Overexpressed A(1) adenosine receptors reduce activation of acetylcholine-sensitive
K(+) current by native muscarinic M(2) receptors in rat atrial myocytes
Circ Res, 86 (6):
643-8 (March 2000)Wellner-Kienitz, M C Bender, K Meyer, T Bunemann, M Pott, L Research
Support, Non-U.S. Gov't United states Circulation research Circ Res.
2000 Mar 31;86(6):643-8..Abstract
In adult rat atrial myocytes, muscarinic acetylcholine (ACh)-sensitive
K(+) current activated by a saturating concentration of adenosine
(I(K(ACh),(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),(ACh)))
via muscarinic M(2) receptors. The half-time of activation of I(K(ACh),(Ado))
on a rapid exposure to agonist was approximately 4-fold longer than
that of I(K(ACh),(ACh)). Furthermore, I(K(ACh),(Ado)) never showed
fast desensitization. To study the importance of receptor density
for A(1)R-I(K(ACh),(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 4 after transfection. Time-matched cells
transfected with only the EGFP vector served as controls. In approximately
30% of EGFP-positive cells (group I), the density of I(K(ACh),(Ado))
was increased by 72%, and its half-time of activation was reduced.
Density and kinetic properties of I(K(ACh),(ACh)) were not affected
in this fraction. In approximately 70% of transfection-positive myocytes
(group II), the density of I(K(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 of I(K(ACh),(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.