Differential coupling of alpha1-adrenoreceptor subtypes to phospholipase C and mitogen activated protein kinase in neonatal rat cardiac myocytes

Role of alpha- and beta-adrenergic receptors in cardiomyocyte differentiation from murine-induced pluripotent stem cells

OBJECTIVES

Induced pluripotent stem cell (iPSC)-derived cardiomyocytes are a promising source of cells for regenerative heart disease therapies, but progress towards their use has been limited by their low differentiation efficiency and high cellular heterogeneity. Previous studies have demonstrated expression of adrenergic receptors (ARs) in stem cells after differentiation; however, roles of ARs in fate specification of stem cells, particularly in cardiomyocyte differentiation and development, have not been characterized.

MATERIALS AND METHODS

Murine-induced pluripotent stem cells (miPSCs) were cultured in hanging drops to form embryoid bodies, cells of which were then differentiated into cardiomyocytes. To determine whether ARs regulated miPSC differentiation into cardiac lineages, effects of the AR agonist, epinephrine (EPI), on miPSC differentiation and underlying signalling mechanisms, were evaluated.

RESULTS

Treatment with EPI, robustly enhanced miPSC cardiac differentiation, as indicated by increased expression levels of cardiac-specific markers, GATA4, Nkx2.5 and Tnnt2. Although β-AR signalling is the foremost signalling pathway in cardiomyocytes, EPI-enhanced cardiac differentiation depended more on α-AR signalling than β-AR signalling. In addition, selective activation of α₁ -AR signalling with specific agonists induced vigorous cardiomyocyte differentiation, whereas selective activation of α₂ - or β-AR signalling induced no or less differentiation, respectively. EPI- and α₁ -AR-dependent cardiomyocyte differentiation from miPSCs occurred through specific promotion of CPC proliferation via the MEK-ERK1/2 pathway and regulation of miPS cell-cycle progression.

CONCLUSIONS

These results demonstrate that activation of ARs, particularly of α₁ -ARs, promoted miPSC differentiation into cardiac lineages via MEK-ERK1/2 signalling.

Cardiac and neuroprotection regulated by α(1)-adrenergic receptor subtypes

Sympathetic nervous system regulation by the α(1)-adrenergic receptor (AR) subtypes (α(1A), α(1B), α(1D)) is complex, whereby chronic activity can be either detrimental or protective for both heart and brain function. This review will summarize the evidence that this dual regulation can be mediated through the different α(1)-AR subtypes in the context of cardiac hypertrophy, heart failure, apoptosis, ischemic preconditioning, neurogenesis, locomotion, neurodegeneration, cognition, neuroplasticity, depression, anxiety, epilepsy, and mental illness.

alpha(1)-Adrenergic receptor subtype function in fetal and adult cerebral arteries

In the developing fetus, cerebral artery (CA) contractility demonstrates significant functional differences from that of the adult. This may be a consequence of differential activities of alpha(1)-adrenergic receptor (alpha(1)-AR) subtypes. Thus we tested the hypothesis that maturational differences in adrenergic-mediated CA contractility are, in part, a consequence of differential expression and/or activities of alpha(1)-AR subtypes. In CA from fetal ( approximately 140 days) and nonpregnant adult sheep, we used wire myography and imaging, with simultaneous measurement of tension and intracellular Ca(2+) concentration ([Ca(2+)](i)), radioimmunoassay, and Western immunoblots to examine phenylephrine (Phe)-induced contractile responses. The alpha(1A)-AR antagonists (5-MU and WB-4101) completely inhibited Phe-induced contraction in adult but not fetal CA; however, [Ca(2+)](i) increase was reduced significantly in both age groups. The alpha(1D)-AR antagonist (BMY-7378) blocked both Phe-induced contractions and Ca(2+) responses to a significantly greater extent in adult compared with fetal CA. In both age groups, inhibition of alpha(1A)-AR and alpha(1B)-AR, but not alpha(1D)-AR, significantly reduced inositol 1,4,5-trisphosphate responses to Phe. Western immunoblots demonstrated that the alpha(1)-AR subtype expression was only approximately 20% in fetal CA compared with the adult. Moreover, in fetal CA, the alpha(1D)-AR was expressed significantly greater than the other two subtypes. Also, in fetal but not adult CA, Phe induced a significant increase in activated ERK1/2; this increase in phosphorylated ERK was blocked by alpha(1B)-AR (CEC) and alpha(1D)-AR (BMY-7378) inhibitors, but not by alpha(1A)-AR inhibitors (5-MU or WB-4101). In conclusion, in the fetal CA, alpha(1B)-AR and alpha(1D)-AR subtypes play a key role in contractile response as well as in ERK activation. We speculate that in fetal CA alpha(1B)-AR and alpha(1D)-AR subtypes may be a critical factor associated with cerebrovascular growth and function.

Interaction of α 1 -Adrenoceptor Subtypes With Different G Proteins Induces Opposite Effects on Cardiac L-type Ca 2+ Channel

We examined the effect of α 1 -adrenoceptor subtype-specific stimulation on L-type Ca 2+ current ( I Ca ) and elucidated the subtype-specific intracellular mechanisms for the regulation of L-type Ca 2+ channels in isolated rat ventricular myocytes. We confirmed the protein expression of α 1A - and α 1B -adrenoceptor subtypes at the transverse tubules (T-tubules) and found that simultaneous stimulation of these 2 receptor subtypes by nonsubtype selective agonist, phenylephrine, showed 2 opposite effects on I Ca (transient decrease followed by sustained increase). However, selective α 1A -adrenoceptor stimulation (≥0.1 μmol/L A61603) only potentiated I Ca , and selective α 1B -adrenoceptor stimulation (10 μmol/L phenylephrine with 2 μ mol/L WB4101) only decreased I Ca . The positive effect by α 1A -adrenoceptor stimulation was blocked by the inhibition of phospholipase C (PLC), protein kinase C (PKC), or Ca 2+ /calmodulin-dependent protein kinase II (CaMKII). The negative effect by α 1B -adrenoceptor stimulation disappeared after the treatment of pertussis toxin or by the prepulse depolarization, but was not attriburable to the inhibition of cAMP-dependent pathway. The translocation of PKCδ and ε to the T-tubules was observed only after α 1A -adrenoceptor stimulation, but not after α 1B -adrenoceptor stimulation. Immunoprecipitaion analysis revealed that α 1A -adrenoceptor was associated with G q/11 , but α 1B -adrenoceptor interacted with one of the pertussis toxin-sensitive G proteins, G o . These findings demonstrated that the interactions of α 1 -adrenoceptor subtypes with different G proteins elicit the formation of separate signaling cascades, which produce the opposite effects on I Ca . The coupling of α 1A -adrenoceptor with G q/11 -PLC-PKC-CaMKII pathway potentiates I Ca . In contrast, α 1B -adrenoceptor interacts with G o , of which the βγ-complex might directly inhibit the channel activity at T-tubules.

Characterization of the alpha1-adrenoceptor subtype activating extracellular signal-regulated kinase in submandibular gland acinar cells

Alpha(1)-Adrenoceptors and extracellular signal-regulated kinases 1 and 2 (ERK1/2) regulate salivary secretion. However, whether alpha(1)-adrenoceptors couple to ERK1/2 activation and the specific alpha(1)-adrenoceptor subtypes involved in salivary glands is unknown. Western blotting of ERK1/2 phosphorylation showed phenylephrine activated ERK1/2 by 2-3-fold in submandibular gland slices and 3-4-fold in submandibular acinar (SMG-C10) cells with an EC(50) of 2.7+/-2 microM. ERK1/2 activation was blocked by either prazosin or HEAT, indicating alpha(1)-adrenoceptors stimulate ERK1/2 in native glands and SMG-C10 cells. Inhibition of [(125)I]HEAT binding by 5-methylurapidil (selective for alpha(1A) over alpha(1B/)alpha(1D)), but not BMY 7378 (selective for alpha(1D) over alpha(1A/)alpha(1B)), was biphasic and best-fit by a two-site binding model with K(i)(H) and K(i)(L) values for 5-methylurapidil of 0.64+/-0.3 and 91+/-7 nM, respectively, in SMG-C10 membranes. From these binding data, we obtained subtype-selective concentrations of 5-methylurapidil to determine the alpha(1)-adrenoceptor subtype/s activating ERK1/2 in SMG-C10 cells. 5-methylurapidil (20 nM) did not affect phenylephrine- or A-61603- (alpha(1A)-selective agonist) induced ERK1/2 activation; whereas, 30 microM chloroethylclonidine (alpha(1B)-selective antagonist) inhibited ERK1/2 activation by phenylephrine, indicating alpha(1B)-adrenoceptors, but not alpha(1A)-adrenoceptors, activate ERK1/2 in submandibular cells. We also examined alpha(1)-adrenoceptor location and dependence on cholesterol-rich microdomains for activating ERK1/2. Sucrose density gradient centrifugation showed 71+/-3% of alpha(1)-adrenoceptor binding sites were in plasma membranes. Cholesterol-disrupting agents filipin and methyl-beta-cyclodextrin inhibited phenylephrine-stimulated ERK1/2. These results show only alpha(1B)-adrenoceptors activate ERK1/2 and suggest subtype-specific ERK1/2 signaling by alpha(1B)-adrenoceptors may be determined by localization to cholesterol-rich microdomains in submandibular cells.

Cardiac-specific transgenic overexpression of alpha1B-adrenergic receptors induce chronic activation of ERK MAPK signalling

Cardiomyocyte-specific overexpression of the wild-type alpha(1B)-adrenergic receptor (alpha(1B)-AR) produces a slowly progressing cardiomyopathy associated with clinical signs of heart failure and premature death around middle age (Lemire et al. 2001). In the heart, alpha(1)-AR activate the extracellular signal-regulated kinase (ERK) MAPK cascade. The aim of this project was to determine if cardiac-specific overexpression of the wild-type alpha(1B)-AR results in sustained activation of the ERK pathway. At 3 and 9 months, ERK activity was increased in alpha(1B)-AR overexpressing hearts relative to non-transgenic animals. Similarly, phosphorylation of MEK and p90(rsk) were also elevated. MAP kinase phosphatases (MKPs), which inactivate MAP kinases, are transcriptionally regulated. MKP2 mRNA levels were reduced at 3 months in alpha(1B)-AR overexpressing hearts. Interestingly, there was a general trend for reduced expression of MKP-1, -2, and -3 with increased age. In addition, expression of the modulatory calcineurin-interacting protein (MCIP) 1, an indicator of calcineurin activity, was elevated 3-fold in alpha(1B)-AR overexpressing hearts at both 3 and 9 months. These results indicate that the overexpression of the wild-type alpha(1B)-AR leads to chronic changes in the activation of signalling pathways previously shown to be associated with the hypertrophic response.

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.

Alpha-adrenergic receptor stimulation produces late preconditioning through inducible nitric oxide synthase in mouse heart

Inducible nitric oxide synthase (iNOS) mediates late preconditioning (PC) induced by ischemia and pharmacological agents. Since alpha -adrenoceptor (alpha -AR) stimulation is one of the key triggers of PC, we hypothesized that activation of this receptor may induce delayed cardioprotective effect via iNOS-sensitive mechanisms. Adult male ICR mice were treated i.p. with either vehicle/inhibitors or phenylephrine (10 mg/kg) and subjected to 30 min of global ischemia and 30 min reperfusion in Langendorff mode 24 h later. 5-Methyl-urapidil (3 mg/kg) and chloroethylclonidine (3 mg/kg) were injected 15 min prior to phenylephrine to block alpha -AR(1A) and alpha -AR(1B) receptors respectively. S-Methylisothiourea (3 mg/kg), an iNOS inhibitor, was given 60 min prior to ischemia in phenylephrine-pretreated mice. Preischemic NO(x) was measured using a chemoluminescence reaction. Phenylephrine treatment reduced infarct size from 31.10 +/- 0.79% (vehicle) to 14.24 +/- 0.84% (P<0.001). Chloroethylclonidine blocked the effect of phenylephrine (infarct size 31.31 +/- 1.69%) but 5-methyl-urapidil (17.72 +/- 1.25%) did not. Phenylephrine-induced delayed cardioprotection was abolished by S-methylisothiourea and absent in iNOS knockout mice. Baseline NO(x) content was significantly increased in phenylephrine and 5-methyl-urapidil+phenylephrine treated hearts, but remained at baseline levels in hearts treated with chloroethylclonidine, 5-methyl-urapidil or S-methylisothiourea. Western blot analysis revealed a 1.8-fold increase in iNOS with phenylephrine, which was inhibited by chloroethylclonidine but not by 5-methyl-urapidil. We conclude that phenylephrine-induced delayed PC is mediated by selective activation of alpha-AR(1B). Enhanced iNOS expression concomitant with increased NO synthesis, as well as pharmacological blockade and absence of cardioprotection in iNOS knockout mice suggests an essential role of NO in phenylephrine triggered late PC.

Adrenergic regulation of cardiac myocyte apoptosis

The direct effects of catecholamines on cardiac myocytes may contribute to both normal physiologic adaptation and pathologic remodeling, and may be associated with cellular hypertrophy, apoptosis, and alterations in contractile function. Norepinephrine (NE) signals via alpha- and beta-adrenergic receptors (AR) that are coupled to G-proteins. Pharmacologic studies of cardiac myocytes in vitro demonstrate that stimulation of beta1-AR induces apoptosis which is cAMP-dependent and involves the voltage-dependent calcium influx channel. In contrast, stimulation of beta2-AR exerts an anti-apoptotic effect which appears to be mediated by a pertussis toxin-sensitive G protein. Stimulation of alpha1-AR causes myocyte hypertrophy and may exert an anti-apoptotic action. In transgenic mice, myocardial overexpression of either beta1-AR or G(alpha)s is associated with myocyte apoptosis and the development of dilated cardiomyopathy. Myocardial overexpression of beta2-AR at low levels results in improved cardiac function, whereas expression at high levels leads to dilated cardiomyopathy. Overexpression of wildtype alpha1B-AR does not result in apoptosis, whereas overexpression of G(alpha)q results in myocyte hypertrophy and/or apoptosis depending on the level of expression. Differential activation of the members of the mitogen-activated protein kinase (MAPK) superfamily and production of reactive oxygen species appear to play a key role in mediating the actions of adrenergic pathways on myocyte apoptosis and hypertrophy. This review summarizes current knowledge about the molecular and cellular mechanisms involved in the regulation of cardiac myocyte apoptosis via stimulation of adrenergic receptors and their coupled effector pathways.

Catecholamines are not linked to myometrial phospholipase C and uterine contraction in late pregnant and parturient mouse

1. We investigated whether catecholamines through activation of alpha(1)-adrenergic receptors (alpha(1)-AR) are involved in mouse uterine contraction at parturition. Myometrial phospholipase C (PLC) activity and uterine contraction were measured in response to noradrenaline (NA), the specific alpha(1)-AR agonist phenylephrine (Phe) and oxytocin (OT). 2. Using the reverse transcription-polymerase chain reaction RT-PCR, we detected the alpha(1a)-AR subtype in late pregnant mouse myometrium. We also detected, by immunoblotting studies, PLCbeta(1), PLCbeta(3) and different alpha-subunits of pertussis toxin-insensitive (Galpha(q/11)) and -sensitive G proteins (Galpha(o/i3), Galpha(i1/2)). 3. Phenylephrine and NA did not alter the myometrial inositol phosphate (InsP) production of late pregnant or parturient mouse. In similar conditions, OT increased InsP production in a dose-dependent manner. Consistent with these results, only OT (10 microM) recruited PLCbeta(1) and PLCbeta(3) to myometrial plasma membranes. The OT-induced InsP response was not altered by pertussis toxin (300 ng ml(-1), 2 h pretreatment), suggesting the involvement of a member of the Galpha(q) family. 4. Noradrenaline and Phe failed to increase uterine contraction at late pregnancy and at parturition. In contrast, OT induced uterine contraction in a dose-dependent manner with maximal increase (400 %) at a concentration of 1 microM. 5. The results indicate that OT receptors (OTR) but not alpha(1)-AR are linked to myometrial PLC activation and uterine contraction in late pregnant and parturient mouse. This discrepancy between mouse and other mammals could be attributed to the alpha(1)-AR subtype expressed in myometrium at this time.

Cardiac-directed overexpression of wild-type alpha1B-adrenergic receptor induces dilated cardiomyopathy

Using transgenesis as a paradigm, we show here that alpha1-adrenergic receptors (alpha1AR) play an important role in cardiac homeostasis. Cardiomyocyte-specific overexpression of the alpha(1B)AR subtype resulted in the development of dilated cardiomyopathy and death at ~9 mo of age with typical signs of heart failure. Histological analyses showed the enlargement of all four cardiac chambers and cardiomyocyte disarray in the failing hearts. Transgenic animals showed increased left ventricular areas, as assessed by echocardiography. In addition, a progressive decrease in left ventricular systolic function was revealed. The abundance and activity of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2) were reduced, and the ratio of phospholamban to SERCA2 was increased. alpha-Myosin heavy chain (MHC) mRNA was less abundant in older transgenic ventricles, whereas beta-MHC was induced in the failing hearts. Titin mRNA abundance was decreased at 9 mo, whereas atrial natriuretic factor mRNA was elevated at all times. This model mimics structural and functional features of idiopathic dilated cardiomyopathy. The results of this study suggest that chronic alpha1AR activity is deleterious for cardiac function.

alpha(1D)-Adrenoceptors do not contribute to phosphoinositide hydrolysis in adult rat cardiac myocytes

We have used the alpha(1D)-adrenoceptor selective antagonist, BMY 7378, to investigate the presence of alpha(1D)-adrenoceptor subtype in adult rat heart by radioligand binding assays. We also determined the role of this subtype in stimulating phosphoinositide (PI) hydrolysis in adult rat cardiac myocytes. BMY 7378 inhibited [(3)H]prazosin binding to cardiac membranes in a biphasic mode with a pK(i) of 9.19 +/- 0.26 for high affinity sites and 6.64 +/- 0.09 for low affinity sites. The inhibition of the adrenaline-induced stimulation of PI hydrolysis by BMY 7378 fitted a one-site model and the calculated pK(b) value (6.92 +/- 0.28) was consistent with the involvement of alpha(1A) and alpha(1B) adrenoceptors. In addition, BMY 7378, at concentrations up to 100 nM, did not significantly affect the concentration-response curves for the adrenaline-induced stimulation of PI hydrolysis. Taken together, these data suggest that alpha(1D)-adrenoceptors are expressed in adult rat heart but this subtype is not involved in the adrenaline-induced stimulation of PI hydrolysis.

alpha(1)-Adrenergic receptors activate NHE1 and NHE3 through distinct signaling pathways in epithelial cells

The Na+/H+ exchanger (NHE) regulates intracellular pH, cell volume, Na+ absorption and H+ secretion in epithelial cells of the renal proximal tubule (PT). alpha(1)-Adrenergic receptors (ARs) increase NHE activity in PT cells. The purpose of this study was to determine the mechanism of alpha(1)-AR activation of NHE isoforms expressed in PT cells. Northern and Western blotting demonstrate transcripts and protein expression of NHE1 and NHE3 in PT cells. An anti-NHE1 antibody predominately labels protein expressed at basal and lateral membranes. In contrast, NHE3 protein is expressed exclusively at the apical membrane. To determine NHE isoforms regulated by alpha(1)-ARs, antisense oligodeoxynucleotides (AS-ODNs) specific for NHE1 and NHE3 isoforms were introduced into cells with streptolysin O permeabilization. Cells incubated with AS-ODNs a total of three times exhibited a reduction in protein expression of ~85%. Na uptake and changes in intracellular pH (pH(i)) were used as measures of NHE activity in PT cells. alpha(1)-AR stimulation increased Na uptake from 8.5 to 13.8 nmol. min(-1). mg protein(-1). AS-ODNs to NHE3 significantly reduced alpha(1)-AR stimulated Na uptake and increases in pH(i); no effect was observed in sense-ODN-treated cells. Inhibition of NHE1 but not NHE3 expression abolishes amiloride-suppressible NHE activity. alpha(1)-AR stimulation of NHE1 is inhibited by the protein kinase C (PKC) inhibitor calphostin C whereas NHE3 activity is abolished by the mitogen-activated protein kinase (MAPK) inhibitor PD-98059. In PT cells transfected with MAPK kinase MEKK1(COOH), a truncated version of MEKK1 that activates MAPK, NHE3 but not NHE1 activity is stimulated. We conclude that alpha(1)-ARs activate distinct signaling pathways to regulate specific NHE isoforms localized on opposite membranes in polarized renal epithelial cells. alpha(1)-AR activation of NHE1 is regulated by PKC whereas NHE3 is controlled by MAPK and serves to separately regulate pH(i), Na absorption, and proton excretion in PT cells.

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.

Adverse effects of constitutively active alpha(1B)-adrenergic receptors after pressure overload in mouse hearts

Cardiac hypertrophy and function were studied 6 wk after constriction of the thoracic aorta (TAC) in transgenic (TG) mice expressing constitutively active mutant alpha(1B)-adrenergic receptors (ARs) in the heart. Hearts from sham-operated TG animals and nontransgenic littermates (WT) were similar in size, but hearts from TAC/TG mice were larger than those from TAC/WT mice, and atrial natriuretic peptide mRNA expression was also higher. Lung weight was markedly increased in TAC/TG animals, and the incidence of left atrial thrombus formation was significantly higher. Ventricular contractility in anesthetized animals, although it was increased in TAC/WT hearts, was unchanged in TAC/TG hearts, implying cardiac decompensation and progression to failure in TG mice. There was no increase in alpha(1A)-AR mRNA expression in TAC/WT hearts, and expression was significantly reduced in TAC/TG hearts. These findings show that cardiac expression of constitutively actively mutant alpha(1B)-ARs is detrimental in terms of hypertrophy and cardiac function after pressure overload and that increased alpha(1A)-AR mRNA expression is not a feature of the hypertrophic response in this murine model.

The alpha(1)-adrenoceptor subtype- and protein kinase C isoform-dependence of Norepinephrine's actions in cardiomyocytes

Catecholamines modulate cardiac function at least in part through alpha(1)-adrenergic receptors linked to the activation of protein kinase C (PKC). This study examines the molecular forms of the alpha(1)-receptor and PKC that mediate norepinephrine's actions in cardiomyocytes; distinct approaches (activation-dependent down-regulation of PKC isoforms) and novel reagents (A61603, an alpha(1A/c)-receptor agonist) are used to resolve this issue which has been the focus of dispute in previous studies. Norepinephrine (NE) induces a rise in diacylglycerol levels which is sustained for 24 h and is associated with the translocation (at 5 min) and down-regulation (at 24 h) of PKC delta and PKC xi (but not PKC alpha). The selective targeting of the alpha(1)-adrenergic receptor to activate novel PKC isoforms is remarkable, given an 8-fold greater abundance of PKC alpha relative to PKC xi in this preparation. NE activates the extracellular signal-regulated protein kinase (ERK) subfamily of mitogen-activated protein kinases through a PKC delta/PKC xi -dependent pathway. WB-4101 (alpha(1A/c)- and alpha(1D)-receptor antagonist) and 5-methylurapidil (alpha(1A/c)-receptor antagonist) inhibit norepinephrine-dependent accumulation of inositol phosphate and diacylglycerol, down-regulation of PKC delta and PKC xi, and activation of ERK. Each of these responses is stimulated by A61603, but not attenuated by high concentrations of chloroethylclonidine (which irreversibly inactivates the alpha(1B)-, and to a lesser extent, the alpha(1D)-receptor) or BMY 7378 (selective alpha(1D)-receptor antagonist). A61603 also activates p38-MAPK and induces hypertrophy. These studies establish that NE's actions in cardiomyocytes can be attributed to the alpha(1A/c)-adrenergic receptor subtype and nPKC isoforms, thereby identifying specific targets for the development of pharmaceuticals to influence cardiac contractile function and/or growth responses.

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.

Constitutively active mutants of the alpha(1a)- and the alpha(1b)-adrenergic receptor subtypes reveal coupling to different signaling pathways and physiological responses in rat cardiac myocytes

Activation of alpha(1)-adrenergic receptors influences both the contractile activity and the growth potential of cardiac myocytes. However, the signaling pathways linking activation of specific alpha(1)-adrenergic receptor (AR) subtypes to these physiological responses remain controversial. In the present study, a molecular approach was used to identify conclusively the signaling pathways activated in response to the individual alpha(1A)- and alpha(1B)-AR subtypes in cardiac myocytes. For this purpose, a mutant alpha(1a)-AR subtype (alpha(1a)-S(290/293)-AR) was constructed based on analogy to the previously described constitutively active mutant alpha(1b)-AR subtype (alpha(1b)-S(288-294)-AR). The mutant alpha(1a)-S(290/293)-AR subtype displayed constitutive activity based on four criteria. To introduce the constitutively active alpha(1)-AR subtypes into cardiac myocytes, recombinant Sindbis viruses encoding either the alpha(1a)-S(290/293)-AR or alpha(1b)-S(288-294)-AR subtype were used to infect the whole cell population with >90% efficiency, thereby allowing the biochemical activities of the various signaling pathways to be measured. When expressed at comparable levels, the alpha(1a)-S(290/293)-AR subtype exhibited a significantly elevated basal level as well as agonist-stimulated level of inositol phosphate accumulation, coincident with activation of atrial natriuretic factor-luciferase gene expression. By contrast, the alpha(1b)-S(288-294)-AR subtype displayed a markedly increased serum response element-luciferase gene expression but no activation of atrial natriuretic factor-luciferase gene expression. Taken together, this study provides the first molecular evidence for coupling of the alpha(1a)-AR and the alpha(1b)-AR subtypes to different signaling pathways in cardiac myocytes.

The importance of being receptive

Neuroendocrine system and immune system can communicate via the use of soluble mediators like hormones, neurotransmitters and cytokines. The level of mediators secreted by either of these systems creates the milieu in which immune and neuroendocrine responses take place. For adequate communication between the systems, receptors for hormones, neurotransmitters and cytokines are required. This review describes the role of regulated expression and function of receptors for hormones and neurotransmitters within the immune system in neuroendocrine-immune communication.

Inverse alpha(1A) and alpha(1D) adrenoceptor mRNA expression during isolation of hepatocytes

It is now well documented that changes in gene expression take place during cell isolation and culture. Here, we report the change in the expression of the mRNAs for alpha(1)-adrenoceptor subtypes, during dissociation of guinea pig liver cells with collagenase. Using Reverse Transcription-Polymerase Chain Reaction (RT-PCR) assays, it was observed that during the isolation procedure, the mRNA for the alpha(1A)-adrenoceptor, normally expressed in whole liver, was degraded and the mRNA for alpha(1D) subtype, barely expressed in whole liver, increased in an actinomycin D-sensitive manner. When the isolation procedure was performed in the presence of cycloheximide, the mRNA for the alpha(1A)-adrenoceptor did not diminish and the induction of the alpha(1D)-adrenoceptor mRNA was even more evident. Our data indicate that cell isolation alters alpha(1)-adrenoceptor mRNA expression.

Alpha1-adrenoceptor subtypes

Alpha1-adrenoceptors are one of three subfamilies of receptors (alpha1, alpha2, beta) mediating responses to adrenaline and noradrenaline. Three alpha1-adrenoceptor subtypes are known (alpha1A, alpha1B, alpha1D) which are all members of the G protein coupled receptor family, and splice variants have been reported in the C-terminus of the alpha1A. They are expressed in many tissues, particularly smooth muscle where they mediate contraction. Certain subtype-selective agonists and antagonists are now available, and alpha1A-adrenoceptor selective antagonists are used to treat benign prostatic hypertrophy. All subtypes activate phospholipase C through the G(q/11) family of G proteins, release stored Ca2+, and activate protein kinase C, although with significant differences in coupling efficiency (alpha1A > alpha1B > alpha1D). Other second messenger pathways are also activated by these receptors, including Ca2+ influx, arachidonic acid release, and phospholipase D. Alpha1-adrenoceptors also activate mitogen-activated protein kinase pathways in many cells, and some of these responses are independent of Ca2+ and protein kinase C but involve small G proteins and tyrosine kinases. Direct interactions of alpha1-adrenoceptors with proteins other than G proteins have not yet been reported, however there is a consensus binding motif for the immediate early gene Homer in the C-terminal tail of the alpha1D subtype. Current research is focused on discovering new subtype-selective drugs, identifying non-traditional signaling pathways activated by these receptors, clarifying how multiple signals are integrated, and identifying proteins interacting directly with the receptors to influence their functions.

Alpha1- and alpha2-adrenoceptor control of sodium transport reverses in developing hypertension

Alpha-Adrenergic receptor (AR) activation enhances sodium retention in certain forms of hypertension. The objective of the present study was to understand the role of alpha-ARs in regulating sodium transport by distal tubules (DT). DT cells were isolated from kidneys of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at 6 weeks, when hypertension is developing, or at 12 weeks, when hypertension is established. The alpha1-AR agonist phenylephrine increased 22Na uptake by 50% into DT cells of 6-week SHR; no effect was observed with WKY cells. The alpha2-AR agonist B-HT 933 increased uptake by only 10%. At 12 weeks, the pattern of alpha-AR regulation was reversed: alpha1-AR-induced sodium uptake was only 15%, whereas alpha2-AR activation increased sodium uptake by 35% in SHR and WKY cells. alpha1-AR-induced sodium uptake in 6-week SHR cells was abolished by prazosin; alpha2-AR-stimulated sodium uptake was blocked by yohimbine in 12-week SHR and WKY. Competitive binding studies were performed with [3H]prazosin and alpha1A-, alpha1B-, and alpha1D-selective antagonists with DT cell membranes from 6- and 12-week SHR and WKY. alpha2-AR subtypes were determined with [3H]rauwolscine and alpha2A- and alpha2B-selective antagonists. Expression of alpha1B-ARs was increased 4-fold in DT cells during the developing phase of hypertension in SHR. No change was detected in alpha2-AR expression. DT cells transiently increase [Ca2+]i in response to alpha1-AR agonists from 6-week but not 12-week SHR. Conversely, alpha2-AR agonists increase [Ca2+]i at 12 weeks. In summary, during developing hypertension, alpha1-ARs increase sodium uptake and [Ca2+]i in SHR cells. Expression of alpha1B-ARs is selectively upregulated during developing hypertension. In established hypertension (and normotension), alpha2-ARs regulate sodium transport and [Ca2+]i in DT cells. We conclude that a molecular switch of alpha1-AR and alpha2-AR signaling occurs in DT cells during the development of hypertension.

Reduced reperfusion-induced Ins(1,4,5)P3 generation and arrhythmias in hearts expressing constitutively active alpha1B-adrenergic receptors

Reperfusion of globally ischemic rat hearts causes the generation of inositol(1,4,5)trisphosphate [Ins(1,4,5)P3] and the initiation of arrhythmias. These responses are mediated by alpha1-adrenergic receptors (ARs), but the subtype of receptor involved has not been identified. Under normoxic conditions, hearts from transgenic animals expressing constitutively active alpha1B-ARs in heart (alpha1B-constitutively active mutant [CAM]) showed higher [3H] inositol phosphate responses to norepinephrine (2.3-fold) than hearts from nontransgenic animals (alpha1B-WT) (1.6-fold). alpha1B-WT hearts responded to 2 minutes of reperfusion after 20 minutes of global ischemia by generation of Ins(1,4,5)P3 (5301+/-1310 to 11 413+/-1597 CPM/g tissue; mean+/-SEM; n=6; P<0.01 in [3H] labeling studies and 3.8+/-0.2 to 6.3+/-0.6 nmol/g by mass analysis, n=6; P<0.05). In contrast to findings in normoxia, hearts from alpha1B-CAM animals showed no Ins(1,4,5)P3 response in early reperfusion. In parallel studies, alpha1B-WT hearts developed ventricular tachycardia and ventricular premature beats (VPB) during 5 minutes of reperfusion after 20 minutes of ischemia. The incidence of these arrhythmias was reduced in the alpha1B-CAM hearts (95% to 62% for VPB and 47% to 12% for ventricular tachycardia; both P<0.05). The resistance of the alpha1B-CAM hearts was not due to alpha1B-AR-mediated preconditioning, as the Ins(1,4,5)P3 response to thrombin receptor activation during reperfusion was not different between the 2 groups. To investigate the possibility of reduced alpha1A-receptor activity in the alpha1B-CAM hearts, expression of the mRNA for alpha1A- and alpha1B-receptors was measured. alpha1B-WT hearts contained mRNA for both receptor subtypes, but the levels of alpha1B-receptor mRNA were 5-fold higher than alpha1A-receptor mRNA. alpha1B-CAM hearts contained very high levels of alpha1B-receptor mRNA (26-fold increase), but the expression of mRNA for the alpha1A-receptors (0.141+/-0.035 amol/ microg RNA; mean+/-SEM; n=6) was reduced by 50% relative to alpha1B-WT controls (0.276+/-0.046 amol/ microg RNA; n=6; P<0.01). The reduction in arrhythmogenic and Ins(1,4,5)P3 responses in alpha1B-CAM hearts provides evidence that these response are not mediated by alpha1B-receptors.