Effects of extraneuronal uptake inhibitors on the positive chronotropic response to isoprenaline and on the accumulation of isoprenaline in perfused rat heart after inhibition of catechol-O-methyl transferase

The effect of entacapone on the disposition and hemodynamic effects of intravenous isoproterenol and epinephrine

BACKGROUND

Entacapone is a potent, selective catechol-O-methyltransferase (COMT) inhibitor. Entacapone could potentiate the hemodynamic effects of exogenously administered catecholamines, which are substrates of the COMT enzyme.

DESIGN AND METHODS

Originally, the study was to follow a placebo-controlled, randomized crossover design. Because of two cases of ventricular arrhythmia, a decision was made to terminate the study before its completion. Six subjects went through the isoproterenol and epinephrine infusions while taking placebo and five other subjects while taking entacapone. The actual design was thus one with two parallel groups with random allocation and double-blind drug administration. The subjects were given either a single dose of 400 mg entacapone or placebo 30 minutes before the start of isoproterenol or epinephrine infusions. Four dosages of epinephrine (1.5, 3, 6, or 12 micrograms/min) and isoproterenol (0.5, 1, 1.5, or 2 micrograms/min) were infused (5 minutes for each level). Heart rate and blood pressure were measured and ECG was monitored. The concentrations of isoproterenol and epinephrine in plasma were determined by HPLC.

RESULTS

The maximal increase in heart rate during isoproterenol infusion after entacapone administration (40 +/- 11 beats/min, mean +/- SD) was statistically greater (p = 0.0496) than after placebo administration (27 +/- 7 beats/min). The increase in heart rate during epinephrine infusion was 25 +/- 13 beats/min after entacapone administration and 14 +/- 9 beats/min after placebo administration (p = 0.127). There were no statistically significant differences between entacapone and placebo in blood pressure or in plasma concentrations of isoproterenol and epinephrine.

CONCLUSION

We conclude that entacapone may potentiate the chronotropic and arrhythmogenic effects of exogenously administered isoproterenol and epinephrine.

Effects of extraneuronal accumulation of isoprenaline on cAMP production in perfused rat heart

The effects of extraneuronal accumulation of isoprenaline on the level of cAMP in perfused rat hearts were investigated. When catechol-O-methyl transferase (COMT) was intact, perfusion with isoprenaline (10(-6) M) for 5 min and 30 min (low accumulation of isoprenaline in the heart) enhanced the cAMP level. Propranolol (10(-6) M) significantly decreased the high level of cAMP produced by the perfusion with isoprenaline for 5 min and 30 min (low accumulation of isoprenaline). When COMT was inhibited by tropolone, perfusion with isoprenaline (10(-6) M) for 5 min (slight accumulation of isoprenaline in the heart) slightly increased the level of cAMP, while perfusion for 30 min (high accumulation of isoprenaline in the heart) did not increase the level of cAMP. Propranolol (10(-6) M) significantly decreased the cAMP level produced by 5 min perfusion with isoprenaline, but did not change the level by 30 min perfusion. The perfusion length (5 min and 30 min) and COMT inhibition by tropolone (10(-4) M) in the absence of isoprenaline did not affect cAMP levels. These results suggest that extraneuronally accumulated isoprenaline may inhibit the adenylate cyclase in perfused rat hearts.

Intracellular accumulation of isoproterenol enhances the lactate production in the perfused rat heart

Effects of intracellular accumulation of isoproterenol (ISO) on lactate production were examined in perfused rat heart. The lactate production during ISO perfusion in rat heart was increased and subsequent addition of an inhibitor of catechol-O-methyl transferase (COMT) further enhanced the production, and the enhanced production was significantly reduced by uptake2 inhibitor. The perfusion with ISO free-medium in the heart with high intracellular accumulation of ISO produced lactate more than that in the low intracellular accumulation. The present experiments demonstrated that the enhanced lactate production is accompanied by intracellular accumulation of ISO in the perfused rat heart, and suggested that the accumulated ISO may activate intracellular beta-adrenoceptors in the rat heart.

Effects of specific alpha-adrenoceptive agents on extraneuronal uptake (uptake2) of isoproterenol in perfused rat heart

Effects of specific alpha-adrenoceptive agents (alpha 1-agonist, alpha 1-antagonist, alpha 2-agonist and alpha 2-antagonist) on the extraneuronal accumulation of 3H-isoproterenol in the perfused rat heart were examined. The extraneuronal accumulation of 3H-isoproterenol in the hearts perfused with 3H-isoproterenol (10(-6)M) under COMT inhibition by tropolone (10(-4)M) was about 6 times higher than that of intact COMT. The increase in the accumulation by COMT inhibition was regarded as 100% and the effects of specific alpha-adrenoceptive agents on the accumulation was evaluated. alpha 1-agonists, methoxamine and phenylephrine, did not affect the accumulation. alpha 1-antagonists, prazosin, bunazosin and YM-12617, significantly decreased the accumulation of 3H-isoproterenol and these IC50 values were 2 x 10(-6)M, 3.5 x 10(-6)M and 2.3 x 10(-5)M, respectively. alpha 2-agonists, clonidine and guanabenz, significantly reduced the accumulation and these IC50 values were 3.4 x 10(-5)M and 2.9 x 10(-7)M, respectively. The alpha 2-antagonist, yohimbine, did not affect the accumulation. The present experiments clearly demonstrated that the tested alpha 1-antagonists and alpha 2-agonists inhibited uptake2 in rat heart but the tested alpha 1-agonists and an alpha 2-antagonist did not inhibit it.

Possible activation of intracellular beta-adrenoceptors by extraneuronally accumulated isoprenaline in perfused rat heart

Two rat hearts were perfused in series by a modified Loewi's method. The recipient heart was perfused with the perfusate collected from the donor heart. 1. After perfusion with 3H-isoprenaline in the presence of tropolone, an inhibitor of catechol-O-methyltransferase, the donor heart was washed out with amine-free medium containing tropolone and corticosterone. The heart rate of the recipient heart increased after the change to the perfusate from the donor heart during the wash-out. After wash-out the heart rate of the donor heart (which had accumulated 43.4 pmol.g-1 3H-isoprenaline) was higher than that of the recipient heart (which had accumulated 0.44 pmol.g-1 3H-isoprenaline), and the rates of efflux of 3H-isoprenaline from both hearts were similar. 2. After perfusion with 3H-isoprenaline and corticosterone in the absence of tropolone, the enhanced heart rate of the donor heart decreased during wash-out with amine-free medium in the presence of corticosterone. The heart rate of the recipient heart increased after the medium change to the perfusate from the donor heart, and the heart rates in both hearts were similar after wash-out. Only small amounts of 3H-isoprenaline remained in both hearts after wash-out, and the rates of efflux of 3H-isoprenaline from both hearts were similar. 3. After perfusion with 3H-isoprenaline in the presence of tropolone, the effects of propranolol and atenolol on the heart rate during wash-out with amine-free medium containing tropolone and corticosterone were compared. The inhibitory effect of propranolol on the heart rate was significantly greater than that of atenolol.(ABSTRACT TRUNCATED AT 250 WORDS)

Extraneuronal accumulation of isoproterenol in atria and ventricle of perfused rat heart

Extraneuronal accumulation of isoproterenol in atria and ventricle of perfused rat heart was investigated. Rat hearts were perfused with various concentrations of 3H-isoproterenol for 30 min in the absence and the presence of catechol-O-methyltransferase (COMT) inhibitor (tropolone). When COMT was intact, the accumulation of 3H-isoproterenol in both atria and ventricle after perfusion with low concentration of 3H-isoproterenol (0.01 to 1 mumol/l) was less than that of perfusing concentration; the tissue/medium ratio (T/M) of isoproterenol for artia was lower than that for ventricle. The T/M of isoproterenol after perfusion with 10 and 20 mumol/l of 3H-isoproterenol were 0.94 and 1.76 for atria and 3.25 and 2.95 for ventricle, respectively. When COMT was inhibited by tropolone, the T/M increased 6.3-9.0 folds for atria and 5.1-6.7 folds for ventricle after perfusion with 3H-isoproterenol (0.01 to 1 mumol/l). From these results, it was concluded that both atria and ventricle of the rat heart have an extraneuronal O-methylating system as reported in rat whole heart, and was suggested that there might be different capacities of extraneuronal uptake and COMT between them.