Naϩ absorption in the colon plays an important role in salt and water homeostasis. Two transport mechanisms involved in Na ϩ absorption have been described, one being an electroneutral pathway mediated by the apical Na ϩ /H ϩ exchanger, which is functionally coupled to the Cl Ϫ /HCO 3 Ϫ exchanger, and the other an electrogenic pathway (generating the short-circuit current, I sc ) mediated by the apical amiloride-sensitive epithelial-type Na ϩ channel (ENaC) [1]. These two Na ϩ transport pathways are under the control of extracellular neurotransmitters, hormones, and paracrine substances.Cyclic AMP is a ubiquitous second messenger produced in cells in response to extracellular stimulants. cAMP production in the colonic mucosa decreases electroneutral Na ϩ absorption mediated by the NaHowever, previous studies concerning the effect of cAMP on electrogenic Na ϩ absorption in the colon have provided conflicting results. In the intestinal cell line, Caco-2, an I sc component inhibitable by an amiloride analogue, is stimulated by secretin, which acts through the cAMP-signaling pathway [5]. In contrast, cAMP has hardly any effect on 22 Na ϩ absorption in the rabbit colon [6]. In the rat colon, forskolin, an adenylate cyclase activator, has no effect on the amiloride-sensitive I sc under normal conditions, but it can enhance the amiloride-sensitive I sc after the tissue is exposed to sonicated suspensions of nucleated cells [7]. Finally, cAMP and isoproterenol cause, at least initially, a decrease in the amiloride-sensitive I sc in the guinea pig and human colon [8,9].The purpose of this study was therefore to determine the role of cAMP in the regulation of electrogenic Na ϩ absorption in the rat distal colon. To this end we have examined the effect of forskolin and the -adrenergic agonist, isoproterenol, on the amiloridesensitive short-circuit current (I sc ) and unidirectional 22 Na ϩ fluxes in the chamber-mounted rat distal colon from animals pretreated with a low Na ϩ diet to enhance electrogenic Na ϩ absorption. Japanese Journal of Physiology, 51, 435-444, 2001 Key words: cAMP-dependent protein kinase, isoproterenol, -adrenergic agonist, amiloride, Na ϩ absorption.
MATERIALS AND METHODS
AnimalsAbstract: Cyclic AMP is a ubiquitous second messenger produced in cells in response to extracellular stimulants. The aim of this study was to examine its role in the regulation of amiloridesensitive electrogenic Na ϩ absorption in the rat distal colon by measuring the short-circuit current (I sc ) and 22
Naϩ fluxes in a chamber-mounted mucosal sheet. Forskolin stimulated the amilorideinhibitable I sc and amiloride-inhibitable 22 Na ϩ absorption. 8Br-cAMP also stimulated the amiloride-inhibitable I sc . Furthermore, isoproterenol, acting via -adrenergic activation, stimulated the amiloride-inhibitable I sc . The isoproterenol-induced increase in the amiloride-sensitive I sc was largely suppressed by H89, a protein kinase A inhibitor. In conclusion, cAMP can upregulate amiloride-sensitive electrogenic Na ϩ absorption in ...