摘要:

Transepithelial transport, intracellular ion activities and membrane potentials are all affected by changes in the conductive properties of the membranes of polarised epithelial cells. Conventional electrophysiological techniques have already determined the major conductances of the apical and basolateral membranes of the various nephron segments. These conductances are presently being studied at the molecular level with the aid of the patch clamp technique. In the case of the amphibian nephron, single-channel studies have been carried out in the proximal and early distal (diluting) segments. Almost all of the channels described so far have been selective for potassium, and the properties of these channels are described in this review. In addition, the basic electrophysiological and transport properties of these two general nephron segments are briefly described. From the physiological stand-point, the results of single-channel studies are providing us with information concerning the regulation of the conductances by intracellular mediators, allowing us to make predictions about the effects of various perturbations on cell membrane conductances. On the other hand, biophysical analysis is giving information ranging from the voltage dependence and ion selectivity of the channels to clues concerning their submicroscopic structure.

ISSN:1420-4096    

DOI:10.1159/000173351

出版商:S. Karger AG    

年代:1990

数据来源: Karger

摘要:

Urinary epithelia separate urine from interstitial fluid. In the mammal, this tight epithelium has a limited transport capacity but is capable of moving sodium from urine to blood through an aldosterone-sensitive cellular pathway. In lower vertebrates, absorption of ions and water from the urine can contribute significantly to fluid and electrolyte homeostasis. Transepithelial ion transport and maintenance of cellular composition are interdependent, requiring a balance between movements across the apical and basolateral plasma membranes through a variety of pathways including electrodiffusion through ion channels. A variety of such channels has been identified in urinary epithelia. Apical membranes contain amiloride-sensitive, highly selective sodium channels of low conductance (≈5-10 pS). There is evidence that in mammalian bladders trypsin-like enzymes in the urine continually degrade these channels, decrease in cation selectivity being followed by loss of the channels from the membrane. New channels stored in the cytoplasm appear to provide a source for replenishment of the membrane. Other channels of higher conductance and lower selectivity have also been described in both mammalian and amphibian bladders, but their physiological significance remains to be established. Basolateral membranes contain potassium channels. In the mammalian bladder, in which chloride appears to be distributed at electrochemical equilibrium, chloride conductance exceeds potassium conductance and patch clamp studies have revealed a chloride channel of conductance ≈60 pS detectable immediately on patch excision and active at normal membrane potentials. In the amphibian bladdder, a variety of findings indicates the presence of a basolateral membrane chloride conductance, but patch clamp data are not yet availa

ISSN:1420-4096    

DOI:10.1159/000173352

出版商:S. Karger AG    

年代:1990

数据来源: Karger