ORIGINAL ARTICLES Effects of Long-Term High-Altitude Hypoxia on

OBJECTIVE: In fetal sheep, we found that the augmentation of cardiac contractility by P-adrenergic receptor (3-AR) stimulation was reduced after exposure to long-term hypoxia. However, cyclic adenosine monophosphate (cAMP) production after E-AR stimulation was higher in long-term hypoxicfetal sheep than in normoxic ones. Therefore, we studied the potential role of changes in myocardial protein kinase A (PK1A) activity and troponin I (TnI) isoforms infetal and nonpregnant sheep exposed to approximately 112 days of hypoxia at high altitude (3820 m). METHODS: Resting and maximally stimulated (by cAMP) PKA activity was measured by phosphor-ylation of the artificial peptide, Kemptide. Specificity was confirmed by inhibition with PKI, a specific PKA inhibitor. For TnI isoforms, sodium dodecyl su~fate-polyacrylamide gel electrophoresis was used to resolve the proteins. We used monoclonal anti-cardiac TnI antibody (clone C5), which also cross-reacted with slow skeletal muscle Tnl, to identify TnI isoforms. RESULTS: For thefetal hearts, resting PKA activity was significantly higher in the high-altitude group than the control group, but total PKA activity was not different between the normoxic and hypoxic groups. In the adult hearts, no significant difference was observed in either resting or total PKA activity between normoxic and hypoxic groups. For both the fetal and adult sheep, the predominant TnI was the cardiac isoform, and hypoxic exposure produced no change in the TnI isoform composition. CONCLUSION: Neither a reduction in PKA activity nor a change in TnI isoforms could explain the reduction in beta-receptor augmentation of cardiac contractility infetal sheep exposed to long-term hypoxia