Effect of chronic hypoxia on myometrial responsiveness in the pregnant rat

Hypoxic conditioning in blood vessels and smooth muscle tissues: effects on function, mechanisms, and unknowns

Hypoxic preconditioning, the protective effect of brief, intermittent hypoxic or ischemic episodes on subsequent more severe hypoxic episodes, has been known for 30 yr from studies on cardiac muscle. The concept of hypoxic preconditioning has expanded; excitingly, organs beyond the heart, including the brain, liver, and kidney, also benefit. Preconditioning of vascular and visceral smooth muscles has received less attention despite their obvious importance to health. In addition, there has been no attempt to synthesize the literature in this field. Therefore, in addition to overviewing the current understanding of hypoxic conditioning, in the present review, we consider the role of blood vessels in conditioning and explore evidence for conditioning in other smooth muscles. Where possible, we have distinguished effects on myocytes from other cell types in the visceral organs. We found evidence of a pivotal role for blood vessels in conditioning and for conditioning in other smooth muscle, including the bladder, vascular myocytes, and gastrointestinal tract, and a novel response in the uterus of a hypoxic-induced force increase, which helps maintain contractions during labor. To date, however, there are insufficient data to provide a comprehensive or unifying mechanism for smooth muscles or visceral organs and the effects of conditioning on their function. This also means that no firm conclusions can be drawn as to how differences between smooth muscles in metabolic and contractile activity may contribute to conditioning. Therefore, we have suggested what may be general mechanisms of conditioning occurring in all smooth muscles and tabulated tissue-specific mechanistic findings and suggested ideas for further progress.

Blood pH and gases in fetuses in preterm labor with and without systemic inflammatory response syndrome

OBJECTIVE

Fetal hypoxemia has been proposed to be one of the mechanisms of preterm labor (PTL) and delivery. This may have clinical implications since it may alter: (i) the method/frequency of fetal surveillance and (ii) the indications and duration of tocolysis to an already compromised fetus. The aim of this study was to examine whether there is a difference in the fetal blood gas analysis [pH, PaO(2) and base excess (BE)] and in the prevalence of fetal acidemia and hypoxia between: (i) patients in PTL who delivered within 72 hours vs. those who delivered more than 72 hours after cordocentesis and (ii) patients with fetal inflammatory response syndrome (FIRS) vs. those without this condition.

STUDY DESIGN

Patients admitted with PTL underwent amniocentesis and cordocentesis. Ninety women with singleton pregnancies and PTL were classified according to (i) those who delivered within 72 hours (n = 30) and after 72 hours of the cordocentesis (n = 60) and (ii) with and without FIRS. FIRS was defined as a fetal plasma concentration of IL-6 > 11 pg/mL. Fetal blood gases were determined. Acidemia and hypoxemia were defined as fetal pH and PaO(2) below the 5th percentile for gestational age, respectively. For comparisons between the two study groups, ΔpH and ΔPaO(2) were calculated by adjusting for gestational age (Δ = observed value - mean for gestational age). Non-parametric statistics were employed.

RESULTS

No differences in the median Δ pH (-0.026 vs. -0.016), ΔPaO(2) (0.25 mmHg vs. 5.9 mmHg) or BE (-2.4 vs. -2.6 mEq/L) were found between patients with PTL who delivered within 72 hours and those who delivered 72 hours after the cordocentesis (p > 0.05 for all comparisons). Fetal plasma IL-6 concentration was determined in 63% (57/90) of fetuses and the prevalence of FIRS was 28% (16/57). There was no difference in fetal pH, PaO(2) and BE between fetuses with and without FIRS (p > 0.05 for all comparisons). Moreover, there was no difference in the rate of fetal acidemia between fetuses with and without FIRS (6.3 vs. 9.8%; p > 0.05) and fetal hypoxia between fetuses with or without FIRS (12.5 vs. 19.5%; p > 0.05).

CONCLUSIONS

Our data do not support a role for acute fetal hypoxemia and metabolic acidemia in the etiology of PTL and delivery.

Functional determination of oxytocin affinity in near-term pregnant rat myometrium: effect of chronic hypoxia

We designed the present study to determine: (1) if phenoxybenzamine can be used as an irreversible blocker for oxytocin receptors, and as such to determine oxytocin affinity, (2) if prolonged hypoxic exposure alters oxytocin receptor coupling efficacy of oxytocin receptors to post-receptor mediated mechanisms in the rat myometrium. Rats were exposed to room air (control), or to continuous hypoxia (10.5% O2) from day 19 through day 21 (2-day exposure). On day 21, one uterine horn was removed and used for in vitro study of myometrial contractile responses to oxytocin, while the other was used for oxytocin receptor analysis. In normoxic tissues, phenoxybenzamine (20 microM) decreased the maximum contractile response (EMAX) to oxytocin (155+/-17 vs. 66+/-19 g s/cm2) and oxytocin binding sites (BMAX: 253+/-35 vs. 114.9+/-21.3 fmol/mg protein). A similar degree of reduction in EMAX and BMAX were observed in hypoxic tissues. The oxytocin dissociation constant (KA) in the normoxic rat was 2.8+/-0.7 nM, which was not different from the chronic hypoxic rat (3.3+/-0.9 nM). Analysis of receptor occupancy-response curves indicated no oxytocin receptor reserve in both normoxic and hypoxic myometrium. However, for a given fraction of the total oxytocin receptors occupied, hypoxic tissue elicited a lower contractile response to oxytocin. We conclude that: (1) phenoxybenzamine is a useful tool to functionally study oxytocin receptor kinetics, (2) prolonged hypoxic exposure does not affect the oxytocin affinity, (3) no spare receptors for oxytocin are present in the rat myometrium, and (4) prolonged exposure to hypoxia decreases oxytocin receptor-effector coupling efficiency in rat myometrium.

Chronic hypoxia suppresses pharmacomechanical coupling of the uterine artery in near-term pregnant sheep

1. The role of inositol 1,4,5-trisphosphate (InsP3) in the reduced vascular responsiveness to 5-hydroxytryptamine (5-HT) caused by chronic hypoxia was examined in uterine arteries obtained from normoxic (control) and chronically hypoxic pregnant sheep (approximately 140 days gestation) maintained at high altitude (3820 m; arterial PO2, 60 mmHg) from 30 days gestation. 2. Chronic hypoxia significantly decreased uterine artery contractile sensitivity in that pD2 (-logEC50) for the contractile response to 5-HT was 7.19 +/- 0.15 and 6.62 +/- 0.12 (P < 0.05) in uterine arteries from normoxic and chronically hypoxic sheep, respectively. The intrinsic efficacy of the agonist was reduced by 75%. Although 5-HT2A receptor density (Bmax) in the uterine artery was not changed in chronically hypoxic sheep compared with normoxic sheep (32.0 +/- 9.8 vs. 31.9 +/- 5.9 fmol (mg protein)-1, respectively) as assessed from the saturation binding of [3H]ketanserin, the agonist binding affinity (pKA, -log of dissociation constant) was decreased from 6.25 +/- 0.07 in normoxic sheep to 5.85 +/- 0.08 in chronically hypoxic sheep (P < 0.05). 3. Chronic hypoxia did not change the time course of 5-HT-induced InsP3 synthesis but decreased its potency in inducing InsP3 synthesis, with the pD2 being 6.09 +/- 0.11 and 5.51 +/- 0.08 (P < 0.05) in uterine arteries from normoxic and chronically hypoxic sheep, respectively. The maximal response of 5-HT-induced InsP3 generation in the uterine artery was decreased from 251.3 +/- 24.2 pmol (mg protein)-1 in normoxic sheep to 146.6 +/- 11.3 pmol (mg protein)-1 in chronically hypoxic sheep (P < 0.05). Furthermore, the ability of the activated 5-HT receptors to couple InsP3 synthesis was significantly decreased in chronically hypoxic compared with normoxic sheep (280 +/- 10 vs. 450 +/- 20 fmol InsP3 (fmol receptor)-1, P < 0.01). In addition, for a given amount of InsP3 generated, the contractile force of the uterine artery was significantly less in chronically hypoxic sheep (0.82 +/- 0.08 g tension (pmol InsP3)-1) than that in normoxic sheep (1.28 +/- 0.05 g tension (pmol InsP3)-1) (P < 0.05). 4. These results suggest that chronic hypoxia suppresses pharmacomechanical coupling of the ovine uterine artery by inhibiting the efficiency of receptor-effector-contraction coupling. This suppression of the InsP3 pathway may play an important role in the adjustment of vascular tone and uterine blood flow in response to the stress of chronic hypoxia in late pregnancy.