Reduced thromboxane receptor affinity and vasoconstrictor responses in placentae from diabetic pregnancies.

Thromboxane has been implicated in the pathogenesis of maternal hypertension in high-risk pregnancies, but potential abnormalities in thromboxane-mediated constriction of fetoplacental vessels has not been examined. Using the isolated perfused fetoplacental cotyledon, we compared the vasoconstrictor responses to a thromboxane mimetic, U46619, in placentae from normal women and women with diabetes mellitus (classes C, D and R). Increases in perfusion pressure in response to bolus injections of U46619 were used to construct dose-response curves. The threshold dose of U46619 to cause a pressor response was similar in placentae from normal and diabetic pregnancies, but the slope of the dose-response curve was decreased by 39 per cent in placentae from diabetic pregnancies compared with normal controls (P < 0.01). To examine the potential contribution of altered thromboxane receptors, equilibrium binding studies were performed using the thromboxane antagonist [3H]-SQ29548 to a 44,000 g fraction of placental homogenate. The affinity of thromboxane receptors was significantly decreased in placentae from diabetic pregnancies compared with normal controls [Kd = 41.9 +/- 7.9, (n = 6) versus control, 21.4 +/- 1.3 nM (n = 26), P < 0.001]. In contrast, the density of thromboxane receptor sites was not significantly changed (diabetes, 176.0 +/- 6.2 versus control, 150.3 +/- 6.5 fmol/mg, P = not significant). Placental production of thromboxane and prostacyclin were measured by the incorporation of [14C]-arachidonic acid into [14C]-thromboxane B2 and [14C]-6-keto-prostaglandin F1 alpha, respectively. Incorporation of [14C]-arachidonic acid into both thromboxane B2 and 6-keto-prostaglandin F1 alpha was similar in placentae from diabetic and normal pregnancies. We conclude that vascular responsiveness to thromboxane is reduced in placentae from mothers with diabetes by a receptor-mediated mechanism. These changes may contribute to abnormalities in the regulation of fetoplacental haemodynamics, growth and development in pregnancies complicated by diabetes mellitus.

Cyclooxygenase inhibitors blunt thromboxane action in human placental arteries by blocking thromboxane receptors.

The effects of cyclooxygenase inhibitors on thromboxane-mediated vasoconstriction in human placental arteries were studied in the isolated perfused fetoplacental cotyledon. The stable thromboxane agonist U-46619 caused a dose-related increase in perfusion pressure in the fetal side of the cotyledon. Meclofenamate (3.3 x 10(-5) M) significantly blunted the pressor response to U-46619, but not to angiotensin II, and inhibited thromboxane B2 formation in placental slices (IC50, 4.80 x 10(-8) M). The mechanism by which meclofenamate prevented thromboxane-induced vasoconstriction was studied using ligand-binding techniques in a membrane fraction prepared from placental cotyledons. Meclofenamate caused a dose-related inhibition of binding of the thromboxane receptor antagonist [3H]SQ 29548 with an IC50 of 2.61 x 10(-5) M. Scatchard analysis of equilibrium binding demonstrated that meclofenamate reduced the number of binding sites without altering the affinity of the receptor, suggesting a noncompetitive mechanism. Indomethacin also caused a dose-related inhibition of thromboxane binding (IC50, 3.27 x 10(-4) M). However, aspirin at a dose of 2.0 x 10(-3) M did not inhibit [3H]SQ 29548 binding. The data indicate that some cyclooxygenase inhibitors blunt thromboxane actions by interfering with binding at thromboxane receptor sites. These studies identify a new mechanism by which cyclooxygenase inhibition by some nonsteroidal anti-inflammatory drugs can prevent thromboxane action in fetoplacental blood vessels in vitro independent of reductions in thromboxane formation.

Plasma angiotensins in anephric humans: evidence for an extrarenal angiotensin system.

The recent identification of messenger RNAs encoding renin and angiotensinogen in nonrenal tissues raises the possibility that angiotensins (Ang) may be formed extrarenally and released into the plasma. The aim of this investigation was to test the hypothesis that plasma angiotensins may originate from extrarenal sites. Twenty-five patients with chronic renal failure (six surgically anephric and 19 with kidneys in situ) were studied prior to and after a standard hemodialysis treatment. Angiotensins were measured by extraction, high-pressure liquid chromatography (HPLC) separation, and radioimmunoassays. In patients with kidneys present, plasma renin activity (PRA) was 3.1 +/- 0.7 ng Ang I/ml/h. Ang I, Ang II, and Ang III levels were 70.6 +/- 9.0, 44.0 +/- 9.8, and 20.2 +/- 3.6 pg/ml, respectively. In all six anephric patients PRA was undetectable (less than 0.1 ng Ang I/ml/h). Ang I and Ang II were detected in four anephric patients, and Ang III was detected in three anephric patients (Ang I, 10.4 +/- 5.2; Ang II, 2.6 +/- 1.2; Ang III, 2.7 +/- 1.5 pg/ml, n = 6). At the completion of dialysis treatments, which reduced body weight by 2.5 +/- 0.2 kg in patients with kidneys and by 2.1 +/- 0.3 kg in anephric patients, there were no significant changes in PRA or plasma angiotensins in either group. Reduction in body water by hemodialysis did not increase the concentration of angiotensins in plasma. We conclude that there is a small but definite component of plasma angiotensin that is produced by nonrenal mechanisms and that is not stimulated by volume depletion.

Endothelin receptors in human placenta: relationship to vascular resistance and thromboxane release.

This investigation was performed to study the potential role of endothelin in the modulation of fetoplacental vascular resistance in the human placenta. Full-term placentas from uncomplicated pregnancies were studied within 30 min of delivery. The umbilical artery and vein to a single placental cotyledon were cannulated and the artery perfused with RPMI media (0.82 ml/min). Endothelin 1 caused a sustained dose-dependent increase in perfusion pressure. Infused endothelin 1 (50 nM) stimulated thromboxane release 2.3-fold compared with basal values. Thromboxane release persisted for 15 min after discontinuation of endothelin. Properties of human placental endothelin 1 receptors were defined in binding studies performed on a crude membrane fraction of placental cotyledons. Binding was saturable, reached steady state by 3 h at 25 degrees C, and was linear with protein concentration. Scatchard analysis of binding data indicated a single class of high-affinity binding sites with a Kd of 36.1 +/- 9.7 pM and a density of 185.4 +/- 9.6 fmol/mg protein (n = 5). The potency order for competitive inhibition of the binding of 125I-labeled endothelin 1 was endothelin 1 greater than endothelin 2 = endothelin 3 = sarafotoxin S6b greater than big endothelin (human) = big endothelin (porcine). Phenylephrine, bradykinin, norepinephrine, atrial natriuretic factor, diltiazem, U46619, and angiotensin II did not displace 125I-endothelin 1 from its receptors. These experiments demonstrate that endothelin 1 is a potent pressor substance in the human fetoplacental cotyledon. Pressor effects of endothelin may be mediated by a combination of direct effects and stimulation of vasoconstrictor prostanoids.

Limbic encephalitis. A rare presentation of the small-cell lung carcinoma.

Two patients with an acute organic brain syndrome and accompanying neurological symptoms are described. Extensive work up showed that both patients suffered from small-cell lung cancer. Cerebral metastases were absent. Following chemotherapy and radiotherapy to the primary tumor one of the two patients showed a complete remission of psychiatric symptoms for one year. A paraneoplastic origin of this syndrome, in the literature known as limbic encephalitis, is postulated. The exact cause of this syndrome is yet unknown. Recent research reveals data indicating an immunological pathogenesis. The major clinical importance of this (neuro)-psychiatric syndrome is that its appearance may serve as a warning sign for an occult malignancy; furthermore, effective treatment of the primary malignancy can reverse the encephalitis. Thus antitumor therapy can result in a prolonged survival and considerably improved quality of life.

Evidence for functional thromboxane A2-prostaglandin H2 receptors in human placenta.

The aim of this investigation was to study the role of thromboxane (TX) A2 in the modulation of human fetoplacental vascular resistance. By use of the isolated perfused fetoplacental cotyledon, TX generation (measured by direct radioimmunoassay of TXB2) was demonstrated on the fetal side of the placental circulation. The stable TX mimetic U-46619 caused a dose-dependent increase in perfusion pressure that was inhibited by the TX receptor antagonist SQ 29548. To further characterize the putative TXA2-prostaglandin H2 receptors, binding studies were performed in placental membranes using [3H]SQ 29548. Kinetic analysis revealed rapid and reversible specific binding of [3H]SQ 29548. Saturation binding and Scatchard analysis indicated radioligand binding to a single class of receptors (dissociation constant, 9.11 +/- 0.60 nM; receptor density, 103 +/- 8 fmol/mg protein, n = 4). Prostaglandins D2, E1, E2, F2a, and I2 did not inhibit the specific binding of [3H]SQ 29548 at concentrations less than or equal to 10 microM. This study demonstrates that the human placenta produces and releases TXA2, which can act locally via specific receptor sites to constrict the fetoplacental vasculature.