Preponderance of beta- over alpha-adrenoceptors in mediating the positive inotropic effect of phenylephrine in the ferret ventricular myocardium

alpha1-adrenergic stimulation induces early afterdepolarizations in ferret Purkinje fibers

The aim of this study was to evaluate whether alpha1-adrenergic stimulation, which prolongs repolarization, could induce early afterdepolarizations (EADs) in ferret Purkinje fibers. We used standard microelectrodes to study the effects of phenylephrine 10(-6)M in the presence of metoprolol 1.5 x 10(-6)M, on action potentials (AP) recorded from isolated ferret Purkinje fibers superfused with normal Tyrode's solution. Phenylephrine induced a time-dependent prolongation of the AP duration at 60 and 90% of full repolarization (APD60, APD90) from 223 +/- 8 and 269 +/- 9 ms, respectively, during control to 279 +/- 11 and 329 +/- 12 ms after 1-h superfusion (n = 29; p < 0.05 vs. control for both parameters) and 334 +/- 13 and 385 +/- 15 ms after 4 h (n = 29, p < 0.05 vs. control and 1-h superfusion for both parameters) without changing the other parameters. About one third of the fibers studied developed EADs that could be either phase 2 or phase 3 EADs. The alpha1-adrenoceptor antagonist WB 4101 (10(-7)M) limited the AP prolongation and prevented the occurrence of EADs. On the other hand, the alpha1-adrenoceptor alkylating agent chloroethylclonidine (10(-7)M) had no effect. The calcium chelator BAPTA [1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid] (2 x 10(-3)M) did not prevent the induction of EADs by phenylephrine, although it suppressed the twitch tension, showing that it did chelate the intracellular calcium. Our results show that alpha1-adrenergic stimulation prolongs repolarization in ferret Purkinje fibers and can induce EADs. This effect is mainly mediated by the WB 4101-sensitive alpha1-adrenoceptors (alpha1A- and/or alpha1C-adrenoceptors) and does not depend on intracellular calcium. alpha1-Adrenergic stimulation may have arrhythmogenic effects in patients with long QT syndrome (LQTS).

Sustained negative inotropism mediated by alpha-adrenoceptors in adult mouse myocardia: developmental conversion from positive response in the neonate

1. Inotropic responses to alpha-adrenoceptor stimulation and the effects of antagonists were examined in isolated ventricular preparations from neonatal and adult mice. 2. Phenylephrine, in the presence of propranolol, produced positive inotropic responses in neonates up to 1 week after birth, while it produced negative inotropic responses in mice older than 3 weeks. 3. Both positive and negative responses to phenylephrine in neonates and adults, respectively, were antagonized by prazosin, WB4101 (2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane) and 5-methylurapidil, but not by atropine, yohimbine or chlorethylclonidine. 4. Noradrenaline (NA) produced positive inotropic responses both in the neonate and adult; the responses were observed in a lower concentration-range in the neonate than in the adult. WB4101 produced a significant leftward shift of the concentration-response curve for noradrenaline in adult preparations while only a slight rightward shift was observed in the neonate. 5. Our results demonstrate the presence of alpha-adrenoceptor-mediated inotropic responses in the mouse ventricular myocardia. The response to phenylephrine changes from a positive to a negative effect during postnatal development. The responses are mediated by alpha 1-adrenoceptors, and modulate the overall inotropic response to NA in the adult.

Demonstration of the suitability of CGP 12177 for in vivo studies of beta-adrenoceptors

1. Positron emission tomography (PET) with appropriate radioligands offers the possibility of studying receptors non-invasively in man. The suitability of CGP 12177, a hydrophilic non-selective beta-adrenoceptor antagonist which can be labelled with the positron emitter 11C, as a ligand for in vivo studies of beta-adrenoceptors was assessed in rats. 2. [3H]-CGP 12177 was injected into the tail veins of restrained conscious rats. Serial blood samples were taken from tail arteries to determine clearance from plasma. Rats were killed and tissues removed to determine tissue uptake. Radioactivity was assessed by liquid scintillation counting. 3. The uptake of (-)-[3H]-CGP 12177 in various tissues was compared to that of (+/-)-[3H]-CGP 12177. Maximum tissue:plasma ratios obtained for the (-)-enantiomer in lung, heart and liver were 170, 42 and 13 compared with 60, 15 and 12 for the racemate. Prior injection of excess unlabelled (+/-)-CGP 12177 blocked the uptake of both (-)- and (+/-)-[3H]-CGP 12177 in lung and heart but not liver, tissue:plasma ratios for both tracers being reduced to 7, 3 and 7 respectively. 4. Clearance of (-)-[3H]-CGP 12177 from plasma was rapid during the first 5 min but showed only small changes during 5 to 90 min. Uptake in lung and heart reached a maximum at 1 to 5 min and showed a slow decrease during 5-90 min. Prior injection of unlabelled (+/-)-CGP 12177 reduced uptake in lung and heart to 10% and 20% respectively. Injection of unlabelled ( +/-)-CGP 12177 at 15 mind is placed ~75% of the radioactivity by 90 min.5. ( +/- )-Propranolol had a similar effect to that of unlabelled ( +/-)-CGP 12177. Prior injection reduced uptake of radioactivity in lung and heart to 15% and 20% respectively. Injection of ( +/- )-propranolol at 15 min displaced ~ 60% of the radioactivity by 90 min indicating that the tracer binds to beta-adrenoceptor sites in vivo.6. In vivo saturation curves, obtained by injection of (-)-[3H]-CGP 12177 with increasing amounts of unlabelled (- )-CGP 12177, gave values of Bmax for lung of ~45 pmol per g wet weight of tissue and for heart of ~6 pmol per g wet weight of tissue. KD could only be expressed as nmol injected per kg bodyweight, that for lung (2.5 nmol kg-1) being greater than that for heart (1.3 nmol kg-1).7. Competition studies carried out by co-injecting (-)-[3H]-CGP 12177 with unlabelled (+/- )-CGP12177 or (-)-propranolol gave similar values for Bmax (lung 44 pmol g-1, heart 6 pmol g-1,). Values of KD for (+/-)-CGP 12177 (lung 4.7 pmol kg-1, heart 2.6 pmol kg-1) were approximately twice those for(-)-CGP 12177. Values of KD for (-)-propranolol (lung 38 nmol kg-1, heart 104 nmol kg-1) were greater.8. The results show that (-)-[3H]-CGP 12177 is a suitable ligand for assessing beta-adrenoceptors in vivo.

Suitability of CGP-12177 and CGP-26505 for quantitative imaging of beta-adrenoceptors

[3H]CGP-12177, a non-selective beta-adrenoceptor antagonist, and [3H]CGP-26505, a beta 1-selective beta-adrenoceptor antagonist, were intravenously administered to rats. 94-97% of the injected radioactivity disappeared from plasma with t1/2 0.2 and 0.5 min. Total/non-specific binding ratios of 5.4 and 6.9 (CGP-12177) or 2.0 and 2.8 (CGP-26505) were maintained in heart and lung from 10 to 40 min post-injection. Labelled plasma metabolites appeared after greater than 20 min (CGP-12177) or within 2 min (CGP-26505). No metabolites were found in the heart. CGP-12177 binds to blood cells, but CGP-26505 does not. CGP-12177 can be used for PET imaging of total (beta 1 and beta 2) adrenoceptors in the heart and lung of experimental animals, but CGP-26505 is less suitable for in vivo analysis of the beta 1-subpopulation.

Activation of ?1-adrenoceptors modulates the inwardly rectifying potassium currents of mammalian atrial myocytes

The selective alpha 1-adrenergic agonist methoxamine (10(-4)-10(-3) M), in the presence of propranolol (10(-6) M), can reduce both the inwardly rectifying K+ background current (IK1) and the muscarinic cholinergic receptor-activated K+ current (IK,ACh) in rabbit atrial myocytes resulting in action potential prolongation during the final phase of repolarization and a depolarization of the resting membrane potential. The reduction of these K+ currents(s) by alpha 1-adrenoceptor stimulation was insensitive to pre-treatment of atrial myocytes with pertussis toxin (0.15-0.5 micrograms/ml) and was irreversible following intracellular dialysis with the non-hydrolysable guanosine triphosphate (GTP) analogue, Gpp(NH)p (1-5 x 10(-3) M). Neither the protein kinase C (PKC) inhibitors, 1((5-isoquinolinesulphonyl)-2-methylpiperoxine (H-7) (5 x 10(-5) M) and staurosporine (1 x 10(-7) M), nor "downregulation" of PKC by prolonged phorbol ester exposure (5 x 10(-7) M, for 7-8 h) had an effect on the alpha 1-adrenergic modulation of this K+ current. Under cell-attached patch-clamp conditions, bath application of methoxamine reversibly decreased acetylcholine-induced single-channel activity, thus confirming the observed reduction of the ACh-induced current under whole-cell voltage clamp. These results demonstrate that the alpha 1-adrenoceptor, once activated, can reduce current through two different inwardly rectifying K+ channels in rabbit atrial myocytes. These current changes are mediated via a pertussis toxin-insensitive GTP-binding protein, and do not appear to involve the activation of PKC.

Synergism of atrial pressure and adrenergic stimulation for ANF release in the rat

The effect of adrenergic stimulation on atrial natriuretic factor (ANF) release was studied in conscious rats. 60 min i.v. infusion of 15 micrograms/kg/min phenylephrine produced an elevation in mean arterial pressure of about 50 mmHg that was associated with an immediate, transitory increase in both central venous (CVP) and left ventricular end-diastolic (LVEDP) pressures. Heart rate was not changed. The elevation in CVP, but not LVEDP, persisted until 5 min, together with a 50-fold increase in plasma C-terminal ANF concentrations (from 19 +/- 5 to 1010 +/- 339 fmol/ml); at 30 min, in the absence of any increases in CVP or LVEDP, plasma ANF was still elevated by 5-fold (114 +/- 35 fmol/ml). It is suggested that adrenergic stimulation 'per se' can induce ANF release, independently of atrial pressure. Furthermore, combined adrenergic stimulation and increased atrial wall tension could result in a potentiation of the ANF secretory response 'in vivo', in the case where both stimuli are present concomitantly.

Central beta 1-adrenergic receptors are involved in CRF-induced defensive withdrawal

Previous studies indicated that peripheral administration of propranolol, a nonselective beta-adrenergic antagonist, attenuated ICV CRF-induced suppression of a conditioned emotional response and defensive withdrawal behavior in rats, suggesting the involvement of a beta-adrenergic receptor in the CRF-induced behavioral changes. The present study was carried out to determine whether central or peripheral beta-adrenergic receptors are involved in CRF-induced defensive withdrawal behavior, and which subtype of beta-adrenergic receptor is involved. l-Propranolol (2.5 mg/kg IP) significantly reversed CRF-induced defensive withdrawal behavior. CGP-12177 (1 mg/kg IP), a beta-adrenergic antagonist with predominant effects on peripheral beta-adrenergic receptors, and ICI 118,551 (0.5 mg/kg IP), a selective beta 2-adrenergic antagonist, had no significant effects on CRF-induced defensive withdrawal. When administered ICV, two selective beta 1-adrenergic antagonists, CGP-20712A (10 micrograms) and atenolol (100 micrograms), significantly antagonized CRF-induced defensive withdrawal. Our results suggest that a central beta 1-adrenergic receptor is involved in CRF-induced defensive withdrawal in rats.