The Effect of Long-term Hypoxia on Tension and Intracellular Calcium Responses Following Stimulation of the Thromboxane A2 Receptor in the Left Anterior Descending Coronary Artery of Fetal Sheep

Abstract

Objective: The purpose of this study was to investigate the mechanisms of tension and intracellular calcium regulation following stimulation with the thromboxane A₂ receptor agonist U46619 in the left anterior descending coronary artery of fetal sheep exposed to long-term hypoxia. We hypothesized that there would be a reduction in intracellular calcium responses in long-term hypoxic left anterior descending coronary artery accompanied by an increase in calcium sensitivity of the contractile mechanism. Methods: Pregnant sheep were kept at altitude (3820 m) from day 30 of gestation until day 140. Fetal hearts from long-term hypoxic and from a control, normoxic group were obtained and the left anterior descending coronary artery of the fetus was dissected, cleaned, and mounted in a bath (Jasco) in which tension and intracellular calcium [Ca²⁺]_i, using Fura-2, could be measured simultaneously following stimulation of the thromboxane A₂ receptor with U46619. The role of intracellular calcium and the Rho kinase and protein kinase C pathways in the tension responses were investigated by maintaining intracellular calcium constant or by using the Rho kinase blocker, Y27632, or the protein kinase C blocker, GF109203-X. Results: There was no difference in the tension dose-response to U46619 between the normoxic fetal and hypoxic fetal left anterior descending, although [Ca²⁺]_i was lower in the hypoxic fetal than normoxic fetal at the highest doses. When [Ca²⁺]_i was maintained constant at baseline levels, U46619 produced the same tension dose-response in both normoxic fetal and hypoxic fetal left anterior descending as when [Ca²⁺]_i was allowed to rise. The tension response was abolished in both groups when the Rho kinase inhibitor, Y27632, was given either during or before stimulation with U46619. The protein kinase C blocker, GF109203-X, had no effect on the tension response in either group. Conclusions: Long-term hypoxia did not alter the tension response to thromboxane A₂ receptor stimulation in fetal left anterior descending. The contractions in response to U46619 were produced apparently completely by changes in calcium sensitivity through the Rho kinase pathway.

Keywords

Fetus smooth muscle calcium sensitivity.

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