This study examined the effects of epinephrine, norepinephrine, AVP and ACh on fluid movement by the lungs of the late-term guinea pig fetus. Catecholamines and AVP are secreted in high amounts by the fetus during delivery, and could be important with respect to fetal lung fluid removal; this event is vital at the time of birth. The lungs were supported in vitro for a duration of three hours, and production rates were measured using a dye-dilution technique. The average resting production rate in terms of ml/kg‧h declined with gestational age (54-67 days gestation; n=171). There was a lesser decline in the average resting production rate in terms of ml/h. The average production rate of untreated preparations in the first hour was 1.60 ± 0.26 ml/kg body weight per hour, and rates did not change significantly during the remaining two hours of experimentation (n=30). This rate is comparable to those reported from chronically catheterized fetal sheep. Treatment was administered during the second hour of experimentation, following an ABA design. Lungs (n=36) were transferred to fresh Krebs-Henseleit saline containing one of the following concentrations of epinephrine: (a) 10‾⁵ M; (b) 10‾⁶ M; (c) 10‾⁷ M; (d) 5 x 10‾⁸ M; (e) 10‾⁸ M; and (f) 10‾⁹ M. With the exception of the top dose, epinephrine treatment caused an immediate reduction in fluid secretion, or fluid reabsorption. Sodium followed the movement of water in all cases. The effect of epinephrine at 10‾⁷ M was maximal, and the threshold dose for epinephrine was calculated at 1.78 x 10‾¹¹ M. Phentolamine and propranolol had no effect in control preparations. However, phentolamine completely blocked the effect of epinephrine, whereas propranolol was ineffective. Isoproterenol had no effect on pulmonary fluid production. Alpha-adrenergic receptors apparently mediate the effect of epinephrine on pulmonary fluid movement in the fetal guinea pig lung. This conclusion is different from that obtained in fetal sheep, in which beta-adrenergic receptors are utilized. A possible synergism between epinephrine and AVP was examined. Lungs (n=12) were transferred to fresh Krebs-Henseleit saline containing either (a) 0.6 mU/ml AVP, or b) 0.6 mU/ml AVP combined with epinephrine at 10‾⁷ M. Treatment with AVP caused a slow, prolonged reduction in fluid production. Treatment with AVP together with epinephrine did not demonstrate synergism. The effect of norepinephrine (NE) was examined. Lungs (n=36) were transferred to fresh Krebs-Henseleit saline containing one of the following concentrations of NE: (a) 1.24 x 10‾⁵ M; (b) 1.24 x 10‾⁶ M; (c) 1.24 x 10‾⁷ M; (d) 5.24 x 10‾⁸ M; (e) 1.24 x 10‾⁸ M; and (f) 1.24 x 10‾⁹ M. In all preparations, treatment with NE resulted in an immediate reduction in fluid production, and reabsorptions were observed at the higher doses. Sodium followed the movement of water in every case. The threshold dose was calculated at 3.16 x 10‾¹⁰ M. Phentolamine blocked the effect of NE, reinforcing the importance of pulmonary alpha-adrenergic receptors in the fetal guinea pig. There was no relationship between age and degree of response with treatment of either epinephrine or NE, but fetuses under 78.0 g did not respond to NE. The effect of ACh was examined. Lungs (n=24) were transferred to fresh Krebs-Henseleit saline containing one of the following concentrations of ACh: (a) 10‾⁴ M; (b) 10‾⁵ M; (c) 10‾⁶ M; and (d) 10‾⁸ M. At the three top doses, immediate and powerful reabsorptions of pulmonary fluid were observed in older fetuses (60 days gestation and above); significant falls were observed in the younger fetuses. This result was unexpected, as it was hypothesized that ACh would stimulate fluid production. The threshold dose for ACh was between 10‾⁶ M and 10‾⁸ M. Phentolamine blocked the effect of ACh. This result suggested that reabsorption is a result of an indirect effect of ACh acting through pulmonary alpha receptors. The results in this study show that epinephrine, NE, AVP and ACh are all important promoters of fetal pulmonary fluid removal in the fetal guinea pig. Pulmonary alpha-adrenergic receptors mediate the effects of epinephrine, NE and ACh (indirectly). The conclusions drawn from this study emphasize the importance of species' comparison in fetal research. LIST OF ABBREVIATIONS AVP Arginine Vasopressin NE Norepinephrine DOPA dihydroxyphenylalanine PNMT Phenylethanolamine n-methyltransferase ACh Acetylcholine / Science, Faculty of / Zoology, Department of / Graduate

This study examined the effects of lung expansion and changes in temperature on fluid movement by the lungs in the initial period after birth. In addition, experiments with amiloride support the belief that fluid reabsorption acts via a sodium transport mechanism. Lungs from fetal guinea pigs (56-67 days of gestation) were supported in vitro for three hours, and lung liquid production rates were measured using a dye dilution technique. The average production rate in the first hour of untreated preparations was 1.30 ±0.22 ml/kg body weight per hour, and this did not change significantly during the remainder of the experiment (n=30). This rate is comparable to secretion rates previously reported from chronically catheterized sheep. In 36 further preparations, the lungs were transferred from 37°C to fresh Krebs-Hanseleit saline at one of the following temperatures, for one hour (an ABA design): (a) 29°C; (b) 32°C; (c) 34°C; (d) 35°C; (e) 36°C; (f) 39°C. In all cases, the temperature change resulted in an immediate and significant fall in secretion. All lungs showed a tendency towards recovery when returned to starting conditions, except those subjected to a temperature increase. Reductions of 2-3°C, those normally seen in the delivery room, had the greatest effect and caused not only a decrease in secretion, but promoted fluid reabsorption. Amiloride at 10⁻⁶M had no effect on control preparations, but completely blocked the reabsorption stimulated by a temperature drop of 2°C. Expansion of the lungs, which occurs naturally as a newborn attempts to take its first breaths, was also examined. Thirty fetal lungs were expanded by one of the following amounts: (a) 18%; (b) 31%; (c) 43%; (d) 50%; (e) 72%. All expansions resulted in a significant fall in secretion rate, with the effect being proportional to the degree of expansion. Amiloride at 10⁻⁶M again blocked the strong reabsorption occurring with 70% expansion. Further studies investigated the possibility that expansion causes reabsorption via the local release of a substance occurring in the lungs. When one set of lungs was expanded in the presence of a second, unexpanded set, both showed a significant decrease in secretion, suggesting that the expanded lung had released some factor which affected the otherwise untreated lung. However, studies with α- and β- adrenergic blockers showed that it is unlikely the expanded lung was liberating either adrenaline or nor-adrenaline. The results of this study show that two changes which are likely to occur in the period immediately after birth, namely a 2-3°C decrease in core temperature, and lung expansion, may be important in promoting the vital reabsorption of fluid. They suggest that expansion may release substances locally in the lungs which stimulate this reabsorption, and that the fluid is removed from the potential air spaces via sodium transport mechanisms. / Science, Faculty of / Zoology, Department of / Graduate