Overexpression of Cyclic Adenosine Monophosphate Effluent Protein MRP4 Induces an Altered Response to β-Adrenergic Stimulation in the Senescent Rat Heart

Background: In the senescent heart, the positive inotropic response to beta-adrenoceptor stimulation is reduced, partly by dysregulation of beta 1- and beta 3-adrenoceptors. The multidrug resistance protein 4 (MRP4) takes part in the control of intracellular cyclic adenosine monophosphate concentration by controlling its efflux but the role of MRP4 in the beta-adrenergic dysfunction of the senescent heart remains unknown. Methods: The beta-adrenergic responses to isoproterenol were investigated in vivo (stress echocardiography) and in vitro (isolated cardiomyocyte by Ionoptix (R) with sarcomere shortening and calcium transient) in young (3 months old) and senescent (24 months old) rats pretreated or not with MK571, a specific MRP4 inhibitor. MRP4 was quantified in left ventricular homogenates by Western blotting. Data are mean +/- SD expressed as percent of baseline value. Results: The positive inotropic effect of isoproterenol was reduced in senescent rats in vivo (left ventricular shortening fraction 120 +/- 16% vs. 158 +/- 20%, P < 0.001, n = 16 rats) and in vitro (sarcomere shortening 129 +/- 37% vs. 148 +/- 35%, P = 0.004, n = 41 or 43 cells) as compared to young rats. MRP4 expression increased 3.6-fold in senescent compared to young rat myocardium (P = 0.012, n = 8 rats per group). In senescent rats, inhibition of MRP4 by MK571 restored the positive inotropic effect of isoproterenol in vivo (143 +/- 11%, n = 8 rats). In vitro in senescent cardiomyocytes pretreated with MK571, both sarcomere shortening (161 +/- 45% vs. 129 +/- 37%, P = 0.007, n = 41 cells per group) and calcium transient amplitude (132 +/- 25% vs. 113 +/- 27%, P = 0.007) increased significantly. Conclusion: MRP4 overexpression contributes to the reduction of the positive inotropic response to beta-adrenoceptor stimulation in the senescent heart.