We studied the developmental changes in the β-adrenergic modulation of L-type calcium current (ICa) in enzymatically isolated adult (AD) and newborn (NB, 1–4-day-old) rabbit ventricular cells using the whole-cell patch-clamp method. ICawas measured as the peak inward current at a test potential of +15 mV by applying a 180–450-msec pulse from a holding potential of −40 mV with Cs+-rich pipettes and a K+-free bath solution at room temperature. In control, ICadensity (obtained by normalizing ICato the cell capacitance) was significantly higher in AD cells (5.5±0.2 [mean±SEM] pA/pF,n=65) than in NB cells (2.6±0.1 pA/pF,n=60). Isoproterenol (ISO, 1 nM-30 μM) increased ICain a dose-dependent manner for both groups. The maximal effect (Emax) of ISO, expressed as percent increase in ICaover control levels, and the concentration for one half of the maximal effect (EC50) were 203% and 51 nM, respectively, for AD cells and 111% and 81 nM, respectively, for NB cells. The effect of ISO (1 μM) on ICawas decreased as the test potential was increased from −10 to +40 mV. However, the ratio of the percent increase in ICafor AD versus NB cells was almost constant (2.09–2.45) at each test potential. Dose-response curves of forskolin (FOR, 0.3–50 μM) gave Emaxand EC50of 268% and 0.74 μM, respectively, for AD cells and 380% and 1.15 μM, respectively, for NB cells. After stimulating ICaby 10 μM ISO, the addition of 10 μM FOR produced a further increase in ICaof only 12±2% in AD cells (n=4) but a further increase of 140±41% in NB cells (n=6). FOR (10 μM) did not produce any increase in ICafor AD and NB cells after stimulating ICaby intracellular application of 200 μM cAMP. ICadensity stimulated by 10 μM ISO (17.8±1.1 pA/pF,n=7), 10 μM FOR (21.0±1.3 pA/pF,n=8), or 200 μM cAMP (18.0±1.3 pA/pF,n=5) was equivalent in AD cells, whereas ICadensity stimulated by 10 μM ISO (5.8±0.6 pA/pF,n=9) was significantly lower than that stimulated by either 10 μM FOR (13.8±1.5 pA/pF,n=7) or 200 μM cAMP (13.4±0.7 pA/pF,n=7) in NB cells. The ICadensity for FOR or cAMP was significantly higher for AD than NB cells. Pretreatment of AD and NB cells with pertussis toxin markedly increased the ISO effect on ICafor NB cells, whereas the enhancement was relatively small for AD cells. We conclude that the effect of ISO to stimulate L-type ICaincreases after birth in rabbit ventricular cells probably as a consequence of the reduction of tonic Giinhibition of adenylate cyclase rather than the postnatal maturation of the β-receptor-adenylate cyclase system.