Effects of Long-Term High-Altitude Hypoxia on Myocardial Protein Kinase A Activity and Troponin I Isoforms in Fetal and Nonpregnant Sheep

Abstract

Objective: In fetal sheep, we found that the augmentation of cardiac contractility by β-adrenergic receptor (β-AR) stimulation was reduced after exposure to long-term hypoxia. However, cyclic adenosine monophosphate (cAMP) production after β-AR stimulation was higher in long-term hypoxic fetal sheep than in normoxic ones. Therefore, we studied the potential role of changes in myocardial protein kinase A (PKA) activity and troponin I (TnI) isoforms in fetal 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 phosphorylation of the artificial peptide, Kemptide. Specificity was confirmed by inhibition with PKI, a specific PKA inhibitor. For TnI isoforms, sodium dodecyl sulfate-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 TnI, to identify TnI isoforms.

Results: For the fetal 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 in fetal sheep exposed to long-term hypoxia.