CHARACTERIZATION OF BRADYKININ B-2 RECEPTORS IN ADULT MYOCARDIUM AND NEONATAL RAT CARDIOMYOCYTES

Specific [I-125-Tyr(8)]bradykinin (BK) binding was observed on myocardial membranes from adult guinea pigs, dogs, rats, and rabbits that was displaced by unlabeled BK with an IC50 between 0.1 and 30 nmol/L. In the adult guinea pig ventricular myocardium, which displays both high- and low-affinity binding, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S; 100 mu mol/L) eliminated high-affinity binding and reduced total specific [2,3-prolyl-3,4-H-3(N)]BK ([H-3]BK) binding by >60%. Agonist competition binding to rat myocardial membranes was characterized as being of one affinity for BK in the nanomolar range, and it was not altered by GTP gamma S. Saturation binding studies with [I-125-Tyr(8)]BK and [H-3]BK, performed on cultured neonatal rat cardiac myocytes, revealed a single class of BK binding sites with a K-d and B-max of 0.24+/-0.04 nmol/L and 18.4+/-1.1 fmol/mg protein, respectively (approximate to 1500 receptors per cell). In competitive binding assays, unlabeled BK, Hoe 140 (a specific BK B-2 receptor antagonist), and des-Arg(9),[Leu(8)]BK (a BK B-1 receptor antagonist) displaced [I-125-Tyr(8)]BK with an IC50 of 4.3, 0.041, and 307 nmol/L, respectively. In the presence of 100 mu mol/L GTP gamma S, [H-3]BK binding to myocyte membranes was reduced by 40%, but the IC50 did not change. Cardiac fibroblasts, evaluated in parallel to the myocytes, contain a single class of [H-3]BK binding sites (248+/-72 fmol/mg) with a 130-fold lower relative affinity (32.4+/-11.3 nmol/L) than that determined in rat cardiomyocytes. BK stimulated the inositol 1,4,5-trisphosphate (IP3) production by cardiomyocytes, which reached a maximum after 20 seconds of stimulation and increased from a baseline of 138.4+/-23.2 pmol/mg protein to 1020.7+/-75.9 pmol/mg with 1 mu mol/L BK (EC(50)=15.3 nmol/L). The effect was significantly blocked by 1 mu mol/L Hoe 140. The IP3 response by cardiomyocytes was fourfold greater and sixfold more sensitive than that by cardiac fibroblasts (EC(50)=92.3 nmol/L). These data suggest the presence of high-affinity BK B-2 receptors on cardiomyocytes, which are functionally coupled via a G protein to the production of IP3.