Pflügers Archiv

, Volume 431, Issue 3, pp 427–434

Crypt base cells show forskolin-induced Cl secretion but no cation inward conductance

  • D. Ecke
  • M. Bleich
  • R. Greger
Original Article

Abstract

Whole-cell patch-clamp studies in base cells of isolated colonic crypts of rats pretreated with dexamethasone were performed to examine the effects of stimulation by forskolin (10 μmol/1). The experiments were designed in order to distinguish between two postulated effector mechanisms: the activation of a non-selective cation channel and the activation of Cl channels. As shown in an accompanying report, forskolin depolarizes the membrane voltage (Vm) by some 40–50 mV and enhances the whole-cell membrane conductance (Gm) substantially in these cells. In this report all experiments were performed in the presence of forskolin. A reduction of the bath Na+ concentration from 145 to 2 mmol/1 led to a hyperpolarization ofVm by some 20–30 mV This hyperpolarization occurred very slowly suggesting that the hyperpolarization produced by the low-Na+ solution was caused indirectly and not by a change in the equilibrium potential for Na+,ENa+. A complete kinetic analysis of the effect on voltage of bath Na+ revealed a saturation-type relation with a high apparent affinity for Na+ of around 5–10 mmol/1. A reduction in bath Cl concentration from 145 to 32 mmol/1 caused a depolarization ofVm from −34 ± 3 to −20 ± 4 mV (n = 13) in the presence of a high bath Na+ concentration, but had the opposite effect at low (5 mmol/1) Na+ concentrations:Vm was hyperpolarized from −46 ± 4 to −62 ± 6 mV (n = 13). If the effect of Na+ onVm was caused by a non-selective cation channel the opposite would have been expected. To test directly whether the Na+2ClK+ cotransporter was responsible for the effects of changes in bath Na+ onVm, the effects of increasing concentrations of several loop diuretics were examined. Furosemide, piretanide, torasemide and burnetanide (up to 0.1–0.5 mmol/1) all hyperpolarizedVm, albeit only by less than 10 mV. Another subclass of loop diuretics containing a tetrazolate in position 1 [e.g. azosemide, no. 19A and no. 20A from Schlatter E, Greger R, Weidtke C (1983) Pflüger Arch 396: 210–217] were much more effective. Azosemide hyperpolarizedVm from −46 ± 3 to −74 ± 2 mV (n = 18) and reducedGm from 11 ± 1 to 4 ± 1 nS (n = 14). These data indicate that forskolin stimulates Cl secretion in these cells by a mechanism fully compatible with the current scheme for exocrine secretion involving the Na+2ClK+ cotransporter.

Key words

Colon Loop diuretics Na+ channel Cl channel Non-selective channel Exocrine secretion 

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Copyright information

© Springer-Verlag 1996

Authors and Affiliations

  • D. Ecke
    • 1
  • M. Bleich
    • 1
  • R. Greger
    • 1
  1. 1.Physiologisches InstitutFreiburgGermany

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