Uterine and peripheral blood concentrations of vasodilator prostaglandins in conscious pregnant rabbits

Mesenteric arterial function in the rat in pregnancy: role of sympathetic and sensory-motor perivascular nerves, endothelium, smooth muscle, nitric oxide and prostaglandins

1. The effects of pregnancy on mesenteric arterial function were examined in constantly perfused (5 ml min-1) mesenteric arterial beds isolated from 21-day pregnant rats. The function of sympathetic and sensory-motor perivascular nerves, endothelium and smooth muscle was examined. The role of nitric oxide and prostaglandins in vasoconstrictor function was tested by use of NG-nitro-L-arginine methyl ester (L-NAME; 100 microM) and indomethacin (10 microM), respectively. 2. Electrical field stimulation (EFS; 4-32 Hz, 1 ms, 90V, 30s) at basal tone elicited frequency-dependent vasoconstriction which was markedly reduced in preparations from pregnant rats at all frequencies. Vasoconstrictor responses to vasopressin and endothelin were also reduced in pregnancy and there was a trend towards a reduction in maximal responses to noradrenaline (NA). In contrast, there was no difference in vasoconstrictor responses to ATP, 5-hydroxytryptamine (5-HT) or angiotension II. 3. L-NAME (100 microM) augmented responses to EFS, NA, ATP and vasopressin in control mesenteric arterial preparations. In contrast, L-NAME augmented responses only to EFS in pregnancy, having no significant effect on responses to NA, ATP and vasopressin. 4. Indomethacin (10 microM) attenuated responses to NA and vasopressin, but not to EFS, in controls and in pregnancy. Responses to ATP were attenuated by indomethacin in controls but not in pregnancy. 5. Mesenteric preparations from pregnant rats were resistant to having tone raised by continuous perfusion with methoxamine. Despite an approximately 10 fold greater concentration of methoxamine, there was a significantly smaller increase in tone in preparations from pregnant, 34.27 +/- 4.8 mmHg (n = 11) compared to control, 65.92 +/- 5.4 mmHg (n = 11), rats. EFS (4-12 Hz, 60 V, 0.1 ms, 30s) in the presence of guanethidine (5 microM) to block sympathetic neurotransmission elicited frequency-dependent vasodilatation due to activation of sensory-motor nerves. Percentage relaxations were similar in preparations from pregnant and non-pregnant rats. 6. Dose-dependent endothelium-dependent vasodilatations to acetylcholine and ATP were similar in preparations from pregnant and non-pregnant rats. Endothelium-independent vasodilatation to sodium nitroprusside and to calcitonin gene-related peptide were also similar between the two groups. 7. There was no significant difference in the basal perfusion pressure of mesenteric arterial beds from control (21.3 +/- 1.0 mmHg, n = 24) and pregnant (20.2 +/- 1.2 mmHg, n = 23) rats. However, a step-wise increase in perfusate flow from 5 to 10, 15, 20 and 24ml min-1 produced smaller increases in perfusion pressure in pregnancy compared to the controls. L-NAME (100 microM) or indomethacin (10 microM) had no significant effect on the relationship between flow and perfusion pressure. 8. The present results show that prejunctional changes are involved in blunted sympathetic vasoconstriction of rat mesenteric arteries in pregnancy. Non-specific postjunctional changes are implicated in the reduced constrictor responses to applied methoxamine, vasopressin and endothelin, but not to ATP. In contrast, sensory-motor nerves and endothelium-dependent and -independent vasodilatation was unchanged. The decrease in receptor-mediated mesenteric arterial constrictor responsiveness in pregnancy does not appear to be due to acute modulation by NO or prostaglandins, but may involve changes in the distensibility of the bed and/or changes in wall thickness.

Haemostatic mechanism in the endometrium: role of cyclo-oxygenase products and coagulation factors

1. The primary mechanism of haemostasis in the endometrium of rat was studied and results were compared to that in the mesenteric artery. 2. The bleeding time of the rat endometrium as assessed by haemoglobin output was significantly decreased after pretreatment of the animals with either indomethacin (5 mg kg-1, i.v.) or meclofenamate (3 mg kg-1, i.v.) whereas the bleeding time was significantly increased in the rat mesenteric artery. 3. The bleeding time of the rat endometrium was unchanged from control values following treatment with prostacyclin (0.5 microgram kg-1 min-1, i.v.) or 1-benzylimidazole (50 mg kg-1, i.v.) whereas the bleeding times were increased in the rat mesentric artery. 4. Administration of heparin (100 units kg-1) increased the bleeding time in the rat mesenteric artery but had no effect on the bleeding time of the endometrium. 5. Superfusion of the endometrium with 16, 16-dimethyl PGE2 (1 microgram ml-1) a vasodilator, increased the bleeding time of the endometrium but superfusion of PGE2 over the mesenteric artery did not affect the bleeding time from this site. 6. Histological studies of the mesenteric artery and the endometrium following haemostatis revealed that the haemostatic plug in the mesenteric artery was mainly composed of platelets and fibrin whereas in the endometrium it was mainly composed of fibrin. 7. These findings suggest that haemostasis in the endometrium may be mediated by the vascular tone and fibrin whereas formation of the platelet plug may be primary mechanism for haemostasis in the mesenteric artery.

The role of nitric oxide in the altered vascular reactivity of pregnancy in the rat

1. Pregnancy is characterized by a decrease in systemic vascular resistance and a blunting of the angiotensin II (AII) pressor response. We studied the role of nitric oxide (NO) and prostanoids in these vascular changes of pregnancy in anaesthesized, ganglion blocked non-pregnant and pregnant rats. 2. Inhibition of NO synthesis with NG-nitro-L-arginine methyl ester (L-NAME) led to an increase in mean arterial pressure (MAP) which was of a significantly greater magnitude in pregnant rats in late gestation than in non-pregnant rats, or rats in mid-gestation. 3. The pressor response to varying doses of AII was attenuated during late pregnancy, and this attenuation was partially reversed by L-NAME. 4. The pressor response to varying doses of a vasoconstrictor, phenylephrine (PE), was also attenuated in late pregnancy. However, this attenuation was not reversed by L-NAME. 5. Inhibition of prostanoid biosynthesis with meclofenamate did not alter basal MAP, nor the pressor response to varying doses of AII or PE in pregnant and non-pregnant animals. 6. It is concluded that (a) increased NO synthesis occurs during late gestation and contributes both to the decrease in systemic vascular resistance, as well as the blunting of the pressor response to AII during pregnancy, and (b) prostaglandins are not important in the maintenance of basal vascular tone, or the blunting of the pressor response to AII during pregnancy.

Eicosanoid production in rabbit vascular tissues and placentas

In pregnant rabbits, systemic levels of prostaglandin E2 (PGE2) are elevated. To evaluate the source of the rise in circulating levels of PGE2, eicosanoid production was determined in rabbit placental cotyledons and the following vascular tissues: mesenteric arteries, main renal arteries, and intrarenal preglomerular vessels (n = 6 or 7 rabbits). Comparisons were made with vascular tissues from nonpregnant rabbits (n = 6 or 7). Freshly isolated tissues were incubated for 15 min at 37 degrees C. Medium aliquots were analyzed for eicosanoids by radioimmunoassay. Maximal net production and release (pg/microgram protein) was determined by subtraction of the quantity of eicosanoids present in medium before incubation from that present at the end of incubation. Net production and release of 6-ketoprostaglandin F1 alpha (PGF1 alpha), a stable metabolite of prostacyclin, and of PGE2 was similar in vascular tissues from pregnant and nonpregnant rabbits. Thromboxane B2 (TxB2) generation in preglomerular vessels from pregnant rabbits was lower (P less than 0.05) than that of nonpregnant rabbits. This could contribute to renal vasodilation in pregnancy. The pattern of eicosanoid production in vascular tissues was 6-keto-PGF1 alpha greater than PGE2 greater than TxB2, and in cotyledons it was PGE2 greater than 6-keto-PGF1 alpha greater than TxB2. In cotyledons, PGE2 and TxB2 production was 56-219 and 2-12 times that in maternal vascular tissues, respectively. These data suggest that physiologically significant quantities of eicosanoids may be released by rabbit placental tissue.

Uteroplacental production of eicosanoids in ovine pregnancy

Dramatic cardiovascular alterations occur during normal ovine pregnancy which may be associated with increased prostaglandin production, especially of uteroplacental origin. To study this, we examined (Exp 1) the relationships between cardiovascular alterations, e.g., the rise in uterine blood flow and fall in systemic vascular resistance, and arterial concentrations of prostaglandin metabolites (PGEM, PGFM and 6-keto-PGF1 alpha) in nonpregnant (n = 4) and pregnant (n = 8) ewes. To determine the potential utero-placental contribution of these eicosanoids in pregnancy, we also studied (Exp 2) the relationship between uterine blood flow and the uterine venous-arterial concentration differences of PGE2, PGF2 alpha, PGFM, 6-keto-PGF1 alpha, and TxB2 in twelve additional late pregnant ewes. Pregnancy was associated with a 37-fold increase in uterine blood flow and a proportionate (27-fold) fall in uterine vascular resistance (p less than 0.01). Arterial concentrations of PGEM were similar in nonpregnant and pregnant ewes (316 +/- 19 and 245 +/- 38 pg/ml), while levels of PGFM and PGI2 metabolite 6-keto-PGF1 alpha were elevated 23-fold (31 +/- 14 to 708 +/- 244 pg/ml) and 14-fold (12 +/- 4 to 163 +/- 78 pg/ml), respectively (p less than 0.01). Higher uterine venous versus uterine arterial concentrations were observed for PGE2 (397 +/- 36 and 293 +/- 22 pg/ml) and 6-keto-PGF1 alpha (269 +/- 32 and 204 +/- 32 pg/ml), p less than 0.05, but not PGF2 alpha or TxB2. Although PGFM concentrations appeared to be greater in uterine venous (1197 +/- 225 pg/ml) as compared to uterine arterial (738 +/- 150 pg/ml) plasma, this did not reach significance (0.05 less than p less than 0.1). In normal ovine pregnancy arterial levels of PGI2 are increased, which may in part reflect increased uteroplacental production. Moreover the gravid ovine uterus also appears to produce PGE2 and metabolize PGF2 alpha.

Arachidonic acid metabolism by rabbit fetal membranes of various gestational ages

The ability of rabbit fetal membranes to convert arachidonic acid to both lipoxygenase and cyclooxygenase products was studied by separation and identification of products derived from incubations of 1-14C-arachidonate with subcellular fractions obtained by differential centrifugation of tissue homogenates. Both amnion and splanchnopleure (the chorion-equivalent of the rabbit) produced a mixture of 11-, 12-, and 15-hydroxyeicosatetraenoic acids when stimulated by calcium ions; these products were produced in greater quantity in middle pregnancy (20-22d) than later (28-30d). Cyclooxygenase products included PGD2, PGE2, TxB2 and PGF2 alpha, all of which were made more actively in late pregnancy than the middle in both amnion (which was more active) and chorion-equivalent. These data suggest that arachidonate metabolism by rabbit fetal membranes in middle pregnancy is directed primarily toward production of monohydroxy fatty acids, but that as pregnancy nears term, the PG-producing enzymes are induced, preparing the uterine smooth muscle for parturition.