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Extracellular nucleotides regulate NaCl transport in some epithelia. However, the effects of nucleotide agonists on NaCl transport in the renal inner medullary collecting duct (IMCD) are not known. The objective of this study was to determine whether ATP and related nucleotides regulate NaCl transport across mouse IMCD cell line (mIMCD-K2) epithelial monolayers and, if so, via what purinergic receptor subtypes. ATP and UTP inhibited Na+ absorption [measured via Na+ short-circuit current[Formula: see text])] and stimulated Cl− secretion [measured via Cl−short-circuit current ([Formula: see text])]. Using selective P2 agonists, we report that P2X and P2Y purinoceptors regulate [Formula: see text] and[Formula: see text]. By RT-PCR, two P2X receptor channels (P2X3, P2X4) and two P2Y G protein-coupled receptors (P2Y1, P2Y2) were identified. Functional localization of P2 purinoceptors suggest that [Formula: see text] is stimulated by apical membrane-resident P2Y purinoceptors and P2X receptor channels, whereas[Formula: see text] is inhibited by apical membrane-resident P2Y purinoceptors and P2X receptor channels. Together, we conclude that nucleotide agonists inhibit[Formula: see text] across mIMCD-K2 monolayers through interactions with P2X and P2Y purinoceptors expressed on the apical plasma membrane, whereas extracellular nucleotides stimulate [Formula: see text]through interactions with P2X and P2Y purinoceptors expressed on the apical plasma membrane.

This essay looks at the historical significance of six APS Classic Papers that are freely available on line: Malnic G, Klose RM, Giebisch G. Micropuncture study of renal potassium excretion in the rat. Am J Physiol 206: 674–686, 1964 ( http://ajplegacy.physiology.org/cgi/reprint/206/4/674 ). Malnic G, Klose RM, Giebisch G. Micropuncture study of distal tubular potassium and sodium transport in rat nephron. Am J Physiol 211: 529–547, 1966 ( http://ajplegacy.physiology.org/cgi/reprint/211/3/529 ). Malnic G, Klose RM, Giebisch G. Microperfusion study of distal tubular potassium and sodium transfer in rat kidney. Am J Physiol 211: 548–559, 1966 ( http://ajplegacy.physiology.org/cgi/reprint/211/3/548 ). Duarte CG, Chomety F, Giebisch G. Effect of amiloride, ouabain, and furosemide on distal tubular function in the rat. Am J Physiol 221: 632–640, 1971 ( http://ajplegacy.physiology.org/cgi/reprint/221/2/632 ). Malnic G, De Mello Aires M, Giebisch G. Potassium transport across renal distal tubules during acid-base disturbances. Am J Physiol 221: 1192–1208, 1971 ( http://ajplegacy.physiology.org/cgi/reprint/221/4/1192 ). Wright FS, Strieder N, Fowler NB, Giebisch G. Potassium secretion by distal tubule after potassium adaptation. Am J Physiol 221: 437–448, 1971 ( http://ajplegacy.physiology.org/cgi/reprint/221/2/437 ).

Electrophysiological and transport properties were studied in isolated and perfused Amphiuma collecting tubules from two groups of animals, one exposed to a high sodium (NA), the other to a high potassium (KA) environment (both conditions known to modulate blood aldosterone levels). The transepithelial lumen-negative potential was significantly larger (-38 +/- 5 mV) in tubules from KA animals than from NA animals (-15 +/- 3 mV). In addition, we observed an increase in the apical amiloride-sensitive sodium conductance and stimulation of the transepithelial sodium current. Although no measurable potassium conductance was found in the apical cell membrane in either group, a potassium selectivity of the paracellular transport pathway was observed in the KA animals. Net potassium secretion was demonstrated in KA tubules (helium-glow photometry and [3H]inulin analysis). Potassium secretion was abolished by luminal amiloride but imposition of a bath-to-lumen potassium gradient induced potassium secretion. We conclude that in contrast to the mammalian cortical collecting tubule in which potassium secretion is largely transcellular, potassium secretion in the Amphiuma collecting tubule is by diffusion through the paracellular pathway.

Initial segments of the inner medullary collecting duct of the rat were perfused in vitro, and the electrophysiological properties of the apical and basolateral membranes were examined with KCl-filled microelectrodes. The fractional resistance of the apical membrane (FRa = Ra/Ra + Rbl) and the transepithelial resistance (RT) were estimated by cable analysis. In control tubules the transepithelial voltage (VT) averaged -2.2 mV, and the voltage across the basolateral membrane (Vbl) averaged -51.1 mV. RT was 11.9 k omega.cm (72.8 omega.cm2), and FRa was 0.94. Pretreatment of the rats with deoxycorticosterone (DOC)-pivalate for 7-10 days did not alter these electrophysiological properties. In control tubules, amiloride in the lumen (10(-5) M) changed VT from -3.0 to +1.4 mV and increased Vbl from -49.4 to -53.8 mV, RT from 12.5 to 13.6 k omega.cm, and FRa from 0.92 to 0.98. Thus the apical membrane is conductive to Na+. An increase of the bath K+ concentration from 4 to 15 mM caused an 18.8 mV depolarization of Vbl: barium in the bath also depolarized Vbl. A fivefold decrease in the [HCO3-] in the bath depolarized Vbl by 13.1 mV. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) blocked this depolarization. Thus the basolateral membrane is conductive to K+ and HCO3-. Experiments with ouabain revealed a Na+-K+-ATPase in the basolateral membrane. Taken together, the results support a model in which electrogenic Na+ absorption is driven by the Na+-K+-ATPase in the basolateral membrane, with passive movement of Na+ occurring through an amiloride-sensitive conductive pathway in the apical membrane.

The mechanisms, of electrolyte transport by isolated and perfused late distal tubules of the salamander, Amphiuma, were investigated by electrophysiological and transport techniques. The tubules absorbed Na+, HCO3-, and Cl- but not K+. The transepithelial voltage (VT) was not different from zero. Amiloride (10(-3) M) in the perfusion fluid reduced sodium absorption by 43% and HCO3- absorption by 49% without changing VT. This and previous data are consistent with the presence of a Na+/H+ antiporter in the apical membrane. Hydrochlorothiazide (HCTZ, 10(-4) M) in the perfusion fluid inhibited Na+ absorption by 48% but had no effect on HCO3- absorption or VT. Thus HCTZ reduced NaCl absorption. Intracellular microelectrode techniques were used to examine the cellular mechanisms of ion transport and sites of action of amiloride and HCTZ. Two cell types were identified by their electrophysiological properties. Neither amiloride nor HCTZ appreciably altered the electrical properties of cell type I, a cell previously identified as being involved in H+ secretion. In contrast, both diuretics hyperpolarized the basolateral membrane voltage (Vbl) of cell type II. Additional studies of cell type II showed that the removal of Cl- from the lumen hyperpolarized Vbl, as did the addition to the lumen of the Cl-/HCO3- exchange inhibitor 4,4'-diisothiocyanostilbene-2-2'-disulphonic acid. Finally, reducing the [HCO3-] of the lumen depolarized Vbl.(ABSTRACT TRUNCATED AT 250 WORDS)

Microperfusion studies were performed on superficial distal tubules to determine the relationship between potassium secretion by the distal tubule and plasma potassium concentrations in rats on a control and a high potassium diet. Potassium was infused in graded doses into animals on a control diet and into animals receiving a high potassium diet. Since potassium loading in rats is known to inhibit proximal tubular sodium and fluid reabsorption and thereby could lead to nonspecific flow-related stimulation of potassium secretion by the distal tubule, continuous microperfusion techniques at constant flow rate were used to measure the rate of potassium secretion. Additionally, plasma aldosterone levels were measured and renal clearance experiments carried out. The results show that potassium secretion, restricted to cells of the late distal tubule, reaches a maximal rate at a plasma potassium concentration of about 6 meq/liter. Potassium adaptation significantly enhances potassium secretion compared with animals on a control diet over a range of plasma potassium from 3.8 to 7.4 meq/liter. Enhanced potassium secretion by the distal tubule, in both control and potassium-adapted animals, is associated with increased aldosterone levels.