Activation of basolateral Cl? channels in the rat colonic epithelium during regulatory volume decrease

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Abstract
Exposure to a hypotonic medium caused an increase in the diameter of isolated crypts from the rat colon. The increase in cell volume was only transient and lasted about 7 min. Despite of the continuous presence of the hypotonic medium, cell volume decreased again. This regulatory volume decrease (RVD) was inhibited by 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), a Cl− channel blocker, and by Ba2+, a K+ channel blocker. Cell-attached patch-clamp recordings revealed that the RVD was associated with the activation of previously silent basolateral channels. These channels were identified after excision of the patch as Cl− channels (28 pS) and as K+ channels (45–60 pS). The RVD was dependent on the presence of external Ca2+. The phospholipase A2 inhibitor, quinacrine, and the lipoxygenase blocker, nordihydroguaiaretic acid, inhibited RVD, while indomethacin had no effect. In Ussing chamber experiments an exposure to hypotonic media caused an initial, transient increase in tissue conductance (G t), followed by a prolonged decrease in short-circuit current (I sc) and the potential difference (V). The height of the electrical response was dependent on the decrease in the osmolarity in a range from 20 mosmol l−1 to 90 mosmol l−1. The increase in G t was blocked by NPPB and Ba2+, whereas the decrease in I sc or V was inhibited by NPPB but enhanced by Ba2+. This suggests that in the later phase the osmotically induced Cl− conductance exceeds the K+ conductance leading to an electrogenic response, while the initial response of the RVD is an opening of Cl− and K+ channels in a ratio of about 1∶1. With respect to the inhibitory efficacy of nordihydroguaiaretic acid and the inefficacy of indomethacin, leukotrienes seem to be involved in the mediation of this response.