Ca²⁺ influx through the L-type calcium channel (LTCC) induces Ca²⁺ release from the sarcoplasmic reticulum (SR) and maintains SR Ca²⁺ loading. Alterations in LTCC properties, their contribution to the blunted adrenergic responsiveness in failing hearts and their recovery after support with LV assist devices (LVAD) were studied. L-type Ca²⁺ current (I_Ca,L) was measured under basal conditions and in the presence of isoproterenol (ISO), dibutyryl-cAMP (db-cAMP), Bay K 8644 (BayK), Okadaic acid (OA, a phosphatase inhibitor), and phosphatase 2A (PP2A) in nonfailing (NF), failing (F), and LVAD-supported human left ventricular myocytes (HVMs). Basal I_Ca,L density was not different in the 3 groups but I_Ca,L was activated at more negative voltages in F- and LVAD- versus NF-HVMs (V_0.5: −7.18±1.4 and −7.0±0.9 versus 0.46±1.1 mV). Both ISO and db-cAMP increased I_Ca,L in NF- and LVAD- significantly more than in F-HVMs (NF >LVAD> F: ISO: 90±15% versus 77±19% versus 24±12%; db-cAMP: 235%>172%>90%). ISO caused a significant leftward shift of the I_Ca,L activation curve in NF- and LVAD- but not in F-HVMs. After ISO and db-cAMP, the I_Ca,L activation was not significantly different between groups. BayK also increased I_Ca,L more in NF- (81±30%) and LVAD- (70±15%) than in F- (51±8%) HVMs. OA increased I_{Ca, L} by 85.6% in NF-HVMs but had no effect in F-HVMs, while PP2A decreased I_{Ca, L} in F-HVMs by 35% but had no effect in NF-HVMs. These results suggest that the density of LTCC is reduced in F-HVMs but basal I_Ca,L density is maintained by increasing in LTCC phosphorylation.