Myosin heavy chain isoforms expression and cyclic AMP concentrations in hypoxia-induced hypertrophied right ventricle in rats.

We have previously demonstrated that the relative expression of myosin heavy chain-beta (MHC-beta) in both ventricles of rats exposed to long-term hypobaric hypoxia correlated significantly with the relative ventricular mass. In the present study, we investigated whether an increased expression of MHC-beta was accompanied by a reduction in cyclic AMP (cAMP) activity in hypoxia-induced hypertrophied right ventricle (RV). We used male Wistar-Kyoto rats born and raised at simulated altitudes (2200 m: H2 group or 4000 m: H4 group) compared to age-matched sea level controls (SC group). There were no significant differences between the groups in basal and forskolin-stimulated adenylyl cyclase (AC) activities. The basal and IBMX-inhibited phosphodiesterase (PDE) activities were slightly higher in both hypoxic groups (p>0.05), except that the H2 group had a higher basal PDE activity than the SC group (p<0.05). The AC/PDE activity ratios were significantly decreased in both hypoxic groups (p<0.05), suggesting that low concentrations of cellular cAMP were maintained in the RV under hypoxic conditions. However, there were no correlations between MHC-beta expression and either AC activity, PDE activity, or AC/PDE activity ratio. These results provided evidence against the causal role for cAMP concentration in the expression of MHC-beta associated with hypoxia-induced ventricular hypertrophy.