B-2 BRADYKININ RECEPTORS IN CULTURED NEONATAL RAT CARDIOMYOCYTES MEDIATE A NEGATIVE CHRONOTROPIC AND NEGATIVE INOTROPIC RESPONSE

Receptors for bradykinin (BK) were characterized in primary cultures o f beating neonatal rat cardiomyocytes using [H-3]BK as radioligand. De gradation studies demonstrated that [H-3]BK was stable for at least 2 h when incubated with cardiomyocytes at 2 and 37 degrees C in the pres ence of bacitracin in combination with captopril or ramiprilat. Withou t these inhibitors, >80% of the [3H]BK was degraded within 2 h at 37 d egrees C. This indicates that angiotensin-concerting enzyme (ACE) is r esponsible for the main BK-degrading activity in cardiomyocytes. Scatc hard plots were linear and gave a K-d of 1.5 +/- 0.8 nmol/l (mean +/- SD, n = 4) and a maximum binding capacity of 55-125 fmol/mg protein. A ssociation and dissociation studies showed that binding of [H-3]BK was saturable and reversible. Binding of [H-3]BK at 37 degrees C led to i nternalization of the ligand. Competition studies with B-1 and B-2 ago nists and antagonists were consistent with a B-2 subtype of receptor. Addition of BK to beating cardiomyocytes (>1 nmol/l) at 37 degrees C g ave a strong but transient negative chronotropic effect. This response was paralleled by changes in the pulsation amplitude, which indicated a simultaneous negative inotropic effect of BK. These results provide a basis for the hypothesis that ACE inhibition exerts its cardioprote ctive effect at the level of a population of cardiomyocytes by virtue of kinin receptor-mediated mechanisms.