytes: perforated patch-clamp recordings

alpha1-Adrenergic stimulation has little effect on L-type Ca2+ channel current (ICa,L) in adult cardiac myocytes measured using conventional whole cell voltage-clamp techniques. In this study using perforated-patch techniques, we reevaluated the effect of alpha1-adrenergic stimulation on ICa,L in adult rat ventricular myocytes. Action potentials and ICa,L were examined in the presence of 1 microM nadolol, a beta-adrenergic antagonist, in myocytes internally dialyzed with Na+- and K+-free solutions (Cs+ and tetraethylammonium as substitutes). Phenylephrine (PE; 30 microM) increased the action potential duration measured at 25 and 70% of repolarization by 104 and 86%, respectively. In the perforated-patch configuration, PE elicited a transient decrease followed by a approximately 60% increase in ICa,L, whereas only the transient decrease in ICa,L was observed in myocytes when the conventional whole cell configuration was used. The PE-induced increase in ICa,L was reversibly blocked by 1 microM prazosin, an alpha1-adrenergic antagonist. These results suggest that alpha1-adrenergic stimulation enhances cardiac ICa,L and that obligatory intracellular mediators for this action are lost during whole cell recordings.

Possible mechanism of the negative inotropic effect of alpha1-adrenoceptor agonists in rat isolated left atria after exposure to free radicals

1. This study was designed to investigate the mechanism(s) of the negative inotropic effects of alpha1-adrenoceptor agonists observed in rat isolated left atria after exposure to free radicals. 2. Ouabain and calphostin C were used in contraction experiments to block the sodium pump and protein kinase C. Methoxamine-induced phospholipase C and Na+/K+ ATPase activities were measured. 3. Methoxamine (300 microM) increased contractile force by 1.6 +/- 0.2 mN in control atria but decreased contractile force in electrolysis-treated atria by 2.0 +/- 0.1 mN (P < 0.05), as determined 10 min after methoxamine addition. In contrast, the positive inotropic effects of endothelin-1 (30 nM) and isoprenaline (10 microM) were reduced from 2.6 +/- 0.3 to 1.3 +/- 0.1 mN and from 2.6 +/- 0.3 to 1.7 +/- 0.2 mN, respectively, by electrolysis treatment (P < 0.05), but not converted into a negative inotropic action. 4. In an inositol phosphate assay we observed that the stimulation of phospholipase C by methoxamine was attenuated by electrolysis when the (electrolyzed) medium from the organ bath was used, but the phospholipase C responses were restored by the use of fresh medium. However, fresh medium did not counteract the negative inotropic effect of methoxamine. Accordingly, the negative inotropic effect of methoxamine is not directly related to the impaired phospholipase C responses seen in atria subjected to electrolysis. 5. Ouabain (10 microM) and the protein kinase C inhibitor calphostin C (50 nM), completely prevented the negative inotropic effect of 300 microM methoxamine in electrolysis-treated atria. 6. Measurement of the Na+/K+ ATPase activity, revealed that in control atria, alpha1-adrenoceptor stimulation with 300 microM methoxamine, decreased the Na+/K+ ATPase activity by 14.4 +/- 7.7%. In contrast, methoxamine increased the Na+/K+ ATPase activity by 48.8 +/- 8.9% (P < 0.05) in electrolysis-treated atria. Interestingly, this increase in Na+/K+ ATPase activity was completely counteracted by calphostin C (1.4 +/- 0.1% over basal). 7. These results indicate that the negative inotropic effects of alpha1-adrenoceptor agonists, observed in rat isolated left atria exposed to free radicals, are likely to be caused by protein kinase C-mediated phosphorylation and subsequent activation of the Na+/K+ ATPase.