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

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.