Active sodium uptake by the skin of foetal sheep and pigs.

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Abstract
Simultaneous measurements of unidirectional Na fluxes across fetal skin incubated in vitro with identical solutions ([Na] = 150 mM) bathing, either side showed a flux ratio (influx/efflux) of 1.40 .+-. 0,08 in 27 sheep skins, which was significantly different from unity (P < 0.001). The gestational ages ranged from 47-98 days (term = 147 days). Similar experiments on 8 fetal pig skins at 58 days gestation (term = 114-118 days) gave a mean flux ratio of 1.10 .+-. 0.03 (P < 0.02). Unidirectional Na fluxes measured with dilute Ringer solution on the outside (mucosal) surface ([Na]o = 100 mM) gave influx to efflux ratios of 0.86 .+-. 0.09 in 17 sheep (P < 0.05) and 1.07 .+-. 0.26 in 5 fetal pigs; the value predicted for passive movement was 0.67. Incubation with the inhibitors, ouabain (10-4 M) or dinitrophenol (DNP 10-4 M) gave a flux ratio for Na which was not significantly different from unity in the absence of a gradient, or from 0.67 when the concentration gradient was applied. Sequential measurement of unidirectional diffusional fluxes of tritiated water across fetal skin gave flux ratios of 0.98 .+-. 0.01 in 6 sheep skins and 1.06 .+-. 0.11 for 4 pig skins in control conditions. When the outside solution was diluted to give an osmotic gradient of 100 m-osmole .cntdot. kg-1 across the skin a flux ratio of 0.95 .+-. 0.07 was obtained for 7 sheep and was not measured in pig skin. Hormones and inhibitors had no effect on the diffusional flux ratio for water in the presence or absence of an osmotic gradient. Lysine vasopressin (ADH) (200 mu[milliunits]/ml) increased influx and efflux of water in the presence and, to a lesser extent in the absence of an osmotic gradient in sheep skin. In pig skin prolactin (1 u/ml) increased both influx and efflux, but ADH had no effect on diffusional water fluxes. ADH increased Na influx in sheep skin slightly but vasotocin (5.5 mu/ml) was more potent, particularly in the presence of an opposing diffusion gradient. Vasotocin (55 mu/ml) reduced Na influx in pig skin, ADH had no effect on influx or efflux and prolactin reduced Na influx and efflux. Ouabain and DNP generally reduced permeability to both Na and water in sheep skin but had no effect in pig skin.