We have previously shown major differences in β-adrenergic and muscarinic modulation of L-type calcium currents (Ica) in newborn and adult rabbit heart. However, little is known about developmental changes in modulation of Icaby phosphodiesterases (PDEs), which also regulate intracellular cAMP concentration by its hydrolysis.Methods and ResultsEnzymatically isolated adult and newborn (1- to 3-day-old) rabbit ventricular myocytes were used to study the effects of PDE inhibitors on Icameasured by the whole-ell patch-clamp method. 3-Isobutyl-1-methyl-xanthine (IBMX), a nonselective PDE inhibitor, increased Icain a dose-dependent manner for both groups. The maximal effect of IBMX, expressed as percentage increase in Icaover control levels, was greater for newborn myocytes than for adult myocytes, but the effects of IBMX applied alone were observed only at concentrations >10 μmol/L. The concomitant use of 0.1 μmol/L isoproterenol produced a significant potentiation of the IBMX effect on Ica, with a significant additive effect of IBMX in newborn myocytes even at 0.05 μmol/L IBMX. The concomitant use of a subthreshold concentration of IBMX (0.1 μmol/L) did not potentiate the dose dependence of adult Icaon isoproterenol but did markedly potentiate the dose dependence of newborn Icaon isoproterenol. The E⊂max and EC50of isoproterenol in the presence of 0.1 μmol/L IBMX on newborn Icawere 235% and 8 nmol/L, respectively, whereas the E⊂max and EC50of isoproterenol in the absence of IBMX on newborn Icawere 111% and 81 nmol/L, respectively. The addition of 50 μmol/L IBMX to 10 μmol/L isoproterenol markedly increased the newborn Ic, density up to a level equivalent to that reached with 200 μmol/L cAMP in the pipette (14.9±1.2 versus 13.4±0.7 pA/pF). Our data suggest that the inhibition constant (Ki) of IBMX for inhibiting PDEs that participate in the regulation of Icais much lower in newborn than in adult myocytes. Milrinone 1 μmol/L, a selective PDE III inhibitor, increased the 0.1 μmol/L isoproterenol-stimulated Icaof adult myocytes but had no significant additive effect for the 0.1 1tmol/L isoproterenol- stimulated Icaof newborn myocytes. Rolipram 1 μmol/L, a selective PDE IV inhibitor, increased the 0.1 μmol/L isoproterenol-stimulated Icafor newborn myocytes but had no significant additive effect for the 0.1 1A/L isoproterenol-stimulated Icafor adult myocytes.ConclusionsThese results suggest that the most important PDE isozyme for regulation of Icaof rabbit myocytes changes from PDE IV to PDE III during the postnatal period.