Constitutive inhibitory G protein activity upon adenylyl cyclase-dependent cardiac contractility is limited to adenylyl cyclase type 6

PURPOSE

We previously reported that inhibitory G protein (Gi) exerts intrinsic receptor-independent inhibitory activity upon adenylyl cyclase (AC) that regulates contractile force in rat ventricle. The two major subtypes of AC in the heart are AC5 and AC6. The aim of this study was to determine if this intrinsic Gi inhibition regulating contractile force is AC subtype selective.

METHODS

Wild-type (WT), AC5 knockout (AC5KO) and AC6 knockout (AC6KO) mice were injected with pertussis toxin (PTX) to inactivate Gi or saline (control).Three days after injection, we evaluated the effect of simultaneous inhibition of phosphodiesterases (PDE) 3 and 4 with cilostamide and rolipram respectively upon in vivo and ex vivo left ventricular (LV) contractile function. Also, changes in the level of cAMP were measured in left ventricular homogenates and at the membrane surface in cardiomyocytes obtained from the same mouse strains expressing the cAMP sensor pmEPAC1 using fluorescence resonance energy transfer (FRET).

RESULTS

Simultaneous PDE3 and PDE4 inhibition increased in vivo and ex vivo rate of LV contractility only in PTX-treated WT and AC5KO mice but not in saline-treated controls. Likewise, Simultaneous PDE3 and PDE4 inhibition elevated total cAMP levels in PTX-treated WT and AC5KO mice compared to saline-treated controls. In contrast, simultaneous PDE3 and PDE4 inhibition did not increase in vivo or ex vivo rate of LV contractility or cAMP levels in PTX-treated AC6KO mice compared to saline-treated controls. Using FRET analysis, an increase of cAMP level was detected at the membrane of cardiomyocytes after simultaneous PDE3 and PDE4 inhibition in WT and AC5KO but not AC6KO. These FRET data are consistent with the functional data indicating that AC6 activity and PTX inhibition of Gi is necessary for simultaneous inhibition of PDE3 and PDE4 to elicit an increase in contractility.

CONCLUSIONS

Together, these data suggest that AC6 is tightly regulated by intrinsic receptor-independent Gi activity, thus providing a mechanism for maintaining low basal cAMP levels in the functional compartment that regulates contractility.

Mental Health and Disability Pension Onset: Changes in Consumption of Antianxiety and Hypnotic Drugs

Introduction:

In Norway, disability pension (DP) has been more prevalent over the later years, with mental disorders being a frequent cause. Previous analyses have questioned whether receiving DP is beneficial for mental health by considering changes in antidepressant drug consumption. To explore this further, we examined changes in antianxiety and hypnotic drug consumption following DP onset.

Methods:

Based on national Norwegian register data, this retrospective study encompassed 8617 working-age individuals (25-50 years) who became DP during 2005 to 2013. We compared their benzodiazepines (BZD) and Z-hypnotic consumption 1 year pre- and postdisability pension onset.

Results:

About 80% of the individuals did not change their altogether benzodiazepine/Z-hypnotic consumption. Among individuals with an initial consumption ≤1 defined daily dose (DDD), 18.9% increased their consumption to above 1 DDD. Individuals in the age-group 45 to 50 versus 24 to 34 years had a lower risk of dose escalation (odds ratio [OR], 0.756, 95% confidence interval [CI]: 0.601-0.957). Individuals who used Z-hypnotics only had a higher risk of dose escalation compared to the joint benzodiazepine/Z-hypnotic user group (OR, 1.594, 95%CI: 1.284-1.970).

Conclusion:

In general, we cannot see that DP is associated with changes in benzodiazepine/Z-hypnotic consumption, but younger users and individuals using Z-hypnotics only had a greater risk of dose escalation compared to the older users and users with combined BZD and Z-hypnotic use.

New benzodiazepine and Z-hypnotic users and disability pension: an eight-year nationwide observational follow-up study

OBJECTIVE

To compare how newly initiated treatment with benzodiazepines, Z-hypnotics or both associates with the reception of disability pension among 40,661 individuals of a working age.

DESIGN

Prescription register study.

SETTING

Norwegian nationwide prescriptions socio-economic and disability status data.

METHODS

Cox regression analyses.

SUBJECTS

New benzodiazepine or Z-hypnotic users.

MAIN OUTCOME MEASURE

Time to receive disability pension given benzodiazepine or Z-hypnotic use or both. Additional analyses focused on the benzodiazepine first redeemed.

RESULTS

Among new users 8.65% of Z-hypnotic users, 12.29% of benzodiazepines users and 13.96% of combined Z-hypnotic and benzodiazepine users became disability pensioners. Z-hypnotic users were weaker associated with becoming disability pensioners (HR = 0.78, CI: 0.73-0.84) and combined users were stronger associated (HR = 1.09, CI: 1.01-1.17), than benzodiazepine users. Women had higher risk than men for becoming disability pensioners. Higher age, lower education, previous drug use and psychiatrist as first prescriber were risk factors. Comparing first benzodiazepine redeemed; clonazepam initiators were stronger associated with becoming disability pensioners than diazepam initiators were (HR = 2.22, CI: 1.81-2.71). No differences between other benzodiazepine users were found.

CONCLUSIONS

Adjusting for known risk factors gave lower risk for Z-hypnotic users compared to benzodiazepine users for receiving disability pension. Combined use increased the risk further. Clonazepam initiators are especially at risk. These findings may be helpful in prescribing situations to identify and guide individuals at risk for becoming disability pensioners.

PDE3 inhibition by C-type natriuretic peptide-induced cGMP enhances cAMP-mediated signaling in both non-failing and failing hearts

We have previously shown that the natriuretic peptide receptor B (NPR-B) agonist C-type natriuretic peptide (CNP) enhances cyclic adenosine 3´,5´-monophosphate (cAMP)-mediated signaling in failing hearts, through cyclic guanosine 3´,5´-monophosphate (cGMP)-mediated phosphodiesterase (PDE) 3 inhibition. As several signaling pathways are importantly changed in failing hearts, it could not be taken for granted that this crosstalk would be the same in non-failing hearts. Thus, we wanted to clarify to which extent this effect of CNP occurred also in non-failing hearts. Inotropic and lusitropic responses were measured in muscle strips and cGMP levels, localized cAMP levels, cAMP-PDE activity and mRNA levels were analyzed in isolated cardiomyocytes from left ventricles of non-failing and failing rat hearts. CNP increased cGMP and enhanced β₁- and β₂-adrenoceptor-mediated inotropic and β₁-adrenoceptor-mediated lusitropic responses, in non-failing and failing hearts. The NPR-A agonist brain natriuretic peptide (BNP) increased cGMP, but did not affect inotropic or lusitropic responses, indicating different compartmentation of cGMP from the two natriuretic peptide receptors. cAMP-PDE activity of PDE3 was concentration-dependently inhibited by cGMP with the same potency and to the same extent in non-failing and failing cardiomyocytes. CNP enhanced β₁-adrenoceptor-induced cAMP increase in living cardiomyocytes in the absence, but not in the presence of a PDE3 inhibitor indicating involvement of PDE3. In summary, CNP sensitizes cAMP-mediated signaling in non-failing as in failing hearts, via NPR-B-mediated increase of cGMP that inhibits the cAMP-PDE activity of PDE3.

Inhibition of phosphodiesterase-3 by levosimendan is sufficient to account for its inotropic effect in failing human heart

BACKGROUND AND PURPOSE

Levosimendan is known as a calcium sensitizer, although it is also known to inhibit PDE3. We aimed to isolate each component and estimate their contribution to the increased cardiac contractility induced by levosimendan.

EXPERIMENTAL APPROACH

Contractile force was measured in electrically stimulated ventricular strips from explanted failing human hearts and left ventricular strips from normal male Wistar rats. PDE activity was measured in a two-step PDE activity assay on failing human ventricle.

KEY RESULTS

Levosimendan exerted a positive inotropic effect (PIE) reaching maximum at 10(-5)  M in ventricular strips from failing human hearts. In the presence of the selective PDE3 inhibitor cilostamide, the PIE of levosimendan was abolished. During treatment with a PDE4 inhibitor and a supra-threshold concentration of isoprenaline, levosimendan generated an amplified inotropic response. This effect was reversed by β-adrenoceptor blockade and undetectable in strips pretreated with cilostamide. Levosimendan (10(-6)  M) increased the potency of β-adrenoceptor agonists by 0.5 log units in failing human myocardium, but not in the presence of cilostamide. Every inotropic response to levosimendan was associated with a lusitropic response. Levosimendan did not affect the concentration-response curve to calcium in rat ventricular strips, in contrast to the effects of a known calcium sensitizer, EMD57033 [5-(1-(3,4-dimethoxybenzoyl)-1,2,3,4-tetrahydroquinolin-6-yl)-6-methyl-3,6-dihydro-2H-1,3,4-thiadiazin-2-one]. PDE activity assays confirmed that levosimendan inhibited PDE3 as effectively as cilostamide.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that the PDE3-inhibitory property of levosimendan was enough to account for its inotropic effect, leaving a minor, if any, effect to a calcium-sensitizing component.

The inotropic effect of the active metabolite of levosimendan, OR-1896, is mediated through inhibition of PDE3 in rat ventricular myocardium

Aims

We recently published that the positive inotropic response (PIR) to levosimendan can be fully accounted for by phosphodiesterase (PDE) inhibition in both failing human heart and normal rat heart. To determine if the PIR of the active metabolite OR-1896, an important mediator of the long-term clinical effects of levosimendan, also results from PDE3 inhibition, we compared the effects of OR-1896, a representative Ca²⁺ sensitizer EMD57033 (EMD), levosimendan and other PDE inhibitors.

Methods

Contractile force was measured in rat ventricular strips. PDE assay was conducted on rat ventricular homogenate. cAMP was measured using RII_epac FRET-based sensors.

Results

OR-1896 evoked a maximum PIR of 33±10% above basal at 1 μM. This response was amplified in the presence of the PDE4 inhibitor rolipram (89±14%) and absent in the presence of the PDE3 inhibitors cilostamide (0.5±5.3%) or milrinone (3.2±4.4%). The PIR was accompanied by a lusitropic response, and both were reversed by muscarinic receptor stimulation with carbachol and absent in the presence of β-AR blockade with timolol. OR-1896 inhibited PDE activity and increased cAMP levels at concentrations giving PIRs. OR-1896 did not sensitize the concentration-response relationship to extracellular Ca²⁺. Levosimendan, OR-1896 and EMD all increased the sensitivity to β-AR stimulation. The combination of either EMD and levosimendan or EMD and OR-1896 further sensitized the response, indicating at least two different mechanisms responsible for the sensitization. Only EMD sensitized the α₁-AR response.

Conclusion

The observed PIR to OR-1896 in rat ventricular strips is mediated through PDE3 inhibition, enhancing cAMP-mediated effects. These results further reinforce our previous finding that Ca²⁺ sensitization does not play a significant role in the inotropic (and lusitropic) effect of levosimendan, nor of its main metabolite OR-1896.

Risk factors for excessive benzodiazepine use in a working age population: a nationwide 5-year survey in Norway

OBJECTIVE

To identify risk factors for becoming an excessive user over time.

SETTING

Prescription database study over five years.

SUBJECTS AND METHOD

Norwegians between 30 and 60 years with a first dispensation of a benzodiazepine during 2006, encompassing 23 227 individuals. A Cox hazard regression model was defined, initially stratifying on gender, age, county, previous relevant drug dispensations, household income, education level, and vocational rehabilitation support.

MAIN OUTCOME MEASURE

The time from the first redemption until excessive use was defined as using more than two DDDs per day on average within a three-month period.

RESULTS

Women's risk was lower than men's for excessive use (HR = 0.42, CI 0.35-0.51). Initial oxazepam, alprazolam, or nitrazepam/flunitrazepam use indicated higher risk compared with diazepam (HR = 1.51, CI 1.24-1.85, HR = 2.75, CI 1.54-4.91, HR = 1.67, CI 1.29-2.16). Previous antidepressants or lithium, antipsychotics or opioids, anti-alcohol and smoke cessation treatment indicated a higher risk compared with no such use (HR = 1.4, CI 1.16-1.69, HR = 1.92, CI 1.54-2.4, and HR = 2.88, CI 2-4.15). Higher education and average or high household income were associated with a low risk compared with low education and income (HR = 0.68, CI 0.57-0.81, HR = 0.58, CI 0.46-0.73, and HR = 0.37, CI 0.26-0.54). Working in the private or public sector was associated with a low risk compared with no registered work (HR = 0.53, CI 0.4-0.71 and HR = 0.57, CI 0.45-0.74).

CONCLUSION

The prevalence of excessive use over a five-year observation period was 2.34%. Risk factors were indications of psychiatric illness, first benzodiazepine choice, low income, and education. Excessive users were also characterized by a more severe disease, indicated by having prescription fulfilments by a psychiatrist and by switching benzodiazepines. Key points Guidelines state that benzodiazepines should be used for a short time and excessive use indicates drug dependency. Of all new benzodiazepine users 2.34% became excessive users, defined as consuming above two defined daily doses (DDDs) per day on average over three months, within a five-year period. Previous use of other psychotropic drugs, opioids and anti-alcohol and smoke cessation drugs, first benzodiazepine prescribed, low household income, and low education were risk factors for excessive use. Excessive users were characterized by switching benzodiazepines and having prescription fulfilments by a psychiatrist suggesting a more severe disease.

Non-classical regulation of β1- and β 2-adrenoceptor-mediated inotropic responses in rat heart ventricle by the G protein Gi

Studies suggest that increased activity of Gi contributes to the reduced β-adrenoceptor-mediated inotropic response (βAR-IR) in failing cardiomyocytes and that β2AR-IR but not β1AR-IR is blunted by dual coupling to Gs and Gi. We aimed to clarify the role of Gi upon the β1AR-IR and β2AR-IR in Sham and failing myocardium by directly measuring contractile force and cAMP accumulation. Contractility was measured ex vivo in left ventricular strips and cAMP accumulation in cardiomyocytes from rats with post-infarction heart failure (HF) or sham operates (Sham). The β2AR-IR in Sham and HF was small and was amplified by simultaneously inhibiting phosphodiesterases 3 and 4 (PDE3&4). In HF, the inotropic response and cAMP accumulation evoked by β1AR- or β2AR-stimulation were reduced. Inactivation of Gi with pertussis toxin (PTX) did not restore the β1AR-IR or β2AR-IR in HF to Sham levels but did enhance the maximal β2AR-IR. PTX increased both β1AR- and β2AR-evoked cAMP accumulation more in Sham than that in HF, and HF levels approached those in untreated Sham. The potency of agonists at β1 and at β2ARs (only under PDE3&4 inhibition) was increased in HF and by PTX in both HF and Sham. Without PDE3&4 inhibition, PTX increased only the maximal β2AR-IR, not potency. We conclude that Gi regulates both β1AR- and β2AR-IR independent of receptor coupling with Gi. Gi together with PDE3&4 tonically restrict the β2AR-IR. Gi inhibition did not restore the βAR-IR in HF despite increasing cAMP levels, suggesting that the mechanism of impairment resides downstream to cAMP signalling.

Gi proteins regulate adenylyl cyclase activity independent of receptor activation

Background and purpose

Despite the view that only β₂- as opposed to β₁-adrenoceptors (βARs) couple to G_i, some data indicate that the β₁AR-evoked inotropic response is also influenced by the inhibition of G_i. Therefore, we wanted to determine if G_i exerts tonic receptor-independent inhibition upon basal adenylyl cyclase (AC) activity in cardiomyocytes.

Experimental approach

We used the G_s-selective (R,R)- and the G_s- and G_i-activating (R,S)-fenoterol to selectively activate β₂ARs (β₁AR blockade present) in combination with G_i inactivation with pertussis toxin (PTX). We also determined the effect of PTX upon basal and forskolin-mediated responses. Contractility was measured ex vivo in left ventricular strips and cAMP accumulation was measured in isolated ventricular cardiomyocytes from adult Wistar rats.

Key results

PTX amplified both the (R,R)- and (R,S)-fenoterol-evoked maximal inotropic response and concentration-dependent increases in cAMP accumulation. The EC₅₀ values of fenoterol matched published binding affinities. The PTX enhancement of the G_s-selective (R,R)-fenoterol-mediated responses suggests that G_i regulates AC activity independent of receptor coupling to G_i protein. Consistent with this hypothesis, forskolin-evoked cAMP accumulation was increased and inotropic responses to forskolin were potentiated by PTX treatment. In non-PTX-treated tissue, phosphodiesterase (PDE) 3 and 4 inhibition or removal of either constitutive muscarinic receptor activation of G_i with atropine or removal of constitutive adenosine receptor activation with CGS 15943 had no effect upon contractility. However, in PTX-treated tissue, PDE3 and 4 inhibition alone increased basal levels of cAMP and accordingly evoked a large inotropic response.

Conclusions and implications

Together, these data indicate that G_i exerts intrinsic receptor-independent inhibitory activity upon AC. We propose that PTX treatment shifts the balance of intrinsic G_i and G_s activity upon AC towards G_s, enhancing the effect of all cAMP-mediated inotropic agents.

SERCA2 activity is involved in the CNP-mediated functional responses in failing rat myocardium

BACKGROUND AND PURPOSES

Myocardial C-type natriuretic peptide (CNP) levels are increased in heart failure. CNP can induce negative inotropic (NIR) and positive lusitropic responses (LR) in normal hearts, but its effects in failing hearts are not known. We studied the mechanism of CNP-induced NIR and LR in failing hearts and determined whether sarcoplasmatic reticulum Ca(2+) ATPase2 (SERCA2) activity is essential for these responses.

EXPERIMENTAL APPROACH

Contractility, cGMP levels, Ca(2+) transient amplitudes and protein phosphorylation were measured in left ventricular muscle strips or ventricular cardiomyocytes from failing hearts of Wistar rats 6 weeks after myocardial infarction.

KEY RESULTS

CNP increased cGMP levels, evoked a NIR and LR in muscle strips, and caused phospholamban (PLB) Ser(16) and troponin I (TnI) Ser(23/24) phosphorylation in cardiomyocytes. Both the NIR and LR induced by CNP were reduced in the presence of a PKG blocker/cGMP analogue (Rp-8-Br-Pet-cGMPS) and the SERCA inhibitor thapsigargin. CNP increased the amplitude of the Ca(2+) transient and increased SERCA2 activity in cardiomyocytes. The CNP-elicited NIR and LR were not affected by the L-type Ca(2+) channel activator BAY-K8644, but were abolished in the presence of isoprenaline (induces maximal activation of cAMP pathway). This suggests that phosphorylation of PLB and TnI by CNP causes both a NIR and LR. The NIR to CNP in mouse heart was abolished 8 weeks after cardiomyocyte-specific inactivation of the SERCA2 gene.

CONCLUSIONS AND IMPLICATIONS

We conclude that CNP-induced PLB and TnI phosphorylation by PKG in concert mediate both a predictable LR as well as the less expected NIR in failing hearts.

Differential regulation of C-type natriuretic peptide-induced cGMP and functional responses by PDE2 and PDE3 in failing myocardium

Recently, we showed C-type natriuretic peptide (CNP)-induced negative inotropic (NIR) and positive lusitropic response (LR) in failing rat heart. We wanted to study whether, and if so, how phosphodiesterases (PDEs) regulate CNP-induced cyclic 3',5'-guanosine monophosphate (cGMP) elevation and functional responses. Inotropic and lusitropic responses were measured in left ventricular muscle strips and cyclic nucleotide levels, PDE activity and phospholamban (PLB) and troponin I (TnI) phosphorylation were measured in ventricular cardiomyocytes from Wistar rats with heart failure 6 weeks after myocardial infarction. CNP-mediated increase in global cGMP was mainly regulated by PDE2, as reflected by a marked amplification of the cGMP increase during PDE2 inhibition and by a high PDE2 activity in cardiomyocytes. PDE3 inhibition, on the other hand, caused no significant cGMP increase by CNP. The functional consequences did not correspond to the changes of cGMP. PDE3 inhibition increased the potency of the CNP-induced NIR and LR, while PDE2 inhibition desensitized the CNP-induced NIR, but not LR. A role for PDE2 on the maximal LR and PDE5 on the maximal NIR to CNP was revealed in the presence of PDE3 inhibition. CNP increased PLB phosphorylation about 25- to 30-fold and tended to increase TnI phosphorylation about twofold. As a whole, CNP-induced functional responses were only modestly regulated by PDEs compared to the cAMP-mediated functional responses to β1-adrenoceptor stimulation, which are highly regulated by PDEs. There is a mismatch between the CNP-induced cGMP increase and functional responses. Global cGMP levels are mainly regulated by PDE2 after CNP stimulation, whereas the functional responses are modestly regulated by both PDE2 and PDE3, indicating cGMP compartmentation by PDEs affecting CNP-induced responses in failing hearts.

A 3-year survey quantifying the risk of dose escalation of benzodiazepines and congeners to identify risk factors to aid doctors to more rationale prescribing

OBJECTIVES

This study investigated and quantified risk factors of dose escalation, as an indication of drug misuse and dependency of benzodiazepines and congeners, among presumably drug naïve patients in the Norwegian drug prescription database, observed over 3 years.

DESIGN

Observational study.

SETTING

Prescription database study.

PARTICIPANTS

We defined an excessive user as one redeeming more than two defined daily doses per day in 3 months.

PRIMARY AND SECONDARY OUTCOME MEASURES

We examined the risk of excessive use over time and the effect of risk factors through multistate logistic regression and scenarios.

RESULTS

Most of the 81 945 patients had zopiclone or zolpidem as the initial drug (63.8%), followed by diazepam (25.3%), oxazepam (6.1%), nitrazepam/flunitrazepam (2.9%), hydroxyzine/buspirone (1.6%) and alprazolam (0.3%). At any time 23% redeemed prescriptions, about 34% did not redeem any prescriptions beyond any 3-month period and 0.9% ended up as excessive users. Patients previously using drugs, such as opioids, antialcohol or smoke cessation treatment, had a higher risk to become excessive users compared to patients who had not. Patients whose first prescription was for oxazepam or nitrazepam/flunitrazepam had a higher risk of becoming an excessive user compared to those who started with diazepam. A specialist in general practice as the first-time prescriber was associated with a lower risk compared to doctors without specialty.

CONCLUSIONS

Most benzodiazepine use occurred according to guidelines. Still, some experienced dose escalation over time, and risk factors were previous use of other psychotropic drugs, long time use, choice of first-time drug and prescriber's specialty. This could incite doctors to have a cessation plan when issuing first-time prescriptions.

Connective tissue growth factor/CCN2 attenuates β-adrenergic receptor responsiveness and cardiotoxicity by induction of G protein-coupled receptor kinase-5 in cardiomyocytes

Myocardial connective tissue growth factor (CTGF/CCN2) is induced in heart failure, a condition associated with diminution of β-adrenergic receptor (β-AR) responsiveness. Accordingly, we aimed to investigate whether CTGF could play a mechanistic role in regulation of β-AR responsiveness. Concentration-response curves of isoproterenol-stimulated cAMP generation in cardiomyocytes from transgenic mice with cardiac-restricted overexpression of CTGF (Tg-CTGF) or cardiomyocytes pretreated with recombinant human CTGF (rec-hCTGF) revealed marked reduction of both β₁-AR and β₂-AR responsiveness. Consistently, ventricular muscle strips from Tg-CTGF mice stimulated with isoproterenol displayed attenuation of maximal inotropic responses. However, no differences of maximal inotropic responses of myocardial fibers from Tg-CTGF mice and nontransgenic littermate control (NLC) mice were discerned when stimulated with supramaximal concentrations of dibutyryl-cAMP, indicating preserved downstream responsiveness to cAMP. Congruent with a mechanism of desensitization of β-ARs, mRNA and protein levels of G protein-coupled receptor kinase 5 (GRK5) were found isoform-selective upregulated in both cardiomyocytes from Tg-CTGF mice and cardiomyocytes exposed to rec-hCTGF. Corroborating a mechanism of GRK5 in CTGF-mediated control of β-AR sensitivity, Chinese hamster ovary cells pretreated with rec-hCTGF displayed increased agonist- and biased ligand-stimulated β-arrestin binding to β-ARs. Despite increased sensitivity of cardiomyocytes from GRK5-knockout (KO) mice to β-adrenergic agonists, pretreatment of GRK5-KO cardiomyocytes with rec-hCTGF, as opposed to cardiomyocytes from wild-type mice, did not alter β-AR responsiveness. Finally, Tg-CTGF mice subjected to chronic exposure (14 days) to isoproterenol revealed blunted myocardial hypertrophy and preserved cardiac function versus NLC mice. In conclusion, this study uncovers a novel mechanism controlling β-AR responsiveness in cardiomyocytes involving CTGF-mediated regulation of GRK5.

The functional activity of inhibitory G protein (G(i)) is not increased in failing heart ventricle

Beta-adrenergic receptor (βAR) inotropic effects are attenuated and muscarinic receptor-mediated inhibition thereof is enhanced in heart failure. We investigated if increased G(i) activity contributes to attenuated βAR-inotropic effects and potentiates muscarinic accentuated antagonism in failing rat ventricle. Contractility was measured in ventricular strips and adenylyl cyclase (AC) activity in ventricular membranes from rats with post-infarction heart failure (HF) or Sham-operated controls (Sham). The maximal βAR-mediated inotropic effect of isoproterenol was reduced by ~70% and basal, βAR- & forskolin-stimulated AC activity was significantly lower in HF vs. Sham. Carbachol-evoked antagonism of the βAR-mediated inotropic response was complete only in HF despite a ~40% reduction in the ability of carbachol to inhibit βAR-stimulated AC. However, neither the relative efficacy (contractility decreased by ~46%) nor the potency of carbachol to inhibit the βAR inotropic response differed between Sham and HF ventricle. Pertussis toxin (PTX) inactivation of G(i) did not increase the maximal βAR inotropic effect or the attenuated basal, βAR- & forskolin-stimulated AC activity in HF, but increased the potency of isoproterenol only in Sham (~0.5 log unit). In HF ventricle pretreated with PTX, simultaneous inhibition of phosphodiesterases 3,4 (PDE3,4) alone produced a larger inotropic response than isoproterenol in ventricle untreated with PTX (84% and 48% above basal respectively). In the absence of PTX, PDE3,4 inhibition evoked negligible inotropic effects in HF. These data are not consistent with the hypothesis that increased G(i) activity contributes to the reduced βAR-mediated inotropic response and AC activity in failing ventricle. The data, however, support the hypothesis that G(i), through chronic receptor independent inhibition of AC, together with PDE3,4 activity, is necessary to maintain a low basal level of contractility.

Prostaglandin E1 facilitates inotropic effects of 5-HT4 serotonin receptors and β-adrenoceptors in failing human heart

Prostaglandins have displayed both beneficial and detrimental effects in clinical studies in patients with severe heart failure. Prostaglandins are known to increase cardiac output, but the mechanism is not clarified. Here, we tested the hypothesis that prostaglandins can increase contractility in human heart by amplifying cAMP-dependent inotropic responses. Contractility was measured ex vivo in isolated left ventricular strips and phosphodiesterase (PDE) and adenylyl cyclase (AC) activity was measured in homogenates or membranes from failing human left ventricles. PGE(1) (1 µM) alone did not modify contractility, but given prior, amplified maximal serotonin (5-HT)-evoked (10 µM) contractile responses mediated by 5-HT(4) receptors several fold (24 ± 7 % with PGE(1) vs. 3 ± 2 % above basal with 5-HT alone). The 5-HT(4)-mediated inotropic response was amplified by the PDE3 inhibitor cilostamide and further amplified in combination with PGE(1) (26 ± 6 vs. 56 ± 12 % above basal). PGE(1) reduced the time to reach 90 % of both the maximal 5-HT- and isoproterenol-evoked inotropic response compared to 5-HT or isoproterenol alone. PGE(1) did not modify PDE activity in the homogenate, either alone or when given simultaneously with PDE3 and/or PDE4 inhibitors. Neither 5-HT- nor isoproterenol-stimulated AC activity was significantly amplified by PGE(1). Sensitivity of ventricular strips to Ca(2+) was not enhanced in the presence of PGE(1). Our results show that PGE(1) can enhance cAMP-mediated responses in failing human left ventricle, through a mechanism independent of PDE inhibition, amplification of AC activity or increasing sensitivity to calcium. This effect of PGE(1) possibly contributes to the increase of cardiac output, independent of decreased afterload, observed after prostaglandin administration in humans.

Prostanoid-mediated inotropic responses are attenuated in failing human and rat ventricular myocardium

Prostanoid-modulatory approaches in heart failure patients have displayed effects which may seem to be mutually incompatible. Both treatment with prostanoids and inhibition of prostanoid synthesis have resulted in increased mortality in heart failure patients. Currently, it is unknown if prostanoids mediate contractile effects in failing human heart and if this can explain some of the clinical effects seen after prostanoid modulatory treatments. Therefore, the objectives of this study were to determine if prostanoids could elicit direct inotropic responses in human ventricle, and if so to determine if they are modified in failing ventricle. Contractile force was measured in left ventricular strips from non-failing or failing human and rat hearts. The ratio of phosphorylated to non-phosphorylated myosin light chain 2 (MLC-2) was measured by Western blotting in myocardial strips, and the levels of prostanoid FP receptor mRNA and protein were measured in rat by real-time RT-PCR and receptor binding assays. In non-failing human hearts, prostanoids evoked a positive inotropic effect and an increase of MLC-2 phosphorylation which was absent in failing human hearts. In failing rat heart, the prostanoid FP receptor-mediated inotropic response and prostanoid FP receptor-density was reduced by ~40-50% compared to non-failing rat heart. Prostanoids mediate a sustained positive inotropic response in non-failing heart, which appears to be down regulated in failing heart. The pathophysiological significance of changes in prostanoid-mediated inotropic support in the failing heart remains to be determined.

Agents increasing cyclic GMP amplify 5-HT4-elicited positive inotropic response in failing rat cardiac ventricle

Activation of 5-HT₄ receptors in failing ventricles elicits a cAMP-dependent positive inotropic response which is mainly limited by the cGMP-inhibitable phosphodiesterase (PDE) 3. However, PDE4 plays an additional role which is demasked by PDE3 inhibition. The objective of this study was to evaluate the effect of cGMP generated by particulate and soluble guanylyl cyclase (GC) on the 5-HT₄-mediated inotropic response. Extensive myocardial infarctions were induced by coronary artery ligation in Wistar rats, exhibiting heart failure 6 weeks after surgery. Contractility was measured in left ventricular preparations. Cyclic GMP was measured by EIA. In ventricular preparations, ANP or BNP displayed no impact on 5-HT₄-mediated inotropic response. However, CNP increased the 5-HT₄-mediated inotropic response as well as the β₁-adrenoceptor (β₁-AR)-mediated response to a similar extent as PDE3 inhibition by cilostamide. Pretreatment with cilostamide eliminated the effect of CNP. Inhibition of nitric oxide (NO) synthase and soluble GC by l-NAME and ODQ, respectively, attenuated the 5-HT₄-mediated inotropic response, whereas the NO donor Sin-1 increased this response. The effects were absent during PDE3 inhibition, suggesting cGMP-dependent inhibition of PDE3. However, in contrast to the effects on the 5-HT₄ response, Sin-1 inhibited whereas l-NAME and ODQ enhanced the β₁-AR-mediated inotropic response. cGMP generated both by particulate (NPR-B) and soluble GC increases the 5-HT₄-mediated inotropic response in failing hearts, probably through inhibition of PDE3. β₁-AR and 5-HT₄ receptor signalling are subject to opposite regulatory control by cGMP generated by soluble GC in failing hearts. Thus, cGMP from different sources is functionally compartmented, giving differential regulation of different G_s-coupled receptors.

Cyclic AMP-dependent inotropic effects are differentially regulated by muscarinic G(i)-dependent constitutive inhibition of adenylyl cyclase in failing rat ventricle

BACKGROUND AND PURPOSE

β-Adrenoceptor (β-AR)-mediated inotropic effects are attenuated and G(i) proteins are up-regulated in heart failure (HF). Muscarinic receptors constitutively inhibit cAMP formation in normal rat cardiomyocytes. We determined whether constitutive activity of muscarinic receptors to inhibit adenylyl cyclase (AC) increases in HF and if so, whether it modifies the reduced β-AR- or emergent 5-HT₄-mediated cAMP-dependent inotropic effects.

EXPERIMENTAL APPROACH

Contractility and AC activity were measured and related to each other in rat ventricle with post-infarction HF and sham-operated (Sham) controls with or without blockade of muscarinic receptors by atropine and inactivation of G(i) protein by pertussis toxin (PTX).

KEY RESULTS

Isoprenaline-mediated inotropic effects were attenuated and basal, isoprenaline- and forskolin-stimulated AC activity was reduced in HF compared with Sham. Atropine or PTX pretreatment increased forskolin-stimulated AC activity in HF hearts. β-AR-stimulated AC and maximal inotropic response were unaffected by atropine in Sham and HF. In HF, the potency of serotonin (5-HT) to evoke an inotropic response was increased in the presence of atropine with no change in the maximal inotropic response. Interestingly, PTX pretreatment reduced the potency of 5-HT to evoke inotropic responses while increasing the maximal inotropic response.

CONCLUSIONS AND IMPLICATIONS

Although muscarinic constitutive inhibition of AC is increased in HF, it does not contribute to the reduced β-AR-mediated inotropic effects in rat ventricle in HF. The data support the hypothesis that there are differences in the functional compartmentation of 5-HT₄ and β-AR AC signalling in myocardium during HF.

Differential regulation of β2 -adrenoceptor-mediated inotropic and lusitropic response by PDE3 and PDE4 in failing and non-failing rat cardiac ventricle

BACKGROUND AND PURPOSE

β-Adrenoceptors play a major role in regulating myocardial function through cAMP-dependent pathways. Different phosphodiesterases (PDEs) regulate intracellular cAMP-pools and thereby contribute to the compartmentalization of cAMP-dependent effects. We explored the involvement of PDEs in limiting the β(2) adrenoceptor-mediated positive inotropic (PIR) and lusitropic (LR) responses in sham-operated (Sham) and failing rat hearts.

EXPERIMENTAL APPROACH

Extensive myocardial infarctions were induced by coronary artery ligation in Wistar rats. Rats developing heart failure were studied 6 weeks after surgery. Contractility was measured in left ventricular strips from failing and Sham hearts. cAMP was quantified by RIA.

KEY RESULTS

In ventricular strips, stimulation of β(2) -adrenoceptors with (-)-adrenaline (300 nM CGP20712A present) exerted a small PIR and LR. In Sham hearts, β(2) -adrenoceptor-mediated as well as β(1) -adrenoceptor-mediated PIR and LR were increased by selective inhibition of either PDE3 (1 µM cilostamide) or PDE4 (10 µM rolipram). In failing rat hearts, PDE3 inhibition enhanced PIR and LR to both β(1) - and β(2) -adrenoceptor stimulation while PDE4 inhibition had no effect on these responses despite a significant increase in cAMP levels. Combined PDE3/4 inhibition further enhanced the PIR and LR of β(2) - and β(1) -adrenoceptor activation both in Sham and failing hearts, compared with PDE3 inhibition alone. PDE4 enzyme activity was reduced in failing hearts.

CONCLUSIONS AND IMPLICATIONS

Both PDE3 and PDE4 attenuated β(2) - and β(1) -adrenoceptor-mediated contractile responses in Sham hearts. In failing hearts, these responses are attenuated solely by PDE3 and thus even selective PDE3 inhibitors may provide a profound enhancement of β-adrenoceptor-mediated responses in heart failure.

Natriuretic peptides increase beta1-adrenoceptor signalling in failing hearts through phosphodiesterase 3 inhibition

AIMS

Whereas natriuretic peptides increase cGMP levels with beneficial cardiovascular effects through protein kinase G, we found an unexpected cardio-excitatory effect of C-type natriuretic peptide (CNP) through natriuretic peptide receptor B (NPR-B) stimulation in failing cardiac muscle and explored the mechanism.

METHODS AND RESULTS

Heart failure was induced in male Wistar rats by coronary artery ligation. Contraction studies were performed in left ventricular muscle strips. Cyclic nucleotides were measured by radio- and enzyme immunoassay. Apoptosis was determined in isolated cardiomyocytes by Annexin-V/propidium iodide staining and phosphorylation of phospholamban (PLB) and troponin I was measured by western blotting. Stimulation of NPR-B enhanced beta1-adrenoceptor (beta1-AR)-evoked contractile responses through cGMP-mediated inhibition of phosphodiesterase 3 (PDE3). CNP enhanced beta1-AR-mediated increase of cAMP levels to the same extent as the selective PDE3 inhibitor cilostamide and increased beta1-AR-stimulated protein kinase A activity, as demonstrated by increased PLB and troponin I phosphorylation. CNP promoted cardiomyocyte apoptosis similar to inhibition of PDE3 by cilostamide, indicative of adverse effects of NPR-B signalling in failing hearts.

CONCLUSION

An NPR-B-cGMP-PDE3 inhibitory pathway enhances beta(1)-AR-mediated responses and may in the long term be detrimental to the failing heart through mechanisms similar to those operating during treatment with PDE3 inhibitors or during chronic beta-adrenergic stimulation.

Effect of piboserod, a 5-HT4 serotonin receptor antagonist, on left ventricular function in patients with symptomatic heart failure

AIMS

Myocardial 5-HT(4) serotonin (5-HT) receptors are increased and activated in heart failure (HF). Blockade of 5-HT(4) receptors reduced left ventricular (LV) remodelling in HF rats. We evaluated the effect of piboserod, a potent, selective, 5-HT(4) serotonin receptor antagonist, on LV function in patients with HF.

METHODS AND RESULTS

This was a prospective, double-blind, parallel group trial in patients with NYHA class II-IV HF and LV ejection fraction (EF) < or =0.35. Patients receiving standard HF treatment were randomized to placebo (n = 70) or piboserod 80 mg (n = 67) for 24 weeks including 4 weeks up titration. The primary endpoint was LVEF measured by cardiac magnetic resonance imaging (MRI). Secondary endpoints were LV volumes, N-terminal pro-brain natriuretic peptide, norepinephrine, quality of life, and 6 min walk test. Piboserod significantly increased LVEF by 1.7% vs. placebo (CI 0.3, 3.2, P = 0.020), primarily through reduced end-systolic volume from 165 to 158 mL (P = 0.060). There was a trend for greater increase in LVEF (2.7%, CI -1.1, 6.6, P = 0.15) in a small subset of patients not on chronic beta-blocker therapy. There was no significant effect on neurohormones, quality of life, or exercise tolerance. Patients on piboserod reported more adverse events, but numbers were too small to identify specific safety issues.

CONCLUSION

Although patients with chronic HF had a small but significant improvement in LVEF when treated with piboserod for 24 weeks, the result was not reflected in significant changes in other efficacy parameters, and its clinical relevance remains uncertain.

Activation of muscarinic receptors elicits inotropic responses in ventricular muscle from rats with heart failure through myosin light chain phosphorylation

BACKGROUND AND PURPOSE

Muscarinic stimulation increases myofilament Ca(2+) sensitivity with no apparent inotropic response in normal rat myocardium. Increased myofilament Ca(2+) sensitivity is a molecular mechanism promoting increased contractility in failing cardiac tissue. Thus, muscarinic receptor activation could elicit inotropic responses in ventricular myocardium from rats with heart failure, through increasing phosphorylation of myosin light chain (MLC).

EXPERIMENTAL APPROACH

Contractile force was measured in left ventricular papillary muscles from male Wistar rats, 6 weeks after left coronary artery ligation or sham surgery. Muscles were also frozen, and MLC-2 phosphorylation level was quantified.

KEY RESULTS

Carbachol (10 micromol.L(-1)) evoked a positive inotropic response only in muscles from rats with heart failure approximating 36% of that elicited by 1 micromol.L(-1) isoproterenol (20 +/- 1.5% and 56 +/- 6.1% above basal respectively). Carbachol-evoked inotropic responses did not correlate with infarction size but did correlate with increased left ventricular end diastolic pressure, heart weight/body weight ratio and lung weight, primary indicators of the severity of heart failure. Only muscarinic receptor antagonists selective for M(2) receptors antagonized carbachol-mediated inotropic effects with the expected potency. Carbachol-evoked inotropic responses and increase in phosphorylated MLC-2 were attenuated by MLC kinase (ML-9) and Rho-kinase inhibition (Y-27632), and inotropic responses were abolished by Pertussis toxin pretreatment.

CONCLUSION AND IMPLICATIONS

In failing ventricular muscle, muscarinic receptor activation, most likely via M(2) receptors, provides inotropic support by increasing MLC phosphorylation and consequently, myofilament Ca(2+) sensitivity. Enhancement of myofilament Ca(2+) sensitivity, representing a less energy-demanding mechanism of inotropic support may be particularly advantageous in failing hearts.

Activation of muscarinic receptors elicits inotropic responses in ventricular muscle from rats with heart failure through myosin light chain phosphorylation

BACKGROUND AND PURPOSE

Muscarinic stimulation increases myofilament Ca(2+) sensitivity with no apparent inotropic response in normal rat myocardium. Increased myofilament Ca(2+) sensitivity is a molecular mechanism promoting increased contractility in failing cardiac tissue. Thus, muscarinic receptor activation could elicit inotropic responses in ventricular myocardium from rats with heart failure, through increasing phosphorylation of myosin light chain (MLC).

EXPERIMENTAL APPROACH

Contractile force was measured in left ventricular papillary muscles from male Wistar rats, 6 weeks after left coronary artery ligation or sham surgery. Muscles were also frozen, and MLC-2 phosphorylation level was quantified.

KEY RESULTS

Carbachol (10 micromol.L(-1)) evoked a positive inotropic response only in muscles from rats with heart failure approximating 36% of that elicited by 1 micromol.L(-1) isoproterenol (20 +/- 1.5% and 56 +/- 6.1% above basal respectively). Carbachol-evoked inotropic responses did not correlate with infarction size but did correlate with increased left ventricular end diastolic pressure, heart weight/body weight ratio and lung weight, primary indicators of the severity of heart failure. Only muscarinic receptor antagonists selective for M(2) receptors antagonized carbachol-mediated inotropic effects with the expected potency. Carbachol-evoked inotropic responses and increase in phosphorylated MLC-2 were attenuated by MLC kinase (ML-9) and Rho-kinase inhibition (Y-27632), and inotropic responses were abolished by Pertussis toxin pretreatment.

CONCLUSION AND IMPLICATIONS

In failing ventricular muscle, muscarinic receptor activation, most likely via M(2) receptors, provides inotropic support by increasing MLC phosphorylation and consequently, myofilament Ca(2+) sensitivity. Enhancement of myofilament Ca(2+) sensitivity, representing a less energy-demanding mechanism of inotropic support may be particularly advantageous in failing hearts.

5-HT4-elicited positive inotropic response is mediated by cAMP and regulated by PDE3 in failing rat and human cardiac ventricles

BACKGROUND AND PURPOSE

The left ventricle in failing hearts becomes sensitive to 5-HT parallelled by appearance of functional G(s)-coupled 5-HT(4) receptors. Here, we have explored the regulatory functions of phosphodiesterases in the 5-HT(4) receptor-mediated functional effects in ventricular muscle from failing rat and human heart.

EXPERIMENTAL APPROACH

Extensive myocardial infarctions were induced by coronary artery ligation in Wistar rats. Contractility was measured in left ventricular papillary muscles of rat, 6 weeks after surgery and in left ventricular trabeculae from explanted human hearts. cAMP was quantified by RIA.

KEY RESULTS

In papillary muscles from postinfarction rat hearts, 5-HT(4) stimulation exerted positive inotropic and lusitropic effects and increased cAMP. The inotropic effect was increased by non-selective PDE inhibition (IBMX, 10 microM) and selective inhibition of PDE3 (cilostamide, 1 microM), but not of PDE2 (EHNA, 10 microM) or PDE4 (rolipram, 10 microM). Combined PDE3 and PDE4 inhibition enhanced inotropic responses beyond the effect of PDE3 inhibition alone, increased the sensitivity to 5-HT, and also revealed an inotropic response in control (sham-operated) rat ventricle. Lusitropic effects were increased only during combined PDE inhibition. In failing human ventricle, the 5-HT(4) receptor-mediated positive inotropic response was regulated by PDEs in a manner similar to that in postinfarction rat hearts.

CONCLUSIONS AND IMPLICATIONS

5-HT(4) receptor-mediated positive inotropic responses in failing rat ventricle were cAMP-dependent. PDE3 was the main PDE regulating this response and involvement of PDE4 was disclosed by concomitant inhibition of PDE3 in both postinfarction rat and failing human hearts. 5-HT, PDE3 and PDE4 may have pathophysiological functions in heart failure.

Prostanoid F receptors elicit an inotropic effect in rat left ventricle by enhancing myosin light chain phosphorylation

AIMS

The aims of this study were to determine if the prostanoid F receptor (FPR)-mediated inotropic effect in rat ventricle is mediated by increased phosphorylation of myosin light chain-2 (MLC-2) and to elucidate the signalling pathway(s) activated by FPRs to regulate MLC-2 phosphorylation.

METHODS AND RESULTS

Contractility was measured in left ventricular strips from adult male rats. Strips were also snap-frozen, and changes in the phosphorylation level of both MLC-2 and myosin phosphatase targeting subunit-2 (MYPT-2) were quantified. FPR stimulation with fluprostenol increased contractility by approximately 100% above basal and increased phosphorylation of both MLC-2 (by approximately 30%) and MYPT-2 (by approximately 50%). The FPR-mediated inotropic effect and MLC-2 phosphorylation were reduced by a similar magnitude in the presence of the myosin light chain kinase (MLCK) inhibitor ML-7 (approximately 60-70%) and an inhibitor of Ca(2+)/calmodulin, W-7 (approximately 35%). Inhibition of Rho-associated kinase by Y-27632 reduced the FPR-mediated inotropic effect and MLC-2 phosphorylation by approximately 40-45% and MYPT-2 phosphorylation by approximately 70%. ML-7 and Y-27632 together reduced contractility and MLC-2 phosphorylation by approximately 70-80%. The FPR-mediated inotropic effect was only modestly affected by high concentrations of the inositol tris-phosphate (IP(3)) receptor blocker 2-APB, but not by the protein kinase C (PKC) inhibitor bisindolylmaleimide.

CONCLUSION

The FPR-evoked inotropic effect is mediated by increasing the phosphorylation of MLC-2 through regulation of both MLCK and myosin light chain phosphatase activities. The second messenger IP(3) and PKC are unlikely to be involved in the signalling cascade of the FPR-mediated positive inotropic effect. Therefore, FPR signalling mechanism(s) regulating MLC-2 phosphorylation likely extend beyond those classically established for G(q/11)-coupled receptors.

Effects of serotonin in failing cardiac ventricle: signalling mechanisms and potential therapeutic implications

Previously, cardioexcitation by serotonin (5-hydroxytryptamine, 5-HT) was believed to be confined to atria in mammals including man, and mediated through 5-HT(4) receptors in pig and man, but 5-HT(2A) receptors in rat. Recent studies, reviewed here, demonstrate that functional 5-HT(4) receptors can be revealed in porcine and human ventricular myocardium during phosphodiesterase inhibition, and that 5-HT(4) receptor mRNA is increased in human heart failure. In rats, functional 5-HT(4) and 5-HT(2A) receptors appear in the cardiac ventricle during heart failure and mediate inotropic responses through different mechanisms. 5-HT(2A) receptor signalling resembles that from alpha(1)-adrenoceptors and causes inotropic effects through increased myosin light chain phosphorylation, resulting in Ca(2+) sensitisation. 5-HT(4) receptor signalling resembles that from beta-adrenoceptors and causes inotropic effects through a pathway involving cAMP and PKA-mediated phosphorylation of proteins involved in Ca(2+) handling, resulting in enhanced contractility through increased Ca(2+) availability. Cyclic AMP generated through 5-HT(4) receptor stimulation seems more efficiently coupled to increased contractility than cAMP generated through beta-adrenoceptor stimulation. Increasing contractility through cAMP is considered less energy efficient than Ca(2+) sensitisation and this may be one reason why beta-adrenoceptor antagonism is beneficial in heart failure patients. Treatment of heart failure rats with the 5-HT(4) antagonist SB207266 (piboserod) resulted in potentially beneficial effects, although small. Further studies are needed to clarify if such treatment will be useful for patients with heart failure.

Cardiac-restricted expression of the carboxyl-terminal fragment of GRK3 Uncovers Distinct Functions of GRK3 in regulation of cardiac contractility and growth: GRK3 controls cardiac alpha1-adrenergic receptor responsiveness

G protein-coupled receptor kinase-2 and -3 (GRK2 and GRK3) in cardiac myocytes catalyze phosphorylation and desensitization of different G protein-coupled receptors through specificity controlled by their carboxyl-terminal pleckstrin homology domain. Although GRK2 has been extensively investigated, the function of cardiac GRK3 remains unknown. Thus, in this study cardiac function of GRK3 was investigated in transgenic (Tg) mice with cardiac-restricted expression of a competitive inhibitor of GRK3, i.e. the carboxyl-terminal plasma membrane targeting domain of GRK3 (GRK3ct). Cardiac myocytes from Tg-GRK3ct mice displayed significantly enhanced agonist-stimulated alpha(1)-adrenergic receptor-mediated activation of ERK1/2 versus cardiac myocytes from nontransgenic littermate control (NLC) mice consistent with inhibition of GRK3. Tg-GRK3ct mice did not display alterations of cardiac mass or left ventricular dimensions compared with NLC mice. Tail-cuff plethysmography of 3- and 9-month-old mice revealed elevated systolic blood pressure in Tg-GRK3ct mice versus control mice (3-month-old mice, 136.8 +/- 3.6 versus 118.3 +/- 4.7 mm Hg, p < 0.001), an observation confirmed by radiotelemetric recording of blood pressure of conscious, unrestrained mice. Simultaneous recording of left ventricular pressure and volume in vivo by miniaturized conductance micromanometry revealed increased systolic performance with significantly higher stroke volume and stroke work in Tg-GRK3ct mice than in NLC mice. This phenotype was corroborated in electrically paced ex vivo perfused working hearts. However, analysis of left ventricular function ex vivo as a function of increasing filling pressure disclosed significantly reduced (dP/dt)(min) and prolonged time constant of relaxation (tau) in Tg-GRK3ct hearts at elevated supraphysiological filling pressure compared with control hearts. Thus, inhibition of GRK3 apparently reduces tolerance to elevation of preload. In conclusion, inhibition of cardiac GRK3 causes hypertension because of hyperkinetic myocardium and increased cardiac output relying at least partially on cardiac myocyte alpha(1)-adrenergic receptor hyper-responsiveness. The reduced tolerance to elevation of preload may cause impaired ability to withstand pathophysiological mechanisms of heart failure.

Substrate specificities of g protein-coupled receptor kinase-2 and -3 at cardiac myocyte receptors provide basis for distinct roles in regulation of myocardial function

The closely related G protein-coupled receptor kinases GRK2 and GRK3 are both expressed in cardiac myocytes. Although GRK2 has been extensively investigated in terms of regulation of cardiac beta-adrenergic receptors, the substrate specificities of the two GRK isoforms at G protein-coupled receptors (GPCR) are poorly understood. In this study, the substrate specificities of GRK2 and GRK3 at GPCRs that control cardiac myocyte function were determined in fully differentiated adult cardiac myocytes. Concentration-effect relationships of GRK2, GRK3, and their respective competitive inhibitors, GRK2ct and GRK3ct, at endogenous endothelin, alpha(1)-adrenergic, and beta(1)-adrenergic receptor-generated responses in cardiac myocytes were achieved by adenovirus gene transduction. GRK3 and GRK3ct were highly potent and efficient at the endothelin receptors (IC(50) for GRK3, 5 +/- 0.7 pmol/mg of protein; EC(50) for GRK3ct, 2 +/- 0.2 pmol/mg of protein). The alpha(1)-adrenergic receptor was also a preferred substrate of GRK3 (IC(50),7 +/- 0.4 pmol/mg of protein). GRK2 lacked efficacy at both endothelin and alpha(1)-adrenergic receptors despite massive overexpression. On the contrary, both GRK2ct and GRK3ct enhanced beta(1)-adrenergic receptor-induced cAMP production with comparable potencies. However, the potency of GRK3ct at beta(1)-adrenergic receptors was at least 20-fold lower than that at endothelin receptors. In conclusion, this study demonstrates distinct substrate specificities of GRK2 and GRK3 at different GPCRs in fully differentiated adult cardiac myocytes. As inferred from the above findings, GRK2 may play its primary role in regulation of cardiac contractility and chronotropy by controlling beta(1)-adrenergic receptors, whereas GRK3 may play important roles in regulation of cardiac growth and hypertrophy by selectively controlling endothelin and alpha(1)-adrenergic receptors.

Expression of mRNA encoding G protein-coupled receptors involved in congestive heart failure--a quantitative RT-PCR study and the question of normalisation

Congestive heart failure (CHF) induces changes in the neurohumoral system and gene expression in viable myocardium. Several of these genes encode G protein-coupled receptors (GPCRs) involved in mechanisms which compensate for impaired myocardial function. We used real-time quantitative RT-PCR (Q-RT-PCR) to investigate the expression of mRNA encoding 15 different GPCRs possibly involved in CHF, and the effect of normalisation to GAPDH mRNA (GAPDH) or 18S rRNA (18S). CHF was induced in rats by coronary artery ligation, with sham-operated controls (Sham). After 6 weeks, mRNA expression in viable left ventricular myocardium was determined using both 18S and GAPDH as the normalisation standard. An apparent 30% reduction in GAPDH mRNA levels vs. 18S in CHF compared to Sham, although not significant in itself, influenced the interpretation of regulation of other genes.Thus, levels of mRNA encoding receptors for angiotensin II (AT(1)), endothelin (ET(A), ET(B)) and the muscarinic acetylcholine (mACh) receptor M(1) increased significantly in CHF only when normalised to GAPDH. Levels of mRNA encoding the mACh receptors M(3) and M(4) and the serotonin receptors 5-HT(2A) and 5-HT(4) increased, whereas alpha(1D)-adrenoceptor mRNA decreased in CHF irrespective of the normalisation standard. No significant change was detected for M2 and M5 mACh receptors or alpha(1A)-, alpha(1B)-, beta(1)- or beta(2)-adrenoceptors. Q-RT-PCR is a sensitive and powerful method to monitor changes in GPCR mRNA expression in CHF. However, the normalisation standard used is important for the interpretation of mRNA regulation.

Effects of treatment with a 5-HT4 receptor antagonist in heart failure

BACKGROUND AND PURPOSE

Positive inotropic responses (PIR) to 5-hydroxytryptamine (5-HT) are induced in the left ventricle (LV) in rats with congestive heart failure (CHF); this is associated with upregulation of the G(s)-coupled 5-HT(4) receptor. We investigated whether chronic 5-HT(4) receptor blockade improved cardiac function in CHF rats.

EXPERIMENTAL APPROACH

Rats were given either the 5-HT(4) antagonist SB207266 (0.5 mg kg(-1) 24h(-1); MI(int)) or placebo (MI(pl)) through mini-osmotic pumps for 6 weeks subsequent to induction of post-infarction CHF. In vivo cardiac function and ex vivo responses to isoprenaline or 5-HT were evaluated using echocardiography and isolated LV papillary muscles, respectively. mRNA levels were investigated using real-time quantitative RT-PCR.

KEY RESULTS

LV diastolic function improved, with 4.6% lower LV diastolic diameter and 24.2% lower mitral flow deceleration in MI(int) compared to MI(pl). SB207266 reduced LV systolic diameter by 6.1%, heart weight by 10.2% and lung weight by 13.1%. The changes in posterior wall thickening and shortening velocity, cardiac output, LV systolic pressure and (dP/dt)(max), parameters of LV systolic function, did not reach statistical significance. The PIR to isoprenaline (10 microM) increased by 36% and the response to 5-HT (10 microM) decreased by 57% in MI(int) compared to MI(pl). mRNA levels for ANP, 5-HT(4(b)) and 5-HT(2A) receptors, MHCbeta, and the MHCbeta/MHCalpha -ratio were not significantly changed in MI(int) compared to MI(pl).

CONCLUSIONS AND IMPLICATIONS

Treatment with SB207266 to some extent improved in vivo cardiac function and ex vivo myocardial function, suggesting a possible beneficial effect of treatment with a 5-HT(4) receptor antagonist in CHF.

Increased expression and function of the myocardial Na–K–2Cl cotransporter in failing rat hearts

Recent studies indicate a role of the Na-K-2Cl cotransporter (NKCC) in regulation of myocardial function. However, potential pathophysiological properties of NKCC in conditions like myocardial infarction (MI) and heart failure have not been explored. We investigated the cellular localization of myocardial NKCC and whether myocardial NKCC levels are changed upon induction of post-infarction heart failure in rats. Immunohistochemical analysis demonstrated extensive distribution of NKCC in normal rat myocardium with fairly strong expression in cardiomyocytes, fibroblasts, vascular endothelial cells, as well as smooth muscle cells. Myocardial mRNA levels of NKCC were investigated at 2, 7 and 28 days after induction of MI or sham operation, but no changes were found. Cardiomyocytes and non-cardiomyocytes were isolated 7 days after induction of MI or sham operation. An approximately 2-fold increase of the NKCC mRNA levels was found in isolated cardiomyocytes from heart failure rats compared to that of sham-operated rats (P < 0.001), whereas a trend towards decreased mRNA levels of NKCC in isolated non-cardiomyocytes was observed. In addition, we found a bumetanide sensitive (86)Rb(+) influx mechanism present in the hearts after induction of MI (P < 0.05). Thus, our data indicate cardiomyocyte specific increase in NKCC mRNA levels and increased NKCC activity in post-infarction heart failure. Our results may indicate a potential role of NKCC during post-infarction remodeling.