β₁ adrenergic receptors (β₁ARs) increase cardiac function through G_s-protein coupling, whereas β₃ARs coupled to G_i-protein decrease contractility (1). We have previously shown that chronic noradrenaline (NA) stimulation induced a decrease in β₁ARs and an increase in β₃ARs at the functional, genomic and protein levels, which can explain the reduction in cardiac function observed in heart diseases (2). Indeed, circulating cathecholamines are increased in heart diseases leading to an alteration of the cardiac function, a decrease in β₁AR density and an elevation in β₃AR level (1). The aim of this work was to study the function and expression of β₁ and β₃ARs in neonatal cardiomyocytes exposed to normoxia (NX), NX and 100μM NA (NX/NA), hypoxia (HX, 0.5% O₂) or HX/NA for 24h. In addition, cell viability was performed using LDH assay following 24h NA or 2μM Cl31 treatment in NX and HX. Functional studies were performed using dobutamine (Dob, β₁ agonist) on cAMP accumulation and Cl 316243 (Cl31, β₃ agonist) on forskolin-induced cAMP response. β₁ and β₃AR mRNA levels were determined by real time PCR. NA pretreatment in NX and HX decreased Dob-induced cAMP accumulation (maximal response: 36±8 vs 167±16%, P<0.001 and 51±10 vs 91±21%, P<0.05, n=6, respectively), without changing the sensitivity. HX downregulated β₁AR response (vs NX, P<0.001) and β₁AR mRNA level (57±3% vs 100%, P<0.01, n=4). The mRNA level was similarly decreased (≈ 60%) following NA treatment in NX (P<0.01) and HX (P<0.05). No functional β₃AR response was observed in NX and HX. NA pretreatment increased Cl31-mediated inhibition of forskolin response in NX and HX, with a higher maximal inhibition in HX/NA (Imax: 43±3% vs 23±4%, P<0.05, n=6). Inversely, the sensitivity decreased (pIC50: HX/NA, 8.16±0.36, vs NX/NA, 9.15±0.26, P<0.05). β₃AR mRNA level was upregulated following chronic NA stimulation in NX (157±52% vs 100%, P<0.05) and HX (193±77% vs 53±16%, P<0.01). Furthermore, HX reduced β₃AR mRNA expression (53±16% vs 100% NX, P<0.05, n=4). Chronic β₃AR stimulation had no effect on LDH level in NX (97±6 vs 100%, n=5) and did not induce cell survival against HX-induced cell death (230±23 vs 254±9%), whereas chronic NA treatment induced an increase in LDH activity in NX (170±18%, P<0.05) and HX (332±23%, P<0.05). In conclusion, the further upregulation of the β₃AR function following chronic NA stimulation in HX in comparison to NX/NA indicates that the β₃AR subtype plays an important role in heart diseases. However, chronic β₃ stimulation did not induce cell survival against cell death by necrosis in our model of ischemic heart disease, although this subtype did not promote cell death.