Decreased type VI adenylyl cyclase mRNA concentration and Mg(2+)-dependent adenylyl cyclase activities and unchanged type V adenylyl cyclase mRNA concentration and Mn(2+)-dependent adenylyl cyclase activities in the left ventricle of rats with myocardial infarction and longstanding heart failure.

Objective: To address the effect of longstanding left ventricular (LV) hypertrophy and failure on LV adenylyl cyclase (AC) gene expression, mRNA concentrations of the main cardiac AC isoforms were measured in the non-infarcted area of LV from rats with myocardial infarction (MI), without (H) or with (F) LV failure, and in control (C) rats. Basal, GTP- and forskolin-stimulated Mg(2+)- and Mn(2+)-dependent AC activities were also measured in F and C rats.
Methods: Two- and six months after MI, steady-state AC mRNA concentrations were assessed by Northern blot analysis and RNase protection assay with isoform-specific cDNA and cRNA probes, respectively. AC activities were assessed on LV microsomal fractions using standard procedures.
Results: Types V and VI, and types IV and VII were the major and minor AC mRNA isoforms in both the LVs of F and C rats. Two months after MI, no difference in LV type V or VI mRNA to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA ratios was observed in rats with H or F compared to C. Six months after MI, no difference in LV type V mRNA concentration was observed between the three rat groups, whether this level was normalized to GAPDH, poly-(A+) or 18S RNAs. In contrast, a 35% decrease in the type VI mRNA to poly-(A+) RNA ratio and a 29% decrease in the type VI mRNA to 18S RNA ratio was observed only in rats with F compared to C (p < 0.05 vs. C for the two comparisons). Two- and six months after MI, basal and forskolin-stimulated Mg(2+)-dependent AC activities were decreased by 30-35% in F rats compared to C (p < 0.05), whereas Mn(2+)-dependent activities were unchanged.
Conclusion: Longstanding LV hypertrophy and failure resulting from MI in rats is not associated with altered expression of the most abundant, type V, AC mRNA isoform, whereas that of type VI is decreased. The lack of change in Mn(2+)-dependent AC activities in the LV of F rats suggests that this decrease has no functional consequence on overall AC activity and that decreased Mg(2+)-dependent activities are related to alterations occurring upstream.