Regulation of the Na+2Cl−K+ co‐transporter—mechanisms in the rectal gland of Squalus acanthias with implications for the thick ascending limb of Henle

Abstract

The mechanism of NaCl transport in the thick rat distal colon [6,7]. Now we addressed the questionascending limb of Henle (TAL) was originally deduced of whether all these mechanisms of co-transporterfrom experiments in isolated perfused tubules of the activation could have one common signal. Time courseshark rectal gland (RGT) [1,2]. Molecular identifica- analysis of transepithelial equivalent short circuit cur-tion of Na+2Cl−K+ co-transport proteins revealed rent (Isc) in isolated perfused RGT and measurementtwo subtypes, namely a luminally expressed kidney of cytosolic Cl− concentration and cell volume bytype (NKCC2, BSC1) and a basolaterally expressed fluorescence methods revealed that transient cellsecretory type (NKCC1, BSC2) [3,4]. These proteins shrinkage is the key event in activation of theare encoded by different genes, share 60% amino acid Na+2Cl−K+ co-transporter [8].identity and show different pharmacodynamic profiles cAMP-dependent stimulation of Cl− secretion infor loop diuretics. In the face of these differences and RGT results in a biphasic development of Isc: a firstalthough co-transport activity in the TAL is constitu- and rapid phase corresponding to the activation oftively high, knowledge of co-transporter regulation in luminal Cl− channels with the respective Cl− currentthe rectal gland might have important implications for from the cytosol into the tubular fluid, and a secondthe understanding of co-transport regulation in the phase which can be completely inhibited by loopTAL. In Cl−-secreting epithelia, NaCl transport is diuretics, reflecting the delayed activation of theunder the control of hormones which converge on Na+2Cl−K+ co-transporter (Figures 1 and 2A).the cAMP and Ca2+ second messenger pathways.Activation of Cl− secretion involves cAMP-dependentphosphorylation of luminal Cl− channels, stimulationof basolateral K+ channels via Ca2+ and cAMP, asecondary increase in (Na++K+)-ATPase activity andstimulation of the Na+2Cl−K+ co-transporter [5].For both secretory and absorptive cell types, it is ademanding task to keep the cell volume constant andto allow at the same time large amounts of trans-epithelial Na+ and Cl− fluxes. This task requires atight coupling of Cl− channel and Na+2Cl−K+co-transporter activity. From experiments in the RGT,it is well known that Cl− secretion starts with thecAMP-dependent activation of a luminal CFTR typeCl− conductance. The regulation of co-transporteractivity, however, is still discussed controversially.Several mechanisms have been proposed forco-transporter regulation: (i) phosphorylation;(ii) increase in cytosolic Ca2+; (iii) fall in cytosolic