EFFECTS OF FORSKOLIN ON CRYPT CELLS OF RAT DISTAL COLON - ACTIVATION OF NONSELECTIVE CATION CHANNELS IN THE CRYPT BASE AND OF A CHLORIDE CONDUCTANCE PATHWAY IN OTHER PARTS OF THE CRYPT

We recently showed that prostaglandin E2 (PGE2) causes depolarization in cells at the base of isolated crypts from rat distal colon by activating nonselective cation channels. In order to investigate whether PGE, acts via intracellular cyclic adenosine monophosphate (cAMP), the effect of forskolin on cell potential and on whole-cell current was investigated using the slow whole-cell patch-clamp method with nystatin. In addition, effects of forskolin in cells at other sites along the crypt were investigated. At the crypt base, the unstimulated cells had a resting potential of -70.6 +/- 1.3 mV (n = 25). When forskolin was added to the bath, the cells depolarized to -21.1 +/- 1.5 mV (n = 25). This depolarization was inhibited by substitution of all Na+ in the bath solution by N-methyl-D-glucamine (NMDG+) or by addition of flufenamic acid (50 mumol/l), a blocker of nonselective cation channels, to the bath. In contrast, the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 50 mumol/l) did not affect the depolarization. Moving along the crypt, the resting potential was -66.8 +/- 1.8 mV (n = 11) in the mid-crypt and -48.1 +/- 2.9 mV (n = 9) in cells of the upper part of the crypt. Forskolin caused a strong depolarization to about -20 mV in all parts of the crypt. In contrast to cells at the base, this depolarization was only partly diminished by substitution of Na+ by NMDG+, whereas substitution of bath Cl- by gluconate caused an initial further depolarization, followed by a repolarization to the cell's resting potential. It can be concluded that forskolin activates nonselective cation channels in the crypt base, whereas in other parts of the crypt mainly a Cl- conductance pathway is induced. Thus, Cl- secretion probably takes place in cells of the crypt wall with the exception of the crypt base. Activation of nonselective cation channels by prostaglandins or forskolin in the crypt base could either be involved in K+ secretion or alternatively, in cell proliferation.