Active potassium transport across guinea-pig distal colon: action of secretagogues.

1. Adrenaline (5 microM) stimulated a K+ secretory current by 2.2 mu equiv h-1 cm-2 in isolated guinea-pig distal colonic epithelium. This secretory activity was inhibited entirely by addition of the loop diuretic bumetanide to the serosal solution. On-going K+ uptake via the absorptive pathway was unaltered by these changes. 2. Prostaglandin E2 (PGE2, 2 microM) stimulated electrogenic K+ secretion and Cl- secretion by 3.0 and 3.6 mu equiv h-1 cm-2, respectively. Serosal addition of bumetanide completely inhibited this K+ secretion but blocked only approximately 70% of Cl- secretion. The bumetanide-insensitive Cl- secretory current was dependent on the presence of Cl- and HCO3- in the bathing solutions. 3. Stimulation of electrogenic K+ secretion by PGE2 occurred with a half-maximal concentration of 4 nM, an affinity approximately 300 times higher than that for stimulation of Cl- secretion by PGE2. 4. Forskolin (10 microM) stimulated Cl- secretion by 4.9 mu equiv h-1 cm-2. The apparent K+ secretory rate was increased by only 1.5 mu equiv h-1 cm-2. A bumetanide-insensitive short-circuit current (ISC) was apparent and of the same size as that stimulated by PGE2. 5. Addition of the Ca2+ ionophore A23187 (10 microM), in the presence of indomethacin (1 microM) to reduce prostaglandin production, inhibited the K+ absorptive pathway by 40% and concurrently stimulated a small rate of electrogenic K+ secretion. 6. Active K+ absorption was inhibited by the addition of ouabain, omeprazole or SCH28080 to the mucosal solution. Both omeprazole and SCH28080 also stimulated a small negative ISC, consistent with electrogenic K+ secretion. 7. Association of K+ absorption, K+ secretion and Cl- secretion is indicated by similarities in transport mechanism and by secretagogue regulation. In particular, maximal rates of K+ secretory current require uptake via apical membrane K+ pumps. Such interrelations support a common cellular locus for these ion transport pathways

Genotoxicity of 2-alkylcyclobutanones, markers for an irradiation treatment in fat-containing food - Part I: cyto- and genotoxic potential of 2-tetradecylcyclobutanone

Previous experiments had indicated a slight genotoxic potential both in rat and in human colon cells of a sample of 2-dodecylcyclobutanone, a compound formed by irradiation of food containing palmitic acid in its triglycerides. Up to date, there is no evidence that 2-alkylcyclobutanones occur in non-irradiated foodstuffs, consequently it is prudent to test several members of the class of 2-alkylcyclobutanones which are produced by treatment of fat-containing food with ionising radiation. In this work, 2-tetradecylcyclobutanone (derived from stearic acid) has been tested for its cytotoxic and genotoxic potential. Human colon tumor cell lines, i.e. HT 29 and HT 29 clone 19A, were employed as models for in vitro experiments for cytotoxicity and genotoxicity tests. Cytotoxicity was measured by tetrazolium salt reduction assays (MTT and WST-1) and genotoxicity by determining DNA damage using the Comet Assay. Neither cytotoxic nor genotoxic effects were induced by 2-TCB in HT 29 or HT 29 cl 19A cells at an incubation time of 30 min at 37degreesC, not even at the highest concentration (400 muM) tested. After prolonged incubation times (1-2 days) at higher concentrations (>50 muM) cytotoxicity did, however, appear. Studies on other 2-alkylcyclobutanones are in progress. (C) 2002 Elsevier Science Ltd. All rights reserved