DIFFERENCES IN ISOPROTERENOL STIMULATION OF CA2+ CURRENT OF RAT VENTRICULAR MYOCYTES IN NEONATAL COMPARED TO ADULT

The developmental changes in the isoproterenol stimulation of the L-ty pe calcium current (I-Ca(L)) were studied in freshly isolated neonatal (3-5-day-old) and adult (2-3-month-old) rat ventricular myocytes usin g whole-cell voltage clamp (at room temperature). Ica(L) was measured as the peak inward current at a test potential of +10 mV (or +20 mV) b y applying a 300 ms pulse from a holding potential of -40 mV. The pipe tte solution was Cs+-rich and Ca2+-free. The external solution was Na-free and K+-free. Isoproterenol stimulated I-Ca(L) in a dose-dependen t manner. The concentrations of isoproterenol for half-maximal effect were 6.8 nM in neonatal and 13.3 nM in adult. The maximal stimulation of I-Ca(L) was 147 +/- 14% in neonatal and 97 +/- 7% in adult. The ste ady-state inactivation curves were not affected by isoproterenol, wher eas the steady-state activation curve was shifted to the left in both neonatal and adult. Forskolin (10 mu M) increased I-Ca(L) by 105 +/- 1 0% in neonatal and 90 +/- 12% in adult. After stimulating I-Ca(L) by f orskolin, the addition of isoproterenol produced a further increase of I-Ca(L) by 99 +/- 27% in neonatal, but only by 19 +/- 3% in adult. Th e presence of an inhibitor of cAMP-dependent protein kinase in the pip ette did not affect this marked difference between neonatal (87 +/- 23 %) and adult (11 +/- 8%). We conclude that, in rat ventricular myocyte s, (1) stimulation of I-Ca(L) by the beta-adrenoceptor agonist, isopro terenol, is already fully developed in the neonatal stage and actually decreases during development; (2) there is evidence for a cAMP-indepe ndent stimulation of Ca2+ channels by isoproterenol, and this is great er in neonatal than in adult. We believe that the cAMP-independent pat hway is the direct pathway mediated by G(s alpha) protein.