Chronic hypoxia modulates relations among calcium, myosin light chain phosphorylation, and force differently in fetal and adult ovine basilar arteries

Abstract

The present study tests the hypothesis that age-related differences in contractility of cerebral arteries from hypoxic animals involve changes in myofilament Ca2+sensitivity. Basilar arteries from term fetal and nonpregnant adult sheep maintained 110 days at 3,820 m were used in measurements of cytosolic calcium concentration ([Ca2+]i), myosin light chain phosphorylation, and contractile tensions induced by graded concentrations of K+or serotonin (5-HT). Slopes relating [Ca2+]ito tension were similar in fetal (0.83 ± 0.07) and adult (1.02 ± 0.08) arteries for K+contractions but were significantly greater for fetal (3.77 ± 0.64) than adult (2.00 ± 0.13) arteries for 5-HT contractions. For both K+and 5-HT contractions, these relations were left shifted in fetal compared with adult arteries, indicating greater Ca2+sensitivity in fetal arteries. In contrast, slopes relating [Ca2+]iand %myosin phosphorylation for K+contractions were less in fetal (0.37 ± 0.08) than adult (0.81 ± 0.07) arteries, and the fetal curves were right shifted. For 5-HT contractions, the slope of the Ca2+-phosphorylation relation was similar in fetal (0.33 ± 0.09) and adult (0.33 ± 0.23) arteries, indicating that 5-HT depressed Ca2+-induced myosin phosphorylation in adult arteries. For slopes relating %myosin phosphorylation and tension, fetal values (K+: 1.52 ± 0.22, 5-HT: 7.66 ± 1.70) were less than adult values (K+: 2.13 ± 0.30, 5-HT: 8.29 ± 2.40) for both K+- and 5-HT-induced contractions, although again fetal curves were left shifted relative to the adult. Thus, in hypoxia-acclimatized basilar arteries, a downregulated ability of Ca2+to promote myosin phosphorylation is offset by an upregulated ability of phosphorylated myosin to produce force yielding an increased fetal myofilament Ca2+sensitivity. Postnatal maturation reprioritizes the mechanisms regulating hypoxic contractility through changes in the source of activator Ca2+, the pathways governing myosin light chain phosphorylation, and its interaction with actin.