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The effects of arginine vasopressin and arginine vasotocin on the movement of water across the isolated amnion and skin of the fetal guinea-pig

University of British Columbia

Thirty-nine amniotic membranes from guinea-pig fetuses between 30 and 68 days of gestation (0.44 - 1.00 of term) were set up in vitro, in apparatus in which net water flow could be measured gravimetrically. Small hydrostatic and osmotic gradients were maintained from the fetal to the maternal side of the membrane; this reproduced in vivo conditions. In the absence of hormone, there was a net transfer of water from the fetal to the maternal side, in the majority of preparations. Addition of arginine vasopressin (AVP) at 100mU/ml (vasopressor activity) to the fetal surface slowed or reversed this flow. The response increased with fetal age, until about 58 days of gestation (0.85 of term). After this time, the effect declined, and was lost; membranes over 64 days (0.94 of term) showed only one weak response in 13 experiments. Electron microscopy of 10 membranes between 28 and 70 days of gestation showed parallel changes in the structure of the amniotic epithelium. The epithelium changed from a relatively simple structure early in gestation, when the response to AVP was low, to one that appeared to be more complex and possibly more specialized in function by about 50 days (0.75 of term). There was an apparent degeneration of epithelial cells between 62 and 64 days, when the amnion ceased to respond to AVP. Electron microscopic studies on a membrane at 38 days revealed that AVP caused the intercellular spaces to dilate. Morphometric analysis showed that the dimensions of the spaces in the AVP treated epithelium were significantly greater than in a control preparation (p<0.001). Studies on 76 guinea-pig fetuses showed that the volume of amniotic fluid increased during the course of gestation, reached a peak, and then declined. The peak coincided roughly with the time at which the amnion showed its maximal response to AVP, and with the time its structure appeared to be compatible with an active role in fluid transport. After approximately 56 - 58 days, both amniotic fluid volume and the response of the amnion to AVP declined. These results are consistent with a physiological role of AVP in supplying fluid to the amniotic sac in the first 80% of gestation. Unlike the situation in most species, the amniotic fluid volume increased again, and reached its maximum value just before delivery. Skin from 35 mid-term fetal guinea-pigs (0.49 - 0.70 of term) was set up in the same gravimetric apparatus used in the amnion experiments. However, there were no gradients in hydrostatic or osmotic pressure. In the absence of hormones, there was little or no net transfer of water in either direction. Arginine vasotocin (AVT) or arginine vasopressin (AVP) added to the serosal surface at 5 - 100mU/ml (vasopressor activity) produced a net uptake of water towards the serosal side (towards the fetus). There was a linear relationship between the log.dose, and the rate of uptake of water, for both peptides. However, AVT was more than twice as potent as AVP (thresholds: AVT, 3.9mU/ml; AVP, 10.4mU/ml). The fine structure of the guinea-pig skin before keratinization appeared to be compatible with an active role in fluid transport. The outer periderm layer resembled the amniotic epithelium; it may be the site of action of neurohypophysial hormones. It is concluded that the gestational changes in the structure and response to AVP of the amnion may be partially reponsible for changes in the volume of amniotic fluid during gestation. The fetal skin, like frog skin, responds to AVT more readily than to AVP. Perhaps it is on the fetal skin that fetal AVT finds its true physiological role.

Text

2010-06-27

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http://hdl.handle.net/2429/26027

Zoology

University of British Columbia

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