Properties of an Inwardly Rectifying ATP-sensitive K � Channel in the Basolateral Membrane of Renal Proximal Tubule

The potassium conductance of the basolateral membrane (BLM) of proximal tubule cells is a critical regulator of transport since it is the major determinant of the negative cell membrane potential and is necessary for pump-leak coupling to the Na�,K�-ATPase pump. Despite this pivotal physiological role, the properties of this conductance have been incompletely characterized, in part due to difficulty gaining access to the BLM. We have investigated the properties of this BLM K� conductance in dissociated, polarized Ambystoma proximal tubule cells. Nearly all seals made on Ambystoma cells contained inward rectifier K� channels (�slope, in � 24.5 � 0.6 pS, �chord, out � 3.7 � 0.4 pS). The rectification is mediated in part by internal Mg2�. The open probability of the channel increases modestly with hyperpolarization. The inward conducting properties are described by a saturating binding–unbinding model. The channel conducts Tl� and K�, but there is no significant conductance for Na�, Rb�, Cs�, Li�, NH4 �, or Cl�. The channel is inhibited by barium and the sulfonylurea agent glibenclamide, but not by tetraethylammonium. Channel rundown typically occurs in the absence of ATP, but cytosolic addition of 0.2 mM ATP (or any hydrolyzable nucleoside triphosphate) sustains channel activity indefinitely. Phosphorylation processes alone fail to sustain channel activity. Higher doses of ATP (or other nucleoside triphosphates) reversibly inhibit the channel. The K� channel opener diazoxide opens the channel in the presence of 0.2 mM ATP, but does not alleviate the inhibition of millimolar doses of ATP. We conclude that this K� channel is the major ATP-sensitive basolateral K� conductance in the proximal tubule. key words: ion channel • kidney • patch-clamp • sulfonylurea • epithelia i n t r o d u c t i o n ductance provides such a pathway and thus steady state intracellular K activity can be maintained in the face of The ability of the renal proximal tubule to maintain holarge transcellular fluxes of salt and water (Sackin and meostatic electrolyte and water reabsorption in the face Boulpaep, 1983). Since a major portion of the transcelof drastic changes in dietary solute and water intake and renal hemodynamics implies that transtubular ion lular Na � flux is reabsorbed across the BLM by the actransport is tightly regulated. Proximal tubule potastion of the Na �,K�-ATPase, a population of BLM K� channels working in concert with the pump would alsium channels, particularly in the basolateral memlow K� to recycle in a regulated fashion. Moreover, hybrane (BLM),1 play pivotal physiologic roles in the regperpolarization secondary to the opening of BLM K� ulation of membrane voltage, potassium recycling, and ultimately in transepithelial solute and water reabsorpchannels enhances the driving force for electrogenic tion. That the basolateral membrane potential of a apical Na � entry and basolateral Cl� efflux, resulting in net NaCl reabsorption. proximal tubule cell is dominated by the BLM K conThe Ambystoma proximal tubule exhibits a large K� ductance is well established (Sackin and Boulpaep, 1983). The Na�,K�-ATPase pump in the BLM provides conductance in the basolateral membrane (Siebens the energy that makes ion transport thermodynamiand Boron, 1987; Sackin and Boulpaep, 1981) that has been shown macroscopically to be sensitive to barium cally favorable, but continuous operation of the pump and pH. However, previous studies of the “macrorequires there be a K� exit pathway. The BLM K conscopic” BLM K� conductance (Boulpaep, 1976) lack the resolution to distinguish whether there is one population of imperfectly selective K� channels or a set of Address correspondence to Alan S. Segal, Department of Medicine, highly selective K� channels coexisting with a populaUniversity of Vermont, 55A South Park Drive, Colchester, VT 05446. tion of nonselective cation channels. More recent “miFax: 802-656-8915; E-mail: asegal@zoo.uvm. edu croscopic” single-channel patch-clamp studies of BLM 1Abbreviations used in this paper: BLM, basolateral membrane; c/a, K� channels have shown diversity in both experimental cell-attached (patch); I-V, current–voltage; i/o, inside-out (patch); KCO, K channel opener; nPo, channel activity represented as the design and findings (Tsuchiya et al., 1992; Hunter, product of the minimum number of channels (n) times the open 1991; Parent et al., 1988; Kawahara et al., 1987; Sackin probability (Po) of the channel; SUR, sulfonylurea receptor. and Palmer, 1987; Gögelein and Greger, 1987a, 1987b), 139 J. Gen. Physiol. © The Rockefeller University Press • 0022-1295/98/01/139/22 $2.00 Volume 111 January 1998 139–160