THE NEGATIVE INOTROPIC EFFECT OF BETA(3)-ADRENOCEPTOR STIMULATION IS MEDIATED BY ACTIVATION OF A NITRIC-OXIDE SYNTHASE PATHWAY IN HUMAN VENTRICLE

beta(1)- and beta(2)-adrenoceptors in heart muscle cells mediate the c atecholamine-induced increase in the force and frequency of cardiac co ntraction, Recently, in addition, we demonstrated the functional expre ssion of beta(3)-adrenoceptors in the human heart. Their stimulation, in marked contrast with that of beta(1)- and beta(2)-adrenoceptors, in duces a decrease in contractility through presently unknown mechanisms . In the present study, we examined the role of a nitric oxide (NO) sy nthase pathway in mediating the beta(3)-adrenoceptor effect on the con tractility of human endomyocardial biopsies. The negative inotropic ef fects of a beta(3)-adrenoceptor agonist, BRL 37344, and also of norepi nephrine in the presence of alpha- and beta(1-2)-blockade were inhibit ed both by a nonspecific blocker of NO, methylene blue, and two NO syn thase (NOS) inhibitors, L-N-monomethyl-arginine and L-nitroarginine-me thyl ester, The effect of the NOS inhibitors was reversed by an excess of L-arginine, the natural substrate of NOS, but not by D-arginine. M oreover, the effects of the beta(3)-adrenoceptor agonist on contractil ity were associated with parallel increases in the production of NO an d intracellular cGMP, which were also inhibited by NOS inhibitors, Imm unohistochemical staining of human ventricular biopsies showed the exp ression of the endothelial constitutive (eNOS), but not the inducible (iNOS) isoform of NOS in both ventricular myocytes and endothelial cel ls. These results demonstrate that beta(3)-adrenoceptor stimulation de creases cardiac contractility through activation of an NOS pathway, Ch anges in the expression of this pathway may alter the balance between positive and negative inotropic effects of catecholamines on the heart potentially leading to myocardial dysfunction.