Activation of K+ channels in renal medullary vesicles by cAMP-dependent protein kinase

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Vasopressin regulation of sodium transport in the distal nephron and collecting duct

Arginine vasopressin (AVP) is released from the posterior pituitary gland during states of hyperosmolality or hypovolemia. AVP is a peptide hormone, with antidiuretic and antinatriuretic properties. It allows the kidneys to increase body water retention predominantly by increasing the cell surface expression of aquaporin water channels in the collecting duct alongside increasing the osmotic driving forces for water reabsorption. The antinatriuretic effects of AVP are mediated by the regulation of sodium transport throughout the distal nephron, from the thick ascending limb through to the collecting duct, which in turn partially facilitates osmotic movement of water. In this review, we will discuss the regulatory role of AVP in sodium transport and summarize the effects of AVP on various molecular targets, including the sodium-potassium-chloride cotransporter NKCC2, the thiazide-sensitive sodium-chloride cotransporter NCC, and the epithelial sodium channel ENaC.

Vasotocin analogues with selective natriuretic, kaliuretic and antidiuretic effects in rats

The aim of the present study was an investigation of mechanisms mediating selective effect of vasotocin analogues on water, sodium, and potassium excretion. We tested vasotocin analogues: Mpa(1)-vasotocin (dAVT), Mpa(1)-Arg(4)-vasotocin (dAAVT) and Mpa(1)-DArg(8)-vasotocin (dDAVT). The effects on water, sodium, and potassium transport were evaluated in experiments using normal and water-loaded Wistar rats. It was shown that all tested peptides exerted antidiuretic activity. Vasotocin and dAVT induced natriuresis and kaliuresis in rats. V1a agonist (Phe(2)-Ile(3)-Orn(8)-vasopressin) reproduced the renal effects of dAVT on sodium and potassium excretion but not on water reabsorption. dAAVT, dDAVT and V2 agonist (desmopressin) induced kaliuresis without any effect on sodium excretion. Natriuresis was associated with increase in cGMP excretion, whereas kaliuresis was correlated with rise of cAMP excretion. V1a antagonist (Pmp(1)-Tyr(Me)(2)-vasopressin) significantly reduced the dAVT-stimulated natriuresis and did not influence on urinary potassium excretion. V2 antagonist (Pmp(1)-DIle(2)-Ile(4)-vasopressin) significantly reduced the dAVT- and dAAVT-induced kaliuresis. It is assumed that effects of the nonapeptides on sodium and potassium transport are independent of their antidiuretic activity and mediated by different subtypes of V receptors (the V1a or V1a-like receptor for natriuretic effect and V2 or V2-like one for kaliuretic). In accordance to the data obtained, there is a possibility of selective regulation of renal water reabsorption and urinary sodium and potassium excretion with involvement of neurohypophysial hormones.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Cl- channels in basolateral TAL membranes. XVI. MTAL and CTAL cells each contain the mRNAs encoding mmClC-Ka and mcClC-Ka

BACKGROUND

Our prior data indicate that two separate but homologous basolateral chloride (Cl-) channels, mmClC-Ka and mcClC-Ka, are the principal mediators of net Cl- absorption in mouse medullary thick ascending limb (MTAL) and cortical thick ascending limb (CTAL) cells, respectively. In the present studies, we evaluated the possibility that there might be translational or post-translational suppression of mmClC-Ka and mcClC-Ka activity in CTAL and MTAL cells, respectively.

METHODS

Polymerase chain reaction (PCR) fragments were prepared that were highly specific for either mmClC-Ka or mcClC-Ka, the cDNAs encoding mmClC-Ka and mcClC-Ka, respectively.

RESULTS

Using reverse transcription (RT)-PCR with these highly specific products, mRNAs specific for non-homologous channel sequences in either mmClC-Ka or mcClC-Ka were present in both MTAL and CTAL cells.

CONCLUSIONS

Both mouse MTAL and CTAL cells contain the mRNAs encoding mmClC-Ka and mcClC-Ka. There may be translational or post-translational suppression of mmClC-Ka activity in CTAL cells, and of mcClC-Ka activity in MTAL cells.

Chloride channels in the loop of Henle

Cl- transport in the loop of Henle is responsible for reclamation of 25-40% of the filtered NaCl load and for the formation of dilute urine. Our understanding of the physiologic and molecular mechanisms responsible for Cl- reabsorption in both the thin ascending limb and thick ascending limb of Henle's loop has increased greatly over the last decade. Plasma membrane Cl- channels are known to play an integral role in transcellular Cl- transport in both the thin and thick ascending limbs. This review focuses on the functional characteristics and molecular identities of these Cl- channels, as well as the role of these channels in the pathophysiology of disease.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Vasopressin and PGE(2) regulate activity of apical 70 pS K(+) channel in thick ascending limb of rat kidney

Vasopressin and prostaglandin E(2) (PGE(2)) are involved in regulating NaCl reabsorption in the thick ascending limb (TAL) of the rat kidney. In the present study, we used the patch-clamp technique to study the effects of vasopressin and PGE(2) on the apical 70 pS K(+) channel in the rat TAL. Addition of vasopressin increased the channel activity, defined as NP(o), from 1.11 to 1.52 (200 pM) and 1.80 (500 pM), respectively. The effect of vasopressin can be mimicked by either forskolin (1-5 microM) or 8-bromo-cAMP/dibutyryl-cAMP (8-Br-cAMP/DBcAMP) (200-500 microM). Moreover, the effects of cAMP and vasopressin were not additive and application of 10 microM H-89 abolished the effect of vasopressin. This suggests that the effect of vasopressin is mediated by a cAMP-dependent pathway. Applying 10 nM PGE(2) alone had no significant effect on the channel activity. However, PGE(2) (10 nM) abolished the stimulatory effect of vasopressin. The PGE(2)-induced inhibition of the vasopressin effect was the result of decreasing cAMP production because addition of 200 microM 8-Br-cAMP/DBcAMP reversed the PGE(2)-induced inhibition. In addition to antagonizing the vasopressin effect, high concentrations of PGE(2) reduced channel activity in the absence of vasopressin by 33% (500 nM) and 51% (1 microM), respectively. The inhibitory effect of high concentrations of PGE(2) was not the result of decreasing cAMP production because adding the membrane-permeant cAMP analog failed to restore the channel activity. In contrast, inhibiting protein kinase C (PKC) with calphostin C (100 nM) abolished the effect of 1 microM PGE(2). We conclude that PGE(2) inhibits apical K(+) channels by two mechanisms: 1) low concentrations of PGE(2) attenuate the vasopressin-induced stimulation mainly by reducing cAMP generation, and 2) high concentrations of PGE(2) inhibit the channel activity by a PKC-dependent pathway.

Cl- channels in basolateral TAL membranes: XIII. Heterogeneity between basolateral MTAL and CTAL Cl- channels

BACKGROUND

Antidiuretic hormone (ADH) or adenosine 3', 5'-cyclic phosphate (cAMP) analogues augment net NaCl absorption in microperfused mouse medullary thick ascending limb (MTAL) segments but not in cortical thick ascending limb (CTAL) segments. This ADH-dependent MTAL effect is due to increased apical Na+/K+/2Cl- admittance and apical K+ recycling accompanied by a rise in calculated intracellular Cl- concentrations and by a threefold rise in basolateral Cl- conductance. rbClC-Ka, a 75.2 member of the ClC family of Cl- channels, mediates net Cl- absorption in the MTAL. The gating characteristics of rbClC-Ka channels from their intracellular surfaces are, to our knowledge, unique among Cl- channels. The channels are activated by small increases in intracellular Cl- (K1/2 = 10 mM Cl-). Adenosine triphosphate plus the catalytic subunit of protein kinase A (ATP + PKA) gate rbClC-Ka when cytosolic Cl- concentrations are 25 mM. Thus, in mouse MTAL segments, ADH-dependent rises in cytosolic Cl- are primarily responsible for basolateral Cl- conductance increases.

METHODS

These experiments compared the properties of Cl- channels fused into bilayers from basolaterally enriched vesicles from cultured mouse CTAL cells with rbClC-Ka channels.

RESULTS

The key findings were that anti-rbClC-Ka, antibody that recognizes and blocks rbClC-Ka, recognized and blocked basolateral Cl- channels in CTAL cells, that the extracellular faces of the CTAL channels were, like rbClC-Ka, substrate gated with a K1/2 of approximately 170 mM Cl-, and that, unlike rbClC-Ka channels, cytosolic faces of basolateral CTAL Cl- channels were not gated by either increasing cytosolic Cl- concentrations or cytosolic (ATP + PKA). This failure of activation of basolateral CTAL Cl- channels was confirmed using excised patch clamp studies. Finally, on Western blots, anti-rbClC-Ka recognized a 74 kDa band on basolateral CTAL vesicles.

CONCLUSIONS

Basolateral CTAL Cl- channels probably share a high degree of structural homology and possibly molecular mass with rbClC-Ka channels. However, significant differences between rbClC-Ka channels and CTAL Cl- channels account for the inability of increasing either cytosolic Cl- or (PKA + ATP) to raise Po in CTAL basolateral Cl- channels.

Vasopressin increases Na-K-2Cl cotransporter expression in thick ascending limb of Henle's loop

To investigate whether the enhancement of thick ascending limb (TAL) NaCl transport in response to long-term increases in circulating vasopressin concentration is associated with increased expression levels of the apical Na-K-2Cl cotransporter in the rat TAL, we have carried out immunoblotting and immunofluorescence studies using affinity-purified, peptide-directed antibodies. Semiquantitative immunoblotting studies demonstrated a marked increase (193% of controls) in Na-K-2Cl cotransporter band density in response to restriction of water intake to 15 ml/day for 7 days. In contrast, the expression levels of two other apical proteins of the TAL (the type 3 Na/H exchanger and Tamm-Horsfall protein) were unchanged in the outer medulla. A 7-day subcutaneous infusion of the V2 receptor-selective vasopressin analog, 1-desamino-[8-D-arginine]vasopressin (DDAVP), to Brattleboro rats also markedly increased Na-K-2Cl cotransporter expression in the outer medulla (183% of controls). Immunofluorescence localization in outer medullary tissue sections confirmed the increase in Na-K-2Cl cotransporter expression in response to DDAVP. We conclude that vasopressin strongly upregulates the expression of the Na-K-2Cl cotransporter of the TAL and that it is likely to play an important role in the long-term regulation of the countercurrent multiplication system.

The A kinase anchoring protein is required for mediating the effect of protein kinase A on ROMK1 channels

In the present study, we have used the two-electrode voltage-clamp and patch-clamp techniques to study the effects of forskolin and cAMP on the ROMK1 channels, which are believed to be the native K+ secretory channels in the kidney. Addition of 1 microM forskolin or 100 microM 8-bromo-cAMP, within 10 min, has no significant effect on the current of ROMK1 channels expressed in Xenopus oocytes. In contrast, application of 1 microM forskolin, within 3 min, significantly increased whole-cell K+ current by 35%, when ROMK1 channels were coexpressed with the A kinase anchoring protein AKAP79, which was cloned from neuronal tissue. Two lines of evidence indicate that the effect of forskolin is mediated by a cAMP-dependent pathway: (i) Addition of 100 microM 8-bromo-cAMP mimics the effect of forskolin and (ii) the effect of forskolin and cAMP is not additive. That AKAP is required for the effect of cAMP is further supported by experiments in which addition of ATP (100 microM) and cAMP (100 microM) restored the activity of run-down ROMK1 channels in inside-out patches in oocytes that coexpressed ROMK1 and AKAP79 but not in those that expressed ROMK1 alone. Moreover, when we used RII, the regulatory subunit of type II protein kinase A, in an overlay assay, we identified a RII-binding protein in membranes obtained from the kidney cortex but not in membranes from oocytes. This suggests that the insensitivity of ROMK1 channels to forskolin and cAMP is due to the absence of AKAPs. We conclude that AKAP may be a critical component that mediates the effect of protein kinase A on the ROMK channels in the kidney.

Cl- channels in basolateral renal medullary membranes. XII. Anti-rbClC-Ka antibody blocks MTAL Cl- channels

Cl- channels in the medullary thick ascending limb (MTAL) studied by either patch-clamp technique or reconstitution into lipid bilayers are activated by increases in intracellular Cl- concentrations. rbClC-Ka, a ClC Cl- channel, may represent this channel. We therefore evaluated the role of rbClC-Ka in transcellular MTAL Cl- transport in two separate ways. First, an antibody was raised against a fusion protein containing a 153-amino acid fragment of rbClC-Ka. Immunostaining of rabbit kidney sections with the antibody was localized to basolateral regions of MTAL and cortical thick ascending limb (CTAL) segments and also to the cytoplasm of intercalated cells in the cortical collecting duct. Second, Cl- uptake and efflux were measured in suspensions of mouse MTAL segments. Cl- uptake was bumetanide sensitive and was stimulated by treatment with a combination of vasopressin + forskolin + dibutyryl adenosine 3',5-cyclic monophosphate (DBcAMP). Cl- efflux was also increased significantly by vasopressin + forskolin + DBcAMP from 114 +/- 20 to 196 +/- 36 nmol.mg protein-1.45 s-1 (P = 0.003). Cl- efflux was inhibited by the Cl- channel blocker diphenylamine-2-carboxylate (154 +/- 26 vs. 70 +/- 21 nmol.mg protein-1.45 s-1, P = 0.003). An anti-rbClC-Ka antibody, which inhibits the activity of MTAL Cl- channels in lipid bilayers, reduced Cl- efflux from intact MTAL segments (154 +/- 28 vs. 53 +/- 14 nmol.mg protein-1.45 s-1, P = 0.02). These results support the view that rbClC-Ka is the basolateral membrane Cl- channel that mediates vasopressin-stimulated net Cl- transport in the MTAL segment.

Renal K+ channels: structure and function

The activity of potassium (K+) channels is intimately linked to several important transport functions in renal tubules. We review recent progress concerning the properties, site along the nephron, and physiological regulation of native K+ channels, and compare their characteristics with those of recently cloned K+ channels. We do not fully cover work on K+ channels in amphibian tubules, cell cultures, and single tubule cells and do not review K+ channels in mesangial cells.

Na(+)-K+(NH4+)-2Cl- cotransport in medullary thick ascending limb: control by PKA, PKC, and 20-HETE

Cell pH was monitored in suspensions of medullary thick ascending limbs (MTALs) of rat kidney to determine possible effects of various transduction pathways on apical Na(+)-K+ (NH4+)-2Cl- cotransport, the activity of which was measured as the bumetanide-sensitive component of cell acidification caused by abrupt exposure to 4 mM NH4Cl. 8-Bromoadenosine 3',5'-cyclic monophosphate stimulated cotransport activity through activation of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA), since the cAMP effect was abolished by N-[2-(p- bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89); stimulation by cAMP (P < 0.02) was observed even when other Na+, Cl-, and K+ carriers were blocked by ouabain, diphenylamine-2-carboxylate, and barium, which indicates that cotransport was directly affected by PKA. Phorbol 12,13-dibutyrate also stimulated cotransport activity (P < 0.03), which was abolished by protein kinase C (PKC) blockade by staurosporine. In contrast, cotransport activity was reduced (P < 0.001) by arachidonic acid or 20-hydroxyeicosatetraenoic acid (20-HETE), as well as by an ionomycin-induced rise in cytosolic Ca2+ ([Ca2+]i). Inhibition by arachidonic acid or ionomycin was abolished by econazole and SKF-525A that inhibit cytochrome P-450-dependent monoxygenase, which produces 20-HETE from arachidonic acid in the MTAL, and the ionomycin effect was prevented when phospholipase A2 (PLA2) was blocked by 4-bromophenacyl bromide or oleyloxyethyl phosphorylcholine. The results demonstrate that MTAL apical Na(+)-K+(NH4+)-2Cl- cotransport is stimulated by PKA and PKC and inhibited by 20-HETE that may be produced after a rise in [Ca2+]i through PLA2 activation.

View of K+ secretion through the apical K channel of cortical collecting duct

The apical small-conductance K+ channel plays an important role in renal K+ secretion, as evidenced by the presence of the extensive modulatory pathways. Figure 3 summarizes the current understanding of the mechanisms that modulate the apical small-conductance K+ channel. Stimulation of adenylate cyclase enhances channel activity and consequently K+ secretion. In contrast, increases in intracellular Ca2+ concentration and activation of Ca(2+)-dependent signal transduction pathways inhibit the K+ channel and thus decrease K+ secretion. The vasopressin-induced stimulation of K+ secretion in CCD results at least in part from cAMP-dependent signal transduction pathways. The Ca(2+)-dependent signal transduction pathway is responsible for modulatory coupling between Na+ pump turnover and apical K+ conductance when the Na+ pump is inhibited.

Purification and characterization of epithelial Ca(2+)-activated K+ channels

Reabsorption of NaCl in the thick ascending limb of Henle's loop in the kidney and in the surface cells in the distal colon involves the integrated function of several membrane transport systems including ion channels, the Na,K,Cl-cotransport system and the Na,K-pump. To determine if their properties are consistent with a role in regulation of transepithelial transport, Ca(2+)-activated K+ channels from the luminal membrane of the TAL cells and from the basolateral membrane of the distal colon cells have been characterized by flux studies in plasma membrane vesicle preparations and by single channel measurements in lipid bilayers. The channels are found to be activated by Ca2+ in the physiological range of concentration with a strong dependence on intracellular pH and the membrane potential. The Ca(2+)-sensitivity of the K+ channels is modulated by phosphorylation and dephosphorylation and the K+ channel protein must be in a phosphorylated state to respond to intracellular concentrations of Ca2+. As a step towards purification of the K+ channel proteins, procedures for solubilization and reconstitution of the K+ channels have been developed. The observation that the epithelial Ca(2+)-activated K+ channels bind calmodulin in the presence of Ca2+ have allowed for partial purification of the K+ channel proteins by calmodulin affinity chromatography. In the sequences for the two cloned Ca(2+)-activated K+ channels, the mSlo channel and the slowpoke channel, putative calmodulin binding regions can be identified.

An ATP-regulated, inwardly rectifying potassium channel from rat kidney (ROMK)

With the cloning of ROMK [31] and IRK1 [32], a new family of inwardly rectifying K+ channels has been identified. ROMK channel isoforms are highly and differentially expressed in distal nephron segments of the mammalian kidney. These channels exhibit many of the characteristics of the low conductance, ATP-sensitive K+ channels found in apical membranes of TAL, macula densa, and principal cells that are involved in potassium secretion. Thus ROMK channel isoforms appear to be involved in the formation of these secretory KATP channels. Further characterization of these channels should provide further evidence for their role in the secretory KATP channels and new insights into the function and regulation of these channels.

Inhibition by fatty acids of cyclic AMP-dependent protein kinase activity in brush border membranes isolated from human placental vesicles

1. The inhibitory effects of arachidonic acid (AA) and a number of structurally related fatty acids on cyclic AMP-dependent protein kinase activity have been investigated in brush border membranes (BBM) prepared from human placental vesicles. 2. BBM vesicles were characterized by electron microscopy and displayed enrichment of the appropriate marker enzymes, alkaline phosphatase and gamma-glutamyltranspeptidase; BBM were prepared by vesicles lysis in hypotonic medium. 3. Cyclic AMP-dependent protein kinase (PKA) activity was measured in BBM. At 1 microM, cyclic AMP stimulated a 4.2 +/- 0.06 fold increase over basal levels of [32P]-phosphate incorporation into the synthetic substrate kemptide and this effect was abolished by a selective PKA inhibitor. By use of synergistic pairs of site-selective cyclic AMP analogues, the kinase was identified as the type II enzyme. 4. Cyclic AMP-stimulated PKA activity was inhibited by 10 microM AA and this effect was significantly enhanced by nordihydroguaiaretic acid (NDGA) + indomethacin (Indo), inhibitors of the lipoxygenase and cyclo-oxygenase pathways of AA metabolism respectively. 5. Oleic acid, elaidic acid, but not caprylic or palmitic acids, also significantly inhibited PKA activity and this effect was again enhanced by NDGA + Indo. While arachidonyl alcohol alone was not inhibitory, in the presence of the metabolic inhibitors a significant reduction in stimulated activity was observed. 6. The commercially available PKA type II holoenzyme (activated by cyclic AMP), but not the free catalytic subunit, was inhibitable by AA, oleic or elaidic acids. 7. These results suggest that PKA localized to the brush border membrane of human placental vesicles is inhibited by fatty acids which may compete with cyclic AMP for binding to the kinase regulatory subunit. The reported inhibition by fatty acids of cyclic AMP-dependent Cl- secretion in epithelial cells may therefore be due in part to negative regulation of a Cl- channel-associated PKA.

Cl- channels in basolateral renal medullary vesicles: V. Comparison of basolateral mTALH Cl- channels with apical Cl- channels from jejunum and trachea

Cl- channels from basolaterally-enriched rabbit outer renal medullary membranes are activated either by increases in intracellular Cl- activity or by intracellular protein kinase A (PKA). Phosphorylation by PKA, however, is not obligatory for channel activity since channels can be activated by intracellular Cl- in the absence of PKA. The PKA requirement for activation of Cl- channels in certain secretory epithelia is, in contrast, obligatory. In the present studies, we examined the effects of PKA and intracellular Cl- concentrations on the properties of Cl- channels obtained either from basolaterally-enriched vesicles derived from highly purified suspensions of mouse medullary thick ascending limb (mTALH) segments, or from apical membrane vesicles obtained from two secretory epithelia, bovine trachea and rabbit small intestine. Our results indicate that the Cl- channels from mTALH suspensions were virtually identical to those previously described from rabbit outer renal medulla. In particular, an increase in intracellular (trans) Cl- concentration from 2 to 50 mM increased both channel activity (Po) and channel conductance (gCl, pS). Likewise, trans PKA increased mTALH Cl- channel activity by increasing the activity of individual channels when the trans solutions were 2 mM Cl. Under the latter circumstance, PKA did not activate quiescent channels, nor did it affect gCl. Moreover, when mTALH Cl- channels were inactivated by reducing cis Cl- concentrations to 50 mM, cis PKA addition did not affect Po. These results are consistent with the view that these Cl- channels originated from basolateral membranes of the mTALH. Cl- channels from apical vesicles from trachea and small intestine were completely insensitive to alterations in trans Cl- concentrations and demonstrated markedly different responses to PKA. In the absence of PKA, tracheal Cl- channels inactivated spontaneously after a mean time of 8 min; addition of PKA to trans solutions reactivated these channels. The intestinal Cl- channels did not inactivate with time. Trans PKA addition activated new channels with no effect on basal channel activity. Thus the regulation of Cl- channel activity by both intracellular Cl- and by PKA differ in basolateral mTALH Cl- channels compared to apical Cl- channels from either the tracheal or small intestine.

Cl- channels in basolateral renal medullary membrane vesicles: IV. Analogous channel activation by Cl- or cAMP-dependent protein kinase

We examined the interactions of cAMP-dependent protein kinase and varying aqueous Cl- concentrations in modulating the activity of Cl- channels obtained by fusing basolaterally enriched renal outer medullary vesicles into planar lipid bilayers. Under the present experimental conditions, the cis and trans solutions face the extracellular and intracellular aspects of these Cl- channels, respectively. Raising the trans Cl- concentration from 2 to 50 mM increased the channel open-time probability, raised the unit channel conductance, and affected the voltage-independent determinant (delta G) of channel activity but not the gating charge (Winters, C.J., Reeves, W.B., Andreoli, T.E. 1990. J. Membrane Biol. 118:269-278). With 2 mM trans KCl, trans addition of the catalytic subunit of PKA (C-PKA) plus ATP increased channel open-time probability and altered the voltage-independent determinant of channel activity without affecting either unit channel conductance or gating charge. The effect was ATP specific, did not occur with (C-PKA plus ATP) addition to cis solutions, and was abolished by denaturing C-PKA. Finally, (C-PKA plus ATP) activation of channel activity was not detected with relatively high (50 mM) trans Cl- concentrations. These data indicate that (C-PKA plus ATP) might modulate Cl- channel activity by phosphorylation at or near the Cl(-)-sensitive site on the intracellular face of these channels.

cAMP-dependent protein kinase inhibits the chloride conductance in apical membrane vesicles of human placenta

The role of adenosine 3',5'-monophosphate (cAMP) dependent protein kinase (PK-A) on the Cl- conductance has been studied in the apical membrane vesicles purified from the chorionic villi of human placenta. In order to phosphorylate the cytosolic side of the membranes, vesicles have been hypotonically lysed, loaded with 100 nM catalytic subunit of PK-A purified from human placenta and 1 mM of the phosphatase resistant adenosine 5'-thiotriphosphate (ATP-gamma-S) and resealed. Cl- conductance has been measured by the quenching of the fluorescent probe 6-methoxy-N-(3-sulfopropyl) quinolinium (SPQ) at 23 degrees C with membrane potential clamped at 0 mV. The actual volume of the resealed vesicles was measured in each experiment by trapping an impermeable radioactive molecule ([14C]-sucrose) and included in each Cl- flux calculation. In 19 independent experiments, the mean Cl- conductance in placental membranes in the absence of phosphorylation was 3.67 +/- 3.18 whereas with the addition of PK-A and ATP-gamma-S it was 1.97 +/- 1.75 nmol.sec-1. (mg protein)-1 (mean +/- SD). PK-A dependent phosphorylation reduced the Cl- conductance in 14/19 experiments. The same protocol applied to the apical membranes of bovine trachea, where PK-A is known to activate the Cl- channels, confirmed that the PK-A dependent phosphorylation increased in Cl- conductance in 11/13 experiments, from 1.01 +/- 0.61 to 1.85 +/- 0.99 nmol.sec-1.(mg protein)-1 (mean +/- SD). These studies indicate that the PK-A dependent phosphorylation inhibits one or more Cl- channel(s) of the apical membranes of human placenta.

Cl- channels in basolateral renal medullary membranes: III. Determinants of single-channel activity

We evaluated the effects of varying aqueous Cl- concentrations, and of the arginyl- and lysyl-specific reagent phenylglyoxal (PGO), on the properties of Cl- channels fused from basolaterally enriched renal medullary vesicles into planar lipid bilayers. The major channel properties studied were the anion selectivity sequence, anionic requirements for channel activity, and the effects of varying Cl- concentrations and/or PGO on the relation between holding voltage (VH, mV) and open-time probability (Po). Reducing cis Cl- concentrations, in the range 50-320 mM, produced a linear reduction in fractional open time (Po) with a half-maximal reduction in Po at cis Cl- approximately 170 mM. Channel activity was sustained by equimolar replacement of cis Cl- with F-, but not with impermeant isethionate. For trans solutions, the relation between Cl- concentration and Po was negatively cooperative, with 50% reduction in po at 10 mM Cl-. Reducing cis Cl- had no effect on the gating charge (Z) for channel opening, but altered significantly the voltage-independent energy (delta G) for channel opening. Phenylglyoxal (PGO) reduced Z and altered delta G for Cl- channel activity when added to cis, but not trans solutions. Furthermore, in the presence of cis PGO, reducing the cis Cl- concentration had no effect on Z but altered delta G. Thus we propose that cis PGO and cis Cl- concentrations affect separate sites determining channel activity at the extracellular faces of these Cl- channels.

The effects of bumetanide, amiloride and Ba2+ on fluid and electrolyte secretion in rabbit salivary gland

1. In order to distinguish between models of anion secretion, the effects of transport inhibitors on saliva flow rate and electrolyte composition were studied during the plateau phase of secretion in rabbit mandibular salivary glands. 2. Bumetanide, an inhibitor of Na+,K+,2Cl- co-transport, inhibited flow rate (by 60%) and reduced Cl- concentration. K+ and HCO3- concentrations were increased. Forskolin, an adenylate cyclase activator which inhibits ductal transport, did not significantly affect this pattern of changes. 3. Amiloride, used at concentrations that would inhibit Na(+)-H+ exchange, inhibited flow rate (by 30%). Cl- concentration was initially increased before subsequently decreasing at the same time as HCO3- concentration increased. These concentration changes can probably be attributed to ductal transport. When amiloride was applied to glands perfused with nominally HCO3- -free solutions, inhibition of flow rate was rapid and almost complete. 4. When amiloride and bumetanide were both present in the perfusate, flow rate was inhibited by 92%. The pattern of electrolyte changes was not significantly different from that observed in the presence of bumetanide alone. 5. Inhibition of K+ channel activity using Ba2+ also inhibited flow rate. Cl- concentration was increased as was K+ concentration. HCO3- concentration was not increased. 6. The anion exchange inhibitor DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) had no effect on either flow rate or electrolyte concentration. It did, however, elicit secretion in the absence of acetylcholine. 7. The data suggest that Na(+)-H+ and Cl- -HCO3- exchangers are unlikely to be involved in fluid and electrolyte secretion in these glands as suggested by some authors. Most of the data can be explained by postulating the existence of non-specific anion channels in the apical membranes of the acinar cells.

Cl- transport in basolateral renal medullary vesicles: I. Cl- transport in intact vesicles

This paper provides the results of studies which characterized conductive 36Cl- flux in basolaterally enriched membrane vesicles prepared from rabbit renal outer medulla. Conductive 36Cl- uptake was studied under two different experimental conditions. In the first, 36Cl- flux was driven by an inside positive voltage created with oppositely directed Cl- and gluconate gradients. In the second, an inwardly direct K+ gradient was used to drive 36Cl- uptake. By these two methods, voltage-sensitive 36Cl- uptake was shown to comprise about 45 and 65%, respectively, of the initial rates of total 36Cl- flux. Separate paired studies demonstrated that the conductive 36Cl- uptake was inhibited by the Cl- channel blocker diphenylamine-2-carboxylate (DPC) with an IC50 for DPC of 154 microM. The voltage-dependent 36Cl- uptake had an activation energy of 6.4 kcal/mole. This 36Cl- conductance had an anion selectivity sequence of I- greater than Cl- greater than or equal to NO3- much greater than gluconate.