Overexpressed A1 adenosine receptors reduce activation of acetylcholine-sensitive K+ current by native muscarinic M2 receptors in rat atrial myocytes

In adult rat atrial myocytes, muscarinic acetylcholine (ACh)-sensitive K+ current activated by a saturating concentration of adenosine (I-K(ACh),I-(Ado)) via A(1) receptors (A(1)Rs) amounts to only 30% of the current activated by a saturating concentration of ACh (I-K(ACh),I-(ACh)) via muscarinic M-2 receptors, The half-time of activation of I-K(ACh),I-(Ado) on a rapid exposure to agonist was approximate to 4-fold longer than that of I-K(ACh),I-(ACh). Furthermore, I-K(ACh),I-(Ado) never showed fast desensitization. To study the importance of receptor density for A(1)R-I-K(ACh),I-(Ado) signaling, adult atrial myocytes in vitro were transfected with cDNA encoding for rat brain A(1)R and enhanced green fluorescent protein (EGFP) as a reporter. Whole-cell current was measured on days 3 and 3 after transfection. Time-matched cells transfected with only the EGFP vector served as controls. In approximate to 30% of EGFP-positive cells (group I), the density of I-K(ACh),I-(Ado) was increased by 72%, and its half-time of activation was reduced. Density and kinetic properties of I-K(ACh),I-(ACh) were not affected in this fraction. In approximate to 70% of transfection-positive myocytes (group II), the density of I-K(ACh),I-(ACh) was significantly reduced, its activation was slowed, and the fast desensitizing component was lost. Adenosine-induced currents were larger in group II than in group I, their activation rate was further increased, and a fast desensitizing component developed. These data indicate that in native myocytes the amplitude and activation kinetics of I-K(ACh),I-(Ado) are limited by the expression of A(1)R. Overexpression of A(1)R negatively interferes with signal transduction via the muscarinic M-2 receptor-linked pathway, which might reflect a competition of receptors with a common pool of G proteins. Negative interference of an overexpressed receptor with physiological regulation of a target protein by a different receptor should be considered in attempts to use receptor overexpression for gene therapy.