Effects of long-term high-altitude hypoxia and troponin I phosphorylation on cardiac myofilament calcium responses in fetal and nonpregnant sheep.
OBJECTIVE
We studied the effects of long-term high-altitude hypoxia and protein kinase A (PKA) phosphorylation on calcium (Ca2+) responses of skinned cardiac papillary muscles from fetal and adult sheep.


METHODS
Fetal and nonpregnant adult sheep were exposed to high-altitude (3820 m), long-term (approximately 110 days) hypoxia. Papillary muscles were isolated and mounted in well-oxygenated, temperature-controlled baths. After the papillary muscles were stimulated electrically to establish the diastolic tension that produced the maximum active contraction, the electrical stimulation was stopped, and the muscles were skinned with 1% vol/vol Triton-X-100. In protocol 1, the skinned muscles were exposed to activating solutions containing different calcium concentrations (pCa; from pCa 8.0 to pCa 4.0), which were prepared by varying the Ca-EGTA/EGTA ratio, and the steady-state tension was measured at each pCa. In protocol 2, the skinned muscles were contracted with activating solution containing a pCa of 5.0. After equilibration, the solution in some baths was changed to activating solution at the same pCa of 5.0 but also containing the catalytic subunit of PKA. The other baths were exchanged with activating solution at a pCa of 5.0 containing no PKA. We then measured the degree of tension reduction caused by PKA until tension reached a new steady state.


RESULTS
In the long-term hypoxic fetal heart, the maximum tension response of right, but not left, ventricular skinned papillary muscle to Ca2+ was significantly less than that in control muscles. In the long-term hypoxic adult heart, the left ventricle, but not the right ventricle, displayed an increased maximum tension response to Ca2+ compared with control. Phosphorylation of troponin I (TnI) with PKA reduced active tension in both fetal ventricles of the long-term hypoxic group more than in hearts from control fetuses. In the adult, phosphorylation with PKA resulted in a larger decrease in tension in the left ventricle and a smaller decrease in tension in the right ventricle in the long-term hypoxic group, although the differences were small.


CONCLUSIONS
In the long-term hypoxic fetal right ventricle, the decreased maximum tension response to Ca2+ is consistent with the decrease in myofibrillar magnesium-activated adenosine triphosphatase activity observed previously. The larger decrease in tension after PKA phosphorylation of TnI in the long-term hypoxic fetal left ventricle indicates a larger reduction in Ca2+ binding to troponin C.
Onishi
Junji
J
Center for Perinatal Biology, Department of Physiology and Obstetrics and Gynecology, Loma Linda University School of Medicine, Loma Linda, California, USA.
Browne
Vaughn A
VA
Kono
Satoshi
S
Stiffel
Virginia M
VM
Gilbert
Raymond D
RD
D008841:Actin Cytoskeleton / Q000187:drug effects / Q000502:physiology; D000531:Altitude; D000818:Animals; D002118:Calcium / Q000008:administration & dosage / Q000494:pharmacology*; D002302:Cardiac Output; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004533:Egtazic Acid / Q000008:administration & dosage; D004558:Electric Stimulation; D005260:Female; D005318:Fetal Heart / Q000187:drug effects* / Q000503:physiopathology; D005311:Fetal Hypoxia / Q000503:physiopathology; D005865:Gestational Age; D006352:Heart Ventricles / Q000187:drug effects / Q000503:physiopathology; D000860:Hypoxia / Q000503:physiopathology*; D010210:Papillary Muscles / Q000187:drug effects* / Q000196:embryology / Q000503:physiopathology; D010766:Phosphorylation; D011247:Pregnancy; D012756:Sheep / Q000196:embryology; D019210:Troponin I / Q000378:metabolism*
D019210:Troponin I; D004533:Egtazic Acid; D017868:Cyclic AMP-Dependent Protein Kinases; D002118:Calcium