AbstractIn adult rat atrial myocytes, muscarinic acetylcholine (ACh)-sensitive K+current activated by a saturating concentration of adenosine (IK(ACh),(Ado)) via A1receptors (A1Rs) amounts to only 30% of the current activated by a saturating concentration of ACh (IK(ACh),(ACh)) via muscarinic M2receptors. The half-time of activation ofIK(ACh),(Ado)on a rapid exposure to agonist was ≈4-fold longer than that ofIK(ACh),(ACh). Furthermore,IK(ACh),(Ado)never showed fast desensitization. To study the importance of receptor density for A1R-IK(ACh),(Ado)signaling, adult atrial myocytes in vitro were transfected with cDNA encoding for rat brain A1R and enhanced green fluorescent protein (EGFP) as a reporter. Whole-cell current was measured on days 3 and 4 after transfection. Time-matched cells transfected with only the EGFP vector served as controls. In ≈30% of EGFP-positive cells (group I), the density ofIK(ACh),(Ado)was increased by 72%, and its half-time of activation was reduced. Density and kinetic properties ofIK(ACh),(ACh)were not affected in this fraction. In ≈70% of transfection-positive myocytes (group II), the density ofIK(ACh),(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 ofIK(ACh),(Ado)are limited by the expression of A1R. Overexpression of A1R negatively interferes with signal transduction via the muscarinic M2receptor–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.