Mobilization of Intracellular Ca²⁺ by Thromboxane A₂ Does Not Affect Ca²⁺-Activated K⁺ Channels in Rat Colonic Crypt Cells

Abstract

The effects of 9,11-epithio-11,12-methano-thromboxane A₂ (STA₂), a stable thromboxane A₂ analogue, and carbachol on colonic Ca²⁺-activated K⁺ channels were studied. In indo-1-loaded single cells in isolated rat colonic crypts, both STA₂ (0.1 μm) and carbachol (10 μm) transiently increased intracellular free Ca²⁺ concentration ([Ca²⁺]_i) by 136 and 155 nm, respectively. In whole-cell current-clamp experiments of the colonic crypt cells with Cl⁻-free solutions, carbachol (10 μm) hyperpolarized the cell by 19.7 mV, while STA₂ (0.1 μm) did not affect the membrane potential. In the isolated colonic mucosa that was permeabilized mucosally by a monovalent ionophore nystatin in the presence of a serosally directed K⁺ gradient, carbachol (10 μm) transiently elicited K⁺ current, but STA₂ (0.1 μm) did not. These results indicate that STA₂ elevates [Ca²⁺]_i in rat colonic crypt cells but does not activate basolateral Ca²⁺-activated K⁺ channels.