The ion conductances of colonic crypts from dexamethasone-treated rats

Whole-cell patch-clamp studies were performed in isolated colonic crypts of rats pretreated with dexamethasone (6 mg/kg subcutaneously on 3 days consecutively prior to the experiment). The cells were divided into three categories according to their position along the crypt axis: surface cells (s.c.); mid-crypt cells (m.c.) and crypt base cells (b.c.). The zero-current membrane voltage (V_m) was -56 ± 2 mV in s.c (n = 34); -76 ± 2 mV in m.c. (n = 47); and -87 ± 1 mV in b.c. (n = 87). The whole-cell conductance (G_m) was similar (8 12 nS) in all three types of cells. A fractional K⁺ conductance accounting for 29-67% of G_m was present in all cell types. A Na⁺ conductance was demonstrable in s.c. by the hyperpolarizing effect on V_m of a low-Na⁺ (5 mmol/l) solution. In m.c. and b.c. the hyperpolarizing effect was much smaller, albeit significant. Amiloride had a concentration-dependent hyperpolarizing effect on V_m in m.c. and even more so in s.c.. It reduced G_m by approximately 12%. The dissociation constant (K_D) was around 0.2 μmol/l. Triamterene had a comparable but not additive effect (K_D = 30 μmol/l, n = 14). Forskolin (10 μmol/l, in order to enhance cytosolic adenosine 3′, 5′-cyclic monophosphate or cAMP) depolarized V_m in all three types of cells. The strongest effect was seen in b.c.. G_m was enhanced significantly in b.c. by 83% (forskolin) to 121% [8-(4-chlorophenylthio)cAMP]. The depolarization of V_m and increase in G_m was caused to large extent by an increase in Cl^- conductance as shown by the effect of a reduction in bath Cl^- concentration from 145 to 32 mmol/l. This manoeuvre hyperpolarized V_m under control conditions significantly by 6-9 mV in all three types of cells, whilst it depolarized V_m in the presence of forskolin in m.c. and in b.c.. These data indicate that s.c. of dexamethasone-treated rats possess mostly a K⁺ conductance and an amiloride- and triamterene-inhibitable Na⁺ conductance, m.c. and b.c. possess little or no Na⁺ conductance; their V_m is largely determined by a K⁺ conductance. Forskolin (via cAMP) augments the Cl^- conductance of m.c. and b.c. but has only a slight effect on s.c.