ISSN:1420-4096    

DOI:10.1159/000173341

出版商:S. Karger AG    

年代:1990

数据来源: Karger

ISSN:1420-4096    

DOI:10.1159/000173342

出版商:S. Karger AG    

年代:1990

数据来源: Karger

ISSN:1420-4096    

DOI:10.1159/000173343

出版商:S. Karger AG    

年代:1990

数据来源: Karger

摘要:

In the plasma membranes of mammalian proximal renal tubules single ion channels were investigated mainly in isolated tubules perfused on one side, in isolated nonperfused (collapsed) tubules and in primary cell cultures. With these techniques, the following results were obtained: in the luminal membrane of isolated one-sided perfused tubules of rabbit and mouse S3 segments, K+-selective channels with single-channel conductance (g) of 33 pS and 63 pS, respectively, were recorded. In primary cultures of rabbit SI segments, a small-conductance (42 pS) as well as a large-conductance (200 pS) K+ channel were observed. The latter was Ca2+- and voltage-sensitive. In cultured cells a Ca2+-activated, nonselective cation channel with g = 25 pS was also recorded. On the other hand, an amiloride-sensitive channel with g = 12 pS, which was highly selective for Na+ over K+, was observed in the isolated perfused S3 segment. In the basolateral membrane of isolated perfused S3 segments, two types of K+ channels with g = 46 pS and 36 pS, respectively, were observed. The latter channel was not dependent on cytosolic Ca2+ in cell-excised patches. A K+ channel with g = 54 pS was recorded in osolated nonperfused SI segments. This channel showed inward rectification and was more active at depolarizing potentials. In isolated perfused S3 segments, in addition to the K+ channels also a nonselective cation channel with g = 28 pS was observed. This channel was highly dependent on cytosolic Ca2+ in cell-free patches. It can be concluded that the K+ channels both in the luminal and contraluminal cell membrane are involved in the generation of the cell potential. Na+ channels in the luminal membrane may participate in Na+ reabsorption, whereas the function of a basolateral cation channel remains unclear. Recently, single anion-selective channels were recorded in membranes of endocytotic vesicles, isolated from rat proximal tubules. Vesicles were enlarged by the dehydration/rehydration method and investigated with the patch clamp technique. The Cl- channel had a conductance of 73 pS, the current-voltage curve was linear and the channel inactivated at high negative clamp potentials. It is suggested that this channel is responsible for charge neutrality during active H+ uptake into the endosomes.

ISSN:1420-4096    

DOI:10.1159/000173344

出版商:S. Karger AG    

年代:1990

数据来源: Karger

摘要:

This review discusses the activation of ion transport pathways during regulatory volume decrease in opossum kidney (OK) cells. OK cells regulate their volume when exposed to a hypotonic medium. The changes in cell volume are caused by activation of ion transport pathways and the accompanying osmotically driven water movement so that the increased cell volume returns toward physiological levels. The reshrinking of hypotonically swollen cells is termed regulatory volume decrease. In OK cells separate K+ and Cl- conductances are activated. The Na+/H+ cotransport system seems not to be involved. The potassium pathway is mediated by a K+ channel with a slope conductance of about 12 pS. The occasionally observed widely distributed Ca2+- and voltage-dependent K+ channel of large unit conductance (120 pS) seems not to be involved. The volume regulatory decrease is accompanied by a cell depolarization from a resting potential of about -60 mV to about -20 mV followed by a repolarization. It will be discussed whether the depolarization is caused by the observed activation of stretch-sensitive ion channels of about 30 and 40 pS, respectively. The transient behavior of the cell volume parallels the time-dependent change of the total membrane current. For both recording techniques the volume regulatory decrease can be blocked by quinine. In addition an inward rectifying K+ channel of about 80 pS has been observed in high KCl solution.

ISSN:1420-4096    

DOI:10.1159/000173345

出版商:S. Karger AG    

年代:1990

数据来源: Karger

摘要:

The thick ascending limb of Henle’s loop (TAL) is polarized with respect to its conductances. The luminal membrane contains a K+ conductance which is made up by the synchronous operation of 60- to 80-pS K+ channels. The basolateral membrane contains a chloride conductance. This conductance corresponds most likely to a 30- to 60-pS Cl- channel present in this membrane. Our knowledge on the properties of the K+ channels of these cells has been increased rapidly by patch clamp studies: these K+ channels are inwardly rectifying. They are highly selective for K+ over Na+, Li+ and many other cations. They do not conduct Rb+, Cs+, NHf or other larger cations. In fact, all these three cations as well as choline, tetraethylammonium, lidocaine, verapamil, diltiazem, quinine, quinidine and Ba2+ inhibit these K+ channels. As apparent from kinetic studies the mechanisms of inhibition are different for the various blockers. The TAL K+ channels are downregulated by increasing cytosolic Ca2+ activity. Cytosolic adenosine trisphosphate (ATP) has a similar effect. This ATP inhibition is Ca2+ dependent. The affinity to ATP is augmented by increasing Ca2+. Cytosolic alkalinity increases the open probability of these channels, and cytosolic acidification has the opposite effect. This pH dependence is very marked. A change by 0.2 pH units leads to a more than twofold change in the open-channel probability. The basolateral chloride conductance reflects the properties of an outwardly rectifying 30- to 60-pS Cl- channel. This channel behaves, in many respects, like the Cl- channels of a multitude of Cl-transporting epithelia. It is characterized by two open and two closed states. It is highly selective for Cl- as compared with larger anions, and it is inhibited reversibly by Cl- channel blockers such as 5-nitro-2-(3-phenylpropylamino)-benzoat

ISSN:1420-4096    

DOI:10.1159/000173346

出版商:S. Karger AG    

年代:1990

数据来源: Karger

摘要:

Na-selective, amiloride-sensitive ion channels are a crucial component of the system for controlled NaCl reabsorption across tight epithelia. A kinetic model for the process of Na conduction through the channel is presented. The model is based on a view of the possible geometry of the pore obtained from an analysis of voltage-dependent block of Na currents by impermeant cations. It is also consistent with the known electrophysiological properties of the channel, including single-channel currents, current-voltage relationships and saturation. The model, while not a unique explanation of the available data, may serve as a starting point for more detailed experimental and theoretical studies.

ISSN:1420-4096    

DOI:10.1159/000173347

出版商:S. Karger AG    

年代:1990

数据来源: Karger

摘要:

Fine control of renal water and electrolyte excretion takes place in the collecting duct, a tubule segment which is also a major site of K+ secretion and hormone action. With the introduction of patch clamp techniques it has been possible to define the contribution of ion channels to K+ transport. Two types of channels have been identified in the cortical collecting tubules of the rabbit and rat: (1) a maxi- or high conductance K+ 80pS) found only in the apical membrane, and (2) smaller conductance K+ channels (single channel conductance < 60 pS) found in both apical and basolateral membranes. The gating properties of the K+ channels with smaller conductances differ in the apical and basolateral cell membranes; whereas the open probability of the small conductance K+ channel in the apical membrane is not voltage-sensitive, that of the basolateral channel increases with hyperpolarization. The maxi-K+ channel, so far only found in the apical cell membrane, is voltage-gated but its open probability increases with cell depolarization. The possible role of these K+ channels in different states of the K+ transport system in collecting ducts is discussed.

ISSN:1420-4096    

DOI:10.1159/000173348

出版商:S. Karger AG    

年代:1990

数据来源: Karger

摘要:

The apical cell membrane of cultured monolayers of collecting duct principal cells was investigated with the patch-clamp technique to study single ion channels. A broad spectrum of channel events was observed which complicated the analysis. Although principal cells absorb mainly Na+ ions and although most patches must have contained an Na+ conductance as evidenced by asymmetric clamp currents in the presence of amiloride, single Na+ channel events could not be identified with certainty, possibly because single-channel conductance was too low (≤5pS). Instead, non-selective cation channels of 21.2 ± 4.0 pS were frequently observed. They were impermeable to choline and anions but did not discriminate between Na+ and K+. These channels, however, do not appear to participate in active Na+ absorption. Different types of K+ channels were observed: a high-conductance Ca2+-activated K+ channel and a bursting low-conductance K+ channel. Since the former channel has been denied a role in K+ secretion/ absorption in native collecting ducts, the latter might be involved. In addition three types of CΓ channels have been observed which will be described separately. At least one of those, a 30-pS outwardly rectifying Cl- channel appears to allow small amounts of Cl- ions to be absorbed across principal ce

ISSN:1420-4096    

DOI:10.1159/000173349

出版商:S. Karger AG    

年代:1990

数据来源: Karger

摘要:

Ion channels in Madin-Darby canine kidney cells serve transepithelial chloride transport and probably cell volume regulation. Three distinct potassium channels and one anion channel have been revealed by patch clamp studies in Madin-Darby canine kidney cells. The potassium channels are activated by an increase in intracellular calcium activity. A number of hormones activate the potassium channels by an increase in intracellular calcium activity. However, under certain conditions the hormones hyperpolarize the cell membrane without increasing intracellular calcium activity sufficiently to activate the calcium-sensitive potassium channels. Thus, the hormones may activate potassium channels via another, as yet undefined, intracellular mechanism. The anion channel is stimulated by cAMP. Another factor modifying channel activity is cell volume: cell swelling leads probably to subsequent activation of potassium and anion channels. The net result is a variable transient hyperpolarization followed by a sustained depolarization of the cell membrane.

ISSN:1420-4096    

DOI:10.1159/000173350

出版商:S. Karger AG    

年代:1990

数据来源: Karger