Biol. Pharm. Bull. 29(9) 1825—1829 (2006)

and water balance, and exhibits significant segmental heterogeneity along its longitudinal axis. The mucosa of the colonic inner surface is a typical electrolyte-transporting epithelium, which is able to move large quantities of ion and water from the mucosal side to the blood side and vice versa. It has been reported that the net absorption of water, sodium and chloride, as well as amiloride-sensitive shortcircuit current (ISC) 4) was dominant in distal colon than that in proximal colon, and the same activators could induce various responses in the different segments of colon. Nobles et al. reported that the distal segments of the rat colon were more sensitive to carbachol (CCh), a Ca pathway activator, as well as forskolin, a cAMP pathway activator which provide the evidences for the segmental specificity with regard to the intracellular signaling pathway responsible for the activation of secretion. Using PCR technique, Greig et al. found that the expression of ENaC (epithelial Na channel) was different between the rat proximal colon and distal colon. Recently, the segmental heterogeneity of endogenous prostaglandin was also reported to contribute to the differences in human colon secretory responses. Ion transport in gastrointestinal tract is under the control of neurotransmitters and hormones. The distribution of various kinds of receptors is also different in both segments of distal and proximal colon. Norepinephrine produced segment-specific effects on ion transport via different kind of receptors. However, most studies were undertaken to investigate the differences between the proximal and distal colon or between the colon and rectum. During our study, we found that even within the rat distal colon, the responses difference to stimulants also existed in the different segments. But there are a limited number of studies performed to compare the distinction. In the present study, the segmental heterogeneity of epithelial responses to different stimulants and its possible mechanism were investigated in rat distal colon.