Amiloride-sensitive Na+ channels constitute a new class of proteins known as the ENaC-Deg family of ion channels. All members in this family share a common protein structure but differ in their ion selectivity, their affinity for the blocker amiloride, and in their gating mechanisms. These channels are expressed in many tissues of invertebrate and vertebrate organisms where they serve diverse functions varying from Na+ absorption across epithelia to being the receptors for neurotransmitters in the nervous system. Here, we review progress made during the last years in the characterization, regulation, and cloning of new amiloride-sensitive Na+ channels.

The epithelial sodium channel (ENaC) provides the rate-limiting step in the reabsorption of sodium by many epithelia. The number of channels at the cell surface is tightly regulated; most cells express only a few channels. We have examined the biosynthesis and cell surface expression of ENaC in Xenopus oocytes. The subunits of ENaC are readily synthesized in the endoplasmic reticulum, but most of them remain as immature proteins in pre-Golgi compartments, where they are degraded by the proteasomal pathway without apparent ubiquitination. Even when the three subunits, ␣, ␤, and ␥, are expressed in the same cell, only a very small fraction of the total channel population leave the endoplasmic reticulum, acquire complex oligosaccharides, and reach the plasma membrane. Overexpression of subunits does not increase the number of channels in the plasma membrane but results in the appearance of cytoplasmic subunits in a form not membrane bound. The data indicate that maturation and assembly of the subunits are slow and inefficient processes, and constitute limiting steps for the expression of functional ENaC channels in the plasma membrane.The amiloride-sensitive epithelial sodium channel (ENaC) 1 is a heteromultimeric protein formed by three homologous subunits, ␣, ␤, and ␥, that associate in a protein complex, whose number of subunits and stoichiometry have not been determined definitively yet (1, 2). Single subunits induce small amiloride-sensitive current and are not detected at the cell surface (3, 4). Even when the three subunits are present, the expression of ENaC at the plasma membrane is of small magnitude; in most tissues only a few copies are found per cell (5, 6), suggesting the presence of highly regulated mechanisms that control its surface expression. Since ENaC is a constitutively active channel with high open probability (7), control of the number of channels at the cell surface is crucial for the regulation of sodium reabsorption. In addition to turnover of channels by endocytosis (8), another mechanism that controls the number of channels in the plasma membrane is the rate of delivery of newly synthesized channels.Xenopus oocytes have become the most frequently used expression system to study structure, function, and regulation of ENaC, and where most of the recent characterizations of wildtype and mutant channels have been performed. Here, we have examined the biosynthesis of wild-type ENaC and of channels with deletions of the carboxyl termini (the latter known to increase the activity of ENaC by 3-5-fold) in Xenopus oocytes (9). Our results show that subunits are readily synthesized in the endoplasmic reticulum (ER) but they remain as core-glycosylated, immature proteins that are degraded in pre-Golgi compartments by a process that requires the activity of the proteasome but without the need for ubiquitination. Co-expression of all three subunits significantly stabilizes the subunits in the ER, but still, only a very small fraction is converted into a mature form that bears complex oligosaccharides...

We have further characterized at the single channel level the properties of epithelial sodium channels formed by coexpression of α with either wild-type β or γ subunits and α with carboxy-terminal truncated β (βT) or γ (γT) subunits in Xenopus laevis oocytes. αβ and αβT channels (9.6 and 8.7 pS, respectively, with 150 mM Li+) were found to be constitutively open. Only upon inclusion of 1 μM amiloride in the pipette solution could channel activity be resolved; both channel types had short open and closed times. Mean channel open probability (P o) for αβ was 0.54 and for αβT was 0.50. In comparison, αγ and αγT channels exhibited different kinetics: αγ channels (6.7 pS in Li+) had either long open times with short closings, resulting in a high P o (0.78), or short openings with long closed times, resulting in a low P o (0.16). The mean P o for all αγ channels was 0.48. αγT (6.6 pS in Li+) behaved as a single population of channels with distinct kinetics: mean open time of 1.2 s and closed time of 0.4 s, with a mean P o of 0.6, similar to that of αγ. Inclusion of 0.1 μM amiloride in the pipette solution reduced the mean open time of αγT to 151 ms without significantly altering the closed time. We also examined the kinetics of amiloride block of αβ, αβT (1 μM amiloride), and αγT (0.1 μM amiloride) channels. αβ and αβT had similar blocking and unblocking rate constants, whereas the unblocking rate constant for αγT was 10-fold slower than αβT. Our results indicate that subunit composition of ENaC is a main determinant of P o. In addition, channel kinetics and P o are not altered by carboxy-terminal deletion in the β subunit, whereas a similar deletion in the γ subunit affects channel kinetics but not P o.

In the placental vasculature, where oxygenation may be an important regulator of vascular reactivity, there is a paucity of data on the expression of potassium (K) channels, which are important mediators of vascular smooth muscle tone. We therefore addressed the expression and function of several K channel subtypes in human placentas. The expression of voltage-gated (Kv)2.1, KV9.3, large-conductance Ca2+-activated K channel (BKCa), inward-rectified K+ channel (KIR)6.1, and two-pore domain inwardly rectifying potassium channel-related acid-sensitive K channels (TASK)1 in chorionic plate arteries, veins, and placental homogenate was assessed by RT-PCR and Western blot analysis. Functional activity of K channels was assessed pharmacologically in small chorionic plate arteries and veins by wire myography using 4-aminopyridine, iberiotoxin, pinacidil, and anandamide. Experiments were performed at 20, 7, and 2% oxygen to assess the effect of oxygenation on the efficacy of K channel modulators. KV2.1, KV9.3, BKCa, KIR6.1, and TASK1 channels were all demonstrated to be expressed at the message level. KV2.1, BKCa, KIR6.1, and TASK1 were all demonstrated at the protein level. Pharmacological manipulation of voltage-gated and ATP-sensitive channels produced the most marked modifications in vascular tone, in both arteries and veins. We conclude that K channels play an important role in controlling placental vascular function.

Two-pore domain K 1 channels are an emerging family of K 1 channels that may contribute to setting membrane potential in both electrically excitable and non-excitable cells and, as such, influence cellular function. The human uteroplacental unit contains both excitable (e.g. myometrial) and non-excitable cells, whose function depends upon the activity of K 1 channels.We have therefore investigated the expression of two members of this family, TWIK (two-pore domain weak inward rectifying K 1 channel)-related acid-sensitive K 1 channel (TASK) and TWIK-related K 1 channel (TREK) in human myometrium. Using RT-PCR the mRNA expression of TASK and TREK isoforms was examined in myometrial tissue from pregnant women. mRNAs encoding TASK1, 4 and 5 and TREK1 were detected whereas weak or no signals were observed for TASK2, TASK3 and TREK2. Western blotting for TASK1 gave two bands of approximately 44 and 65 kDa, whereas TREK1 gave bands of approximately 59 and 90 kDa in myometrium from pregnant women. TASK1 and TREK1 immunofluorescence was prominent in intracellular and plasmalemmal locations within myometrial cells. Therefore, we conclude that the human myometrium is a site of expression for the two-pore domain K 1 channel proteins TASK1 and TREK1.Reproduction (