The mammalian urinary bladder exhibits transepithelial Na+ absorption that contributes to Na+ gradients established by the kidney. Electrophysiological studies have demonstrated that electrogenic Na+ absorption across the urinary bladder is mediated in part by amiloride-sensitive Na+ channels situated within the apical membrane of the bladder epithelium. We have used a combination of in situ hybridization, Northern blot analysis, and immunocytochemistry to examine whether the recently cloned epithelial Na+ channel (ENaC) is expressed in the rat urinary bladder. In situ hybridization and Northern blot analyses indicate that α-, β-, and γ-rat ENaC (rENaC) are expressed in rat urinary bladder epithelial cells. Quantitation of the levels of α-, β-, and γ-rENaC mRNA expression in rat urinary bladder, relative to β-actin mRNA expression, indicates that, although comparable levels of α- and β-rENaC subunits are expressed in the urinary bladder of rats maintained on standard chow, the level of γ-rENaC mRNA expression is 5- to 10-fold lower than α- or β-rENaC mRNA. Immunocytochemistry, using an antibody directed against α-rENaC, revealed that ENaCs are predominantly localized to the luminal membrane of the bladder epithelium. Together, these data demonstrate that ENaC is expressed in the mammalian urinary bladder and suggest that amiloride-sensitive Na+ transport across the apical membrane of the mammalian urinary bladder epithelium is mediated primarily by ENaC.
The epithelial sodium channel (ENaC) plays a major role in the transepithelial reabsorption of sodium in the renal cortical collecting duct, distal colon, and lung. ENaCs are formed by three structurally related subunits, termed α-, β-, and γENaC. We previously isolated and sequenced cDNAs encoding a portion of mouse α-, β-, and γENaC (α-, β-, and γmENaC). These cDNAs were used to screen an oligo-dT-primed mouse kidney cDNA library. Full-length βmENaC and partial-length α- and γmENaC clones were isolated. Full-length α- and γmENaC cDNAs were subsequently obtained by 5′-rapid amplification of cDNA ends (5′-RACE) PCR. Injection of mouse α-, β-, and γENaC cRNAs into Xenopus oocytes led to expression of amiloride-sensitive ( K i = 103 nM), Na+-selective currents with a single-channel conductance of 4.7 pS. Northern blots revealed that α-, β-, and γmENaC were expressed in lung and kidney. Interestingly, αmENaC was detected in liver, although transcript sizes of 9.8 kb and 3.1 kb differed in size from the 3.2-kb message observed in other tissues. A partial cDNA clone was isolated from mouse liver by 5′-RACE PCR. Its sequence was found to be nearly identical to αmENaC. To begin to identify regions within αmENaC that might be important in assembly of the native heteroligomeric channel, a series of functional experiments were performed using a construct of αmENaC encoding the predicted cytoplasmic NH2 terminus. Coinjection of wild-type α-, β-, and γmENaC with the intracellular NH2 terminus of αmENaC abolished amiloride-sensitive currents in Xenopus oocytes, suggesting that the NH2 terminus of αmENaC is involved in subunit assembly, and when present in a 10-fold excess, plays a dominant negative role in functional ENaC expression.
Kadsurenone, a specific receptor antagonist of platelet-activating factor (PAF), and its analogues were prepared from derivatives of cinnamyl alcohol and (allyloxy)phenol. Racemic kadsurenone, resolvable by a Chiralpak column at low temperatures, has an IC50 value of 2 X 10(-7) M, which is about 50% of the activity of the natural product (IC50 = 1 X 10(-7) M). The structural specificity of kadsurenone was further demonstrated by the low PAF-receptor-blocking activities of denudatin B, mirandin A, desallylkadsurenone, and the 2-epimer of kadsurenone.
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