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

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Abstract
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.

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