Mechanisms of ion transport across the loop of Henle : Regulation by hormones and drugs

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It is well known that the thick ascending limb of Henle's lop (TAL) consists of two distinct types of cells having smooth surface (S cell) or rough surface (R cell). This project was designed to clarify the functional significance of this cell heterogeneity. For this purpose, we made histometrical analyzes, electrophysiological studies and ion flux measurements. The histometrical analysis of the TAL along the renal axis revealed that R-cells are abundant in the cortical portion (cortical thick ascending limb, CAL), whereas S-cells are rich in the medullary portion (medullary thick ascending limb, MAL). Random cell puncture showed that there are two cell populations having different basolateral K conductance ; the high basolateral conductance cell (HBC) and the low basolateral conductance cell (LBC). In combination with K flux studies, we proposed the following schema : S cell^=HBC cell^=K absorptive cell ; and R cell^=LBC cell^=K secretory cell. After andrenalectomy, the basolateral membrane conductance of HBC cells of rat MAL tended to reduce. Replacement of glucocorticoid (dexamethasone) improved this abnormality, whereas that of mineralocorticoid (aldosterone) was without any effects. K absorption was reduced in the MAL obtained from adrenalectomized animals, and recovered by administration of dexamethasone, but not by administratin of aldosterone. Based on these observations, we concluded that glucocorticoids act on R cell to increase the basolateral mambrane K conductance, and thereby increase K absorption across this segment. It is possible that morphological heterogeneity along the thick ascending limb has functional significance with respect to K transport in the kidney.