 -Adrenergic Receptor (  -AR) Subtypes Have Opposing Effects on Survival and Cardiac Function in MLP Cardiomyopathy Mingming Zhao, Giovanni Fajardo and Daniel Bernstein Department of Pediatrics, Stanford University, Stanford, CA Alterations in  -AR signaling have been shown to modulate severity of a genetic cardiomyopathy (Muscle LIM Protein, MLP-/- mice).  2 -AR overexpression increases mortality, whereas  ARKct partially and phospholamban ablation fully rescues MLP-/- mice. To dissect the role of  1 vs  2 -ARs in modulating MLP cardiomyopathy, we crossbred MLP-/- with  1 -/- or  2 -/- mice. Ablation of  1 vs  2 -AR had opposite effects on early survival: MLP-/-/  2 -/- (91.2%, n=57), MLP-/- (74.0%, n=27), MLP-/-/  1 +/- (62.3%, n=53), MLP-/-/  1 -/- (3%, n=31). Nearly all MLP-/-/  1 -/- mice died in utero between ED10 and ED15. MLP-/-/  2 -/- rescued LV function in 80% of MLP-/-/  2 -/- mice (F/S 39.6±12.1%, LVEDD 4.6±1.1mm, n=15 ) vs MLP- /- (F/S 18.3±4.7%*, LVEDD 5.7±0.7mm*, *:p<0.05, n=10) and MLP-/-/  1 +/- (F/S 22.9±9.9%*, LVEDD 5.8±1.3mm*, n=11). MLP-/-/  2 -/- mice ran longer (22.7±3.0min) on a graded treadmill protocol vs. MLP-/- (14.6±2.0 min*) and MLP-/-/  1 +/- (13.3±5.2 min*). MLP-/-/  2 -/- mice had normal heart size (HW/BW 4.7±1.0) vs MLP-/- (7.6±2.5*) and MLP-/-/  1 +/- (8.3±3.4*). In conclusion,  -AR subtypes play critical but differential roles during the development of a genetic (MLP) cardiomyopathy. The  1 -AR positively modulates survival and cardiac function whereas the  2 -AR has an opposite effect. Total ablation of the  1 -AR is embryonic lethal in the absence of MLP. ABSTRACT INTRODUCTION CONCLUSIONS  -AR subtypes play critical but differential roles during the development of a genetic (MLP) cardiomyopathy. The   -AR positively modulates survival and cardiac function whereas the   -AR has the opposite effect. Total ablation of the   -AR is embryonic lethal in the absence of MLP.  -AR modulation of phospholamban may play a role in the severity of heart failure in MLP-/- mice. MLP (Muscle LIM Protein) is a regulator of myogenic differentiation and is involved in cytoarchitecture organization. MLP deficient mice develop dilated cardiomyopathy in two phases: heart failure occurs in 50-70% within 10d and in the remainder by 3-6 months (Arber et al. Cell 88:393, 1997).. Previous studies have shown that  2 -AR overexpression increases mortality, whereas the  ARK1 inhibitor (  ARKct) partially rescues MLP-/- mice (Rockman et al. PNAS 12:7000, 1998). Ablation of phospholamban (PLB), an inhibitor of the SR Ca 2+ ATPase (SERCA), rescues MLP-/- mice (Minamisawa et al. Cell 99:313, 1999). To further investigate the role of  -AR subtypes in the development of MLP cardiomyopathy. FIGURE 5. At 6 mos. HW/BW and LW/BW ratios were increased in MLP-/- and MLP-/-/  1 +/- mice, but not in MLP-/-/  2 -/- mice. PURPOSE FIGURE 6. Phospholamban phosphorylation was decreased in MLP-/- and MLP-/-/  1 +/- mice but was normal in MLP-/-/  2 -/- mice. WT MLP-/- MLP-/-/  1 +/- MLP-/-/  2 -/- RESULTS FIGURE 1. Ablation of  1 vs  2 -ARs had opposite effects on early (2 wks) and late (6 mos) survival. 97% of MLP-/-/  1 -/- mice died between ED METHODS FIGURE 2. At 6 mos. MLP-/- and MLP-/-/  1 +/- had decreased exercise capacity whereas MLP-/-/  2 -/- mice had normal exercise capacity.   1 -/- or  2 -/- mice were mated to MLP-/- mice (courtesy Dr. K. Chien) and pre- and post-natal survival was recorded.  Genotyping of newborn and adult mice was performed by PCR.  Genotyping of mouse embryos was performed at ED10, ED15 and ED18.  Cardiac function (%FS, LVEDD by echo) and treadmill exercise capacity were assessed at 6 months.  Mice were sacrificed at 7 months and heart and lung weights were recorded and normalized to body weight.  Western blots were performed on LV samples for PLB phosphorylation * * FIGURE 3. At 6 mos. LV %FS was decreased and LVEDD was increased in MLP-/- and MLP-/-/  1 +/- mice but not in MLP-/-/  2 -/- mice. FIGURE 4. At 6 mos. MLP-/- and MLP-/-/  1 +/- developed cardiomegaly, whereas MLP-/-/  2 -/- did not. MLP-/-/   +/-MLP-/-/   -/- * * * * * * * * * p< 0.05 by ANOVA