Epinephrine Activates Both Gsand GiPathways, but Norepinephrine Activates Only the GsPathway through Human β2-Adrenoceptors Overexpressed in Mouse Heart

Abstract

Isoproterenol increases and decreases contractile force at low and high concentrations, respectively, through β₂-adrenoceptors overexpressed in transgenic mouse heart (TG4), consistent with activation of both G_s and G_i proteins. Using TG4 hearts, we demonstrated that epinephrine behaves like isoproterenol, but norepinephrine does not. Epinephrine both increased (-log EC₅₀M = 9.4) and decreased (-log EC₅₀M = 6.5) left atrial force. Pertussis toxin (PTX) abolished the negative inotropic effects of epinephrine, consistent with mediation through G_i protein. Norepinephrine only increased contractile force (-log EC₅₀M = 7.5). Norepinephrine (10-100 μM) prevented the positive inotropic effects but hardly affected the negative inotropic effects of epinephrine. Cardiodepressive epinephrine concentrations (1-10 μM) antagonized the positive inotropic effects of norepinephrine. In the free wall of TG4 right ventricle, norepinephrine and low epinephrine concentrations caused positive inotropic effects, and high epinephrine concentrations caused PTX-sensitive negative inotropic effects, as observed in the left atrium. Epinephrine (10 nM), a concentration causing maximum increase in contractile force, and norepinephrine (1 and 100 μM) increased cAMP-dependent protein kinase activity in TG4 left ventricle. Cardiodepressive concentrations of epinephrine (1 and 100 μM) did not increase cAMP-dependent protein kinase activity. The inotropic results were simulated with a model of two β₂-adrenoceptor sites. For one site involved in receptor coupling to G_s, both epinephrine and norepinephrine compete. The other site, recognized by epinephrine but not by norepinephrine, leads to receptor G_i coupling.