The cystic fibrosis transmembrane conductance regulator (CFTR) is thought to modulate the function of ENaC and its responsiveness to cAMP in various tissues including colon (Kunzelmann et al. 2000). We investigated the effects of dietary Na⁺ restriction (to increase plasma aldosterone (P_aldo)) and of forskolin (to increase intracellular cAMP) on transepithelial ion transport in late distal colon of CF mice (Cftr^tm2cam ▓Dgr│F508 (Colledge et al. 1995)) using short-circuit current (I_SC) measurements. Pooled data from wild-type and heterozygous (WT/HE) mice were used as controls. Animals were killed in accordance with Home Office regulations. Data are given as means ± S.E.M. and significance was evaluated using the appropriate Student’s t tests.

In WT/HE and CF animals kept on regular diet (Na⁺ content, 0.32 %) P_aldo averaged 0.51 ± 0.10 nM (n = 19) and 0.75 ± 0.25 nM (n = 14), respectively. Apical application of amiloride (10 µM) revealed an amiloride-sensitive I_SC (▓Dgr│I_SC-ami) averaging 89 ± 18 µA cm^-2 in WT/HE and 77 ± 20 µA cm^-2 in CF animals.

In animals kept for 2 weeks on a low-salt diet (Na⁺ content, 0.01 %) P_aldo significantly increased to 1.13 ± 0.28 nM in WT/HE (n = 23; P < 0.05) and 4.02 ± 0.39 nM in CF animals (n = 15, P < 0.001). P_aldo of low-salt CF animals was 3.7-fold higher than that of the low-salt WT/HE animals (P < 0.001). Low-salt diet increased ▓Dgr│I_SC-ami to 192 ± 24 µA cm^-2 in WT/HE (n = 19; P < 0.01) and to 223 ± 20 µA cm^-2 in CF mice (n = 11; P < 0.001).

In WT/HE animals kept on regular or low-salt diet, apical application of forskolin (FSK; 1 µM) increased I_SC by 77 ± 10 µA cm^-2 or by 75 ± 6 µA cm^-2, respectively. The FSK-induced increase was largely preserved in the presence of amiloride, while apical application of 1 mM DPC had a substantial inhibitory effect. These findings indicate that FSK stimulates a Cl^– secretory I_SC component probably via cAMP-mediated activation of apical CFTR. In the presence of FSK, ▓Dgr│I_SC-ami increased to 116 ± 6 % (n = 18; P < 0.05) of its size prior to FSK application in animals on regular diet, while it was not significantly increased in animals on low-salt diet averaging 110 ± 4 % (n = 19). In any case, FSK did not inhibit ▓Dgr│I_SC-ami in WT/HE animals.

In CF animals FSK did not stimulate a DPC-sensitive Cl^– secretory I_SC component consistent with the lack of functional CFTR channels. However, FSK increased ▓Dgr│I_SC-ami to 130 ± 9 % (regular diet; n = 12; P < 0.05) and to 123 ± 5 % (low-salt diet; n = 11; P < 0.001) of its value prior to FSK application. Thus there was a clear stimulatory effect of FSK on ▓Dgr│I_SC-ami in CF animals.

We conclude that in CF mice low-salt diet stimulates a larger increase in P_aldo than in control animals. This suggests that an increase in P_aldo may contribute to enhanced ENaC activity in CF tissues. In mouse distal colon stimulation of CFTR by cAMP was not associated with reciprocal inhibition of ▓Dgr│I_SC-ami. A stimulatory effect of cAMP on ▓Dgr│I_SC-ami was prominent in distal colon of CF mice.

This work was supported by The Wellcome Trust.

Colledge, W.H., Abella, B.S., Southern, K.W., Ratcliff, R., Jiang, C., Cheng, S.H., MacVinish, L.J., Anderson, J.R., Cuthbert, A.W. & Evans, M.J. (1995). Nature Gen. 10, 445-452.

Kunzelmann, K., Schreiber, R., Nitschke, R. & Mall, M. (2000). Pflƒgers Arch. 440, 193-201.