DEVELOPMENTAL-CHANGES IN MODULATION OF CALCIUM CURRENTS OF RABBIT VENTRICULAR CELLS BY PHOSPHODIESTERASE INHIBITORS

Background We have previously shown major differences in beta-adrenerg ic and muscarinic modulation of L-type calcium currents (I-Ca) in newb orn and adult rabbit heart. However, little is known about development al changes in modulation of I-Ca by phosphodiesterases (PDEs), which a lso regulate intracellular cAMP concentration by its hydrolysis. Metho ds and Results Enzymatically isolated adult and newborn (1- to 3-day-o ld) rabbit ventricular myocytes were used to study the effects of PDE inhibitors on I-Ca measured by the whole-cell patch-clamp method. 3-Is obutyl-1-methyl-xanthine (IBMX), a nonselective PDE inhibitor, increas ed I-Ca in a dose-dependent manner for both groups. The maximal effect of IBMX, expressed as percentage increase in I-Ca over control levels , was greater for newborn myocytes than for adult myocytes, but the ef fects of IBMX applied alone were observed only at concentrations >10 m u mol/L. The concomitant use of 0.1 mu mol/L isoproterenol produced a significant potentiation of the IBMX effect on I-Ca, with a significan t additive effect of IBMX in newborn myocytes even at 0.05 mu mol/L IB MX. The concomitant use of a subthreshold concentration of IBMX (0.1 m u mol/L) did not potentiate the dose dependence of adult I-Ca on isopr oterenol but did markedly potentiate the dose dependence of newborn I- Ca on isoproterenol. The E(max) and EC(50) of isoproterenol in the pre sence of 0.1 mu mol/L IBMX on newborn I-Ca were 235% and 8 nmol/L, res pectively, whereas the E(max) and EC(50) of isoproterenol in the absen ce of IBMX on newborn I-Ca were 111% and 81 nmol/L, respectively. The addition of 50 mu mol/L IBMX to 10 mu mol/L isoproterenol markedly inc reased the newborn I,density up to a level equivalent to that reached with 200 mu mol/L cAMP in the pipette (14.9+/-1.2 versus 13.4+/-0.7 pA /pF). Our data suggest that the inhibition constant (K-i) of IBMX for inhibiting PDEs that participate in the regulation of I-Ca is much low er in newborn than in adult myocytes. Milrinone 1 mu mol/L, a selectiv e PDE III inhibitor, increased the 0.1 mu mol/L isoproterenol-stimulat ed I-Ca of adult myocytes but had no significant additive effect for t he 0.1 mu mol/L isoproterenol-stimulated I-Ca of newborn myocytes. Rol ipram 1 mu mol/L, a selective PDE IV inhibitor, increased the 0.1 mu m ol/L isoproterenol-stimulated I-Ca for newborn myocytes but had no sig nificant additive effect for the 0.1 mu mol/L isoproterenol-stimulated I-Ca for adult myocytes. Conclusions These results suggest that the m ost important PDE isozyme for regulation of I-Ca of rabbit myocytes ch anges from PDE IV to PDE III during the postnatal period.