MECHANISM OF BETA-ADRENERGIC-RECEPTOR UP-REGULATION INDUCED BY ACE-INHIBITION IN CULTURED NEONATAL RAT CARDIAC MYOCYTES - ROLES OF BRADYKININ AND PROTEIN-KINASE-C

Background-Although bradykinin is thought to contribute to the effects of ACE inhibitors on the cardiovascular system, its precise role rema ins to be elucidated. Evidence suggests that bradykinin might be impor tant in the upregulation of beta-adrenergic receptors (beta-ARs) induc ed by ACE inhibitors, and the role of bradykinin in this effect has no w been investigated with cultured neonatal rat cardiac myocytes. Metho ds and Results-The density of beta-ARs on the myocyte surface was dete rmined with a binding assay with [H-3]CGP-12177. Incubation of culture d myocytes for 24 hours with the ACE inhibitor captopril (1 mu mol/L) increased beta-AR density by 35% and enhanced the response of cells to isoproterenol but not to forskolin. Neither an angiotensin-II type 1 (AT(1)) receptor antagonist, CV-11974, nor angiotensin-I affected beta -AR density. However, the bradykinin B-2 receptor antagonist Hoe 140 a bolished the effect of captopril on beta-AR upregulation in a dose-dep endent manner. The protein kinase C inhibitor staurosporine (20 nmol/L ) but neither indomethacin nor L-NAME also inhibited captopril-induced upregulation of beta-ARs. Exogenous bradykinin increased the spontane ous beating frequency of cultured myocytes and Hoe 140 abolished this effect. Bradykinin level in the medium increased 1.4-fold by the treat ment of cultured myocytes with captopril for 24 hours. Conclusions-The results suggest that captopril enhances beta-AR responsiveness by ind ucing P-AR upregulation and that the latter effect is mediated by acti vation of bradykinin B-2 receptors and protein kinase C. These observa tions also offer insight into the different roles of ACE inhibitors an d AT(1) receptor antagonists in the treatment of heart failure.