Nitric oxide regulation of corticotropin-releasing hormone release from the human perfused placenta in vitro.

We have investigated the regulatory role of nitric oxide (NO) in corticotropin-releasing hormone (CRH) release from the human perfused placental lobule in vitro. The effects of the NO donor sodium nitroprusside, the NO synthase inhibitor N omega-nitro-L-arginine, and the NO substrate L-arginine on human (h) placental CRH secretion have been studied. Single lobules of term placentae were bilaterally perfused with Krebs solution (5 mL/min; 95% O2-5% CO2; 37 C; pH 7.3). Fetal and maternal perfusates were collected at 4 C every 30 min for 3 h. CRH immunoreactivity (CRH-IR) in perfusates was measured by RIA using the 41-residue synthetic CRH as standard, 125I-labeled Tyr-hCRH as tracer, and a rabbit anti-CRH antibody Y2BO. The sensitivity of the assay was 0.13 pmol/L. Under basal conditions, human perfused placentae in vitro continuously secreted CRH-IR, which diluted in parallel to a synthetic hCRH-(1-41) standard curve. Size-exclusion chromatography of placental perfusates using a Sephadex G-50 column indicated that placental CRH-IR predominately coeluted with hCRH-(1-41) standard. Basal maternal perfusate CRH-IR levels (27 +/- 4 pmol/L) released from perfused placental lobules were nearly 10-fold greater than fetal perfusate CRH-IR levels (3.4 +/- 0.7 pmol/L; P < 0.05). Infusion of sodium nitroprusside (30-100 mumol/L) into the maternal and fetal placental circulations inhibited CRH-IR release into maternal perfusate in a concentration-dependent manner, but did not inhibit CRH-IR release into the fetal perfusate. N omega-nitro-L-arginine (100 mumol/L) increased placental CRH-IR secretion into fetal perfusate, and this effect was reversed by the infusion of L-arginine (100 mumol/L), which also reduced release below basal levels. In contrast, maternal perfusate CRH-IR levels were not affected by N omega-nitro-L-arginine or L-arginine. These results indicate that the human perfused placenta in vitro releases a substance of similar mol wt and hCRH-IR. Moreover, modulators of the NO signaling pathway differentially affect placental secretion of CRH-IR into the maternal and fetal perfusates. These data are consistent with the involvement of NO in the regulation of placental CRH release during pregnancy.

Adrenocorticotropin causes vasodilatation in the human fetal-placental circulation.

During human pregnancy, ACTH is produced by both the placenta and fetal pituitary. ACTH has been shown to cause vasodilatation in the adrenal cortex in vitro. In this context we have investigated the vasoactive effects of ACTH in the human fetal-placental circulation. Single lobules of term human placentas were bilaterally perfused in vitro with Krebs solution (maternal and fetal, 5 mL/min; 95% O2-5% CO2; 37 C; pH 7.3), and changes in fetal placental arterial perfusion pressure (FAP) were measured. ACTH (40-4000 pmol/L; n = 5) caused a dose-dependent reduction of both KC1 and PGF2alpha-induced increases in FAP in the fetal placental circulation. The reductions were of a similar magnitude in the presence of either constrictor agent. ACTH was 187.4 (95% confidence limits, 162.7-215.9) times more potent than prostacyclin (PGI2; 1.2-1180 nmol/L; n = 6), which is a known vasodilator of the fetal-placental circulation. The threshold concentrations for ACTH and PGI2 were 40 pmol/L and 1.2 nmol/L, respectively. ACTH-induced reductions in PGF2alpha-induced increases in FAP in the fetal placental circulation were not inhibited by the nitric oxide synthase inhibitor, N omega-nitro-L-arginine (100 micromol/L; n = 5), the cyclooxygenase inhibitor indomethacin (3 micromol/L; n = 5), or a guanylate cyclase inhibitor LY83583(1 micromol/L; n = 5). The inhibitory effect of ACTH was attenuated by the antagonist, ACTH-(7-38) (240 pmol/L; n = 4), and a polyclonal ACTH antiserum (1:1000 dilution; n = 4). We have demonstrated that ACTH causes a reduction in fetal placental vascular resistance in the human fetal-placental circulation in vitro. The mechanism by which it exerts these effects has not been defined, but neither nitric oxide nor PG-mediated pathways appear to be involved.

Vasodilator actions of urocortin and related peptides in the human perfused placenta in vitro.

Urocortin, is a recently isolated peptide belonging to the CRH family that binds with high affinity to the CRH2 receptor. Like CRH, urocortin causes hypotension in the rat, but its vasoactive actions have not yet been studied in the human. We have compared the vasoactive properties of urocortin, CRH, and urotensin-1 in the human fetal placental vasculature in vitro. Single placental lobules were bilaterally perfused (maternal and fetal sides, 5 mL/min each; 95% O2-5% CO2; 37 C), and changes in fetal arterial perfusion pressure were recorded. Submaximal vasoconstriction was induced by PGF2alpha (4+/-0.7 micromol/L), which increased perfusion pressure from 19.6+/-1.4 to 100.7+/-3.1 mm Hg (n=38; P < 0.001). Subsequent fetal arterial infusion of urocortin (0.001-1 nmol/L) caused concentration-dependent vasodilatation. Urocortin was equipotent with urotensin-1 and 25 times more potent than CRH in causing vasodilatation. Nevertheless, the maximum vasodilator responses to each of the peptides were similar (P > 0.05). The CRH receptor antagonist, alpha-helical CRH-(9-41) (0.2 nmol/L) significantly attenuated the vasodilatation produced by urocortin, urotensin-1, and CRH (P < 0.05). These results indicate a possible physiological role for urocortin in the modulation of human fetal placental vascular tone by activation of CRH2-like receptors.

Corticotropin-releasing hormone-induced vasodilatation in the human fetal placental circulation.

The vasoactive effects of corticotropin-releasing hormone (CRH) in the human fetal-placental circulation in vitro have been investigated. Single lobules of term placentae were bilaterally perfused with constant flows of Krebs' solution (maternal and fetal, 5 ml/min, 95% O2, 5% CO2, 37 degrees C, pH 7.3) and changes in fetal-placental arterial perfusion pressure measured. Effects of human (hCRH) and ovine (oCRH) CRH were examined during submaximal vasoconstriction (100-120 mmHg) of the fetal-placental vasculature induced by prostaglandin F2 alpha (PGF2 alpha), (0.7-2 mumol/L). During infusion of hCRH or oCRH (24-7000 pmol/L) a concentration-dependent vasodilatation was observed. Human CRH and oCRH were equipotent as vasodilator agents (regression analysis; P > 0.05; n = 5). The vasodilator response curves to human and ovine CRH were compared to prostacyclin (PGI2) (1.2-1180 nmol/L). Human and oCRH were 53 times more potent than PGI2 (regression analysis, P < 0.05; n = 5). These results indicate that CRH has powerful vasodilator properties in the human fetal-placental circulation and may play a role in control of placental vascular resistance to blood flow.

Corticotropin-releasing hormone-induced vasodilatation in the human fetal-placental circulation: involvement of the nitric oxide-cyclic guanosine 3',5'-monophosphate-mediated pathway.

This study has used an in vitro perfusion method to investigate the mechanism by which CRH causes vasodilatation in the human fetal-placental circulation. In normal term placentas, vasodilatory responses to human CRH (24-7000 pmol/L) were examined during submaximal vasoconstriction (100-120 mm Hg) of the fetal-placental vasculature induced by prostaglandin F2 alpha (0.7-2 mumol/L), KCl (50-100 mmol/L), or the thromboxane A2 mimetic, U46619 (0.05-0.5 mumol/L). Infusion of CRH caused a concentration-dependent vasodilatation that was similar in the presence of each constrictor agent (P > 0.05). The CRH antagonist, alpha-helical CRH-(9-41) (200 pmol/L), and a polyclonal CRH antiserum significantly inhibited CRH-induced vasodilatation during constriction with prostaglandin F2 alpha (P < 0.05). Vasodilatory responses to CRH were attenuated by the nitric oxide synthase inhibitor, N omega-nitro-L-arginine (100 mumol/L; P < 0.05), and the guanylate cyclase inhibitor, LY 83583 (1 mumol/L; P < 0.05), but not by the cyclooxygenase inhibitor, indomethacin (3 mumol/L; P > 0.05). In placentas of women with increased fetal vascular resistance, as demonstrated by Doppler ultrasound waveforms in vivo, CRH-induced vasodilatation was significantly reduced (P < 0.05). These results indicate that in the human fetal-placental circulation, CRH causes a vasodilatory response via a nitric oxide-/cGMP-dependent pathway. CRH may play a role in the control of vascular resistance to blood flow in the normal human placenta, and there may be a deficiency in the CRH signaling pathway of placentas with increased fetal vascular resistance.

U46619-mediated vasoconstriction of the fetal placental vasculature in vitro in normal and hypertensive pregnancies.

OBJECTIVES: To measure in-vitro responses to the thromboxane A2 (TxA2) mimetic U46619 in the fetal placental vasculature of human placentae from normotensive women and those with pre-eclampsia. Furthermore, to compare fetal vascular responses to endothelin-1,5-hydroxytryptamine, potassium chloride (KCl) and prostacyclin (PGI2) in placentae from normal or pre-eclamptic pregnancies. METHODS: Single placental lobules of intact placentae were bilaterally perfused in situ (fetal and maternal) with constant flows of Krebs' solution. Changes in fetal arterial perfusion pressure during intra-arterial infusion of vasoactive agents were recorded. Fetal placental vasoconstrictor concentration response curves were obtained to U46619 (0.01-300 nmol/l), endothelin-1 (0.4-160 nmol/l), KCl (3-300 mmol/l) and 5-hydroxytryptamine (0.03-30 mumol/l). In addition, vasodilator concentration response curves were obtained for PGI2 (1.2-350 nmol/l) in the fetal placental circulation during submaximal increases in perfusion pressure with prostaglandin F2 alpha (PGF2 alpha; 0.7-2.0 mumol/l). RESULTS: The maximum increase in perfusion pressure caused by U46619 in placentae from normotensive women was 194 +/- 25 mmHg. The maximum response to U46619 was significantly reduced in the placentae from women with pre-eclampsia (104 +/- 21 mmHg). In contrast, there were no differences in constrictor responses to endothelin-1,5-hydroxytryptamine and KCl, or in dilator responses to PGI2 in placentae obtained from either normotensive women or those with pre-eclampsia. CONCLUSION: TxA2 receptor-mediated vasoconstriction is reduced in the fetal vasculature of placentae from women with pre-eclampsia, possibly to compensate for the increased levels of TxA2 seen in these conditions.

Converting enzyme inhibition in the rat by captopril is accompanied by potentiation of carrageenin-induced inflammation.

Hind paw oedema in rats, measured by plethysmography or extravasation of Evans Blue dye into the skin, after subplantar injection of submaximal doses of carrageenin (1-100 micrograms) was significantly increased for 4 h during kininase II inhibition with captopril (1 mg kg-1, s.c.). Submaximal oedema, as assessed by paw swelling, after subplantar bradykinin (0.1-1.0 microgram) was also significantly increased after subcutaneous administration of this dose of captopril, whereas that in response to either histamine (2-20 micrograms) or prostaglandin E2 (2 micrograms) was unchanged. The pain threshold of the paw, injected with carrageenin (1 microgram) was lowered significantly after subcutaneous administration of captopril (1 mg kg-1). Potentiation by captopril (1 mg kg-1, s.c.) of paw swelling in response to intraplantar carrageenin (100 micrograms) or bradykinin (1 microgram) was reduced by prior subcutaneous administration of indomethacin (5 mg kg-1). It is suggested that normally, tissue kininase II activity is sufficient to decrease the inflammatory response of the hind paw to carrageenin or bradykinin. After inhibition of kininase II with captopril, bradykinin levels are increased and interact with concomitantly released prostaglandins to potentiate inflammation.

Effects of clonidine and guanethidine on peripheral sympathetic nerve function in the pithed rat.

Clonidine, in low intravenous doses, inhibited the increased heart rate of pithed rats caused by peripheral sympathetic nerve stimulation. The magnitude of this effect was greatest at low frequencies of nerve stimulation, responses to high frequencies being little affected by the drug. In contrast, guanethidine reduced cardiac responses to both low and high rates of nerve stimulation. The difference between the depressant effects of the two drugs on responses to various frequencies of sympathetic nerve traffic may contribute to the differences known to occur between their properties as hypotensive agents.

Cigarette smoke inhalation specifically inhibits depressor responses to prostacyclin in the rat.

Studies have been made of the effects of prior exposure to cigarette smoke on cardiovascular responses of the anaesthetized rat to arachidonic acid, prostaglandin E2 (PGE2), PGF2 alpha and PGI2. When compared with a control group, falls in systolic and diastolic blood pressure and tachycardia following intravenous PGI2 were significantly reduced in those animals exposed to smoke 1 and 24 h previously. Responses 48 h after exposure were not significantly different. Pressor effects of PGF2 alpha and depressor responses to arachidonic acid and PGE2 were not significantly affected these times. It is suggested that the specific and long lasting attenuation of the effects of PGI2 which occurs following cigarette smoke inhalation could contribute to both acute cardiovascular changes and the circulatory diseases associated with smoking.

Changes in cardiovascular sensitivity of alloxan-treated diabetic rats to arachidonic acid.

Arachidonic acid (AA, 0.125-2.0 mg kg-1) administered intravenously to male Wistar rats produced a dose-dependent fall in diastolic blood pressure. However AA (0.125-1.0 mg kg-1) injected into the autoperfused hindquarters via the aorta produced a dose-dependent increase in perfusion pressure. Both these responses to AA were inhibited by indomethacin (5 mg kg-1). The thromboxane A2 receptor antagonist AH23848 (5 mg kg-1, i.v.) inhibited pressor responses to AA in the autoperfused hindquarters, but potentiated depressor responses to AA (0.125-0.5 mg kg-1) in the whole animal. Alloxan-treated diabetic rats (14 days after a single s.c. injection of alloxan, 175 mg kg-1) displayed reduced sensitivity to the depressor effects of AA (1-2 mg kg-1) in the whole animal, increased sensitivity to the pressor effects of AA (0.5-1.0 mg kg-1) in the perfused hindquarters, and reduced sensitivity to the pressor effects of the thromboxane A2 mimetic U46619 (0.5-8.0 micrograms kg-1, i.a.) in the perfused hindquarters. These results suggest that AA can be predominantly converted to either pressor or depressor metabolites depending on the vasculature. In the diabetic state the ratio of the metabolites formed appears to change favouring a major pressor metabolite, which is probably thromboxane A2.

Methoxyphenamine inhibits basal and histamine-induced nasal congestion in anaesthetized rats.

1. Nasal resistance in anaesthetized rats was assessed by measuring air overflow during ventilation of the nasal passages at constant pressure. Nasal basal resistance was reduced in a dose-dependent manner by methoxyphenamine hydrochloride (0.01-30 mg kg-1, i.v.), pseudoephedrine hydrochloride (0.03-3 mg kg-1, i.v.) and adrenaline bitartrate (0.01-3 micrograms kg-1, i.v.). Both methoxyphenamine and pseudoephedrine were less potent and less efficacious than adrenaline but caused longer-lasting responses. 2. Nasal congestion induced by histamine (0.2% nebulised solution passed into the nasal passages for 15 s) was inhibited by i.v. administration of methoxyphenamine, pseudoephedrine, adrenaline, methoxamine or tyramine: the ID50s against 0.2% histamine-induced nasal congestion were 1.16 (95% confidence limits; 0.5, 1.8) mg kg-1, 0.25 (0.19, 0.33) mg kg-1, 0.037 (0.018, 0.06) micrograms kg-1, 8.12 (6.74, 9.65) micrograms kg-1 and 30.6 (26.1, 35.8) micrograms kg-1 respectively. 3. The inhibitory effects of both methoxyphenamine and tyramine on histamine-induced nasal congestion were reduced after administration of desmethylimipramine (0.1 and 1 mg kg-1, i.v.) or prazosin (0.1 and 0.3 mg kg-1, i.v.). Similarly, the inhibitory effects of methoxamine were reduced after prazosin (0.1 and 0.3 mg kg-1). 4. These results indicate that methoxyphenamine (1 mg kg-1, i.v.) inhibits histamine-induced nasal congestion in the rat. This action, at least in part, is probably indirect being mediated by release of neuronal noradrenaline which then acts on alpha 1-adrenoceptors.

Vascular actions of purines in the foetal circulation of the human placenta.

1. The vasoactive effects of adenosine triphosphate (ATP), adenosine and other purines in the foetal circulation of the human placenta were examined. Single lobules of the placenta were bilaterally perfused in vitro with Krebs buffer (maternal and foetal sides 5 ml min-1 each, 95% O2:5% CO2, 37 degrees C). Changes in foetal vascular tone were assessed by recording perfusion pressure during constant infusion of each purine. To allow recording of the vasodilator effects, submaximal vasoconstriction was induced by concomitant infusion of prostaglandin F2 alpha (0.7-2.0 mumol l-1). 2. ATP (1.0-100 mumol l-1) usually caused concentration-dependent reductions in perfusion pressure. However, biphasic with initial transient increases, or only increases in pressure were sometimes observed. Falls in pressure caused by ATP were significantly reduced by addition to the perfusate of NG-nitro-L-arginine (L-NOARG) (100 mumol l-1) but not NG-nitro-D-arginine (D-NOARG) (100 mumol l-1). They were not influenced by addition of indomethacin (10 mumol l-1) or L-arginine (100 mumol l-1). 3. Adenosine (0.01-1.0 mmol l-1) consistently caused concentration-dependent reductions in perfusion pressure, this effect not being influenced by indomethacin. L-NOARG, but not D-NOARG, reduced the potency of adenosine approximately three fold. L-Arginine, but not D-arginine enhanced its potency by a similar amount. 4. 2-Methylthio-ATP, a selective P2 gamma agonist was approximately 50 times more potent than ATP as a vasodilator agent, always causing decreases in perfusion pressure. 5. Beta-gamma-Methylene ATP, a selective P20 agonist, was approximately 100 times more potent than ATP as a vasoconstrictor, but only caused transient increases in perfusion pressure.6. The rank order of vasodilator potencies of a selection of adenosine receptor agonists was, 2-chloroadenosine>>5-(N-cyclopropyl)-carboxamidoadenosine, >5-N-ethylcarboxamidoadenosine, >2-chloro-N6-cyclopentyladenosine, >CGS-21680 > N6-cyclohexyladenosine = adenosine. Vasodilatation due to adenosine was inhibited by the PI-A2 receptor antagonist 3,7-dimethyl-l-propargylxanthine(DMPX).7. These results suggest that ATP may cause an endothelium-dependent vasodilatation in the foetal vessels of the human placenta via activation of a P2y receptor linked to the formation of nitric oxide(NO). Vasodilatation caused by ATP may mask an accompanying vasoconstrictor effect mediated, via a P2X receptor, in the villous vascular smooth muscle. Adenosine acting on P1-A2 receptors, which are also present in the foetal vasculature, may require synergistic interaction with NO to achieve a maximal vasodilator response.

Release of prostaglandins during contraction of the human umbilical vein on reduction of temperature.

Flow rate was measured through the vein of the human isolated umbilical cord perfused at constant pressure (40 mmHg) at 37.5 degrees C and 20 degrees C. At the latter temperature the flow was decreased by 50.9% when compared with a mean of 201 ml/min at 37.5 degrees C indicating venospasm. Indomethacin (10 microgram/g) effected a highly significant reduction in the venous spasm caused by lowering the temperature. After indomethacin pretreatment, changing the cord temperature from 37.5 degrees C to 20 degrees C caused a mean decrease in flow of only 3.1%. When the effluent from the vein was passed over rat isolated stomach fundus and colonic strips, cooling of the cord was accompanied by contractions of the isolated tissues characteristic of prostaglandins. These results suggest that prostaglandins are involved in temperature-induced closure of the human umbilical vein after birth.

The effects of N-(cyclopropylmethyl)-19-isopentylnororvinol (M320), a potent agonist at kappa- and mu-opiate receptors, on urine excretion of rats.

The effects of N-(cyclopropylmethyl)-19-isopentylnororvinol hydrochloride (M320) on urine excretion by rats were investigated. Further studies, using rabbit isolated vas deferens, investigated its interactions with kappa-opiate receptors. The output of urine for a 2 h period after M320, administered subcutaneously to normally hydrated Long Evans rats, showed a bell-shaped dose-response relationship, the maximum effect occurring after 10 micrograms kg-1. Urinary retention contributed to but did not fully account for the weaker diuresis after high doses. Attenuation of the ascending portion of the dose-response curve to M320 occurred after 1 and 10 mg kg-1 but not 0.1 mg kg-1 of naltrexone intraperitoneally. M320 in low doses (3-10 micrograms kg-1) caused a small but significant increase in sodium excretion. M320 (30 micrograms kg-1) reduced both sodium and potassium excretion. M320 (10 micrograms kg-1 s.c.) did not increase the volume of urine voided in 2 h by Brattleboro rats showing diabetes insipidus, even when urine excretion was reduced to normal by 1 week of vasopressin replacement. The volume of urine voided in 4 h by Brattleboro rats was progressively reduced to zero by M320 (10-100 micrograms kg-1 s.c.). Urinary retention contributed to but did not account for this reduction. Plasma levels of immunoreactive arginine vasopressin (ir-AVP) were reduced in both normal and dehydrated Long Evans rats after doses greater than 1 microgram kg-1 M320 s.c. In vitro, M320 caused persistent inhibition of twitches of the electrically stimulated rabbit vas deferens (IC50 1.7 nM). 8 These data suggest that M320 has potent opioid agonist activity at K-receptors and at higher concentrations stimulates mu- receptors. In the rat, its activity on K-receptors is associated with diuresis and suppression of plasma vasopressin levels. The antidiuresis seen after high doses may be due to its activity on mu-receptors, possibly at a central site.

Acetylcholine output and foetal vascular resistance of human perfused placental cotyleda.

The foetal villous vessels of single cotyleda of human placentae have been perfused with a constant flow of Krebs solution, recording inflow pressure and passing the venous perfusate in cascade over guinea-pig ileum and rat stomach strip preparations in vitro. Each cotyledon released for at least 4 h a substance that was probably acetylcholine. The perfusate caused contractions of both preparations which were inhibited by atropine or hyoscine and potentiated by physostigmine. Contractile activity was destroyed after incubation at 37 degrees C of perfusate with acetylcholinesterase but not with acetylcholinesterase plus physostigmine. When the perfusion temperature was lowered to 34 degrees C or below, acetylcholine output was reduced, the extent depending on the fall in temperature. No change in resistance of the villous vessels occurred during the changes in temperature or in the presence at 37 degrees C of atropine, hyoscine, hexamethonium, (+)-tubocurarine, hemicholinium-3 or bretylium. Submaximal vasoconstrictor responses of the villous vessels to the thromboxane A2-mimetic U46619 were not affected by reduction of the perfusion temperature to 30 degrees C, which lowered acetylcholine-like output by approximately 70%. Responses to U46619, at 37 degrees C, were unchanged during the presence of atropine or hyoscine. Acetylcholine is released into the foetal circulation of the human placenta but no evidence could be obtained that it affects villous vascular smooth muscle tone or vasoconstrictor responses.

Effects of morphine, H-Tyr-D-Arg-Phe-Lys-NH2 (DALDA) and B-HT920 on non-cholinergic nerve-mediated bronchoconstriction in pithed guinea-pigs.

1. Electrical stimulation (1 ms, 5 Hz, 80 V) for 5 or 15 s at the level of C4-T1 in the spinal canal of artificially respired pithed guinea-pigs (which had received intravenously (i.v.) (+)-tubocurarine chloride 2 mg kg-1, atropine sulphate 2 mg kg-1 and pentolinium tartrate 5 mg kg-1) caused constriction of airways, indicated by increased insufflation pressure. 2. This non-cholinergic constriction was inhibited by morphine (1-3 mg kg-1, i.v.), the peripherally acting mu-receptor agonist, H-Tyr-D-Arg-Phe-Lys-NH2 (DALDA, 0.1-1 mg kg-1, i.v.) or the alpha 2-adrenoceptor agonist B-HT920 (1-3 mg kg-1, i.v.). 3. The effects of either morphine (3 mg kg-1, i.v.) or DALDA (1 mg kg-1, i.v.) were inhibited by naloxone (3 mg kg-1, i.v.). Idazoxan (3 mg kg-1, i.v.) inhibited the anti-constrictor effect of B-HT920 (3 mg kg-1, i.v.), but not that of DALDA (0.1 mg kg-1, i.v.). 4. Thus activation of peripheral mu-opioid receptors or alpha 2-adrenoceptors inhibits airways constriction induced by non-cholinergic nerve stimulation in the pithed guinea-pig. This preparation therefore provides a further method for the in vivo examination of the effects of drugs on non-cholinergic tracheobronchial constrictor nerve function.

Plasma levels of atrial natriuretic peptides during pregnancy and post partum in the rat.

Plasma concentrations of atrial natriuretic peptides (ANP) in female Wistar rats were measured by radioimmunoassay at oestrus, during pregnancy, during parturition and between 3 h and 4 days post partum. Concentrations of ANP in rats on days 10, 15 and 17 of pregnancy were not significantly different from those in non-pregnant animals in oestrus (32.5 +/- 2.2 pmol/l; mean +/- S.E.M., n = 9), but levels near term (days 20 and 21 of pregnancy) were reduced by approximately 50%. However, plasma concentrations of ANP at 6, 12 and 24 h post partum were approximately twice those of non-pregnant animals in oestrus, but returned to normal levels within 4 days after parturition. Maternal plasma volume increased significantly during pregnancy, and fell 15-20% 6-24 h post partum. These results suggest that the relationship between plasma volume and the plasma concentration of ANP is reset during pregnancy and changes rapidly post partum. The results do not necessarily, however, imply any changes in the relationship between atrial pressure and the concentration of ANP.

The effects of Al3+, Cd2+ and Mn2+ on human erythrocyte choline transport.

The effects of Al3+, Cd2+ and Mn2+ on human erythrocyte choline transport, Na-K-ATPase, Ca-Mg-ATPase and intracellular K+ levels were examined. The concentrations used were below the levels which caused significant haemolysis (less than or equal to 300 microM). All three cations inhibited concentrative choline accumulation over 3 hr [IC50 values at 1 microM choline were 35 microM (AlCl3), 250 microM (CdCl2) and 300 microM (MnCl2)] but at the concentrations tested, none decreased initial rates of choline uptake. The effects of Cd2+ and Mn2+ (but not Al3+) on choline accumulation were reversed by removing the cations from the extracellular medium by washing. All three cations also inhibited efflux of choline, at 1 microM choline, 30% inhibition being produced by 33 microM AlCl3, 81 microM CdCl2 and 111 microM MnCl2. At subhaemolytic concentrations, only CdCl2 inhibited Na-K-ATPase, (IC50 = 147 microM) and none of the cations significantly inhibited Ca-Mg-ATPase. Intracellular K+ levels were only reduced by the highest concentration of AlCl3 used (100 microM). These results suggest that inhibition of choline accumulation and efflux in erythrocytes by Al3+, Cd2+ and Mn2+ is not explicable solely in terms of either inhibition of Ca-Mg-ATPase, or inhibition of Na-K-ATPase causing reduced intracellular K+. Our conclusions are similar to those previously obtained using synaptosomes and provide support for the hypothesis that inhibition of choline transport by Al3+ may contribute to a number of disease states.

Effects of variation in oxygen tension on responses of the human fetoplacental vasculature to vasoactive agents in vitro.

The human placenta perfused in vitro with Krebs' solution has been used to examine the effects of low oxygen tension on the vasoreactivity of the fetal placental vessels to several vasodilator and vasocontrictor autacoids. Increases in fetal arterial perfusion pressure (FAP) produced by endothelin-1 (ET-1, human), the thromboxane A2-mimetic U46619, 5-hydroxytryptamine (5-HT), angiotensin II (A II) and bradykinin (BK) were examined under conditions of high ( >or= 450 mmHg) and low

Effects of opioids on acetylcholine output of perfused human placental cotyleda.

(1) Output of acetylcholine (ACh) from fetal vessels of Krebs-perfused human placental cotyleda was estimated by bioassay using the rat stomach strip. (2) Infusion of high concentrations of the opioids pethidine, morphine or ethylketocyclazocine, but not [D-Ala2]-methionine enkephalinamide (0.1-300 mumol/l), reduced ACh output. Fifty per cent inhibition of output occurred in the presence of concentrations of greater than or equal to 300 mumol/l (morphine and ethylketocyclazocine) and 338 (317, 353; 95 per cent c.l., n = 6) mumol/l (pethidine). (3) ACh output was inhibited by infusion of 100 mumol/l naloxone or 10 mumol/l naltrexone, but was not affected by lower concentrations of either antagonist. (4) These results suggest that the therapeutic concentrations of morphine and pethidine likely to occur in vivo would not affect placental ACh output into fetal vessels. The finding that high concentrations of synthetic opioids or opioid antagonists were required to inhibit output suggests that they may not be acting specifically, and provides no evidence for the hypothesis that endogenous opioids play a role in control of ACh release into fetal vessels of human placentae.