Pertussis toxin reduces the antiadrenergic effect of 2-chloroadenosine on papillary muscle and the direct negative inotropic effect of 2-chloroadenosine on atrium

beta(1)- and beta(2)-adrenoceptor responses in cardiomyocytes derived from human embryonic stem cells: comparison with failing and non-failing adult human heart

BACKGROUND AND PURPOSE

Characterization of human embryonic stem cell-derived cardiomyocytes (hESC-CM) in relation to adult myocytes is essential for their future use in transplantation or as a model system. The beta-adrenoceptor pathways, which are known to be effective early in hESC-CM development, are of major importance because of their control of rate and force of beating, arrhythmia generation and apoptosis/necrosis. We have therefore performed detailed pharmacological analysis of the beta-adrenoceptor responses in developing hESC-CM.

EXPERIMENTAL APPROACH

hESC-CMs were differentiated from H7 ESCs and studied up to 79 days of differentiation. Rate of beating and time course of contraction and relaxation were measured in superfused preparations.

KEY RESULTS

Responses to the mixed beta(1)- and beta(2)-adrenoceptor agonist isoprenaline were evident from day 10 to day 79. Stability of the responses during an application, for repeated applications on the same experimental day and over the time of development, was determined. Concentrations for half-maximal response (12.9 nM) were similar to those from adult human heart, but closer to those obtained from failing rather than normal ventricle. Acceleration of both contraction and relaxation was quantitatively similar to that in adult ventricular myocytes, as was sensitivity to muscarinic inhibition. Use of specific antagonists showed that both beta(1)- and beta(2)-adrenoceptors contributed to contractile responses, as seen with adult myocytes.

CONCLUSIONS AND IMPLICATIONS

These data show the compatibility of hESC-CM with adult human myocardium in terms of beta-adrenoceptor response. The experiments described here also confirm the utility of the hESC-CM preparation for detailed pharmacological analysis.

Roles of PKA, PI3K, and cPLA2 in the NO-mediated negative inotropic effect of β2-adrenoceptor agonists in guinea pig right papillary muscles

Although β2-adrenoceptors represent 15–25% of β-adrenoceptors in the guinea pig heart, their functionality is controversial. We assessed the inotropic effects of β2-adrenoceptor partial agonists in right papillary muscles. Salbutamol induced a small but significant concentration-dependent negative inotropic effect (NIE, −5% at 60 nM) followed by a moderate positive inotropic effect (+36% at 6 μM) due to activation of β1-adrenoceptors. In the presence of 4 μM atenolol, the concentration-dependent NIE (−12% at 6 μM) was biphasic, best described by a double logistic equation with respective EC50 values of 3 and ∼420 nM, and was insensitive to SR59230A. In muscles from pertussis toxin-treated guinea pigs, the salbutamol-induced positive inotropic effect was sensitive to low concentrations of ICI-118551 in an unusual manner. Experiments in reserpinized animals revealed the importance of the phosphorylation-dephosphorylation processes. PKA inhibition reduced and suppressed the effects obtained at low and high concentrations, respectively, indicating that its activation was a prerequisite to the NIE. The effect occurring at nanomolar concentrations depended upon PKA/phosphatidylinositol 3-kinase/cytosolic phospholipase A2 (cPLA2) activations leading to nitric oxide (NO) release via the arachidonic acid/cyclooxygenase pathway. NO release via PKA-dependent phosphorylation of the receptor was responsible for the inotropic effect observed at submicromolar concentrations, which is negatively controlled by cPLA2. The possibility that these effects are due to an equilibrium between different affinity states of the receptor (Gs/Gi coupled and Gi independent with different signaling pathways) that can be displaced by ICI-118551 is discussed. We conclude that β2-adrenoceptors are functional in guinea pig heart and can modulate the inotropic state.

Beta-adrenoceptor subtype dependence of chronotropy in mouse embryonic stem cell-derived cardiomyocytes

Cardiomyocytes derived from embryonic stem cells (ESCM) have potential both as an experimental model for investigating cardiac physiology and as a source for tissue repair. For both reasons it is important to characterise the responses of these cells, and one of the key modulators of contraction is the beta-adrenergic system. We therefore undertook a detailed study of the response of the spontaneous beating rate of ESCM to beta-adrenoceptor (betaAR) stimulation. Embryoid bodies (EBs) were generated from murine ES line E14Tg2a by the hanging drop method, followed by plating. Spontaneously beating areas were seen starting from 9-14 days after differentiation: the experiments described here were performed on EBs between developmental day 19 and 48. Beating cell layers were seeded with charcoal to allow tracking of movement by a video-edge detection system. Experiments were performed in physiological medium containing 1 mM Ca2+ at 37 degrees C. Isoprenaline (Iso) increased beating rate with an EC50 value of 52 nM. Iso (0.3 microM) increased basal rate from 67 +/- 7 beats per minute (bpm) to 138 +/- 18 bpm, P < 0.001, n = 22. At earlier developmental time points the response to Iso was not maintained through 5 min exposure; this spontaneous desensitisation only being observed before day 36. A repeat application of Iso after a wash period of 20 min produced reproducible effects on beating rate. Subtype dependence of the betaAR response was determined by comparing an initial response with a second in the presence of selective beta1- or beta2AR antagonists. In the presence of the specific beta1AR-blocker CGP 20712A (300 nM) the increase in rate with Iso was reduced from 207 +/- 42% of basal to 128 +/- 13%, P < 0.01. With the beta2AR-blocker ICI 118,551 (50 nM) there was no significant change in Iso response. Exposure to the muscarinic agonist, carbachol (10 microM), inhibited the increase in frequency mediated by isoprenaline, but had mixed stimulatory and inhibitory effects on basal rate. This study extends the characterisation of ESCM as a preparation for studying receptor pharmacology, and indicates that the beta1AR is the predominant subtype mediating increases in contraction rate in murine ESCM.

Effect of aging on the negative chronotropic and anti-beta-adrenergic actions of adenosine in the rat heart

The effect of aging on the antiadrenergic actions of adenosine was studied in vitro and in vivo by using adult (6-month-old) and old (24-month-old) male Fischer 344 rats. In anesthetized animals, adenosine (0.01-0.1 micromol/kg), given as a rapid bolus into the right atrium, exerted a negative chronotropic effect manifested by a dose-dependent transient prolongation of sinus cycle length (SCL). This effect was similar in both age groups (n = 6, each; i.e., the percentage maximal prolongation of SCL (%deltaSCL) ranged from 12 +/- 2% to 63 +/-14% in the adult and from 20 +/- 7% to 57 +/- 15% in the old rats. In the presence of isoproterenol (0.2 microg/kg/min), the negative chronotropic action of adenosine was potentiated in the adult rats much more than in the old rats [i.e., %deltaSCL ranged from 60 +/- 28% to 183 +/- 48% vs. 40 +/- 12% to 70 +/- 13%, respectively (p < 0.05, adult vs. old)]. In the isolated perfused hearts, isoproterenol (1 microM for 1 min) exerted similar chronotropic and inotropic effects in adult (n = 9) and old hearts [n = 6; i.e., heart rate, left ventricular pressure (LVP), and LVdp/dt increased by 56 +/- 3%, 17 +/- 1%, and 37 +/- 2%, and 57 +/- 2%, 17 +/- 1%, and 35 +/- 3%, respectively, in the absence of, and by 27 +/- 2%, 7 +/- 1%, and 19 +/- 2% and 41 +/- 3%, 12 +/- 1%, and 25 +/-2% in the presence of adenosine (5 microM for 1 min)]. Adenosine administration after isoproterenol caused only an insignificant increase in coronary blood flow. Finally, the adenosine attenuation of either isoproterenol- or forskolin-induced production of 3',5'-cyclic adenosine monophosphate (cAMP) was significantly less in atrial membranes isolated from old versus adult rats (n = 6, each). It was concluded that in the old Fischer 344 rat hearts, the antiadrenergic action of adenosine is attenuated as compared with its action in adult rat hearts.

The effect of pertussis toxin on beta-adrenoceptor responses in isolated cardiac myocytes from noradrenaline-treated guinea-pigs and patients with cardiac failure

1. A decreased responsiveness to the positive inotropic effects of beta-adrenoceptor agonists is a characteristic of human heart failure. We have investigated the involvement of inhibitory guanine nucleotide binding proteins (G-proteins) in this process using pertussis toxin treatment of isolated cardiac myocytes. 2. Myocytes isolated from failing human myocardium had a reduced maximum contractile response to isoprenaline relative to that for maximally stimulating concentrations of calcium, giving an isoprenaline/calcium ratio of 0.71 +/- 0.06 (n = 7 patients). This was significantly lower than in myocytes from non-failing myocardium, where the isoprenaline/calcium ratio was 1.16 +/- 0.07 (n = 6, P less than 0.001). Responses to high calcium were unchanged. 3. Myocytes were treated with pertussis toxin for 3-5 h at 35 degrees C. Successful inactivation of inhibitory G-proteins by pertussis toxin treatment was indicated by a loss of responsiveness to 10 microM adenosine (in the presence of submaximal isoprenaline). 4. Following pertussis toxin treatment of the myocytes from failing human heart the isoprenaline/calcium ratio increased to 1.43 +/- 0.27 (n = 7, P less than 0.05). Pertussis toxin treatment had no effect upon the maximum calcium contraction. The isoprenaline/calcium ratio in myocytes from non-failing human ventricle was not affected by the toxin treatment (n = 3). These observations support the hypothesis that increased inhibitory G-protein levels or activities contribute to beta-adrenoceptor desensitization in human heart failure. 5. beta-Adrenoceptor desensitization in human heart failure is thought to be secondary to raised noradrenaline levels in these patients. Experiments were repeated on myocytes isolated from hearts of guinea-pigs which had been chronically infused with noradrenaline. The isoprenaline/calcium ratio of these myocytes was reduced below that of myocytes from sham-operated animals (0.65 0.04, n = 6 compared with 0.88 +/- 0.02, n = 7, P<0.001).6. Pertussis toxin treatment (2 h at 35 degrees C) increased the isoprenaline/calcium ratio to 1.02 0.02 (n = 6,P<0.01) in myocytes from noradrenaline-treated guinea-pigs. This effect of pertussis toxin treatment was not seen in myocytes from sham-operated guinea-pig hearts.7. Incubation at 35 degrees C for similar periods in the absence of pertussis toxin also restored the isoprenaline/calcium ratio towards normal in the myocytes from both failing human and noradrenaline-treated guinea-pig hearts, although the effect was significantly smaller than that produced by the toxin. Myocytes kept at room temperature (22 degrees C) showed no such evidence of resensitization over periods up to 6h.8. These observations are consistent with the hypothesis that raised catecholamine levels result in increased inhibitory G-protein levels and functional P-adrenoceptor desensitization in heart failure.

Adenosine as a vasodilator in primary pulmonary hypertension

BACKGROUND

The acute administration of vasodilator drugs to patients with primary pulmonary hypertension has been advocated to identify those with reversible pulmonary vasoconstriction. Unfortunately, the usefulness of the drugs currently available is limited by accompanying systemic hypotension. A vasodilator with effects confined to the pulmonary circulation would therefore be advantageous in such patients.

METHODS AND RESULTS

The purine nucleoside adenosine was infused into the pulmonary artery in seven patients with primary pulmonary hypertension (baseline pulmonary vascular resistance [PVR], 442-1,295 dyne/cm/sec-5) to determine its effect on PVR. In all patients, there was a dose-dependent and significant reduction (mean maximal percent decrease from baseline, 38.9%; p less than 0.001) in PVR mediated through a decrease in pulmonary artery pressure and an increase in cardiac output. Systemic vascular resistance (SVR) also decreased, but the ratio of PVR to SVR decreased (maximal mean percent decrease from baseline) by 10.5% (p less than 0.025), indicating that adenosine has a preferential vasodilator effect on the pulmonary circulation when administered in this manner.

CONCLUSIONS

Because of its pharmacokinetic and vasodilator properties, adenosine may have a specific role in the investigation of primary pulmonary hypertension.

Comparison between the cardiac effects induced by muzolimine and furosemide in guinea-pig atria

Muzolimine (10-500 microM) induced a concentration-dependent reduction of both the contractile force and frequency in spontaneously beating atria and in electrically driven left atrium from reserpine-treated guinea pigs. This negative inotropic response was unaffected by the addition of atropine to the perfusion fluid, and it was highly sensitive to changes in external Ca2+ concentration. Both in spontaneously beating and in electrically driven atrium, muzolimine (50-400 microM) antagonized, in an apparently competitive manner, the increase in contractile force induced by cumulative addition of CaCl2 (0.68-9.59 mM) to the bathing fluid. Muzolimine (50-100 microM) reduced the inotropic response to low (5-30 nM), but not high (50-100 nM) concentrations of Bay K 8644, a calcium-channel agonist. The inotropic effects of 8-phenyltheophylline and of ouabain were antagonized by muzolimine (10-100 microM) in a noncompetitive manner, while the response to noradrenaline was not altered. Similar to muzolimine, verapamil at a concentration suitable to block calcium channels inhibited, in a noncompetitive way, the inotropic effect induced by 8-phenyltheophylline and by ouabain without altering the contractile response to noradrenaline. Furosemide (10 and 100 microM) did not influence the contractile force or the frequency of spontaneously beating atria, nor the inotropic effect induced by CaCl2, 8-phenyltheophylline, ouabain, or noradrenaline. These results indicate that the influence of muzolimine on guinea-pig atria originates from an inhibition of Ca2+ influx into cardiac cells and that furosemide does not mimic the effect of muzolimine at this level.

In vivo pertussis toxin treatment attenuates some, but not all, adenosine A1 effects in slices of the rat hippocampus

In order to examine the involvement of G-proteins in mediating the different effects of adenosine A1-receptor stimulation in rat hippocampus we injected pertussis toxin (PTX) intraventricularly close to the hippocampus and examined its effect in slices 48-60 h later. The in vivo PTX treatment caused a partial (50 +/- 5%) inhibition of the [32P]ADP ribosylation produced by PTX added together with [32P]NAD in vitro. Such PTX treatment eliminated the electrophysiologically determined gamma-amino-n-butyric acid (GABA)B receptor response in the hippocampal CA1 region, but GABAA effects were unaffected. The adenosine (50 microM)-mediated hyperpolarization and decrease in input resistance as well as the adenosine-mediated inhibition of low calcium-induced bursting in pyramidal CA1 neurons were virtually abolished. The same was true for the decrease in [3H]cyclic AMP accumulation that is produced by the adenosine analogue R-N6-phenylisopropyl adenosine (R-PIA) in forskolin-treated hippocampal slices. As far as modulation of transmitter release was concerned, the R-PIA (1 microM)-induced inhibition of release of both [3H]noradrenaline (NA) and [3H]acetylcholine (ACh) evoked by field stimulation in hippocampal slices was affected hardly or not at all by pertussis toxin treatment. The inhibitory effect of adenosine on field excitatory postsynaptic potential (EPSP)s evoked in the CA1 region was unaltered by PTX pretreatment. The present results show that in vivo pertussis toxin treatment can inhibit some but not all A1-adenosine-receptor effects. This strongly suggests that closely similar A1 receptors might be coupled to G-proteins that differ in their sensitivity to PTX treatment.

Alinidine antagonizes the myocardial effects of adenosine

The bradycardiac drug alinidine (2.8-182.3 microM) antagonized the negative inotropic effect of adenosine in isolated left rat atria in a non-competitive fashion. A 50% reduction of the maximal adenosine effect was achieved with 64.6 microM alinidine. The shortening of the action potential duration and the suppression of slow action potentials in guinea pig atria by 1-10 microM adenosine could be abolished by 5.7 microM alinidine. Adenosine (1 mM) exerted a negative inotropic effect on isolated strips from the right ventricle of rat heart exposed to 0.36 microM isoproterenol, an effect which was fully antagonized by 91.1 microM alinidine. Addition of 1 mM adenosine to spontaneously beating isolated rat atria induced pronounced bradycardia. Under these conditions, the bradycardiac agent alinidine increased the contractile rate concentration dependently (11.4-182.3 microM), while the chemically unrelated bradycardiac compound UL-FS 49 did not accelerate the beat frequency. As alinidine antagonized all the cardiac actions of adenosine that we tested, it may be concluded that the drug interferes with A1-receptor-mediated effects.