The ion conductances of colonic crypts from dexamethasone-treated rats

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Abstract
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% ofG m was present in all cell types. A Na+conductance was demonstrable in s.c. by the hyperpolarizing effect onV m of a low-Na+ (5 mmol/1) solution. In m.c. and b.c. the hyperpolarizing effect was much smaller, albeit significant. Amiloride had a concentration-dependent hyperpolarizing effect onV m in m.c. and even more so in s.c.. It reducedG 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) depolarizedV 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 ofV m and increase inG 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/1. This manocuvre hyperpolarizedV m under control conditions significantly by 6–9 mV in all three types of cells, whilst it depolarizedV 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 Tramterene-inhibitable Na+ conductance. m.c. and b.c. possess little or no Na+ conductance; theirV 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.