Type V and VI adenylyl cyclase mRNAs are the two main cyclase isoforms expressed in the mammalian heart. A recent report has shown that their expression is differentially regulated during ontogenic development, but the accumulation of the two mRNA species and their concentration ratio have not been determined. We thus determined the accumulation and the relative amounts of type V and VI adenylyl cyclase mRNA in fetal, neonatal and adult rat hearts, using a sensitive ribonuclease protection assay. In 18-day-old fetuses, the two adenylyl cyclase mRNA isoforms were weakly expressed in approximately equal amounts (type V mRNA/type VI mRNA = 0.93 +/- 0.09). Further development was characterized by a sharp increase in type V adenylyl cyclase mRNA (x 1.9 in neonates v fetuses, P < 0.01; x 2.4 and x 4.5 in adults v neonates and fetuses, respectively, P < 0.01 for both comparisons) and a slight, non-significant fall in type VI mRNA (P = 0.16). As a result, the type V mRNA/type VI mRNA ratio was 2.86 +/- 0.57 and 9.09 +/- 1.21 in neonatal hearts and adult ventricles, respectively (P < 0.01 v ratio in fetal hearts for both comparisons; P < 0.01 for ratio in adult ventricles v ratio in neonatal hearts), and the overall amount of the two mRNA isoforms was 2.3 times greater in adult than in fetal hearts (P < 0.01). This increase was paralleled by an increase in basal and isoproterenol- and forskolin-stimulated adenylyl cyclase activities in adult hearts compared to fetal and neonatal hearts (P < 0.01 for the three comparisons). Our results demonstrate that type V adenylyl cyclase mRNA accumulates in the rat heart after birth to become the highly predominant isoform in the adult heart. They further suggest that the increase in cardiac adenylyl cyclase activity observed during rat development is due to this accumulation.