Lack of a role for cyclic nucleotide gated cation channels in lung liquid absorption in fetal sheep

1. Late gestation fetal sheep were chronically catheterised in utero to allow measurement of the rate of production of lung liquid (J(v)) from 132-143 days gestation (term, 147 days), and to test the hypothesis that cyclic nucleotide gated cation channels mediate a component of fetal lung liquid absorption. 2. In eight experiments, 0.5 μg min^-1 adrenaline caused a significant (P < 0.005) reduction in J(v) from +18.12 ± 3.52 to -10.27 ± 5.26 ml h^-1. Dichlorobenzamil (a blocker of cyclic nucleotide gated cation channels) at 1.5 x 10^-5 M did not significantly inhibit the adrenaline-induced lung liquid absorption (J(v) dichlorobenzamil, -5.77 ± 2.78 ml h^-1; P > 0.1) when the data were grouped, but did exert a significant gestational effect (r = 0.90, P < 0.01). Subsequent addition of 10^-4 M amiloride (a blocker of epithelial sodium channels) abolished the adrenaline-induced absorption of lung liquid (mean J(v) amiloride, +6.45 ± 1.59 ml h^-1; P < 0.01 relative to J(v) adrenaline and P < 0.005 relative to J(v) dichlorobenzamil). 3. In seven experiments, 0.5 μg min^-1 adrenaline caused a significant (P < 0.0005) reduction in J(v) from +18.95 ± 2.98 to -10.08 ± 3.75 ml h^-1. Amiloride (10^-4 M) inhibited the adrenaline response (J(v) amiloride, +5.46 ± 1.09 ml h^-1; P < 0.005). However, subsequent addition of 1.5 x 10^-5 M dichlorobenzamil had no additive effect to that of amiloride (J(v) dichlorobenzamil, +4.58 ± 0.93 ml h^-1; P > 0.1). 4. In six experiments, the cGMP analogue 8-Br-cGMP at 10^-4 M caused a significant (P < 0.05) reduction in J(v) from +15.20 ± 2.81 to +11.63 ± 1.71 ml h^-1. Amiloride (10^-4 M) did not block the effect of 8-Br-cGMP (J(v) amiloride, +14.00 ± 2.49 ml h^-1; not significantly different from 8-Br-cGMP). Subsequent addition of 1.5 x 10^-5 M dichlorobenzamil also did not block the effect of 8-Br-cGMP (J(v) dichlorobenzamil, +11.37 ± 1.22 ml h^-1; not significantly different from either J(v) amiloride or J(v) 8-Br-cGMP). 5. We conclude that, in fetal sheep, neither adrenaline nor cGMP stimulate lung liquid absorption by actions on cyclic nucleotide gated cation channels, and that the effect of cGMP on fetal lung liquid secretion is minor and does not involve epithelial sodium channels. The effect of dichlorobenzamil, when given before amiloride, was probably due to an action on amiloride sensitive epithelial sodium channels.