EFFECTS OF GLUCOCORTICOID AND MINERALOCORTICOID ON POTASSIUM-TRANSPORT IN THE RAT MEDULLARY THICK ASCENDING LIMB OF HENLES LOOP

Potassium transport in the thick ascending limb is conducted by hetero geneous cells to opposite directions. Effects of glucocorticoids and m ineralocorticoids on potassium transport of the rat medullary thick as cending limb (MTAL) were examined by in vitro microperfusion technique by measuring net potassium flux (J(K)), and apparent potassium conduc tances in the apical and basolateral membranes of two cell populations . Segments of MTAL were obtained from four groups of rats: sham operat ed control rats, adrenalectomized rats (ADX), adrenalectomized rats tr eated with dexamethasone (DEX), and adrenalectomized rats treated with aldosterone (ALDO). Fractional urinary potassium excretion was reduce d by ADX and partially recovered by either DEX or ALDO. J(K) of the is olated perfused MTAL was markedly decreased by ADX from 5.13 to 1.94 p mol/min/mm. It was partially recovered by DEX (3.36 pmol/min/mm), but not by ALDO (2.07 pmol/min/mm). Random impalement of MTAL cells with a microelectrode in the control group revealed two cell populations; 76 % was high basolateral conductance cell (HBC) and 24% was low basolate ral conductance cell (LBC). In the ADX group, the basolateral potassiu m conductance of the HBC cell was markedly reduced, whereas the apical membrane potassium conductance of the HBC cell was increased. These c hanges were recovered in the DEX group, but not in the ALDO group. Pot assium conductances of the apical and basolateral membranes in the LBC were unchanged. These findings suggest that potassium transport in th e MTAL, is regulated mainly by glucocorticoids which may predominantly act on the HBC. However, it remains to be established whether ADX cau ses a conversion of HBC to LBC or alters the basolateral K+ conductanc e of the HBC.