Axial heterogeneity of potassium transport across hamster thick ascending limb of Henle's loop

Abstract

Functional significance of morphological heterogeneities along the thick ascending limb of Henle's loop of hamsters was explored by the in vitro microperfusion technique with special reference to K+ transport. The transmission electron microscopic study confirmed that there are two types of cells, with smooth surface (S-cell) and rough surface (R-cell), respectively, and that the former is abundant in the medullary thick ascending limb (MTAL), whereas the latter is in the cortical portion (CTAL). The electrophysiological study revealed that in both segments there are two cell populations, one having high basolateral and low apical membrane K+ conductances (HBC) and the other having low basolateral and high apical K+ conductances (LBC). Random cell puncture revealed that the ratios of HBC/LBC were 24/7 (77%/23%) in the MTAL and 7/22 (24%/76%) in the CTAL, suggesting that HBC corresponds to S-cell, whereas LBC corresponds to R-cell. Net K+ transport was determined in two segments by measuring K+ concentration in the collected and perfused fluid by ultramicroflame photometry. In all six tubules of MTAL, net K+ flux had a direction to reabsorption with a mean of 4.87 +/- 0.46 pmol.min-1.mm-1. In marked contrast, in all six tubules of CTAL, we observed K+ secretion with a mean of -3.81 +/- 0.49 pmol.min-1.mm-1. The transmural voltage was positive in both segments and was significantly higher in the CTAL (7.8 +/- 0.5 mV) than in the MTAL (2.5 +/- 0.2 mV). From these observations, we conclude that the S-cell corresponding to the HBC cell reabsorbs K+, whereas the R-cell corresponding to the LBC cell secrets K+.(ABSTRACT TRUNCATED AT 250 WORDS)

Thick Ascending Limb of the Loop of Henle

“…Increases in basolateral K 1 cause Cl 2 -dependent cell swelling in the Amphiuma early distal tubule, an analog of the mammalian TAL. In Amphiuma LBC cells with low basolateral conductance, analogous to mammalian LBC cells (3,5), this cell swelling was not accompanied by changes in basolateral membrane voltage or resistance (47), consistent with electroneutral K 1 -Cl 2 transport. By extension, KCC4 may play an important role in the basolateral chloride transport of LBC cells.…”

“…The TAL contains two morphologic subtypes: a rough-surfaced cell type (R cells) with prominent apical microvilli and a smooth-surfaced cell type (S cells) with an abundance of subapical vesicles (3,4) (see Figure 3). In the hamster TAL, cells can also be separated into those with high apical and low basolateral K 1 conductance and a weak basolateral Cl 2 conductance (LBC cells), versus a second population with low apical and high basolateral K 1 conductance, with high basolateral Cl 2 conductance (HBC cells) (3,5). The relative frequency of the morphologic and functional subtypes in the cortical and medullary TAL suggests that HBC cells correspond to S cells and LBC cells to R cells (3).…”

“…In the hamster TAL, cells can also be separated into those with high apical and low basolateral K 1 conductance and a weak basolateral Cl 2 conductance (LBC cells), versus a second population with low apical and high basolateral K 1 conductance, with high basolateral Cl 2 conductance (HBC cells) (3,5). The relative frequency of the morphologic and functional subtypes in the cortical and medullary TAL suggests that HBC cells correspond to S cells and LBC cells to R cells (3). Molecular characterization of this heterogeneity is still rudimentary; however, R and S cells clearly differ in the expression pattern of EGF (6) and NKCC2 (4).…”

“…The 30-pS channel is completely absent from the apical membrane of mice with homozygous deletion of the gene encoding ROMK (29), providing genetic evidence that ROMK mediates this 30-pS conductance. Notably, not all cells in the TAL are labeled with ROMK antibody (28), suggesting that ROMK might be absent in the HBC cells with high basolateral Cl 2 conductance and low apical/high basolateral K 1 conductance (also see above) (3,5). HBC cells are thought to correspond to the smoothsurfaced morphologic subtype of TAL cells (S cells) (3); however, the relative expression of ROMK protein by immunoelectron microscopy in R and S cells has not yet been published.…”

“…Apical, carbonic anhydrase-dependent (62) bicarbonate reabsorption is accomplished by Na 1 /H 1 exchange, primarily mediated by the Na 1 /H 1 exchanger NHE3 (63) (see Figure 4). The apical exit of an H 1 ion is accompanied by basolateral exit of HCO 3 2 (i.e., bicarbonate absorption). There are several basolateral exit mechanisms for bicarbonate in the TAL (64), including Cl 2 /HCO 3 2 exchange, K 1 -HCO 3 2 cotransport (likely mediated by the K 1 -Cl 2 cotransporter KCC4), and Na 1 /H 1 exchange.…”