Effects of substrate and inhibition of oxidative metabolism on contraction and myosin phosphorylation in ASM

Key role of glycogen storage in high K+-induced contraction of the smooth muscles of the bovine trachea

To elucidate the role of glycogen in the contraction of tracheal smooth muscle, we investigated the changes in the glycogen contents of the bovine trachea during contractions induced by high K(+) and hypoxia (achieved by bubbling N(2) instead of O(2)), either in a glucose-free condition or in the presence of iodoacetic acid (IAA), an inhibitor of glycolysis. Hyperosmotic addition of 65 mM KCl (H-65 K(+)) induced a sustained contraction. A glucose-free condition did not affect H-65 K(+)-induced contraction. However, hypoxia slightly inhibited the contraction, and glucose-free PSS with hypoxia or IAA remarkably inhibited the H-65 K(+)-induced contraction. H-65 K(+) induced a sustained increase in reduced pyridine nucleotide (PNred) fluorescence, representing glycolysis activity. Hypoxia alone slightly enhanced PNred fluorescence, and when combined with a glucose-free condition, it remarkably enhanced the H-65 K(+)-induced PNred fluorescence. IAA inhibited PNred fluorescence. In the presence of H-65 K(+), a glucose-free condition, hypoxia and the combination of glucose-free PSS and hypoxia decreased the glycogen contents. However, IAA had no effect on glycogen contents. Although hypoxia or glucose-free PSS did not affect PCr and ATP contents, the combination of hypoxia and glucose-free PSS or IAA induced a gradual decrease of PCr content. In conclusion, we suggest that endogenous glycogen was utilized to increase the activity of glycolysis for maintaining high K(+)-induced contraction of the bovine trachea in the glucose -free and/or hypoxic condition.

Ca2+-independent hypoxic vasorelaxation in porcine coronary artery

To demonstrate a Ca(2+)-independent component of hypoxic vasorelaxation and to investigate its mechanism, we utilized permeabilized porcine coronary arteries, in which [Ca(2+)] could be clamped. Arteries permeabilized with beta-escin developed maximum force in response to free Ca(2+) (6.6 microm), concomitant with a parallel increase in myosin regulatory light chain phosphorylation (MRLC-P(i)), from 0.183 +/- 0.023 to 0.353 +/- 0.019 MRLC-P(i) (total light chain)(-1). Hypoxia resulted in a significant decrease in both force (-31.9 +/- 4.1% prior developed force) and MRLC-P(i) (from 0.353 to 0.280 +/- 0.023), despite constant [Ca(2+)] buffered by EGTA (4 mm). Forces developed in response to Ca(2+) (6.6 microm), Ca(2+) (0.2 microm) + GTPgammaS (1 mM), or in the absence of Ca(2+) after treatment with ATPgammaS (1 mM), were of similar magnitude. Hypoxia also relaxed GTPgammaS contractures but importantly, arteries could not be relaxed after treatment with ATPgammaS. Permeabilization with Triton X-100 for 60 min also abolished hypoxic relaxation. The blocking of hypoxic relaxation after ATPgammaS suggests that this Ca(2+)-independent mechanism(s) may operate through alteration of MRLC-P(i) or of phosphorylation of the myosin binding subunit of myosin light chain phosphatase. Treatment with the Rho kinase inhibitor Y27632 (1 microm) relaxed GTPgammaS and Ca(2+) contractures; but the latter required a higher concentration (10 microm) for consistent relaxation. Relaxations to N(2) and/or Y27632 averaged 35% and were not additive or dependent on order. Our data suggest that the GTP-mediated, Rho kinase-coupled pathway merits further investigation as a potential site of this novel, Ca(2+)-independent O(2)-sensing mechanism. Importantly, these results unambiguously show that hypoxia-induced vasorelaxation can occur in permeabilized arteries where the Ca(2+) is clamped at a constant value.

Maternal dexamethasone treatment alters myosin isoform expression and contractile dynamics in fetal arteries

We tested the hypothesis that maternal glucocorticoid treatment modulates 17-kDa myosin light chain (myosin LC17) isoform expression and contractile dynamics in fetal ovine carotid arteries. In the single course group, ewes received 6 mg dexamethasone or placebo over 48 h. In the repeated course group, ewes received 6 mg dexamethasone or placebo weekly for 5 wk. In response to 1 microM phenylephrine, arteries from fetuses of dexamethasone-treated ewes exhibited biphasic contractions, characterized by an intermediate relaxation phase. The relaxation rate constant was significantly higher in arteries from the fetuses of dexamethasone than placebo-treated ewes. The observed biphasic contractions suggest the appearance of functional sarcoplasmic reticulum in the arteries from the fetuses of dexamethasone-treated ewes. The myosin LC17(a) isoform expression was lower in the arteries from the fetuses of the placebo-treated ewes than in those from the ewes. Repeated maternal administration of dexamethasone induced an almost twofold increase in myosin LC17(a) isoform expression in the fetal arteries. In contrast, maternal myosin LC17a isoform expression was not affected by dexamethasone treatment. We speculate that dexamethasone-induced increases in fetal myosin LC17(a) isoform expression represent accelerated differentiation of a subpopulation of vascular smooth muscle cells from the fetal to adult phenotype.