Heterogenous vascular effects of AP5A in different rat resistance arteries are due to heterogenous distribution of P2X and P2Y(1) purinoceptors

In the accompanying article, we showed that AP5A displayed heterogenous vasoactive effects in rat resistance arteries. It induced a stable vasoconstriction in the superior epigastric artery (SEA) and a transient vasoconstriction in the mesenteric resistance artery (MrA). In the phenylephrine-precontracted MrA AP5A induced a marked vasorelaxation. In this study the noncompetitive inhibition of the AP5A-induced vasoconstriction with pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid was found to be significantly stronger in MrA than in SEA. The nonselective P2 purinoceptor antagonist suramin inhibited AP5A-induced vasoconstriction in MrA only. The vasoconstriction by the P2X purinoceptor agonist alpha,beta-methylene ATP was inhibited by with pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid and suramin similarly to that induced by AP5A. Thus, the AP5A-induced vasoconstriction is due to P2X receptor activation, but two different P2X receptors seem to be operational in the two different vessels. The AP5A-induced vasorelaxation of phenylephrine-precontracted MrA was inhibited by the P2Y(1) receptor antagonist ADP3'5'. The vasorelaxation induced by ADPbetaS (P2Y(1) agonist) also was inhibited by ADP3'5'. These findings suggest that AP5A-induced vasorelaxation of MrA is caused by P2Y(1) receptor activation. The P1 (A(2)) receptor antagonist 3, 7-dimethyl-1-propargylxanthine only slightly inhibited AP5A-induced vasorelaxation at high concentrations. Adenosine and the A(2) receptor agonist CGS21680 failed to produce significant vasorelaxation. Therefore, vasorelaxation in MrA does not involve A(2) purinoceptor activation. AP5A-induced vasorelaxation was not inhibited by Ca(2+)- or ATP-dependent K(+) channel blockade with clotrimazole, apamin, or glibenclamide. These data indicate that vasoconstriction in MrA and SEA by AP5A is due to different P2X receptors, and vasorelaxation in precontracted MrA is due to P2Y(1) receptor activation.

Influence of purinoceptor antagonism on diadenosine pentaphosphate-induced hypotension in anesthetized rats

Diadenosine polyphosphates (ApnA; n = 3-6) are potent vasoactive agents in isolated vessels. Information on effects of ApnA in vivo is still limited despite the fact that these compounds are starting to be used in humans. This study was designed to compare the effects of ApnA and their possible metabolites on blood pressure in vivo and to functionally identify purinoceptors involved in their action. All four ApnA and their degradation products induced a sustained drop of mean arterial blood pressure during i.v. infusion, which was fully reversible. The rank order of potency was Ap4A > or = Ap6A > Ap5A = Ap3A = ATP = ADP > AMP > or = adenosine, suggesting that the hypotensive effect is predominantly evoked by the original dinucleotides and not by their degradation products. The hypotensive effect of Ap5A was reduced by the P2X and P2Y(1) purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid, the A(1) purinoceptor antagonist 8-cyclopentyl-1,3-dipropylxanthine, and the A(2) purinoceptor antagonist 3, 7-dimethyl-1-propargylxanthine. The hypertensive effect by the prototype P2X receptor agonist alphabeta-methylene ATP was inhibited by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid, too. Purinoceptor antagonists reduced the maximal effects of the agonists indicating a noncompetitive inhibition. In summary, the reported vasocontractile effect of ApnA seems to be limited to isolated preparations under resting tone conditions; however, the systemic cardiovascular effects of all four ApnA are hypotensive, also making them candidates for blood pressure reduction in humans. These effects are fast in onset and easily reversible. Activation of different purinoceptors in the vasculature (most probably P2Y(1) and A(2) receptors) contributes to the Ap5A-induced decrease of mean arterial blood pressure.

Diadenosine polyphosphates cause contraction and relaxation in isolated rat resistance arteries

The effects of diadenosine polyphosphates (APnA; n = 3-6) and adenine nucleotides on contractile reactivity of isolated rat mesenteric resistance arteries (MrA) and superior epigastric arteries (SEA), which display a dense and sparse autonomic innervation, respectively, were evaluated. All agonists examined, except adenosine and AMP, induced contractions. The rank order of potency was similar in both arteries: alpha,beta-methylene ATP (alpha,beta-meATP) > AP5A > AP6A > AP4A > ATP > ADP > AP3A. Contractions were stable during several minutes in SEA but highly transient in MrA. They were reduced after exposure to 10 microM alpha,beta-meATP and by 10 microM of the P2X antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid. During phenylephrine (10 microM)-induced contractions, the agonists induced a further contraction in SEA. In MrA, however, further contraction was followed by marked relaxation. The rank order of relaxing potency was comparable to that of the contractile potency of agonists. Also, the relaxing effects of APnA were blunted by 10 microM pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid and after exposure to alpha,beta-meATP. In vitro and in vivo sympathectomy with 6-hydroxydopamine and removal of the endothelium did not modify the effects of APnA in MrA. Thus, the contractile effects of APnA in resistance arteries 1) are due to a P2X purinoceptor-mediated stimulation of the smooth muscle; 2) depend on the length of the phosphate chain; and 3) are followed by endothelium-independent relaxing effects in MrA but not SEA, which may involve receptors that are similar to those mediating contraction. The regional heterogeneity of APnA effects cannot be attributed to a direct neurogenic influence.

Diadenosine polyphosphates and atrial natriuretic peptide are antiproliferative in rat mesangial cells

Modulation of cell proliferation by vasoactive hormones and growth factors involves also changes in the activity of pH-regulatory transporters. In a preceeding paper (1) we examined the influence of such factors on cellular pH Here the influence of the same factors, diadenosine polyphosphates (ApnA), atrial natriuretic peptide, the growth factor PDGF and the Ca2+-ATPase inhibitor thapsigargin, on proliferation of cultured rat mesangial cells was examined by quantification of [3H]-thymidine incorporation. Mesangial cells were synchronised and growth reduced (0.5% FCS for 24 h) before experiments were started, Incubation with Ap3A, Ap4A, Ap5A or Ap6A (all 10 microM) for 24 h all reduced cell proliferation by 30 to 45%. At 0.1 and 1 microM the effects of Ap4A, Ap5A and Ap6A did not reach significance The antimitogenic effect of Ap5A was not significantly different when cells were incubated for 24, 48 or 72 h. In addition there was no significant difference between the antiproliferative effect of Ap5A in cells of the second, sixth or thirteenth passage. The growth factor PDGF-BB (0.25 nM) resulted man approximately 3-fold increase in [3H]-thymidine incorporation. This increase in proliferation could be significantly reduced by coincubation with 10 microM Ap5A. The mitogenic effect of PDGF was completely abolished in the presence of the Ca2-ATPase inhibitor thapsigargin (1 nM), which also significantly reduced basal cell proliferation by approximately 40%. Incubation of mesangial cells with 10 nM ANP for 24 h reduced basal [3H]-thymidine incorporation slightly by approximately 20% and decreased the PDGF-induced stimulation. The antimitogenic effects of these agonists is especially pronounced when cells are stimulated.

The affinity of the organic cation transporter rOCT1 is increased by protein kinase C-dependent phosphorylation

Members of the organic cation transporter (OCT) family are mainly expressed in kidney, liver, intestine, and brain. The regulation of the OCT type 1 from rat (rOCT1) stably transfected in HEK293 cells was examined using a fluorimetric technique, 1-[(3)H]methyl-4-phenylpyridinium uptake studies, and fast-whole-cell patch-clamp recordings. For the fluorescence measurements, the cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP(+)) was used as substrate. Uptake of ASP(+) via rOCT1 was electrogenic, and its inhibition by other organic cations was consistent with previously reported radioactive tracer flux measurements. The inhibitor quinine was not translocated by the organic cation transporter in contrast to tetraethylammonium. Stimulation of diacyl glycerol-dependent protein kinase C (PKC) by sn-1,2-dioctanoyl glycerol (1 microM) resulted in an increase in initial ASP(+) uptake rate by 216 +/- 28% (n = 29). The effect was completely antagonized by the PKC inhibitor tamoxifen (20 microM, n = 22). Forskolin (1 microM), which activates adenylate cyclase and thereby protein kinase A (PKA), stimulated the initial rate of ASP(+) accumulation by 51 +/- 6% (n = 19). This effect was inhibited by the specific PKA inhibitor KT5720 (1 microM, n = 12). Inhibition of tyrosine kinases by aminogenestein (10 microM) reduced ASP(+) uptake by 63 +/- 7% (n = 7), while genestein or tyrphostin AG1295 (each 10 microM) were without significant effects. Incubation of the cells with sn-1, 2-dioctanoyl glycerol (1 microM) increased the affinities of the transporter to tetraethylammonium, tetrapenthylammonium, and quinine by a factor of 58, 14.5, and 2.4, respectively. Western blot analysis revealed that rOCT1 protein was phosphorylated at a serine residue upon stimulation of PKC. In conclusion, it has been demonstrated that the organic cation transport by rOCT1 is stimulated by PKC, PKA, and endogenous tyrosine kinase activation. The PKC phosphorylates rOCT1 and leads to a conformational change at the substrate binding site.

In vivo effects of diadenosine polyphosphates on rat renal microcirculation

BACKGROUND

Diadenosine polyphosphates (APXA) are vasoactive nucleotides that elicit effects via purinoceptors. Recent data suggest differential effects of APXA on kidney vasculature.

METHODS

The in vivo effects of AP3A, AP5A, and adenosine on renal microvessels and the role of purinoceptors were investigated by the application of agonists to the hydronephrotic rat kidney and preincubation with respective antagonists.

RESULTS

The addition of the agonists (10-7 mol/L up to 10-4 mol/L) resulted in a concentration-dependent transient vasoconstriction [interlobular artery (ILOB): adenosine 30 +/- 7%, N = 7, AP3A 35 +/- 10%, N = 5; AP5A 66 +/- 19%, N = 5; 10-5 mol/L each] lasting up to one minute, followed by a concentration-dependent vasodilation (ILOB: adenosine 10 +/- 3%, N = 6; AP3A 19 +/- 4%, N = 5; AP5A 12 +/- 5%, N = 6; 10-5 mol/L each). In ILOB and in the afferent arteriole (AFF), the constrictory effects of AP5A were more pronounced than those of AP3A and adenosine. In the efferent arteriole (EFF), vascular tone was only slightly affected by all agonists. The dilatory potency was comparable for all agonists in ILOB and EFF. No significant vasodilation occurred in AFF. The application of the selective A1 receptor antagonist DPCPX (10-5 mol/L) completely abolished the adenosine-induced vasoconstriction, whereas the A2 receptor antagonist DMPX and the P2 purinoceptor antagonists PPADS and A3P5P (all 10-5 mol/L) did not affect adenosine-induced constriction. The AP3A-induced constriction was abolished by DPCPX and was partially inhibited by PPADS. The constriction induced by AP5A was less sensitive to DPCPX but more sensitive to PPADS. In ILOB and EFF, DMPX or A3P5P abolished dilation after the addition of the agonists. The dilation after AP5A was not significantly reduced. In AFF, no significant dilation was observed with these agonists alone, but it was clearly visible in the presence of DPCPX or PPADS.

CONCLUSIONS

APXA evoke transient constrictions in vessels of the hydronephrotic rat kidney, which are mediated by A1 and P2 purinoceptors. The length of the phosphate chain determines the degree of vasoconstriction and the extent to which the substances exert effects on the P2 purinoceptor subtypes. ILOB and AFF are more potently affected by APXA than EFF. Afferent vasodilation is partially overridden by sustained vasoconstriction.

Natriuretic peptides and diadenosine polyphosphates modulate pH regulation of rat mesangial cells

Modulation of cell proliferation has often been thought to be connected to changes in the activity of pH-regulatory transporters and consequently intracellular pH (pH(i)). The influence of natriuretic peptides, diadenosine polyphosphates, adenosine and ATP as well as platelet-derived growth factor (PDGF) on pH(i) regulation of cultured rat mesangial cells was examined with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. The inhibitors of Na(+)/H(+) exchange, amiloride and HOE694, blocked pH(i) recovery completely in the absence of and by approximately 50% in the presence of HCO(3)(-)/CO(2). Natriuretic peptides (ANP, BNP, CNP, urodilatin) completely inhibited pH(i) recovery in the absence of and by approximately 40% in the presence of HCO(3)(-)/CO(2). These effects were abolished by the cGMP-dependent protein kinase inhibitor KT5823. Diadenosine polyphosphates (Ap3A-Ap6A), ATP and adenosine also inhibited pH(i) recovery completely in the absence of and partially (30-40%) in the presence of HCO(3)(-)/ CO(2). The effect of adenosine was abolished in the presence of the cAMP-dependent protein kinase inhibitor KT5720, and that of Ap5A by the protein kinase C inhibitor calphostin C. PDGF activated acid extrusion in these cells by approximately 40%. From the four cloned isoforms of the Na(+)/H(+) exchanger in the rat, only transcripts of NHE-1 were found in these mesangial cell cultures using RT-PCR analysis. These data suggest that in these rat mesangial cells the Na(+)/H(+) exchanger, specifically the NHE-1 isoform, accounts for around 50% of pH(i) recovery from an acid load under physiological conditions, and that Na(+)/H(+) exchange stimulated by acidification can be inhibited by activation of PKG, PKA, and PKC and stimulated by PDGF after acute exposition to these agonists.

A novel cGMP-regulated K+ channel in immortalized human kidney epitheliall cells (IHKE-1)

1. K+ channels from the apical membrane of immortalized human kidney epithelial (IHKE-1) cells were investigated in the cell-attached membrane configuration as well as in excised membranes using the patch clamp technique. 2. In cell-attached membrane patches the open probability (Po) of the K+ channel was 0.42 +/- 0.06 (mean +/- s.e.m. , n = 22) and its conductance was 94 +/- 5 pS with 145 mM K+ in the pipette (n = 25). In excised membrane patches the Po of the channel was 0.55 +/- 0.03 (n = 86) and its conductance was 65 +/- 2 pS (n = 68) with 145 mM K+ on one side of the membrane and 3.6 mM K+ on the other. The I-V curve of the K+ channel was not rectifying. 3. The channel was inhibited by several blockers of K+ channels such as 1 mM Ba2+ (cell-attached membrane: 78 +/- 8 %, n = 9; excised: 80 +/- 4 %, n = 26), 10 mM TEA+ (excised inside-out: 48 +/- 5 %, n = 34; excised outside-out: 100 +/- 0 %, n = 26), 0.1 mM verapamil (excised: 73 +/- 9 %, n = 12), and 10 nM charybdotoxin (excised outside-out: 67 +/- 9 %, n = 9). 4. The K+ channel was activated by depolarization and rising cytosolic Ca2+. Half-maximal activity occurred at a cytosolic Ca2+ concentration of 200 nM. In the cell-attached membrane configuration the K+ channel was inhibited in a concentration-dependent manner by atrial natriuretic peptide (ANP). Powas blocked equally well by 10 nM ANP (52 +/- 7 %, n = 10), brain natriuretic peptide (BNP; 37 +/- 11 %, n = 6) and C-type natriuretic peptide (CNP; 44 +/- 13 %, n = 8). 8-Bromoguanosine 3',5' cyclic monophosphate (8-Br-cGMP, 0.1 mM) also inhibited Poof this K+ channel, by 70 +/- 10 % (n = 5). 5. In excised membrane patches cGMP inhibited Po of this K+ channel in a concentration-dependent manner. The first significant effects were measured at a concentration of 1 microM (22 +/- 7 %, n = 6), and greatest effects were obtained at 0.1 mM (34 +/- 5 %, n = 15). cAMP (0.1 mM, n = 5) as well as GTP (0.1 mM, n = 5) had no significant effects on Po of this K+ channel. ATP (0.1 mM) had a weak inhibitory effect (17 +/- 5 %, n = 14). Addition of Mg-ATP to cGMP did not increase the inhibitory effect (30 +/- 4 %, n = 14). KT5823 (1 microM), a specific inhibitor of cGMP-dependent protein kinases, did not significantly alter the cGMP-induced reduction in Po of the K+ channel in three excised membrane patches. 6. The results present the first electrophysiological characterization of a mammalian K+ channel that is directly regulated by cGMP.

cGMP-dependent and -independent inhibition of a K+ conductance by natriuretic peptides: molecular and functional studies in human proximal tubule cells

In immortalized human kidney epithelial (IHKE-1) cells derived from proximal tubules, two natriuretic peptide receptors (NPR) were identified. In addition to NPR-A, which is bound by atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and urodilatin (URO), a novel form of NPR-B that might be bound by C-type natriuretic peptide (CNP) was identified using PCR. This novel splice variant of NPR-B (NPR-Bi) was also found in human kidney. Whereas ANP, BNP, and URO increased intracellular cGMP levels in IHKE-1 cells in a concentration-dependent manner, CNP had no effect on cGMP levels. To determine the physiologic responses to these agonists in IHKE-1 cells, the membrane voltage (Vm) was monitored using the slow whole-cell patch-clamp technique. ANP (10 nM), BNP (10 nM), and URO (16 nM) depolarized these cells by 3 to 4 mV (n = 47, 7, and 16, respectively), an effect that could be mimicked by 0.1 mM 8-Br-cGMP (n = 15). The effects of ANP and 8-Br-cGMP were not additive (n = 4). CNP (10 nM) also depolarized these cells, by 3+/-1 mV (n = 28), despite the absence of an increase in cellular cGMP levels, indicating a cGMP-independent mechanism. In the presence of CNP, 8-Br-cGMP further depolarized Vm significantly, by 1.6+/-0.3 mV (n = 5). The depolarizations by ANP were completely abolished in the presence of Ba2+ (1 mM, n = 4) and thus can be related to inhibition of a K+ conductance in the luminal membrane of IHKE-1 cells. The depolarizations attributable to CNP were completely blocked when genistein (10 microM, n = 6), an inhibitor of tyrosine kinases, was present. These findings indicate that natriuretic peptides regulate electrogenic transport processes via cGMP-dependent and -independent pathways that influence the Vm of IHKE-1 cells.

Regulation of organic cation transport in IHKE-1 and LLC-PK1 cells. Fluorometric studies with 4-(4-dimethylaminostyryl)-N-methylpyridinium

The regulation of transport of the fluorescent organic cation 4-(4-dimethylaminostyryl)-N-methylpyridinium (ASP+) by renal proximal tubular organic cation transport was studied in IHKE-1 and LLC-PK1 cells with a recently established fluorometric technique (Stachon et al., 1996, 1997). Stimulation of Ca++/diacylglycerol-dependent protein kinase by 1,2-dioctanoyl glycerol (DOG; 0.01-1 mumol/l, n = 7), ATP (0.1 mmol/l, n = 9), oxytocin (0.1 mumol/l, n = 6) and bradykinin (1 mumol/l, n = 7) resulted in an increase of ASP+ accumulation in IHKE-1 cells by 35 +/- 9% (DOG), 65 +/- 30% (ATP), 66 +/- 14% (bradykinin) and 70 +/- 20% (oxytocin) as compared with basal conditions, whereas ASP+ accumulation was slightly reduced in LLC-PK1 cells after stimulation with DOG (1 mumol/l, -20 +/- 7%, n = 10) and angiotensin II (0.1 nmol/l, -20 +/- 5%, n = 6). ASP+ accumulation in IHKE-1 cells also was increased by 0.5 mumol/l (20 +/- 8%, n = 8) and 1 mumol/l forskolin (35 +/- 13%, n = 19), and by 8-bromo-cAMP (1 mumol/l, 125 +/- 25%, n = 9), both activators of the cAMP-dependent protein kinase (PKA). Activation of the cGMP-dependent protein kinase (PKG) by human atrial natriuretic peptide (10 nmol/l, n = 10) or 8-bromo-cGMP (0.1 mmol/l, n = 12) resulted in an increase of 35 +/- 5% and 28 +/- 6%, respectively. Activation of PKA and PKG had no influence on ASP+ transport in LLC-PK1 cells. Regulation of ASP+ uptake by these two cell lines may be caused by direct phosphorylation of the organic cation transporters involved or by regulation of trafficking of the transporters to the membrane. Differences in the organic cation transporter isoforms or alternatively, in the trafficking may contribute to the distinct regulation of ASP+ transport in IHKE-1 and LLC-PK1 cells.

Guanylin stimulates regulated secretion from human neuroendocrine pancreatic cells

BACKGROUND & AIMS

Gastroenteropancreatic neuroendocrine cells secrete chemical messengers in a calcium-dependent fashion. So far, other second messenger systems involved in regulated secretion have gained little attention. The aim of this study was to characterize guanosine 3',5'-cyclic monophosphate (cGMP)-mediated vesicular secretion in pancreatic neuroendocrine cells.

METHODS

In a human pancreatic cell line, BON, cyclic nucleotide levels and chromogranin A release were monitored with specific immunoassays. Uptake and release of gamma-aminobutyric acid were measured. Intracellular Ca2+ concentration was monitored with fura-2. Guanylyl cyclase C was analyzed by reverse-transcription polymerase chain reaction.

RESULTS

Guanylin increased cGMP concentrations in BON cells via guanylyl cyclase C. Stimulation of the cGMP pathway by guanylin or Escherichia coli heat-stable enterotoxin increased the release of chromogranin A and gamma-aminobutyric acid from BON cells. This effect was mimicked by the cGMP analogue 8-bromo-cGMP.

CONCLUSIONS

Guanylin and STa stimulate the regulated secretion from BON cells via guanylyl cyclase C and cGMP. Our study yields novel information about secretory properties of guanylin, mediated via a signal transduction pathway, increasing cGMP and leading to regulated secretion of neuroendocrine cells.

Renal and blood pressure effects of moxonidine and clonidine in spontaneously hypertensive rats

Recently we could demonstrate that the imidazoline receptor agonist moxonidine exerts specific renal effects in Sprague Dawley rats [Hohage et al. 1997]. Interestingly, the effects of this compound are attenuated in one kidney-one clip hypertensive rats [Li et al. 1994]. In this study, we therefore investigated the effects of moxonidine as compared to clonidine in genetically determined spontaneously hypertensive rats. Moxonidine in a concentration of 0.5 mg/kg b.w.i.v. induced a significant and long-lasting increase of both urine flow from 11.9 +/- 2.1 microliters/min x 100 g b.w. to 50.3 +/- 12.5 microliters/min x 100 g b.w. and of Na(+)-excretion from 2.2 +/- 0.5 mumol/min x 100 g b.w. to 8.4 +/- 1.9 mumol/min x 100 g b.w. In contrast to moxonidine, the effects of clonidine (0.5 mg/kg b.w.i.v.) on urine flow and Na(+)-excretion were negligible. The antagonists idazoxan, effaroxan and rauwolscine abolished the effects of moxonidine on urine flow and Na(+)-excretion, whereas 4-aminopyridine, phenformine and 1,2,3,4-tetrahydro-9-aminoacridine, which have been described to interact with imidazoline binding sites, had no effect. Addition of the antagonists idazoxan, effaroxan and rauwolscine attenuated the initial blood pressure increase immediately after intravenous application, whereas 4-aminopyridine, phenformine and 1,2,3,4-tetrahydro-9-aminoacridine had no influence on this side-effect. Our results provide further evidence that imidazoline receptor agonists such as moxonidine exhibit renal effects, different from the modulation in urine flow and Na(+)-excretion following renal alpha 2 adrenoceptor stimulation. An upregulation of imidazoline receptors in hypertension may contribute to the effects observed.

A simplified method for isolation of large numbers of defined nephron segments

We describe a simplified method for the isolation of large numbers of nephron segments from rat and rabbit kidneys. In contrast to most previous protocols, the kidneys are not perfused. After removal from the animal, the kidney is sliced and torn in pieces that are subsequently digested in culture medium containing 0.5 mg/ml of collagenase at 37 degrees C. If the preparation is agitated only very gently and infrequently, then the tissue gradually falls apart into a suspension containing long nephron fragments, often consisting of multiple connected segments. These are easily sorted into homogeneous segment populations that can be used for enzyme assays, protein extraction for immunoblotting, and RNA extraction for reverse transcription-polymerase chain reaction, all of which have been done successfully in our laboratory. For comparison, we have also examined cortical collecting tubule segments and cells prepared by the more rigorous protocol described previously (E. Schlatter, U. Fröbe, and R. Greger. Pflügers Arch. 421: 381-387, 1992). Even after the isolation of single cells in a Ca2+-free medium, the cells maintain their normal architecture and a distinct separation of apical and basolateral membranes.

Receptors for bradykinin and prostaglandin E2 coupled to Ca2+ signalling in rat cortical collecting duct

In freshly isolated rat cortical collecting ducts (CCD) we measured intracellular Ca2+ activity ([Ca2+]j) with the Fura-2 method. Bradykinin (BK) induced a transient and biphasic increase in [Ca2+]j. This increase was concentration dependent and was half maximal at a concentration of 15 nM. The B2 receptor antagonist HOE 140 (100 nM, n = 6) completely abolished BK (100 nM) induced increase in [Ca2+]j. The B1 receptor agonist des-Arg9-bradykinin (100 nM, n = 4) had no effect on [Ca2+]j. In the absence of extracellular Ca2+, the maximal increase in [Ca2+]j, induced by BK was diminished and the secondary plateau phase was completely abolished. Prostaglandin E2 (PGE2) elevated [Ca2+]j, also concentration-dependently and biphasically. A half maximal effect was reached with 1 nM PGE2. The secondary plateau phase was absent when extracellular Ca2+ was removed. Sulprostone (100 nM, n = 6) mimicked the PGE2 (100 nM) induced increase in [Ca2+]j. The effect of BK (100 nM) on [Ca2+]j was not inhibited by the cyclooxygenase inhibitor indomethacin (10 microM, n = 5). Dopamine (1 microM, n = 4) did not significantly alter [Ca2+]j. BK and PGE2 regulate [Ca2+]j in the rat CCD via release of Ca2+ from intracellular Ca2+ stores as well as via Ca2+ influx from extracellular space. BK directly modulates [Ca2+]j, through B2 receptors. EP1 receptors are most likely to be responsible for the PGE2 induced increase in [Ca2+]j.

Moxonidine inhibits Na+/H+ exchange in proximal tubule cells and cortical collecting duct

The imidazoline receptor agonist moxonidine has been recently introduced as an antihypertensive therapy. Imidazoline specific binding sites have also been found in the kidney. Moxonidine induced natriuresis and diuresis in clearance studies in rats. Related substances such as various guanidinium derivatives have been shown to inhibit Na+/H+ exchange in several preparations. We therefore examined whether the renal effects of moxonidine could be mediated by an inhibition of the Na+/H+ exchanger. Intracellular pH (pHi) was measured microfluorimetrically with BCECF in proximal LLC-PK1 cells and in the principal cells of rat cortical collecting ducts (CCD). In LLC-PK1 cells moxonidine (10 mumol/liter) had no effect on the basal pH1; however, it reduced the Na+/H+ activity reversibly by 43 +/- 4% (N = 26) when the exchanger was activated by cellular acidification. In rat CCD cells moxonidine slightly decreased basal pHi by 0.08 +/- 0.03 pH units (N = 12). After acidification the recovery rate of pHi was reduced with moxonidine by 45 +/- 6% (N = 18). The effects of moxonidine could be mimicked in both cell types by inhibitors of the Na+/H+ exchanger (HOE 694, amiloride). In the presence of the imidazoline receptor antagonist idazoxan (10 mumol/liter) the effects of moxonidine were almost completely inhibited. The alpha 2-antagonist yohimbine (10 mumol/liter) did not significantly alter the effects of moxonidine in both cell types. These data suggest that in LLC-PK1 and in rat CCD cells, Na+/H+ is inhibited by moxonidine via an activation of the imidazoline receptor.

Ca(2+)-dependent K+ channels in the cortical collecting duct of rat

In the cortical collecting duct of the rat two Ca(2+)-dependent K+ channels have been described so far. In the luminal membrane a maxi K+ channel with a single channel conductance of 139 +/- 3 pS in excised membrane patches (n = 91) at 0 mV clamp voltage and asymmetrical KCl-concentrations in pipette and bath was found, while in the basolateral membrane an intermediate conductance K+ channel (85 +/- 1 pS, n = 53) and a small K+ channel (28 +/- 2 pS, n = 15) was described. All these K+ channels had similar pharmacological properties since all could be blocked by the K+ channel inhibitors Ba2+, TEA+, and charybdotoxin. Verapamil, known as a L-type Ca2+ channel blocker, was also capable of inhibiting these K+ channels. While the maxi K+ channel from the luminal membrane was upregulated by intracellular Ca2+ (EC50: 5 microM), the small and the intermediate K+ channel from the basolateral membrane were downregulated (IC50: 10 microM). When the cytosolic Ca(2+)-activity was in the physiological range below 1 microM the activity of the maxi K+ channel was low and regulated via intracellular pH and ATP. Furthermore, when CCD cells were strongly depolarized and under hypoosmotic stress, Ca2+ rose and activated this K+ channel, indicating that this channel is involved in volume regulation. Like the maxi K+ channel the intermediate conductance K+ channel from the basolateral membrane was also sensitive to intracellular changes of pH where acidic pH inhibited while alkaline pH activated this channel. But unlike the K+ channels from the luminal membrane the K+ channel from the basolateral membrane is not regulated by ATP up to 5 mM. The activity of the K+ channels from the basolateral membrane decreased steadily after excision of the membrane. This decrease could be prevented by applying cGMP and MgATP to the bath and thus, activating a membrane-bound cGMP-dependent protein kinase (PKG). The activation of the PKG could be reversed by its specific inhibitor KT5823 (1 microM). Due to the opposite regulation via intracellular Ca2+ and the involvement of different protein kinases a specific and independent regulation of K+ secretion and Na+ reabsorption is possible in the CCD of the rat.

Effects of moxonidine and clonidine on renal function and blood pressure in anesthetized rats

Using classical clearance techniques, the renal effects of moxonidine and clonidine were studied in the rat. Moxonidine (0.25 and 0.5 mg/kg body weight i.v.) increased transiently fractional fluid and Na+ excretion. Similarly, clonidine in the same i.v. doses caused a sustained increase in fractional fluid excretion, but only a short lasting increase in fractional Na+ excretion that was similar to the effect of moxonidine. Water diuresis experiments showed that the agonists mostly exert their actions within the proximal tubule. Antagonists such as idazoxan and yohimbine did not change fractional fluid excretion and Na+ excretion by their own. Both, the nonselective imidazoline/alpha 2-adrenoceptor antagonist idazoxan and the pure alpha 2-adrenoceptor antagonist yohimbine attenuated the moxonidine induced effects on fractional fluid excretion and Na+ excretion. The clonidine induced increase in fractional fluid and Na(+)-excretion was inhibited by yohimbine and desmopressin only. The effects on systemic blood pressure after moxonidine or clonidine application were similar. The initial blood pressure increases after moxonidine application, however, was less than with clonidine, whereas the hypotensive potency was more pronounced. Similar to the effects on kidney function, idazoxan and yohimbine had no influence on systemic blood pressure by themselves. Immediately after moxonidine application, a short lasting increase in renal plasma flow and glomerular filtration rate could be observed. Our results demonstrate, that moxonidine exerts renal effects, which are different from those of clonidine. At present time, renal effects mediated by imidazoline receptors and alpha 2-adrenoceptors cannot be clearly distinguished.

Functional evidence for the regulation of cytosolic Ca2+ activity via V1A-receptors and beta-adrenoceptors in rat CCD

In freshly isolated rat CCD segments, the effects of arginine vasopressin (AVP), oxytocin (OT), adrenaline (Ad), and their specific receptor agonists and antagonists on the intracellular Ca2+ activity ([Ca2+]i) were measured using the Ca2+ sensitive dye Fura-2 as fluorescence indicator. We observed that AVP, the V1-receptor agonist [Phe2Orn8] vasotocin ([Phe2]OVT), and OT increased [Ca2+]i biphasically. AVP (n = 9) and OT (n = 8) induced increases in [Ca2+]i were completely blocked by the V1A-receptor antagonist d(CH2)5Tyr(Me)2AVP. However, neither the V2-receptor agonist [Val4-D-Arg8]AVP (100 nM, n = 5), nor the OT-receptor agonist [Thr4,Gly7]OT (100 nM, n = 5) nor forskolin (1 microM, n = 4 and 10 microM, n = 5) did significantly change [Ca2+]i. Ad and the beta-adrenoceptor agonist isoproterenol (ISO) increased [Ca2+]i, which was not mimicked by the alpha 2-adrenoceptor agonist clonidine (1 microM, n = 10) or the alpha 1-adrenoceptor agonist phenylephrine (1 microM, n = 5). The beta-adrenoceptor antagonist propranolol (1 microM) completely blocked this Ad (1 microM, n = 4) induced [Ca2+]i increase. Insulin (INS 10 nM, n = 8), endothelin (ET 1 microM, n = 6), and angiotensin II (Ang II 1 pM to 10 nM; each n = 4) had no significant effect on [Ca2+]i. Considering the present results we propose a V1A-receptor and beta-adrenoceptor dependent modulation of [Ca2+]i in rat CCD.

Effects of sodium nitroprusside in the rat cortical collecting duct are independent of the NO pathway

Recently we described K+ channels in the basolateral membrane of principal cells of rat cortical collecting duct (CCD) which are regulated by a cGMP-dependent protein kinase (Pflugers Arch 429:338-344, 1995). We examined the effects of the NO-liberator sodium nitroprusside (SNP) on single channel activity and membrane voltage (Vm) in principal cells of rat CCD, and on transepithelial voltage, lumen-to-bath Na+ fluxes, and osmotic water permeability in isolated perfused rat CCD tubules. While in patch clamp experiments SNP (10 microM) hyperpolarized principal cells from -54 +/- 10 mV to -71 +/- 5 mV (N = 5) and increased the activity of the described K+ channels from 0.05 +/- 0.03 to 0.45 +/- 0.14 (N = 5) in cell-attached and from 0.04 +/- 0.02 to 0.25 +/- 0.05 (N = 4) in excised patch clamp experiments, it had no effect on basal or AVP-dependent transepithelial voltage, Na+ fluxes, or the osmotic water permeability. In addition, neither 50 microM SIN-1, another liberator of NO, nor 1 mM L-NAME, an inhibitor of the NO-synthase, changed Vm significantly. Furthermore, in cGMP-assays SNP failed to increase intracellular cGMP in CCD segments. Thus, we conclude that in the rat CCD transport is not regulated via the NO-pathway and that SNP acts as an cGMP independent activator of K+ channels in the basolateral membrane of these cells.

cGMP-activating peptides do not regulate electrogenic electrolyte transport in principal cells of rat CCD

K+ channels in the basolateral membrane of rat cortical collecting duct (CCD) are regulated by a cGMP-dependent protein kinase (J. Hirsch and E. Schlatter. Pfluegers Arch. 429: 338-344, 1995). Conflicting data exist on the effects of cGMP-activating agonists on Na+ transport in these cells. Thus we tested members of the family of peptides that increase intracellular cGMP [cardiodilatin/atrial natriuretic peptide (CDD/ANP), brain natriuretic peptide, C-type natriuretic peptide, urodilatin, guanylin, and uroguanylin], as well as bradykinin +/- CDD/ANP on membrane voltages (Vm) of principal cells of isolated rat CCD using the slow whole cell patch-clamp technique (E. Schlatter, U. Fröbe, and R. Greger. Pfluegers Arch. 421: 381-387, 1992). None of the agonists tested changed Vm significantly. There was also no effect of dibutyryl guanosine 3',5'-cyclic monophosphate (DBcGMP) on AVP-dependent lumen-to-bath Na+ flux, transepithelial voltage, or osmotic water permeability in isolated perfused rat CCD. Finally, CDD/ANP increased intracellular cGMP only in glomeruli but not in CCD. Thus the findings provide no evidence for control of electrogenic electrolyte transport by these natriuretic peptides in principal cells of rat CCD, and the agonist that physiologically regulates the cGMP-dependent K+ channels remains to be identified.

Effects of diadenosine polyphosphates on renal function and blood pressure in anesthetized Wistar rats

In this study, the effects of diadenosine polyphosphates on kidney function were examined. Intravenous application of diadenosine hexaphosphate (AP6A) led to a significant threefold increase in both urine flow (from 2.45 +/- 0.2 to 13.8 +/- 0.74 microL/min per 100 g body wt (P < 0.05)) and Na+ excretion (from 0.41 +/- 0.12 to 1.52 +/- 0.28 mumol/min per 100 g body wt at a dose of 1.0 mg/kg body wt). In contrast, diadenosine triphosphate dose-dependently reduced urine flow (from 3.74 +/- 0.3 to 2.57 +/- 0.1 microL/min per 100 g body wt (P < 0.05)) and Na+ excretion (from 0.45 +/- 0.1 to 0.13 +/- 0.1 mumol/min per 100 g body wt at a dose of 1.0 mg/kg body wt). ATP and the P2y purinoceptor agonist gamma-S-ATP did not significantly modulate urine flow and Na+ excretion. alpha, beta-methylene-ATP, a P2x purinoceptor agonist, significantly increased urine flow from 1.74 +/- 0.5 to 4.07 +/- 1.51 microL/min per 100 g body wt, whereas Na+ excretion was unaffected. The effects were independent of alterations in GFR. Pretreatment with indomethacin (2.0 mg/kg body wt iv) completely abolished the effects of AP6A on urine flow and Na+ excretion. Similarly, pretreatment with the endothelin antagonist bosentan abolished the effects of AP6A on both urine flow and Na+ excretion, whereas suramin had no effects on the AP6A-induced increase in urine flow. In conclusion, diadenosine polyphosphates exert specific actions on urine flow and Na+ excretion that are different from the effects of ATP. AP6A may partially influence renal function by stimulating prostaglandin and endothelin release.

Natriuretic peptides increase a K+ conductance in rat mesangial cells

Mesangial cells (MC) are a main target of natriuretic peptides in the kidney and are thought to play a role in regulating glomerular filtration rate. We examined the influence of cGMP-generating (i.e. guanosine 3',5'-cyclicmonophosphate) peptides on membrane voltages (Vm) of rat MC by using the fast whole-cell patch-clamp technique. The cGMP-generating peptides were tested at maximal concentrations ranging from 140 to 300 nmol/l. Whereas human CNP (C natriuretic peptide), rat guanylin and human uroguanylin had no significant effect on Vm these cells, human BNP (brain natriuretic peptide), rat CDD/ANP-99-126 (cardiodilatin/atrial natriuretic peptide) and rat CDD/ANP-95-126 (urodalatin) hyperpolarized Vm significantly by 1.6 +/- 0.4 mV (BNP, n=8), 3.7 +/- 0.3 mV (CDD/ANP-99-126, n=25) and 2.8 +/- 0.4 mV (urodilatin, n=9), respectively. The half-maximally effective concentration (EC50) for the latter two was around 400 pmol/l each. This hyperpolarization could be mimicked with 0.5 mmol/l 8-bromo-guanosine 3',5'-cyclic monophosphate (8-Br-cGMP) and was blocked by 5 mmol/l Ba2+. The K+ channel blocker 293 B (100 micromol/l) depolarized basal Vm by 4.3 +/- 0.4 mV (n=8), but failed to inhibit the hyperpolarization induced by CDD/ANP-99-126 (160 nmol/l) (n=8). The K+ channel opener cromakalim (10 micromol/l) neither influenced basal Vm nor altered the hyperpolarization induced by 160 nmol/l CDD/ANP-99-126 (n=8). Adenosine (100 micromol/l) hyperpolarized Vm by 13.4 +/- 1.3 mV (n=16). At 100 micromol/l, 293 B did not inhibit the adenosine-induced hyperpolarization (n=6). At 160 nmol/l, CDD/ANP-99-126 enhanced the adenosine-induced hyperpolarization significantly by 1.5 +/- 0.6 mV (n=10). CDD/ANP-99-126 (160 nmol/l) failed to modulate the value to which Vm depolarized in the presence of 1 nmol/l angiotensin II (n=10), but accelerated the repolarization to basal Vm by 49 +/- 20% (n=8). These results indicate that the natriuretic peptides CDD/ANP-99-126, CDD/ANP-95-126 and BNP hyperpolarize rat MC probably due to an increase of a K+ conductance. This effect modulates the voltage response induced by angiotensin II. The natriuretic-peptide-activated conductance can be blocked by Ba2+, but not by 293 B and cannot be activated by cromakalim. This increase in the K+ conductance seems to be additive to that inducable by adenosine, indicating that different K+ channels are activated by these hormones.

Correlation between intracellular activities of Ca2+ and Na+ in rat cortical collecting duct--A possible coupling mechanism between Na+-K+-ATPase and Basolateral K+ conductance

In principal cells of rat cortical collecting ducts (CCD) the K+ conductance of the basolateral membrane is functionally coupled to the Na(+)-K(+)-ATPase. Inhibition of the Na(+)-K(+)-ATPase by ouabain resulted in a decrease of this conductance. This inhibition was absent in the presence of amiloride. In the present study we attempted to measure the activities of intracellular Na+ ([Na+]i) and intracellular Ca2+ ([Ca2+]i) fluorimetrically, and discuss their role for the functional coupling of the activity of the Na(+)-K+ pump and the recently identified K+ channels in the basolateral membrane [Na+]i and [Ca2+]i were measured in isolated CCD segments using the Na(+)-sensitive dye sodium-binding benzofuran isophthalate (SBFi) and the Ca2+-sensitive dye fura-2 as fluorescence indicators. Basal [Na+]i and [Ca2+]i were 22 +/- 4 mM (n = 23) and 84 +/- 13 nM (n = 28), respectively. With amiloride (10 microM) [Na+]i and [Ca2+]i decreased by 5 +/-1 mM (n = 18) and by 19 +/- 9 nM (n = 21), respectively. With ouabain (0.5 mM), [Na+]i and [Ca2+]i increased by 30 +/- 7 mM (n = 7) and by 25 +/- 10 nM (n = 13), respectively. In the presence of amiloride, ouabain increased [Na+]i by only 8 +/- 3 mM (n = 7), while [Ca2+]i did not change significantly (delta = -2 + 3 nM, n = 13). The observed parallel changes in [Ca2+]i and [Na+]i are compatible with the function of the Na+/Ca2+ exchanger present in the basolateral membrane of the CCD. The K+ channels in the basolateral membrane of rat CCD are inhibited by increases in [Ca2+]i. These These data suggest the changes in [Na+]i and consecutive changes in [Ca2]i as possible functional link between the K+ conductance of the basolateral membrane and the Na(+)-K(+)-ATPase of rat CCD.

Regulation of Na+/H+ exchange by diadenosine polyphosphates, angiotensin II, and vasopressin in rat cortical collecting duct

In principal cells of rat cortical collecting ducts (CCD) cellular pH (pHi) is regulated by basolateral Na+/H+ exchange. The influence of various agonists on pHi and cellular Ca2+ activity ([Ca2+]i) in freshly isolated CCD cells was examined with BCECF and fura-2 fluorescence ratios. The recovery of pHi per minute (delta pH/min) after an acid load was 0.26 +/- 0.03 (N = 53) in control conditions and was increased by the diadenosine polyphosphates Ap4A, Ap5A, Ap6A, the phorbol ester phorbol 12-myristat 13-acetate (PMA) (each 5 mumol/L) and angiotensin II (100 nmol/L) by 0.05 +/- 0.02 (N = 10), 0.11 +/- 0.05 (N = 13), 0.09 +/- 0.02 (N = 24), 0.10 +/- 0.03 (N = 7), and 0.09 +/- 0.03 (N = 8), respectively. Vasopressin (10 nmol/L) decreased delta pH/min by 0.11 +/- 0.03 (N = 9); ATP and Ap3A (each 5 mumol/L) had no significant effect. The increase in delta pH/min with Ap6A was abolished in the presence of an inhibitor of protein kinase C, calphostin C (0.1 mumol/l, N = 8). Fura-2 fluorescence ratio was not significantly changed with angiotensin II, Ap3A, or Ap4A but increased with vasopressin, ATP, Ap5A, and Ap6A by 0.08 +/- 0.02 (N = 13), 0.04 +/- 0.02 (N = 13), 0.03 +/- 0.01 (N = 14), and 0.03 +/- 0.01 (N = 10), respectively. These data indicate that Na+/H+ exchange in rat CCD is activated by the stimulation of a Ca(2+)-independent protein kinase C and inhibited by protein kinase A.

Effects of diadenosine polyphosphates, ATP and angiotensin II on cytosolic Ca2+ activity and contraction of rat mesangial cells

Diadenosine polyphosphates (Apn A) are known to influence cellular Ca2+ activity ([Ca2+]i) in several cells. Their vasoactive potency has been described in various systems including the kidney. We examined the effects of diadenosine polyphosphates, adenosine 5'-triphosphate (ATP) and angiotensin II (Ang II) on cytosolic Ca2+ activity of mesangial cells (MC) in culture obtained from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. [Ca2+]i was measured as a fluorescence ratio F340/F380 with the fura-2 technique using three excitation wavelengths (340 nm, 360 nm and 380 nm) and a photon counting tube. Resting [Ca2+]i was not significantly different in MC from WKY and SHR rats and was measured as 132 +/- 9 nmol/l (n = 65) and 114 +/- 12 nmol/l (n = 36), respectively. Diadenosine polyphosphates (Ap3A-Ap6A) increased [Ca2+]i transiently with an initial peak and a secondary plateau phase comparable to the effects of ATP or Ang II. Increases in [Ca2+]i induced by all these agonists were not significantly different between MC of WKY and SHR rats. ATP, Ap3A, Ap4A, Ap5A, Ap6A (each 5 micromol/l) increased the fura-2 fluorescence ratio initially by 0.66 +/- 0.09 (n = 33), 0.52 +/- 0.08 (n = 18), 0.25 +/- 0.05 (n = 16), 0.09 +/- 0.06 (n = 7), 0.09 +/- 0.04 (n = 11), respectively. A half-maximal initial increase in the fura-2 fluorescence ratio was reached at 22 nmol/l, 0.9 micromol/l, 2.0 micromol/l and 4.0 micromol/l with Ang II, Ap3A, ATP and Ap4A, respectively. Ap4A (100 micromol/l, n = 18) led to a reversible contraction of MC. Diadenosine polyphosphates increase [Ca2+]i in rat MC, in a similar manner to ATP or Ang II and lead to a contraction of MC, suggesting that these nucleotides are also involved in the control of glomerular haemodynamics.

Effects of diadenosine polyphosphates, ATP and angiotensin II on membrane voltage and membrane conductances of rat mesangial cells

Diadenosine polyphosphates have been shown to influence renal perfusion pressure. As mesangial cells may contribute to these effects we investigated the effects of diadenosine triphosphate (Ap3A), diadenosine tetraphosphate (Ap4A), diadenosine pentaphosphate (Ap5A) and diadenosine hexaphosphate (Ap6A) on membrane voltage (Vm) and membrane conductance (gm) in mesangial cells (MC) of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats in primary and long-term culture. We applied the patch-clamp technique in the fast-whole-cell configuration to measure Vm and gm. To compare the effects of diadenosine polyphosphates with hitherto known agonists we also tested adenosine 5&acute;-triphosphate (ATP) and angiotensin II (Ang II). As there was no significant difference in the Vm values in MC of WKY (-42 +/- 1 mV, n = 70) and SHR rats (-45 +/- 2 mV, n = 99) as well as in the agonist-induced changes of Vm, all data were pooled. The Vm of all the cells was -44 +/- 1 mV (n = 169) and gm was 15.9 +/- 1.8 nS (n = 141). Ion-exchange experiments showed the presence of a K+ and a non-selective cation conductance in resting MC whereas a Cl- conductance or a Na+-selective conductance could not be observed. Ap3A, Ap4A, Ap5A, AP6A and ATP each at a concentration of 5 micromol/l, led to a significant depolarization of Vm by 5 +/- 2 mV (n = 14), 7 +/- 1 mV (n = 25), 3 +/- 1 mV (n = 23), 2 +/- 1 mV (n = 16), and 14 +/- 2 mV (n = 23), respectively. For Ap4A, the most potent diadenosine polyphosphate, we determined the half-maximally effective concentration (EC50) as 6 micromol/l (n = 5-25), for ATP as 2 micromol/l (n = 9-37), and for Ang II as 8 nmol/l (n = 6-18). Ap4A 100 micromol/l increased gm significantly by 55 +/- 20% (n = 16), 100 micromol/l ATP by 135 +/- 60% (n = 18). The diadenosine polyphosphates examined were able to depolarize Vm (Ang II > ATP > Ap4A > Ap3A > Ap5A > Ap6A) by activation of a Cl- conductance and a non-selective cation conductance, as do ATP or Ang II.

Effect of NH4+/NH3 on cytosolic pH and the K+ channels of freshly isolated cells from the thick ascending limb of Henle's loop

The conductance properties of the luminal membrane of cells from the thick ascending limb of Henle's loop of rat kidney (TAL) are dominated by K+. In excised membrane patches the luminal K+ channel is regulated by pH changes on the cytosolic side. To examine this pH regulation in intact cells of freshly isolated TAL segments we measured the membrane voltage (Vm) in slow-whole-cell (SWC) recordings and the open probability (Po) of K+ channels in the cell-attached nystatin (CAN) configuration, where channel activity and part of Vm can be recorded. The pipette solution contained K+ 125 mmol/l and Cl- 32 mmol/l. Intracellular pH was determined by 2',7'bis(2-carboxyethyl)-5,(6)-carboxyfluorescein (BCECF) fluorescence. pH changes were induced by the addition of 10 mmol/l NH4+/NH3 to the bath. In the presence of NH4+/NH3 intracellular pH acidified by 0.53 +/- 0.11 units (n = 7). Inhibition of the Na+2Cl-K+ cotransporter by furosemide (0.1 mmol/l) reversed this effect and led to a transient alkalinisation by 0.62 +/- 0.14 units (n = 7). In SWC experiments Vm of TAL cells was -72 +/- 1 mV (n = 70). NH4+/NH3 depolarised Vm by 22 +/- 2 mV (n = 25). In 11 SWC experiments furosemide (0.1 mmol/l) attenuated the depolarising effect of NH4+ from 24 +/- 3 mV to 7 +/- 3 mV. Under control conditions the single-channel conductance of TAL K+ channels in CAN experiments was 66 +/- 5 pS and the reversal voltage for K+ currents was 70 +/- 2 mV (n = 35). The Po of K+ channels in CAN patches was reduced by NH4+/NH3 from 0.45 +/- 0.15 to 0.09 +/- 0.07 (n = 7). NH4+/NH3 exposure depolarised the zero current voltage of the permeabilised patches by -9.7 +/- 3.6 mV (n = 5). The results show that TAL K+ channels are regulated by cytosolic pH in the intact cell. The cytosolic pH is acidified by NH4+/NH3 exposure at concentrations which are physiologically relevant because Na+2Cl-K+(NH4+) cotransporter-mediated import of NH4+ exceeds the rate of NH3 diffusion into the TAL. K+ channels are inhibited by this acidification and the cells depolarise. In the presence of furosemide TAL cells alkalinise proving that NH4+ uptake occurs by the Na+2Cl-K+ cotransporter. The findings that, in the presence of NH4+/NH3 and furosemide, Vm is not completely repolarised and that K+ channels are not activated suggest that the respective K+ channels may in addition to their pH regulation be inhibited directly by NH4+/NH3.

Effects of cell differentiation on ion conductances and membrane voltage in LLC-PK1 cells

LLC-PK1 cells serve as a widely used model for the renal proximal tubule. Until now, little has been found out about their membrane voltage (Vm) and ionic conductances (g). Several studies have shown changes in cell properties during differentiation and ageing. The aim of this study was to examine the relationship between Vm or g and the age of these cells. Therefore, we investigated single cells, subconfluent and confluent monolayers of LLC-PK1 cells aged 1-8 days with the slow-whole-cell patch-clamp technique. The Vm of all cells was -34 +/- 2 mV (n = 75) and the membrane conductance (gm) was 2.3 +/- 0.3 nS (n = 30). Vm in cells aged up to 2 days was -24 +/- 3 mV (n = 22) whereas Vm in cells aged 5-8 days was -50 +/- 3 mV (n = 15). An increase of extracellular K+ from 3.6 to 18.6 mmol/l led to a depolarization in all cells of 4 +/- 1 mV (n = 31) and an increase of gm by 17 +/- 13% (n = 15). Complete replacement of extracellular Na+ by N-methyl-D-glucamine (NMDG) led to a hyperpolarization of 19 +/- 2 mV (n = 38) and gm was lowered by 27 +/- 14% (n = 17). A reduction in extracellular Cl- from 147 to 32 mmol/l showed no significant effect on Vm (n = 16) or gm (n = 11).(ABSTRACT TRUNCATED AT 250 WORDS)

K+ channels in the basolateral membrane of rat cortical collecting duct are regulated by a cGMP-dependent protein kinase

The basolateral membrane of the rat cortical collecting duct (CCD) principal cell is K+ conductive. Recently, two different K+ channels have been described, namely a small- and an intermediate-conductance K+ channel (s-K+ and i-K+) which most likely are responsible for the macroscopic K+ conductance. K+ channel activity was investigated at the single-channel level using the patch-clamp technique. Patch-clamp recordings were obtained from enzymatically isolated CCD segments and freshly isolated CCD cells using conventional cell-free, cell-attached, cell-attached-nystatin and slow-whole-cell methods. Both K+ channels showed rundown behaviour after excision. In an excised inside-out oriented membrane, K+ channels could be activated by simultaneous addition of 0.1 mmol/l (cyclic guanosine monophosphate (cGMP) and 0.1 mmol/l MgATP to the bath. The i-K+ was activated in 13 out of 45, the s-K+ in 15 out of 45, cases. No activation of either channel was observed with cGMP alone (0.1 mmol/l), MgATP alone (0.1 mmol/l), cGMP and guanosine triphosphate (GTP) (0.1 mmol/l each) or cyclic adenosine monophosphate (cAMP) and MgATP (0.1 mmol/l each) n = 15, 11, 7, 8, respectively). The activated s-K+ could be blocked by KT 5823 (n = 8), a specific inhibitor of a cGMP-dependent protein kinase (PKG). An inhibition of the activated i-K+ was seen in seven cases. The membrane potential hyperpolarized significantly after application of dibutyryl-cGMP (0.1 mmol/l, n = 6) or nitroprusside (10 mumol/l, n = 5), which is known to liberate NO and thus increase the intracellular cGMP level.(ABSTRACT TRUNCATED AT 250 WORDS)

pH dependence of K+ conductances of rat cortical collecting duct principal cells

The K+ channels of the principal cells of rat cortical collecting duct (CCD) are pH sensitive in excised membranes. K+ secretion is decreased with increased H+ secretion during acidosis. We examined whether the pH sensitivity of these K+ channels is present also in the intact cell and thus could explain the coupling between K+ and H+ secretion. Membrane voltages (Vm), whole-cell conductances (gc), and single-channel currents of K+ channels were recorded from freshly isolated CCD cells or isolated CCD segments with the patch-clamp method. Intracellular pH (pHi) was measured using the pH-sensitive fluorescent dye 2'-7'-bis(carboxyethyl)-5-6-carboxyfluorescein (BCECF). Acetate (20 mmol/l) had no effect on Vm, gc, or the activity of the K+ channels in these cells. Acetate, however, acidified pHi slightly by 0.17 +/- 0.04 pH units (n = 19). Vm depolarized by 12 +/- 3 mV (n = 26) and by 23 +/- 2 mV (n = 66) and gc decreased by 26 +/- 5% (n = 13) and by 55 +/- 5% (n = 12) with 3-5 or 8-10% CO2, respectively. The same CO2 concentrations decreased pHi by 0.49 +/- 0.07 (n = 15) and 0.73 +/- 0.11 pH units (n = 12), respectively. Open probability (Po) of all four K+ channels in the intact rat CCD cells was reversibly inhibited by 8-10% CO2. pHi increased with the addition of 20 mmol/l NH4+/NH3 by a maximum of 0.64 +/- 0.08 pH units (n = 33) and acidified transiently by 0.37 +/- 0.05 pH units (n = 33) upon NH4+/NH3 removal. In the presence of NH4+/NH3 Vm depolarized by 16 +/- 2 mV (n = 66) and gc decreased by 26 +/- 7% (n = 16). The activity of all four K+ channels was also strongly inhibited in the presence of NH4+/NH3. The effect of NH4+/NH3 on Vm and gc was markedly increased when the pH of the NH4+/NH3-containing solution was set to 8.5 or 9.2. From these data we conclude that cellular acidification in rat CCD principal cells down-regulates K+ conductances, thus reduces K+ secretion by direct inhibition of K+ channel activity. This pH dependence is present in all four K+ channels of the rat CCD. The inhibition of K+ channels by NH4+/NH3 is independent of changes in pHi and rather involves an effect of NH3.

Analysing ion channels with hidden Markov models

Ion channel current amplitudes (mu) and open probabilities (Po) have been analysed so far by defining a 50% threshold to distinguish between open and closed states of the channels. With this standard method (SM) it is very difficult or even impossible to analyse channels of different size in one membrane patch correctly. A stochastical model, named the hidden Markov model (HMM), separates between observation noise and the stochastic process of opening and closing of ion channels. The HMM allows the independent analysis of mu, Po, and mean dwell times (tau) of different channels in one membrane patch, without defining threshold levels. Using this method errors in the analysis are not summarized like in the SM because all different analysing procedures (e.g. filtering, setting of threshold, fitting processes) are done in one step. Two different K+ channels in excised basolateral membranes of the cortical collecting duct of rat (CCD) were analysed by the SM and the HMM. The mu value of the intermediate-conductance K+ channel (i-K+) was 3.9 +/- 0.1 pA (SM) and 3.8 +/- 0.2 pA (HMM) for 11 observations. The Po value of this channel was 10.2 +/- 4.2% (SM) and 10.1 +/- 4.0% (HMM). The mean tau values were 5.4 +/- 0.6 ms for the open state and 9.6 +/- 2.2 ms and 145 +/- 21 ms for the closed states (SM) and 7.8 +/- 1.1 ms, 7.7 +/- 0.9 ms and 148 +/- 24 ms (HMM), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

K+ channels in the basolateral membrane of rat cortical collecting duct

Impalement studies in isolated perfused cortical collecting ducts (CCD) of rats have shown that the basolateral membrane possesses a K+ conductive pathway. In the present study this pathway was investigated at the single-channel level using the patch-clamp technique. Patch-clamp recordings were obtained from enzymatically isolated CCD segments and freshly isolated CCD cells with the conventional cell-free, cell-attached and the cell-attached nystatin method. Two K+ channels were found which were highly active on the cell with a conductance of 67 +/- 5 pS (n = 18) and 148 +/- 4 pS (n = 21) with 145 mmol/l K+ in the pipette. In excised patches the first channel had a conductance of 28 +/- 2 pS (n = 15), whereas the second one had a conductance of 85 +/- 1 pS (n = 53) at 0 mV clamp voltage with 145 mmol/l K+ on one side and 3.6 mmol/l K+ on the other side of the membrane. So far it has not been possible to characterize the smaller channel further. Excised, and with symmetrical K+ concentrations of 145 mmol/l, the intermediate channel had a linear conductance of 198 +/- 19 pS (n = 5). After excision in the inside-out configuration the open probability (Po) of this channel was low (0.18 +/- 0.05, n = 13) whereas in the outside-out configuration this channel had a threefold higher Po (0.57 +/- 0.04, n = 12). Several inhibitors were tested in excised membranes. Ba2+ (1 mmol/l), tetraethylammonium (TEA+, 10 mmol/l) and verapamil (0.1 mmol/l) all blocked this channel reversibly.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of NaPi-1, a Na-Pi cotransporter, in rabbit kidney proximal tubules. I. mRNA localization by reverse transcription/polymerase chain reaction

We have recently isolated from a rabbit cortex cDNA library a cDNA clone (NaPi-1), which, after in vitro transcription (cRNA) and injection into Xenopus laevis oocytes, expresses Na-dependent Pi uptake [Werner A, et al. (1991) Proc Natl Acad Sci USA 88:9608-9612]. The aim of the present work was to study the nephron location of the NaPi-1-related mRNA(s) by combining nephron microdissection procedures, reverse transcription (RT) and amplification of the resultant cDNA by the polymerase chain reaction (PCR). RT-PCR using NaPi-1-specific primers (different combinations) and either total kidney cortex RNA or microdissected proximal tubule segments resulted in two PCR products, both of approximately the expected length (but differing by about 30 base pairs). Restriction-enzyme analysis and nucleotide sequencing confirmed that both PCR products are related to NaPi-1 and that the "longer" PCR product has an insert of 26 base pairs containing an AluI restriction site. Nephron microdissection documents expression of NaPi-1-related mRNA(s) in superficial and deep proximal tubules (S1, S2 and S3 segments) and their absence in glomeruli, thin descending limb and thick ascending limbs of Henle's loop, distal convoluted tubules and cortical and inner medullary collecting ducts. These experiments suggest a "microheterogeneity" of NaPi-1-related mRNA(s) (which is not detected in Northern blot analysis) and proximal tubular expression of NaPi-1.

Effect of various diuretics on membrane voltage of macula densa cells. Whole-cell patch-clamp experiments

The tubuloglomerular feedback mechanism is inhibited by diuretics such as furosemide. For the macula densa (MD) cells similar transport systems, as present in thick ascending limb (TAL) cells, have been suggested. To examine this further, membrane voltages (Vm) of MD cells were recorded with the fast or slow whole-cell patch-clamp method. The effects of diuretics on voltages and the conductance properties of these cells were examined. Vm of MD cells measured with the whole-cell patch-clamp method were as high as those in TAL cells: -72 +/- 1 mV (n = 21). An increase in the extracellular K+ concentration by 15 mmol/l depolarized Vm of MD cells by 11 +/- 1 mV (n = 18). Ba2+ (1 mmol/l) reversibly depolarized MD cells by 10 +/- 2 mV (n = 10). Thus, MD cells possess a K+ conductance that could allow for the recycling of K+ taken up by the Na(+)-2 Cl(-)-K+ cotransporter. MD cells hyperpolarized reversibly upon addition of the loop diuretics furosemide, piretanide and torasemide, whereas muzolimine and hydrochlorothiazide, neither one acting on this cotransport system in other preparations including the TAL, had no effect on Vm. MD cells most likely possess the same cotransport system as the TAL cells, which drives NaCl reabsorption in the TAL and serves as sensor for the tubular NaCl concentration in MD cells.

Cation specificity and pharmacological properties of the Ca(2+)-dependent K+ channel of rat cortical collecting ducts

The luminal membrane of principal cells of rat cortical collecting duct (CCD) is dominated by a K+ conductance. Two different K+ channels are described for this membrane. K+ secretion probably occurs via a small-conductance Ca(2+)-independent channel. The function of the second, large-conductance Ca(2+)-dependent channel is unclear. This study examines properties of this channel to allow a comparison of this K+ channel with the macroscopic K+ conductance of the CCD and with similar K+ channels from other preparations. The channel is poorly active on the cell. It has a conductance of 263 +/- 11 pS (n = 36, symmetrical K+ concentrations) and of 139 +/- 3 pS (n = 91) with 145 mmol/l K+ on one side and 3.6 mmol/l K+ on the other side of the membrane. Its open probability is high after excision (0.71 +/- 0.03, n = 85). The channel flickers rapidly between open and closed states. Its permeability in the cell-free configuration was 7.0 +/- 0.2 x 10(-13) cm3/s (n = 85). It is inhibited by several typical blockers of K+ channels such as Ba2+, tetraethylammonium, quinine, and quinidine and high concentrations of Mg2+. The Ca2+ antagonist verapamil and diltiazem also inhibit this K+ channel. As is typical for the maxi K+ channel, it is inhibited by charybdotoxin but not by apamin. The selectivity of this large-conductance K+ channel demonstrates significant differences between the permeability sequence (pK > pRb > pNH4 > pCs = pLi = pNa = pcholine = 0) and the conductance sequence (gK > gNH4 > gRb > gLi = gcholine > gCs = gNa = 0). The only other cations that are significantly conducted by this channel besides K+ (gK at Vc = infinity is 279 +/- 8 pS, n = 88) re NH+4 (gNH4 = 127 +/- 22 pS, n = 10) and Rb+ (gRb = 36 +/- 5 pS, n = 6). The K+ currents through this channel are reduced by high concentrations of choline+, Cs+, Rb+, and NH+4. These properties and the dependence of this channel on Ca2+ and voltage classify it as a "maxi" K+ channel. A possible physiological function of this channel is discussed in the accompanying paper.

Regulation and possible physiological role of the Ca(2+)-dependent K+ channel of cortical collecting ducts of the rat

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)

Regulation of ion channels in the cortical collecting duct

Properties of ion channels of the cortical collecting duct (CCD) and their relevance for the macroscopic conductances are discussed. Although the regulation of the Na+ conductance by various hormones is widely studied, the relevant Na+ channel appears to be extremely difficult to be detected in the intact preparation. Three different K+ channels with slope conductances of about 30, 140 and 80 pS (excised, 145 mmol/l KCL and 145 mmol/l NaCl on both sides of the membrane) have been found in principal cells of the CCD so far. The first two channels are located in the luminal, the latter one in the basolateral membrane. The two luminal channels are highly sensitive to the cytosolic pH and are also inhibited by cytosolic ATP. The small luminal K+ channel, highly active on the cell, is most likely responsible for K+ secretion. The large luminal K+ channel is involved in the volume regulation. The basolateral K+ channel, again highly active on the cell, is probably responsible for the recirculation of K+ across this membrane. The physiological role of the observed Cl- channels is still unknown.

Ion conductances of isolated cortical collecting duct cells

The study of ion conductances in the intact cortical collecting duct (CCD) with the patch-clamp method is rather difficult. An optimized method to isolate CCD cells from rat kidneys using an in vivo followed by an in vitro enzyme digestion is described. Individual CCD segments were collected after this digestion and incubated in EGTA-buffered medium. This procedure resulted in single cells or cell clusters. These freshly isolated CCD cells were studied with different modifications of the patch-clamp method. Membrane voltages measured in the cell-attached-nystatin configuration were -74 +/- 1 mV (n = 13) and -68 +/- 3 mV (n = 22) in cells isolated from normal and mineralocorticoid-treated rats respectively. These values and those measured with the nystatin-perforated slow-whole-cell configuration (-79 +/- 1 mV, n = 23) are comparable to those measured in principal cells of isolated CCD segments. The cells hyperpolarized after the addition of amiloride and depolarized with the addition of adiuretin to the bath. The amiloride effect was enhanced when cells were isolated from deoxycorticosterone-acetate-treated rats. The cells were strongly depolarized upon elevation of the extracellular K(+)-concentration and did not demonstrate a measurable Cl- conductance. A large-conductance K+ channel (174 pS, n = 5, cell-attached, 145 mmol/l K+ in the pipette; 140 pS, n = 12, cell-free, 3.6 mmol/l K+ in the bath) was seen. It had a very low activity on the cell, but a high open probability when excised into a solution with 1 mmol/l Ca2+ on the cytosolic side.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of the potassium conductances of principal cells of rat cortical collecting ducts

In this study we examined by impalement techniques properties of the macroscopic K+ conductances in the luminal and basolateral membrane of principal cells from isolated perfused cortical collecting ducts (CCD) of the rat. Both membranes possess a dominating K+ conductance. Compared to their behaviour with K+, both membranes appear much less permeable to NH+4 and Rb+, and the K+ conductances of both membranes are inhibited by these cations. In light of these findings, it is very unlikely that significant amounts of NH+4, which is secreted in the CCD, cross the principal cells as NH+4. Several inhibitors with known effects on K+ channels in patch-clamp studies have been examined. Tetraethylammonium, which inhibits the excised K+ channels of these cells, has no effect on the macroscopic K+ conductances of either membrane. Verapamil, which inhibits the K+ channels in the luminal membrane, acts predominantly on the basolateral membrane K+ conductance in the intact tubule. Therefore, some of the macroscopic properties of the K+ conductances are distinct from those of single channels thus far observed in patch-clamp studies.

[Regulation of ion conductance in the cortical collecting duct]

In the following, factors will be discussed which regulate the ionic conductances of the luminal membrane of principal cells of the cortical collecting duct. Transport of sodium and potassium, which is mediated via selective sodium- and potassium-channels, is chronically or acutely adjusted to the needs of the salt balance by the hormones aldosterone and adiuretin. In addition, the potassium conductance and thus potassium secretion can be modified by the cellular pH, which results in a direct and reciprocal coupling between potassium and proton excretion in the cortical collecting duct.