Objective:To determine the contribution of decreased calcium responsiveness of fetal coronary arteries to decreased contractile responses to potassium and the thromboxane A 2 analogue U46619 in these arteries after exposure to chronic hypoxemia.Methods:Concentration-response curves to Ca 2 + in β-escin-permeabilized left circumflex (LCx), left anterior descending (LAD), and right coronary artery (RCA) rings from high-altitude (HA) and control (CON) fetuses were measured. In a second set of β-escin-permeabilized coronary artery rings, the effect of U46619 on Ca 2 + sensitivity was tested.Results:Maximum Ca 2 + -activated force (T m a x ) was decreased in HA LCx (CON 0.091 +/- 0.010 versus HA 0.057 +/- 0.006 g/cm 2 ; P < .05) and HA LAD (CON 0.065 +/- 0.012 versus HA 0.031 +/- 0.007 g/cm 2 ; P < .05). No significant difference was observed in the RCA. There was no change in the pD 2 (-log EC 5 0 ) values between CON and HA coronary rings. The Ca 2 + sensitizing effect of U46619 on submaximal Ca 2 + -activated force was lower only in the HA LCx (CON 0.044 +/- 0.010 versus HA 0.023 +/- 0.006 g/cm 2 at 10 - 5 mol/L; P < .05).Conclusion:These results indicate that maximum tension development in response to Ca 2 + was decreased in the HA LCx and LAD but not the RCA; however, Ca 2 + sensitivity of the contractile apparatus was unaltered in all of them. Decreased Ca 2 + responsiveness may partially explain the decreased contractile capability of fetal LCx and LAD during long-term, high-altitude intrauterine hypoxemia.