Characterization of bradykinin B2 receptors in adult myocardium and neonatal rat cardiomyocytes

Abstract

Specific [125I-Tyr8]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 mumol/L) eliminated high-affinity binding and reduced total specific [2,3-prolyl-3,4-3H(N)]BK ([3H]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 [125I-Tyr8]BK and [3H]BK, performed on cultured neonatal rat cardiac myocytes, revealed a single class of BK binding sites with a Kd and Bmax of 0.24 +/- 0.04 nmol/L and 18.4 +/- 1.1 fmol/mg protein, respectively (approximately 1500 receptors per cell). In competitive binding assays, unlabeled BK, Hoe 140 (a specific BK B2 receptor antagonist), and des-Arg9,[Leu8]BK (a BK B1 receptor antagonist) displaced [125I-Tyr8]BK with an IC50 of 4.3, 0.041, and 307 nmol/L, respectively. In the presence of 100 mumol/L GTP gamma S, [3H]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 [3H]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.(ABSTRACT TRUNCATED AT 250 WORDS)