Role of Ca2+-activated K+ channel in regulation of NaCl reabsorption in thick ascending limb of Henle's loop

BK channels in the kidney: role in K(+) secretion and localization of molecular components

Although it is generally accepted that ROMK is the K(+) secretory channel in the mammalian distal nephron, recent in vitro and in vivo studies have provided evidence that large-conductance Ca(2+)-activated K(+) channels (BK, or maxi K) also secrete K(+) in renal tubules. This review assesses the current evidence relating BK channels with K(+) secretion. We shall consider the component proteins of the BK channel, their localization with respect to segment and cell type, and the electrophysiological forces involved in K(+) secretion. Although the majority of studies have focused on a role for BK channels in flow-mediated K(+) secretion, this review also considers a potential role for BK channels in high-K diet-induced K(+) secretion. The division of workload between ROMK and BK is discussed as a mechanism for ensuring a constant plasma K(+) concentration.

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.

Investigation of the intestinal spherocrystals of different Cephalobaenida (Pentastomida)

The midgut cells of cephalobaenid pentastomids contain spherocrystals varying in size and appearance between the genera. Iron was detected in the vicinity of the crystals in all three genera, being considerably fainter in Reighardia sternae than in the other species. Ultrastructurally a distinct lamination of the spherocrystals was evident, being faint in R. sternae but clearly expressed in Raillietiella hemidactyli and Cephalobaena tetrapoda. According to the species, their diameter ranged from 1.3 to 6.25 microns. The size and number of the crystals were highest in R. hemidactyli and lowest in R. sternae. The inclusions were formed in the endoplasmic reticulum and migrated toward the cellular apex, accumulating there and being expelled into the midgut lumen. As determined by energy-dispersive X-ray analysis, calcium turned out to be the main component of the crystals. Furthermore, small amounts of chlorine and iron could be traced in the crystals. The relevance of the crystals is regarded as a kind of storage excretion such as that present in many other arthropods.

Papaverine reduces the sodium permeability of the apical membrane and the potassium permeability of the basolateral membrane in isolated frog skin

The effect of papaverine, an inhibitor of the phosphodiesterase responsible for breakdown of cAMP, on the transepithelial sodium transport across the isolated frog skin was investigated. Serosal addition of papaverine caused initially an increase in the short-circuit current (SCC), a doubling of the cellular cAMP content and a depolarization of the intracellular potential under SCC conditions (Vscc). The initial increase in the SCC was followed by a pronounced decrease both in the SCC and in the natriferic action of antidiuretic hormone (ADH), but papaverine had no inhibitory effect on the ability of ADH to increase the cellular cAMP content. As SCC declines, no hyperpolarization was observed. The I/V relationship across the apical membrane during the inhibitory phase, revealed that papaverine reduces the sodium permeability of the apical membrane (PNaa) as well as intracellular sodium concentration. These observations and the previously noted effect of papaverine on Vscc indicates that papaverine must have an effect on the cellular Cl or K permeability. The basolateral Na,K,2Cl cotransporter was blocked with bumetanide, which should bring the cellular chloride in equilibrium. Bumetanide had no effect on basal SCC and Vscc. When papaverine was added to skins preincubated with bumetanide, the effect of papaverine on SCC and Vscc was unchanged. Therefore, the depolarization of Vscc, observed during the papaverine-induced inhibition of the SCC, must be due to a reduction in the cellular K permeability. In conclusion, it is suggested that papaverine reduces the sodium permeability of the apical membrane and the potassium permeability of the basolateral membrane of the frog skin epithelium.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

The luminal K+ channel of the thick ascending limb of Henle's loop

In vitro perfused rat thick ascending limbs of Henle's loop (TAL) were used (n = 260) to analyse the conductance properties of the luminal membrane applying the patch-clamp technique. Medullary (mTAL) and cortical (cTAL) tubule segments were dissected and perfused in vitro. The free end of the tubule was held and immobilized at one edge by a holding pipette kept under continuous suction. A micropositioner was used to insert a patch pipette into the lumen, and a gigaohm seal with the luminal membrane was achieved in 455 instances out of considerably more trials. In approximately 20% of all gigaohm seals recordings of single ionic channels were obtained. We have identified only one single type of K+ channel in these cell-attached and cell-excised recordings. In the cell-attached configuration with KCl or NaCl in the pipette, the channel had a conductance of 60 +/- 6 pS (n = 24) and 31 +/- 7 pS (n = 4) respectively. In cell-free patches with KCl either in the patch pipette or in the bath and with a Ringer-type solution (NaCl) on the opposite side the conductance was 72 +/- 4 pS (n = 37) at a clamp voltage of 0 mV. The permeability was 0.33 +/- 0.02 . 10(-12) cm3/s. The selectivity sequence of this channel was: K+ = Rb+ = NH4+ = Cs+ greater than Li+ much greater than Na+ = 0; the conductance sequence was K+ much greater than Li+ much greater than Rb+ = Cs+ = NH4+ = Na+ = 0. In excised patches Rb+, Cs+ and NH4+ when present in the bath at 145 mmol/l all inhibited K+ currents out of the pipette. The channel kinetics were described by one open (9.5 +/- 1.5 ms, n = 18) and by two closed (1.4 +/- 0.1 and 14 +/- 2 ms) time constants. The open probability of this channel was increased by depolarization. The channel open probability was reduced voltage dependently by Ba2+ (half maximal inhibition at 0 mV: 0.07 mmol/l) from the cytosolic side. Verapamil, diltiazem, quinine and quinidine inhibited at approximately 1 mumol/l -0.1 mmol/l from either side. Similarly, the amino cations lidocaine, tetraethylammonium and choline inhibited at 10-100 mmol/l. The channel was downregulated in its open probability by cytosolic Ca2+ activities greater than 10(-7) mol/l and by adenosine triphosphate greater than or equal to 10(-4) mol/l. The open probability was downregulated by decreasing cytosolic pH (2-fold by a decrease in pH by less than or equal to 0.2 units).(ABSTRACT TRUNCATED AT 400 WORDS)