Regulation and possible physiological role of the Ca2-dependent K+ channel of cortical collecting ducts of the rat

Abstract

In the luminal membrane of rat cortical collecting duct (CCD) a big Ca(2+)-dependent and a small Ca(2+)-independent K+ channel have been described. Whereas the latter most likely is responsible for the K+ secretion in this nephron segment, the function of the large-conductance K+ channel is unknown. The regulation of this channel and its possible physiological role were examined with the conventional cell-free and the cell-attached nystatin patch-clamp techniques. Patch-clamp recordings were obtained from the luminal membrane of isolated perfused CCD segments and from freshly isolated CCD cells. Intracellular calcium was measured using the calcium-sensitive dye fura-2. The large-conductance K+ channel was strongly voltage- and calcium-dependent. At 3 mumol/l cytosolic Ca2+ activity it was half-maximally activated. At 1 mmol/l it was neither regulated by cytosolic pH nor by ATP. At 1 mumol/l Ca2+ activity the open probability (Po) of this channel was pH-dependent. At pH 7.0 Po was decreased to 4 +/- 2% (n = 9) and at pH 8.5 it was increased to 425 +/- 52% (n = 9) of the control. At this low Ca2+ activity the Po of the channel was reduced by 1 mmol/l ATP to 8 +/- 4% (n = 6). Cell swelling activated the large-conductance K+ channel (n = 14) and hyperpolarized the membrane potential of the cells by 9 +/- 1 mV (n = 23). Intracellular Ca2+ activity increased after hypotonic stress. This increase depended on the extracellular Ca2+ activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Apical sorting of a voltage- and Ca 2+ -activated K + channel α-subunit in Madin-Darby canine kidney cells is independent of N-glycosylation

“…The ␤-subunits are N-glycosylated and, with the exception of a ␤3 isoform, endow the channel with altered kinetics, calcium͞voltage, and toxin sensitivities (23)(24)(25). In a diversity of epithelial cells, from kidney (27)(28)(29), male reproductive tract (30), oviduct (31), and vestibulum (32), K V,Ca channels have been electrophysiologically detected at the apical domain. However, they have been also reported in the basolateral domain of enterocytes (33) and epithelial cells from acinar glands (34).…”

Modulation of the Ca2+-Activated Large Conductance K+ Channel by Intracellular pH in Human Renal Proximal Tubule Cells.

“…pH i -sensitivity of the BK channels has been reported in several tissues, including renal tubule cells such as rabbit medullary thick ascending limb [14] and rat cortical collecting duct cells [8]. Namely, lowering pH i suppressed channel activity, and raising pH i enhanced it.…”

“…Our preliminary kinetic analysis also supported this notion. [Japanese Journal of Physiology, 52, 267-276, 2002] 26], rabbit connecting tubule [27], and rat cortical collecting ducts [8,28]. Furthermore, we recently demonstrated the presence of a BK channel with ATPsensitivity in cultured human renal proximal tubule epithelial cells (RPTECs) [29].…”

“…Furthermore, we recently demonstrated the presence of a BK channel with ATPsensitivity in cultured human renal proximal tubule epithelial cells (RPTECs) [29]. Among these channels, pH i -sensitive activity was shown in rabbit medullary thick ascending limb [14] and rat cortical collecting ducts [8]. However, pH i -sensitivity of the BK channel in human renal tubules has not been reported.…”

“…Besides Ca 2ϩ and voltage, other factors such as ATP [8-11], pH [8, 9, 12-21], and Mg 2ϩ [19,[22][23][24] were also reported to modulate BK channels. Among these factors, intracellular pH (pH i ) is one of the significant factors that affect channel activity [8,9,[12][13][14][15][16][17][18][19][20][21], i.e., channel activity is inhibited by intracellular acidification and stimulated by its alkalization. Although the mechanism for pH i effects on channel activity was not precisely understood, two different mechanisms have been proposed.…”