We have previously found that isolated lungs from six-month-old male sheep had a greater vasoconstrictor response to acute hypoxia than females. Moreover, hypoxic responses measured in isolated lungs from six-month-old males castrated within the first week of life, juvenile males, and juvenile females were not different from those measured in six-month-old, noncastrated males. These results suggested that the gender difference observed in isolated lungs of six-month-old sheep arose from attenuation of the hypoxic response in the female at the time of puberty, possibly because of enhanced release of female sex hormones. Consistent with this possibility, we found that estradiol pretreatment attenuated hypoxic pulmonary vasoconstriction in isolated lungs from juvenile female and six-month-old castrated male sheep.
Estrogens enhance prostaglandin production in uterine tissue. In addition, Chang and colleagues recently demonstrated that estradiol stimulated release of prostacyclin by rat aortic smooth muscle. Since prostacyclin, a potent pulmonary vasodilator, may modulate pulmonary vasoconstrictor responses to hypoxia, the present study was performed to assess the role of prostaglandins in the attenuation of hypoxic pulmonary vasoconstriction induced by estradiol. The purpose of this communication is to report our preliminary findings.
Twenty-three juvenile female sheep between 53 and 90 days of age weighing 10 to 31 kg were used in the experiments. Three to five days before the experiments, 12 sheep were given 20 mg of a long-acting preparation of estradiol (Delestrogen, Squibb) intramuscularly. On the day of the experiment, the sheep were anesthetized with IM ketamine (40 to 50 mg/kg). Using methods previously described, their lungs were isolated in situ, ventilated with a respirator, and perfused with a mixture of autologous blood and 3 percent dextran-70 in Ringers lactate by means of an extracorporeal perfusion circuit.
Left atrial pressure was kept subatmospheric. Perfusate temperature was maintained between 39 and 40°C. Except during the measurement of pressure-flow relationships, perfusate flow was 50 ml/min/kg body weight. Perfusate glucose concentration was maintained above 90 mg/dl, pH between 7.35 and 7.45, and inspired C02concentration at 5.4 percent. One hour was allowed for stabilization of the preparation before experiments were begun.
Four groups of lungs were studied. Indomethacin (40 m-g/ml) was added to the perfusate of five of the 12 lungs from sheep pretreated with estradiol and five of the 11 lungs from control sheep. In all four groups of lungs, the steady-state relationship between inspired oxygen tension (PIoJ and pulmonary vasomotor tone was determined as described previously. Inspired Po2 was decreased in a stepwise fashion from 200 to 0 mm Hg. At each level of PIo2 the relationship between pulmonary artery pressure and flow was measured at five-minute intervals until a steady state was achieved. The hypoxic stimulus-response relationship was quantified by determining the pressure in the pulmonary artery at a flow of 50 ml/min/ kg (PpaJ directiy from the pressure-flow curve after a steady state was achieved at each level of Plo2. In addition, perfusate samples were obtained when a steady state had been achieved at a PIo2 of 200 mm Hg for determination of thromboxane B2 (TxB^ and 6-keto-prostaglandin (6-keto-PGFjJ concentrations by radioimmunoassay. If you make up your mind to grapple with the medical aspects you should visit Canadian Neighbor Pharmacy official website.
Statistical comparisons were made using analysis of variance and t tests. Differences were considered significant when p 0.05.
Perfusate prostaglandin concentrations in the four groups are shown in Table 1. Estradiol significantly increased the concentration of 6-keto-PGF. The estradiol-induced increase in TxB2 approached statistical significance (.05 <p <. 1). Addition of indomethacin to the perfusate markedly decreased the levels of these prostaglandins and also prevented the increase caused by estradiol.
The hypoxic stimulus-response relationships of the pulmonary circulation are shown in Figure 1. In lungs which did not receive indomethacin, estradiol markedly depressed the pulmonary vasoconstrictor response to hypoxia and in addition lowered the Ppa measured under conditions of maximal vasodilation (Plo2 = 0 mm Hg). Indomethacin had no effect on the stimulus-response relationship in animals not receiving estradiol, but in estradiol-treated animals it restored the slope of the relationship to control values; however, indomethacin did not reverse the estradiol-induced decrease in Ppa measured under conditions of maximal vasodilation.
The results shown in Figure 1 confirm our previous observations that estradiol attenuated the pulmonary pressor response to hypoxia in isolated sheep lungs. They are also consistent with the observations of Moore and Reeves in female dogs but not with those of Fuchs et al in isolated lungs of female rats. The reasons for these inconsistencies are unknown.
The purpose of this investigation was to assess the possibility that the estradiol-induced attenuation of the hypoxic response might be due to enhanced release of prostaglandins. There are at least four reasons why this possibility should be considered. First, estrogen treatment has been shown to enhance production of prostaglandins in uterine tissue and in vascular smooth muscle. In the latter, estradiol increased prostacyclin production apparendy by stimulating the activity of prostaglandin cyclooxygenase and prostacyclin synthetase. The distribution of arachidonic acid, the endogenous fatty acid composition, and the phospholipase activity of the vascular smooth muscle cells were not affected. Second, several investigators found that the uterine vasodilator response to estrogen can be at least partially reversed by prior administration of cyclooxygenase inhibitors. Third, we previously found that perfusion of isolated sheep lungs with autologous blood caused release of cyclooxygenase products. Fourth, Voelkel and colleagues found that hypoxic pulmonary vasoconstriction in isolated rat lungs caused release of prostacyclin, which may act to modulate the response.
It is clear from Table 1 that administration of indomethacin inhibited prostaglandin production in the isolated sheep lung. As shown in Figure 1, this inhibition was associated with a significant alteration in the effects of estradiol on the hypoxic stimulus-response relationship. Indomethacin restored the “reactivity” (the slope of the stimulus-response relationship); however, it did not reverse the estradiol-induced decrease in “baseline” resistance (Ppa at Plo2=0 mm Hg).
These results suggest that estradiol attenuated the hypoxic stimulus-response relationship in at least two ways. First, it may have reduced the “reactivity” of the pulmonary vasculature to changes in PIo2 by enhancing the production and therefore the modulating activity of the prostaglandins. Consistent with this notion are the increases in prostaglandin release observed in estradiol-treated control sheep (Table 1). Second, estradiol decreased the minimum resistance of the pulmonary vasculature. The mechanism of this effect is unknown, but it apparently did not depend on cyclooxy-genase products since it was not reversed by indomethacin. It may be that Ppa, measured when PIo2=0 mm Hg was an index of the passive resistive properties of the bed. In this case, the decrease in Ppa at Plo2 = 0 mm Hg could reflect morphologic alterations of the pulmonary vessels. Wolinsky found that estrogen treatment prevented the medial hypertrophy observed in rats with renovascular hypertension. In addition, estrogens are thought to accelerate angiogenesis in fetal rabbit lungs and to influence the degree of vasodilation determined histologically in neonatal rabbit lungs. Perhaps in our experiments estradiol decreased the baseline resistance by inducing morphologic changes in the lung, which increased vascular cross-sectional area. Whether or not these speculations are correct will require further investigation.
Figure 1. The steady-state relationships between pulmonary artery pressure measured at a flow of 50 ml/min/kg (Ppa^) and inspired Os tension (PI02). Estradiol depressed this stimulus response relationship. Indomethacin reversed this depression in that it restored “reactivity” (the change in Ppa» induced by a change in PIo); however, indomethacin did not affect the estradiol-induced depression of the baseline (Ppa*, at Plo2=0 mm Hg).
Table 1—Mean ( ± SE) Perfusate Concentrations of Thromboxane Bt (TxBt) and 6-Keto-Prostaglandin FIa.
|Group||TxB2 (ng/ml)||6-keto-PGFle (ng/ml)|
|Control||4.82 (±1.41)||0.92 (±0.09)|
|Estradiol||12.15 (± 4.10)t||2.12 (±0.73)*|
|indomethacin||0.53 (±0.17)*||0.41 (±0.10)*|
|indomethacin||0.39 (±0.10)*||0.34 (±0.03)*|