COUPLING OF M-CHOLINOCEPTORS AND A(1) ADENOSINE RECEPTORS IN HUMAN MYOCARDIUM

This study investigated the coupling of M-cholinoceptors and A(1) adenosine receptors in human myocardium. Carbachol reduced force of contraction in atria and ventricles by 60 and 35% (in the presence of 0.03 mu M isoprenaline), respectively. Addition of (-)-N-6-(2-phenylisopropyl)adenosine (R-PIA) in the presence of carbachol did not further reduce force of contraction. R-PIA reduced force of contraction less than carbachol in atria (ventricles) by 35% (25%), but addition of carbachol after R-PIA reduced force of contraction further by 35% (15%). Carbachol increased S-35-labeled guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) binding maximally 14-fold only when GDP was present by an excess of > 100 times [S-35]GTP gamma S, while R-PIA increased binding only 2-fold. Activation of [S-35]GTP gamma S binding by A(1) adenosine receptor and M-cholinoceptor stimulation was antagonized by theophylline and atropine, respectively. Addition of R-PLA. to carbachol did not further increase [S-35]GTP gamma S binding. Activation of high-affinity [S-35]GTP gamma S binding by agonists showed that carbachol activated [S-35]GTP gamma S binding to 20 pmol/mg protein [S-35]GTP gamma S binding sites, i.e., 25% of the total number of binding sites (80 pmol/mg protein). With R-PIA, activation of high-affinity [S-35]GTP gamma S binding could not reliably be detected with this technique. From the number of M-cholinoceptors (atria, 360 fmol/mg protein; ventricle, 270 fmol/mg protein), it is estimated that 1 M-cholinoceptor activates similar to 50-80 G protein ol-subunits in atria and ventricles, respectively. It is concluded that stimulation of M-cholinoceptors but not A(1) adenosine receptors is able to maximally activate a pool of G protein alpha-subunits. A catalytic activation of G protein alpha-subunits by M-cholinoceptors was demonstrated. Because A(1) adenosine receptors mediate 40-50% of the maximal effects produced by M-cholinoceptors but their density is only 8-10% of the M-cholinoceptor density, A(1) adenosine receptors are Likely to be coupled more efficiently than M-cholinoceptors in the human heart.