MYOCARDIAL SIGNALING DEFECTS AND IMPAIRED CARDIAC-FUNCTION OF A HUMANBETA(2)-ADRENERGIC RECEPTOR POLYMORPHISM EXPRESSED IN TRANSGENIC MICE

A threonine to isoleucine polymorphism at amino acid 164 in the fourth transmembrane spanning domain of the beta(2)-adrenergic receptor (bet a(2)AR) is known to occur in the human population, The functional cons equences of this polymorphism to catecholamine signaling in relevant c ells or to end-organ responsiveness, however, are not known, To explor e potential differences between the two receptors, site-directed mutag enesis was carried out to mimic the polymorphism, Transgenic FVB/N mic e were then created overexpressing wild-type (wt) beta(2)AR or the mut ant Ile-164 receptor in a targeted manner in the heart using a murine alpha myosin heavy chain promoter, The functional properties of the tw o receptors were then assessed at the level of in vitro cardiac myocyt e signaling and in vivo cardiac responses in intact animals, The expre ssion levels of these receptors in the two lines chosen for study were approximate to 1200 fmol/mg protein in cardiac membranes, which repre sents a approximate to 45-fold increase in expression over endogenous beta AR, Myocyte membrane adenylyl cyclase activity in the basal state was significantly lower in the Ile-164 mice (19.5 +/- 2.7 pmol/min/mg ) compared with wt beta(2)AR mice (35.0 +/- 4.1 pmol/min/mg), as was t he maximal isoproterenol-stimulated activity (49.8 +/- 7.8 versus 77.1 +/- 7.3 pmol/min/mg), In intact animals, resting heart rate (441 +/- 21 versus 534 +/- 17 bpm) and dP/dt(max) (10,923 +/- 730 versus 15,308 +/- 471 mmHg/sec) were less in the Ile-164 mice as compared with wt b eta(2)AR mice, Similarly, the physiologic responses to infused isoprot erenol were notably less in the mutant expressing mice, Indeed, these values, as well as other contractile parameters, were indistinguishabl e between Ile-164 mice and nontransgenic littermates. Taken together, these results demonstrate that the Ile-164 polymorphism is substantial ly dysfunctional in a relevant target tissue, as indicated by depresse d receptor coupling to adenylyl cyclase in myocardial membranes and im paired receptor mediated cardiac function irt vivo, Under normal homeo static conditions or in circumstances where sympathetic responses are compromised due to diseased states, such as heart failure, this impair ment may have important pathophysiologic consequences.