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The metabolic clearance of arginine vasopressin in the amniotic sac of the fetal guinea pig
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|Title:||The metabolic clearance of arginine vasopressin in the amniotic sac of the fetal guinea pig|
|Authors:||Uyehara, Catherine F.T.|
Hormones -- Metabolism
Guinea pigs -- Physiology
|Abstract:||A route for arginine vasopressin (AVP) clearance in the fetus has yet to be established. Ervin et ale (Am. J. Physiol. 250:E253-8, 1986) reported that AVP may be absorbed in the ovine fetal gut and remain biologically active. Thus, an AVP clearance problem could exist in the fetus since fetal urinary AVP may be recirculated. The results of the present study indicate that in the fetal guinea pig AVP is metabolized in the amniotic sac. When fetal urine and amniotic fluid (AF) were fractionated on HPLC, AVP was identified only in fetal urine. Moreover, when AVP and inulin were injected into the AF in vivo during the last week of gestation, the AVP/inulin ratio decreased (regression, p=0.01) over a 2 hour period, indicating a swallowing-independent disappearance of AVP. Also, the rate of AVP disappearance increased (regression, p<0.01) with increasing initial AF AVP concentration. When tritiated AVP was similarly injected, the radioactivity/inulin ratio remained constant, suggesting that AVP was not diffusing out of the amniotic sac. Two hours after the injection the AF was collected and fractionated by HPLC, and only 42% of the total radioactivity was intact AVP. The remainder was identified in 2 other products (M1 and M2). Tritiated AVP incubated with AF and amnionic membrane in vitro produced both Ml and M2. Tritiated AVP incubated with amnionic membrane in artificial amniotic solution produced Ml with no M2. Tritiated AVP incubated in only AF produced M2 with no MI. Thus, neither metabolite was necessary for the production of the other, Ml was amnionic membrane dependent and M2 was AF dependent. The amnionic membrane metabolic system was functioning as early as 34 days of gestation (term = 68 days) whereas the AF enzyme system was not effective until 54 days of gestation, further support that the 2 metabolic systems are independent of each other. M2 comigrates with des-glycinamide AVP in three different HPLC buffers and crossreacts only with an AVP antiserum that detects des-glycinamide AVP. M2 production in the amniotic fluid can be blocked with the trypsin inhibitor aprotinin, suggesting that the amniotic fluid enzyme responsible for M2 production is trypsin-like. In establishing the guinea pig as an animal model, amniotic fluid osmolality (OSM), and sodium (Na), potassium (K) and cortisol concentrations were measured in the second and third trimesters of gestation. Amniotic fluid OSM did not change and K concentration increased (regression, p<0.01) with gestational age in contrast to reported human values in which OSM decreased and K remained constant with age. Amniotic fluid Na levels decreased (regression, p<0.01) and cortisol levels increased (regression, p<0.01) with age in the guinea pig, similar to results reported in humans. Fetal plasma and urine OSM, Na and K were also measured in the late third trimester. Fetal urine was found to be slightly hypertonic compared to fetal plasma (328 ± 8 m0sm/kg H2O vs. 308 ± 3 mOsm/kg H2O, p<0.05), and urinary Na concentration was lower than fetal plasma (127.2 ± 1.0 mEq/l vs. 139.9 ± 0.4 mEq/l, p<0.05), which indicated that the guinea pig fetal kidney or bladder may be involved with the reabsorption of water and Na. Some other undetermined osmotically active substances besides Na and K are responsible for most of the fetal urine osmotic activity since Na, K, and attendant anions account for less than 15% of the urine osmolality. A difference was discovered between Na and K levels in the amniotic fluid and fetal urine: although fetal urine was hypertonic to amniotic fluid (328 ± 8 mOsm/kg H2O vs. 287 + 2 mOsm/kg H2O, p<0.05), urinary Na and K concentrations were lower than amniotic fluid concentrations (by 115.8 mEq/l and 6.3 mEq/l, respectively). Therefore, fetal urine is modified in the amniotic sac. Amniotic fluid OSM measured in fetuses in the late third trimester was constant (287±2 mOsm/kg H2O) despite a large variation in urine osmolalities (288 to 393 mOsm/kg H2O). The regulation of amniotic fluid OSM is not well understood but a hormonal regulation of water and electrolyte movement across the amnion is believed to be involved. The present study was unable to demonstrate an influence of amniotic fluid AVP on amniotic fluid OSM. In conclusion, the amniotic sac may play a role in the clearance of AVP, as well as the clearance of other proteins, from the fetal system. Amniotic fluid OSM appears to be tightly regulated and the mechanisms behind this regulation remain unresolved.|
Bibliography: leaves 158-169.
xviii, 169 leaves, bound ill. 29 cm
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|Appears in Collections:||Ph.D. - Biomedical Sciences (Physiology)|
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