Pharmacological characteristics of adenosine-induced inhibition of dog ventricular contractility: dependence on the pre-existing level of beta-adrenoceptor activation

Inotropic effects of diadenosine tetraphosphate in isolated canine cardiac preparations

We studied the effects of diadenosine tetraphosphate (AP4A) on the force of contraction in canine preparations. The force of contraction was measured in isolated electrically driven (1 Hz) atrial and ventricular cardiac trabeculae from adult dogs. AP4A (100 microM) alone and after prestimulation with 10 nM isoproterenol reduced force of contraction in atrial preparations by approximately 24%. Moreover, AP4A (100 microM) alone and after prestimulation with 10 nM isoproterenol reduced the force of contraction in ventricular preparations by 29 and 29%, respectively. The negative inotropic effects of AP4A were abolished by the A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX). In summary, in canine myocardium, AP4A alone and after prestimulation with a beta-adrenoceptor agonist exerts negative inotropic effects, which are probably mediated via A1-adenosine receptors.

Deferoxamine blocks interactions of fluoride and carbachol in isolated mammalian cardiac preparations

In papillary muscles, carbachol reduced the positive inotropic effects of isoprenaline (10 nmol/l). The negative inotropic effects of carbachol in isoprenaline-stimulated guinea pig papillary muscles were attenuated by additionally applied sodium fluoride (3 mmol/l). These effects of sodium fluoride were blocked by deferoxamine (200 micromol/l). In guinea pig left atria, sodium fluoride alone greatly reduced force of contraction. These effects in atria were blocked by 200 micromol/l deferoxamine, and positive inotropic effects of sodium fluoride were observed. It is suggested that the cardiac effects of muscarinic M2 receptor agonists in the ventricle involve, at least in part, the activation of phosphatases which are blocked by fluoride and reactivated by deferoxamine.

Sodium fluoride attenuates the negative inotropic effects of muscarinic M2 and adenosine receptor agonists

Sodium fluoride increased the force of contraction in isolated guinea-pig papillary muscles concentration dependently, starting at 3 mmol/1. Sodium fluoride inhibited phosphorylase phosphatase activity in homogenates from guinea pig hearts, starting at 1 mmol/1. The positive inotropic effect of 3 mmol/1 sodium fluoride was not accompanied by an increase in cAMP content in guinea-pig papillary muscles. In papillary muscles, carbachol or (-)-N(6)-phenylisopropyladenosine reduced the positive inotropic effect of isoprenaline (10 nmol/1) or the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (60 mu mol/1). These negative inotropic effects of carbachol and (-)-N(6)-phenylisopropyladenosine were attenuated by additional sodium fluoride (3 mmol/l). It is concluded that sodium fluoride can impair the signal transduction of muscarinic M2 (carbachol) and adenosine receptor (-)-N(6)-phenylisopropyladenosine) agonists. This effect of sodium fluoride could support the hypothesis that the cardiac effects of muscarinic M2 and adenosine receptor agonists involve, at least in part, the activation of phosphatases.

Effects of adenosine on Ca2+ transients and tension in aequorin-injected ferret papillary muscles

The effects of adenosine on Ca2+ transients and tension in ferret papillary muscles were investigated using the aequorin method. Adenosine (0.01-1 mM) reduced the peak of Ca2+ transients and caused a slight concentration-dependent decrease in tension. Adenosine prolonged the decay time of Ca2+ transients but did not alter the time course of tension. In isoproterenol (0.1 microM)-treated preparations, adenosine decreased the peak of Ca2+ transients but did not alter the peak of tension. Adenosine prolonged the isoproterenol-shortened decay time of Ca2+ transients. The effects of adenosine on Ca2+ transients were antagonized by the selective A1 receptor antagonist 8-cyclopentyl-1,3,-dipropylxanthine. In the presence of isoproterenol, adenosine (0.1 mM) shifted the intracellular [Ca2+]/tension relation to the left. These results can be explained by the hypothesis that adenosine inhibits the activity of adenylate cyclase via stimulation of the A1 receptor, other mechanisms however cannot be overlooked. The prolongation of the decay time of Ca2+ transients and the increase in the Ca2+ sensitivity of the contractile elements are the underlying mechanisms of adenosine which maintain developed tension in twitch response, although adenosine decreases the peak of Ca2+ transients.

Antiadrenergic effect of carbachol but not of adenosine on contractility in the intact human ventricle in vivo

OBJECTIVES

The purpose of this study was to investigate the antiadrenergic effects of adenosine and carbachol on beta-adrenoceptor-stimulated human ventricular contractility in vivo. In addition, the antiadrenergic effects of adenosine and carbachol were compared in vitro.

BACKGROUND

Adenosine is reported to exhibit an antiadrenergic negative inotropic response in the beta-adrenergic-stimulated ventricular myocardium in vitro. The effect of adenosine is similar to the antiadrenergic effect of m-cholinoceptor stimulation in vitro.

METHODS

The inotropic response in vivo was assessed in seven healthy volunteers by M-mode echocardiography and simultaneous blood pressure monitoring. It was calculated as the increase in the rate-corrected velocity of circumferential fiber shortening and in the systolic pressure/dimension ratio. All volunteers received pretreatment with 450 mg of dipyridamole/day for 48 h. In addition, the effects of adenosine and carbachol in the presence of 0.03 mumol/liter of isoproterenol on cumulative concentration-response curves of isolated, electrically driven human ventricular muscle strips were compared in vitro (n = 13).

RESULTS

The positive inotropic response to continuous infusion of 20 ng/kg per min of isoproterenol (increase of rate-corrected velocity of circumferential fiber shortening [10.2 +/- 2.1% x square root of beats/min per ms] and increase of systolic pressure/dimension ratio 1.09 +/- 0.3 mm Hg/mm) was significantly (p < 0.01) reduced by 3.6 micrograms/kg body weight of intravenous carbachol (4.2 +/- 1.2% x square root of beats/min per ms, 0.21 +/- 0.18 mm Hg/mm) but not by 50 micrograms/kg of intravenous adenosine (8.2 +/- 3.1% x square root of beats/min per ms, 1.35 +/- 0.42 mm Hg/mm), although adenosine induced a significant negative dromotropic effect. In vitro comparison of force of contraction with cumulative concentration-response curves in the presence of 0.03 mumol/liter of isoproterenol demonstrated an EC50 value (concentration producing half-maximal effect) for adenosine 466 times higher than that for carbachol (65.3 vs. 0.14 mumol/liter, p < 0.001).

CONCLUSIONS

In contrast to carbachol, adenosine does not attenuate the catecholamine-induced increase in contractility in the human ventricle in vivo. These differences between the A1-adenosine receptor- and m-cholinoceptor-mediated effects could be due to fewer A1-adenosine receptors or a less efficient receptor-effector coupling, or both.

Pronounced direct inhibitory action mediated by adenosine A1 receptor and pertussis toxin-sensitive G protein on the ferret ventricular contraction

An adenosine A1 receptor agonist R-N6-phenylisopropyladenosine (R-PIA) elicited a pronounced negative inotropic effect with the EC50 value of 0.69 mumol/l in the presence of a beta-adrenoceptor blocking agent bupranolol (0.3 mumol/l) in the isolated ferret papillary muscle. The negative inotropic effect of R-PIA was not associated with changes in cyclic AMP level. Adenosine and other A1 receptor agonists also elicited a negative inotropic effect. DPCPX (1,3-dipropyl-8-cyclopentyl xanthine) antagonized the negative inotropic effect of R-PIA in a competitive manner (pA2 value = 8.4). The inhibitory action of R-PIA was markedly attenuated in the ventricular muscle preparation isolated from ferrets pretreated with pertussis toxin that caused ADP-ribosylation of 39 kDa proteins in the membrane fraction. In the membrane fraction derived from the ferret ventricle, [3H]-DPCPX bound to a single binding site in a saturable and reversible manner with high affinity (Kd value = 1.21 +/- 0.41 nmol/l; Bmax = 12.8 +/- 3.02 fmol/mg protein; n = 7). The binding characteristics of [3H]-DPCPX in the rat ventricle (Kd value = 1.51 +/- 0.09 nmol/l; Bmax = 12.7 +/- 1.47 fmol/mg protein; n = 5) were similar to those in the ferret. On the other hand, the content of G(o), a major pertussis toxin-sensitive G protein in the ferret heart, was much higher in the ferret than in the rat ventricle. The present results indicate that adenosine receptors may play an important role in the inhibitory regulation of ventricular contractility in the ferret in contrast to other mammalian species. The signal transduction process subsequent to agonist binding to A1 receptors including the pertussis toxin-sensitive G protein and ion channels may be responsible for the unique inhibitory action of adenosine in this species.