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

This study investigated the coupling of M-cholinoceptors and A(1) aden osine receptors in human myocardium. Carbachol reduced force of contra ction in atria and ventricles by 60 and 35% (in the presence of 0.03 m u M isoprenaline), respectively. Addition of (-)-N-6-(2-phenylisopropy l)adenosine (R-PIA) in the presence of carbachol did not further reduc e force of contraction. R-PIA reduced force of contraction less than c arbachol in atria (ventricles) by 35% (25%), but addition of carbachol after R-PIA reduced force of contraction further by 35% (15%). Carbac hol increased S-35-labeled guanosine 5'-O-(3-thiotriphosphate) (GTP ga mma S) binding maximally 14-fold only when GDP was present by an exces s 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 rec eptor and M-cholinoceptor stimulation was antagonized by theophylline and atropine, respectively. Addition of R-PLA. to carbachol did not fu rther 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 bind ing 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 bi nding could not reliably be detected with this technique. From the num ber of M-cholinoceptors (atria, 360 fmol/mg protein; ventricle, 270 fm ol/mg protein), it is estimated that 1 M-cholinoceptor activates simil ar to 50-80 G protein ol-subunits in atria and ventricles, respectivel y. It is concluded that stimulation of M-cholinoceptors but not A(1) a denosine receptors is able to maximally activate a pool of G protein a lpha-subunits. A catalytic activation of G protein alpha-subunits by M -cholinoceptors was demonstrated. Because A(1) adenosine receptors med iate 40-50% of the maximal effects produced by M-cholinoceptors but th eir density is only 8-10% of the M-cholinoceptor density, A(1) adenosi ne receptors are Likely to be coupled more efficiently than M-cholinoc eptors in the human heart.