Overexpressed A(1) adenosine receptors reduce activation of acetylcholine-sensitive K+ current by native muscarinic M-2 receptors in rat atrial myocytes

In adult rat atrial myocytes, muscarinic acetylcholine (ACh)-sensitive K+ c urrent activated by a saturating concentration of adenosine (I-K(ACh),I-(Ad o)) via A(1) receptors (A(1)Rs) amounts to only 30% of the current activate d 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 exp osure 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 s tudy the importance of receptor density for A(1)R-I-K(ACh),I-(Ado) signalin g, adult atrial myocytes in vitro were transfected with cDNA encoding for r at 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-m atched cells transfected with only the EGFP vector served as controls. In a pproximate 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 th is fraction. In approximate to 70% of transfection-positive myocytes (group II), the density of I-K(ACh),I-(ACh) was significantly reduced, its activa tion was slowed, and the fast desensitizing component was lost. Adenosine-i nduced currents were larger in group II than in group I, their activation r ate was further increased, and a fast desensitizing component developed. Th ese data indicate that in native myocytes the amplitude and activation kine tics of I-K(ACh),I-(Ado) are limited by the expression of A(1)R. Overexpres sion of A(1)R negatively interferes with signal transduction via the muscar inic M-2 receptor-linked pathway, which might reflect a competition of rece ptors with a common pool of G proteins. Negative interference of an overexp ressed receptor with physiological regulation of a target protein by a diff erent receptor should be considered in attempts to use receptor overexpress ion for gene therapy.