Spermatogonial exchange involving the X but not the y chromosome inDrosophila Melanogaster

Summary The distal uninverted portion of In(1)sc⁸, which carriesy ⁺ andac ⁺, is occasionally lost during spermatogonial divisions. This is accomplished by exchange between the protion of the proximal heterochromatin that has been removed distally by the inversion and some other heterochromatin in the complement (see alsoLindsley 1955b).. The majority of the recombiants recovered from males carrying In(1)sc⁸ arise through exchange with the Y chromosome (12/15). The majority of the recombinants recovered from males carring In(1)sc^8L, EN^R, which is characterized by a heterochromatic second arm, do not arise through exchange with the Y chromosome (18/22). The absolute frequencies of Y involvement with In(1)sc⁸ (7/105067) and In(1)sc^8L, EN^R,(2/38588), however, are comparable. The heterochromatic constitution of the recombinants examined is consistent with the hypothesis that an observed excess of recombinants recoverred from In(1)sc^8L, EN^R as compared with In(1)sc⁸ is accounted for by Y independent recombinants and is the consequence of exchange between the second heterochromatin arm of In(1)EN^R and the distal heterochromatin of In(1)sc^8L. A maximum of six different regions of exchange between these two regions may be inferred from the constitution of the recombinants. This inference is considered to support the hypothesis that pairing and exchange between heterochromatic regions are not strictly homologous.With 6 Figures in the TextOperated by Union Carbide Nuclear Company for the U.S. Atomic Energy Commission.Part of the material was presented to the Graduate School of the California Institute of Technology in partial fulfillment of requirements for the degree of Doctor of Philosophy supported by an Atomic Energy Commission predoctoral fellowship. Further experimentation has been pursued under a National Research Council postdoctoral fellowship at the University and under a National Science Foundation postdoctoral fellowship at the University of Missouri. Experimentation was completed at Oak Ridge.     

Effect of somatostatin on electrogenic ion transport in the duodenum and colon of the mouse, Mus domesticus

In this study, we have used the mouse intestine and the Ussing short circuit technique to compare the effects and mechanism of action of somatostatin (SST, 0.1 μM) on cAMP- and Ca²⁺-mediated ion secretion in the duodenum and colon of the Swiss-Webster mouse. The cAMP-dependent secretagogues, prostaglandin E₂ (1 μM) and dibutyryl-cAMP (150 μM) increased short circuit current (I_sc) in both regions, but only the colonic response was inhibited by SST. This inhibition was independent of enteric nerves, suggesting a direct action on the epithelial cells. The Ca²⁺-dependent secretagogue carbachol (10 μM) stimulated a transient increase in I_sc in both intestinal segments. In the duodenum, SST partially inhibited this increase in I_sc and both the responses to carbachol and SST were independent of enteric nerves. In the colon, while SST inhibited the carbachol induced increase in I_sc, pre-treatment with tetrodotoxin (750 nM) profoundly inhibited the carbachol induced increase in I_sc, thus markedly reducing the inhibitory effect of SST. This indicates an involvement of the enteric nervous system in the response to carbachol and the action of SST in the colon. These data indicate marked regional differences within the mouse intestine of the effects of SST on ion secretion and demonstrate different mechanisms of action of SST in the duodenum and colon.     

Identification of a V-type proton pump in the outer mantle epithelium of Anodonta cygnea

The dissected outer mantle epithelium (OME) of Anodonta cygnea, when mounted in Ussing type chambers, generated a spontaneous potential difference of 22.0±12.6 mV and, when short-circuited, a positive current (I_sc) of 30.0±11 μA/cm² towards the shell side and a conductance of 1.1±0.4 mS/cm². When in contact with the shell side, Bafilomycin A₁ and Concanamycin A, specific inhibitors of V-proton pumps, induced 90% inhibition of I_sc in 5 and 35 min, respectively. They had no effect in the current from the hemolymph side. Both drugs induced a dramatic fall in conductance from the shell side. Tributyltin oxide (TBTO) inhibited 90% of the I_sc in 3 min and induced a fall in conductance only from the shell side. Two observations suggest a direct effect on the proton pump: it was effective only from the shell side and the time course of the effect was identical to that of Bafilomicyn A₁. Dicyclohexycarbodiimide (DCCD) and N-ethylmaleimide (NEM) inhibited I_sc from both sides but very slowly and there was a delay of the effect from the hemolymph side in relation to the shell side. Taken together, the results suggest the presence of a V-type proton pump located at the apical (shell side) barrier of the OME.     

Bivalent behavior in Drosophila melanogaster males containing the In(1)sc 4Lsc8RX chromosome

The sex chromosome bivalent was examined in Drosophila melanogaster males possessing the In(1)sc ^4Lsc^8R X chromosome. Three-dimensional reconstructions from electron micrographs of serially cut thin sections were made. A large proportion of the kinetochores of In(1)sc ^4Lsc^8R/Y bivalents did not face opposite poles during metaphase I and anaphase I. This suggests that In(1)sc ^4Lsc^8R/Y bivalents may have difficulty achieving bipolar stability. Delay in achieving bipolar stability could contribute to the nondisjunctional behavior found in In(1)sc ^4Lsc^8R/Y males.     

Effects of aldosterone on Na transport in the toad bladder II. The anaerobic response

The action of aldosterone on active Na⁺ transport was assessed under aerobic and anaerobic conditions in the isolated urinary bladder of the toad, BUfo marinus. Aldesterone augmented the short-circuit current (I_sc) under rigorous anaerobiosis. Four lines of evidence indicate that the increase in anaerobic I_sc does not represent an equivalent increase in active Na⁺ transport: 1. Net Na⁺ transport, determined by isotopic fluxes, was the same in the aldosterone-treated and control quarter-bladders, and significantly greater than the simultaneously measured I_sc. 2. Amiloride, an inhibitor of the apiral entry of Na⁺, did not reduce the steroid-dependent increase in the anaerobic I_sc. 3. Substitution of choline for Na⁺ in the mucosal medium reduced the magnitude of the anaerobic I_sc values did not eliminate the effect of aldosterone. 4. Addition of ouabain, a potent inhibitor of the Na⁺ pump, partially inhibited the effect of aldosterone on the anerobic I_sc but a significant hormonal increment remained. The source of the anaerobic I_sc was not identified; an effort was made, however, to determine the dependence of this current on glycolysis. During anaerobics, aldosterone increased the integral I_sc by 42% but did not alter lactate production. These results suggest that the steroid-dependent increase in the anaerobic I_sc may involve effects on permeability properties of the epithelium rather than on active tranport systems.     

Autonomy of the wingless mutation in Drosophila melanogaster

Summary T(Y;2) translocations were used to cytologically localise the wingless locus of Drosophila melanogaster. We found that an existing T(Y;2), which is an insertion of a segment of 2L into the Y chromosome, has wg ⁺ within this insert. This Y chromosome was used to generate an attached XY chromosome containing wg ⁺. The mutation claret-nondisjunctional (ca ^nd) was used to induce the loss of this XY chromosome and thus generate gynandromorphs with wg ¹/wg ¹ male tissue and wg ⁺/wg ¹/wg ¹ female tissue. Analysis of these gynanders demonstrated that a genotypically wingless mutant hemithorax is usually also phenotypically mutant in these half body mosaics; thus wg ¹ is discautonomous. This observation is of interest as it is known that wg is not cell autonomous.     

Control of gene action in the synthesis of anthocyanin in maize

Summary R ^st is an unstable allele of R that exhibits a mosaic phenotype in the aleurone, consisting of heavily pigmented spots on a colorless background. This variagated phenotype is presumably caused by the frequent somatic reversion of R from an inactive to an active form. Data are presented showing that such a reversion can take place at different times during the plant ontogenesis. Various stippled derivatives have been selected that differ in the number and size of dots formed in the endosperm. The reversion rate of R ^st and derivatives toward self-colored (R ^sc) has been estimated in the germinal and somatic tissues. This analysis indicates that some of the stippled derivatives differ in their capacity to revert toward R ^sc in both the somatic and germ cells. The effect of internal factors on the R ^st reversion rate has been measured. On the basis of these data the possible mechanism causing genetic instability of R ^st are briefly discussed.This investigation has been partially supported by Consiglio Nazionale delle Ricerche, Roma.     

Cytogenetical localization of Zygotic hybrid rescue (Zhr), a Drosophila melanogaster gene that rescues interspecific hybrids from embryonic lethality

Hybrid females from crosses between Drsophila melanogaster males and females of its sibling species, D. simulans, D. mauritiana, or D. sechellia die as embryos. This lethality is believed to be caused by incompatibility between the X chromosome of D. melanogaster and the maternal cytoplasm. Zygotic hybrid rescue (Zhr) prevents this embryonic lethality and has been cytogenetically mapped to a proximal region of the X chromosome of D. melanogaster, probably in the centromeric heterochromatin. We have carried out high resolution cytological mapping of Zhr using deficiencies and duplications of the X heterochromatin. Deletions of the Zhr ⁺ gene from the hybrid genome exhibit the Zhr phenotype. On the contrary, addition of the wild-type gene to the hybrid genome causes embryonic lethality, regardless of sex. The Zhr locus has been narrowed down to the region covered by Dp(1;f)1162 but not covered Dp(1;f)1205, a chromosome carrying a duplication of heterochromatin located slightly distal to the In(1)sc ⁸ heterochromatic breakpoint.     

Some genetic aspects of meiosis in males of Drosophila melanogaster,carriers of In(1)sc 4 sc 8

In order to verify the theory of non-homologous pairing in the male meiotic metaphase of Drosophila melanogaster, particular crosses were carried out with In(1)sc ⁴ sc ⁸ males, with or without the inversion In(2)SM ₅, Cy on the second chromosome. The results seem to indicate the existence of the above mechanisms, which would appear to contribute, not infrequently, to the overall non-disjunction of the In(1)sc ⁴ sc ⁸+Cy males.The work has been financially supported by the Consiglio Nazionale delle Ricerche (CNR).     

Genetic analysis of the het and nif genes in the blue-green alga Nostoc muscorum

Summary Two multiply marked complementary strains namely Het ⁺ Nif⁺ Str-R and Het ^- Nif^- Ery-R MSO-R were constructed and crossed under conditions counterselective for the Het ⁺ Nif⁺ Str-R parent and selective only for recombinants of Str-R and Ery-R or Str-R and MSO-R constitution. The results of the recombinant analysis with regard to the selected and unselected markers suggested that the Het ^- Nif^- Ery-R MSO-R parent acted as a recipient and the Het ⁺ Nif⁺ Str-R parent as donor of the genetic markers in the cross. The joint inheritance of Het ⁺ and Nif ⁺ unselected markers among the recombinants was found to occur more frequently than the inheritance of the Het ⁺ or Nif ⁺ markers alone. The observed joint inheritance of Het ⁺ and Nif ⁺ markers among the recombinants probably results from the inheritance of the regulatory gene(s) required for the activation of latent het and nif genes. This interpretation is fully supported by (a) the frequency distribution of unselected Het ⁺ and Nif ⁺ markers and (b) the reversion frequency of Het ^- Nif ^- strains to Het ⁺ Nif⁺ prototrophy. Accordingly the apparent close genetic linkage of het and nif genes is not due to their organization in a single operon but to their common regulation by regulatory gene(s) of a positive control nature. The Het ⁺ Nif⁺ wild type, mutant, revertant, and recombinant strains all appear similar in their NO ₃ ^- repression of both heterocyst and nitrogenase. The Het ⁺ Nif^- and Het ^- Nif⁺ recominants also show similar NO ₃ ^- repression of their heterocyst and nitrogenase respectively. The presence of only microaerobic acetylene reducing activity in Het ^- Nif⁺ recombinants clearly indicates the heterocyst to be an organ for protection of nitrogenase against oxygen toxicity.Abbreviations CFU Colony forming units - Ery erythromycin - Ery-R erythromycin resistance - het genotypic designation of genes required for heterocyst differentiation - Het phenotype designation of genes required for heterocyst differentiation - MSO l-Methionine-dl-sulfoximine - MSO-R MSO-resistance - N₂ medium Chu 10 medium without combined nitrogen - NH ₄ ⁺ medium basic mineral medium with ammonium nitrogen - nif genotype designation of genes required for N₂ fixation - Nif phenotype designation of genes required for N₂ fixation - NO ₃ ^- medium Chu 10 medium supplemented with KNO₃ - NTG N-methyl-N-nitro-N-nitrosoguanidine - r gene(s) regulatory gene(s) - Str streptomycin - Str-R streptomycin resistance - Str-S streptomycin sensitive     

Na and Cl transport and short-circuit current in rabbit ileum

Summary Na and Cl fluxes and short-circuit current (I _sc) in rabbit ileum have been studied as a function of ionic concentrations in HCO₃-free solutions. Both net Na flux (J _net ^Na ) andI _sc show similar saturation functions of [Na] at fixed [Cl]. They show no significant difference between zero and 112mm Na but at 140mm NaI _sc is significantly greater than theJ _net ^Na . Net Cl transport, secretion, is observed only at 140mm Na and is approximately equivalent to the difference between theI _sc andJ _net ^Na . The transcellular mucosa-to-serosa Na fluxes measured at 140 and 70mm Na do not differ significantly from the correspondingI _sc. The net Cl flux varies with [Cl] at fixed [Na] whileI _sc is virtually not affected by [Cl]. These results suggest that the absorptive Na transport process is electrogenic and responsible for theI _sc and that the secretory fluxes of Na and Cl are coupled, require high [Na], vary with [Cl], and do not contribute toI _sc. K-free solution abolishes theI _sc after a prolonged lag. Finally, the effect of a low resistance shunt pathway on active Na absorption is examined with a four-compartment model.Deceased (October 16, 1974).     

K<Superscript>+</Superscript> transport in the caterpillar intestine epithelium: role of osmolytes for the K<Superscript>+</Superscript>-secretory capacity of the tobacco hornworm midgut

The midgut of the tobacco hornworm, Manduca sexta, actively secretes potassium ions. This can be measured as short-circuit current (I_sc) with the midgut mounted in an Ussing chamber and superfused with a high-K⁺ saline containing as its major osmolyte 166 mM sucrose. Iso-osmotic substitution of sucrose by non-metabolisable compounds (mannitol, urea, NaCl and the polyethylene glycols 200, 400 and 600) led to a dramatic, though reversible, drop in the current. Acarbose, a specific inhibitor of invertase (sucrase) in vertebrates and insects, had no detectable influence on I_sc. Unexpectedly, after replacing sucrose iso-osmotically with the saccharides glucose, fructose, trehalose or raffinose, the K⁺ current could no longer be supported. However, all osmolytes smaller than sucrose (except for NaCl), metabolisable or not, initiated an immediate, quite uniform but transient, increase in I_sc by about 20%, before its eventual decline far below the control value. Hypo-osmotic treatment by omission of sucrose also transiently increased the K⁺ current. Small osmolytes substituted for sucrose caused no transient I_sc stimulation when the epithelium had been challenged before with hypo-osmolarity; however, the eventual decline in I_sc could not be prevented. Our data seem inconsistent with a role of sucrose as energiser or simple osmolyte. Rather, we discuss here its possible role as analogous to that of sucrose in lower eukaryotes or plants, as an extra- and/or intracellular compatible osmolyte that stabilises structure and/or function of the proteins implicated in K⁺ transport.Communicated by G. Heldmaier     

Studies of partially repressed mutants at the tamA and areA loci in Aspergillus nidulans

Summary Mutants, designated tamA ^r, have been isolated on the basis of simultaneous resistance to toxic analogues thiourea, aspartate hydroxamate and chlorate with L-alanine as the sole nitrogen source. tamA ^r mutants are also resistant to methylammonium. This resistance of tamA ^r mutants is correlated with partially repressed activity of a number of enzyme and transport systems regulated by ammonium. Furthermore, tamA ^r mutants have low NADP-glutamate dehydrogenase (NADP-GDH) activity and also efflux ammonium under certain growth conditions.Mutants at the areA locus (areA ^r) have also been isolated on the basis of resistance to these analogues, with nitrate or L-aspartate as the nitrogen source. These, similar to tamA ^r lesions, result in resistance to methylammonium and are partially repressed for ammonium repressible systems, but in contrast to tamA ^r, areA ^r alleles have wild-type NADP-GDH activity and normal ammonium efflux. tamA ^r and areA ^r mutants grow as wild type on all nitrogen or carbon sources tested, are recessive, and appear to be epistatic to all other mutations (gdhA1, meaA8 and meaB6) which result in derepressed levels of ammonium regulated system. Whereas tamA ^r and areA ^r phenotypes are additive, tamA ^r is epistatic to areA ^d phenotype.     

Complex alterations of the ribosomal gene spacers in mutant sc 8 of Drosophila melanogaster

Extreme changes in the proportions of the rDNA intergenic spacers (IGSs) have previously been demonstrated by us in the X-chromosomal rDNA array of the Drosophila melanogaster mutant sc ⁸ , where nearly all IGSs were found to be reduced in size. We have cloned different structural variants of the Xsc ⁸ ribosomal units and mapped their spacers by restriction endonuclease analysis. Most of the IGSs exhibited complex rearrangements within the subrepeated region at their 5′ terminus. The sequence data revealed the presence of an additional 100 bp subrepeat. In addition, extended deletions/substitutions down to the 18S gene were also found. Examination of the Ysc ⁸ IGS polymorphism indicates that some of the X-chromosomal IGS variants are probably the result of X-Y interchanges. The rarity of alternative recombination products implies that the overabundant deleted spacers in sc ⁸ are generated by nonreciprocal recombination. Our results demonstrate nonrandom distribution of deletional breakpoints within the “1900” region in sc ⁸ and show that the deletions do not truncate the internal IGS subregions at either breakpoint but eliminate them as discrete blocks. Received: 15 April 1997; in revised form: 8 August 1997 / Accepted: 22 August 1997     

Aldosterone regulation of active sodium chloride transport in the lizard colon (Gallotia galloti)

Summary Bioelectrical parameters and unidirectional sodium and chloride fluxes were measured under voltageclamp conditions in groups of lizards submitted to single or chronic aldosterone treatment. Both acute (AT) and chronic (CT) treatment induced significant increases in the short-circuit current (I _sc), as well as in the mucosa-to-serosa (J _m-s ^Na ) and net sodium flux (J _net ^Na ). In AT tissues, aldosterone did not change net chloride flux (J _net ^Cl ) but did so in CT tissues. Amiloride reduced the aldosterone-increased I _sc in AT and CT tissues, inhibited J _net ^Na in AT tissues and abolished it in CT colons. J _net ^Cl was also reduced by the diuretic in the group of AT colons, whereas no changes were observed in the CT tissues. Addition of luminal DIDS reduced Na⁺ absorption and totally inhibited Cl^- absorption in the AT tissues, but did not change I _sc. However, in CT tissues neither Na⁺ nor Cl^- transport were affected by DIDS. A good relationship between I _sc and J _m-s ^Na was apparent after DIDS treatment in AT tissues. In this group, simultaneous addition of DIDS and amiloride totally abolished J _net ^Na and reduced I _sc to untreated control values. Addition of serosal ouabain abolished I _sc and Na⁺ absorption in AT and CT colons, but Cl^- absorption was only altered in AT tissues. These results support the hypothesis that aldosterone induces an electrogenic, amiloride-sensitive sodium absorption, and in a dose-dependent fashion suppresses electroneutral NaCl absorption in the lizard colon.Abbreviations AT acutely treated - CT chronically treated animals - DIDS 4-4-diisothiocyanatostibene-2-2-disulfonic acid - DMSO dimethylsulphoxide - G _t tissue conductance - I _sc short circuit current - PD transepithelial potential difference - SITS 4-acetamido-4-isothiocyanatostilbene-2-2-disulfonic acid - UC untreated controls Preliminary results of this paper were presented at the X ^th meeting of the European Intestinal Transport Group (EITG), Askov Hojskole, Denmark, 16–19 September 1990     

Role of Zeste in transvection at the Bithorax locus of Drosophila

Summary We have re-examined the effect of the mutation z^a in the zeste locus on the phenotype of the bx ^34e /Ubx trans combination of bithorax mutations, in the presence and absence of rearrangements which presumably affect homologous pairing in the bithorax region. Our observations suggest the possibility of the zeste ⁺gene product being needed for transvection at the bithorax locus     

Mechanisms of Na and Cl absorption across the distal colon epithelium of the pig

Summary Porcine distal colon epithelium was mounted in Ussing chambers and bathed in plasma-like Ringer solution. Tissue conductances ranged from 10 to 15 mS and the short-circuit current (I_sc) ranged from-15 to 220 A·cm^-2. Variations in basal I_sc resulted from differences in the amount of amiloride (10M mucosal addition)-sensitive Na⁺ absorption. Ion substitution and transepithelial flux experiments showed that 10 M amiloride produced a decrease in the mucosal-to-serosal (M-S) and net Na flux, and that this effect on I_sc was independent of Cl^- and HCO ₃ ^- replacement. When the concentration of mucosal amiloride was increased from 10 to 100 M, little change in I_sc was observed. However, increasing the concentration to 1 mM produced a further inhibition, which often reversed the polarity of the I_sc. The decrease in I_sc due to 1 mM amiloride was dependent on both Cl^- and HCO ₃ ^- , and was attributed to reductions in the M-S and net Na⁺ fluxes as well as the M-S unidirectional Cl^- flux. Ion replacement experiments demonstrated that Cl^- substitution reduced the M-S and net Na fluxes, while replacement of HCO ₃ ^- with HEPES abolished net Cl^- absorption by reducing the M-S unidirectional Cl^- flux. From these data it can be concluded that: (1) Na⁺ absorption is mediated by two distinct amiloride-sensitive transport pathways, and (2) Cl^- absorption is completely HCO ₃ ^- -dependent (presumably mediated by Cl^-/HCO ₃ ^- exchange) and occurs independently of Na⁺ absorption.Abbreviations G_t tissue conductance - HEPES tris (hydroxymethyl) aminomethane - (tris) N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - I_sc short-circuit current - J_r residual flux - M-S mucosal-to-scrosal - S-M serosal-to-mucosal - TTX tetrodotoxin     

Mutations affecting regulation of the anabolic argF and the catabolic aru genes in Pseudomonas aeruginosa PAO

Summary The nucleotide sequence required for a fully functional promoter and operator of the Pseudomonas aeruginosa argF gene (argF _po), the arginine-repressible gene for anabolic ornithine carbamoyltransferase, was defined within a 160 by region. The streptomycin (Sm) resistance genes strAB of plasmid RSF1010 were fused to argF _po. This construct in P. aeruginosa strain PAO conferred resistance to Sm. Mutants of strain PAO were selected which were resistant to Sm in the presence of arginine due to constitutive expression of argF _po —strAB. These mutants were designated argR. They were unable to grow or grew poorly on arginine or ornithine as the sole carbon and nitrogen source. This growth defect (Aru^–/Oru^– phenotype) was correlated with a reduced level of N-succinylornithine aminotransferase, an enzyme participating in the major aerobic pathway for arginine and ornithine catabolism in this organism. The argR mutants were classified into four groups by transduction analysis and three argR mutations were mapped on the PAO chromosome. argR9901 and argR9902 were co-transducible with car-9 (at 1 min) and thus close to the oru-310 locus; argR9906 was localized in the oruI (=aru) gene cluster (67 min). Some aru mutants, which have been isolated previously and which produce very low amounts of all enzymes in the arginine succinyltransferase pathway, were unable to repress the argF gene in an arginine medium. Thus, P. aeruginosa PAO appears to have multiple genes that are involved in the regulation of both the anabolic argF and the catabolic aru genes.Abbreviations Arg^– arginine auxotrophy - Aru arginine utilization - Oru ornithine utilization     

Effects of tryptamine on active sodium and chloride transport in the isolated bullfrog cornea

The effects of the serotonin analogue, tryptamine, on the active transepithelial transport of Na⁺ and Cl⁻ in the in vitro bullfrog cornea were studied. Tryptamine, 1 mM, inhibited both the short-circuit current (I_sc) and potential difference (PD) of corneas transporting either Na⁺ alone or both Na⁺ and Cl⁻. The electrical resistance, R, increased in all cases. Both unidirectional Na⁺ and Cl⁻ fluxes were decreased by tryptamine and these changes accounted for the inhibitory effects on the I_sc. The effects of tryptamine were considered along with with those of 2 mM theophylline and 0.1 mM ouabain. Tryptamine inhibited the I_sc and both undirectional Cl⁻ fluxes which were previously stimulated by theophylline. Theophyline addition, after tryptamine preincubation, increased the Cl⁻ undirectional fluxes but did not restore the inhibited I_sc. The inhibitory effects of tryptamine on active Na⁺ and Cl⁻ transport were different from those of ouabain. While both drugs inhibited the forward Na⁺ and Cl⁻ fluxes, their backfluxes decreased with tryptamine and increased with ouabain. The addition to the bathing solution of tryptamine after ouabain preincubation reduced the ouabain-increased backward Cl⁻ flux and further increased the electrical resistance. These results are analyzed in terms of an electrical model from which it appears that tryptamine's mechanism of action was to decrease cellular permeability to the transepithelial movement of Na⁺ and Cl⁻.