Recent evidence has suggested that gonadal steroid hormones have specific and rapid effects by acting at the plasma membrane of different tissues, including smooth muscle from blood vessels and reproductive organs (Valverde et al. 1999; Nadal et al. 2001). Until recently, the intestinal muscle has not been considered as a target for these rapid actions of oestrogen or antioestrogens, in spite of increasing evidence pointing to a direct effect of these compounds on crucial proteins involved in excitation- contraction coupling (Díaz, 2002). In the present study we have analysed the effects of different oestrogens and steroidal antioestrogens on the contractile activity of mouse duodenal muscle. Assays included spontaneous peristalsis, basal tone, response to calcium and KCl-induced depolarisation. 17β-Oestradiol causes a dose-dependent relaxation of spontaneous contractile activity, reduces basal tone and depresses the responses to CaCl₂ and KCl. Application of either 17β-E2 or the inactive estereoisomer 17α-E2, but not the steroidal antioestrogen ICI182, 780 (10 µM), relaxed duodenal muscle and abolished spontaneous activity within min after exposure. Dose-response analyses showed that 17β-E2 was more potent than 17α-E2, relaxing duodenal muscle and reducing CaCl₂– and KCl-induced contraction. The effects of oestrogens could be partially reversed by addition of Bay K 8644 (1 µM) to the bath, which suggests the involvement of L-type Ca²⁺ channels in the response to oestrogen. In addition, 17β-E2-relaxed tissues could be contracted by depolarisation with KCl (33 mM), TEA (5 mM) and charybdotoxin (38 nM), but not glibenclamide (10-50 µM), thereby indicating that oestrogen had also activated calcium-dependent K⁺ channels (Maxi-K⁺). The relaxing effects of oestradiol were mimicked by addition of 8-bromo-cyclicGMP (8-Br-cGMP, 100 µM) or forskolin (FSK, 20 µM). However, while the effect of 8-Br-cGMP could be reversed by KCl or TEA, the effect of FSK was not. The relaxing effects of 17β-E2 were not affected by preincubation of duodenal muscle with L-NAME (100-300 µM) nor KT5823 (0.3-1 µM), ruling out the possible participation of protein kinase G (PKG) in the final step of oestrogen-induced relaxation. We hypothesise that oestrogen rapidly activates Maxi-K⁺ channels and inhibits L-type calcium channels to relax duodenal muscle.

This work was supported by SAF2001-3614-C03-01/02.