A method for the estimation of gene flow parameters from a population structure caused by restricted gene flow and genetic drift

Summary A method has been developed which enables the estimation of the plant gene flow parameters _p (pollen dispersal), _s (seed dispersal) and t (outcrossing rate) from a selection-free continuously structured population in equilibrium. The method uses Wright's F-coefficients and introduces a new F-function which describes the genetic similarity as a function of the spatial distance. The method has been elaborated for wind pollinated plant species but can be modified for insect pollination and for animal species. In practice allozymes will provide for the necessary neutral genetic variation. The more loci used and the more intermediate the gene frequencies, the more reliable the results. For the estimation of _p and t together (when the outcrossing rate is not known) at least two chromosomally unlinked loci are required. The method for estimating _s depends on whether the plant species is annual or perennial. The mechanism of selfing has been analysed by the explanation of the value of t by three components: population density (d), pollen flow (_p) and relative fertilization potential of own pollen (Z). The concepts of neighbourhood size and isolation by distance, developed by Wright, who used a single gene flow parameter , have been extended to the situation which is realistic for seed plants, using all three parameters _p, _s and t. When _p is large with respect to _s, _s largely determines the value of the neighbourhood size, whereas _p is the most dominating factor in isolation by distance. The use of local effective population size and mean gene transport per generation instead of neighbourhood size and neighbourhood area, respectively, is proposed to avoid confusion. Computer simulations have been carried out to check the validity and the reliability of the method. Populations of 200 plants, using two or three loci with intermediate allele frequencies, gave good results in the calculation of _p with known value of t and of _s and N_e. With unknown t, especially with lower values of t, larger populations of at least 1,000 plants are necessary to obtain reasonably accurate results for _p and mean gene transport per generation M.Grassland Species Research Group Publication No. 81     

Foam behavior of biological media

Summary The surface tension and foaminess of (a) unlimited, (b) substrate limited, and (c) oxygen transfer limited growth media of Hansenula polymorpha were measured using methanol, ethanol or glucose as a substrate.The time dependence of can be described by the Avrami-Überreiter relationship: log (2.3 log V)=n log t+log b, where V = (_O – _eq/(_t – _eq, and _O, _t and _eq are at t_M=0, t_M=t and t_M (equilibrium value).The constants n and b are functions of the fermentation time t_F as long as the growth is unlimited but they are constant in the state of limited growth. With glucose substrate, the foaminess can be presented as a definite function of the time, t_DG, which is necessary to attain _eq. With alcohol as a substrate no definite (t_DG) function was found.Symbols b constant in Eq. (1) - n constant in Eq. (1) - S substrate concentration - T temperature - t_M time h (measured from the beginning of the determination of the surface tension ) - t_F cultivation time h (measured from the time of inoculation) - t_DG time (min) necessary to attain the equilibrium surface tension ) - X dry biomass concentration (gl^–1) - V (_O – _eq)/(_t – _eq) - V_S equilibrium volume of the foam (cm³) - V_G volumetric gas flow rate during the estimation of (cm³ s^–1) - vvm volumetric gas flow rate with regard to the volume of the medium (min^–1) - w_SG superficial gas velocity (cm s^–1) - _m maximum specific growth rate (h^–1) - V_S/V_G foaminess (s) - surface tension, mMm^–1 (milli Newton m^–1) - _O at t_M=0 - _eq equilibrium surface tension ( at t_M) - _t at t_M=t - HP probes from Hansenula polymorpha cultivation - NLG non limited growth - OTLG oxygen transfer limited growth - SLG substrate limited growth     

RNA polymerase sigma-related proteins in Escherichia coli: detection by antibodies against a synthetic peptide

Summary Antibodies were raised against a synthetic tetradecameric peptide with an amino acid sequence, DLIQEGNIGLMKAV, which corresponds to the most highly conserved region of bacterial RNA polymerase factors. In a Western-blot analysis of total Escherichia coli proteins, the antiserum reacted specifically with at least three proteins with apparent molecular weights of 75 kDa, 27 kDa and 23 kDa, in addition to the known factors (⁷⁰ and ³²). The majorities of ⁷⁰ and ³² were recovered as associated forms with the RNA polymerase on glycerol gradient centrifugation, while the other cross-reacting proteins were not. Unambiguous evidence was obtained which indicated that the intracellular level of ³² increased rapidly upon heatshock, at least in the strain containing high copy numbers of the rpoH gene.     

Hormonal regulation of gene expression in the “slender” mutant of barley (Hordeum vulgare L.)

The slender mutant of barley resembles a normal barley plant treated with high doses of gibberellic acid (GA₃). Expression of GA₃-regulated and abscisic acid (ABA)-regulated mRNAs was studied in the endosperm and roots of mutant and wild-type (WT) plants.Production of -amylase (EC 3.2.1.1) by WT embryoless half-grains was dependent on the presence of GA₃, and was prevented by ABA. In contrast, -amylase was produced by half-grains of the slender mutant in the absence of added GA₃, although it was still reduced by ABA. The spectrum of -amylase mRNAs in slender embryoless half-grains incubated in the absence of added GA₃ was the same as in WT endosperm half-grains incubated in the presence of GA₃. These results indicate that the endosperm of the slender mutant exhibits similar properties to WT endosperm treated with GA₃.In roots the expression of an ABA-inducible mRNA was similar in slender and WT seedlings either treated with exogenous ABA or exposed to dehydration. This result, and the effect of ABA on -amylase production by the endosperm, indicate that the slender plants retain sensitivity to ABA.Abbreviations ABA abscisic acid - AMV avian myeloblastosis virus - GA gibberellin - GA₁ gibberellin A₁ - GA₃ gibberellic acid - WT wild-type     

Theoretical studies on the necessary number of components in mixtures

Summary Theoretical studies on the optimal numbers of components in mixtures (for example multiclonal varieties or mixtures of lines) have been performed according to phenotypic yield stability (measured by the parameter variance). For each component i, i = 1, 2,..., n, a parameter u_i with 0 u_i 1 has been introduced reflecting the different survival and yielding ability of the components. For the stochastic analysis the mean of each u_i is denoted by u ₁ and its variance by _i ² For the character total yield the phenotypic variance V can be explicitly expressed dependent on 1) the number n of components in the mixture, 2) the mean of the _i ² 3) the variance of the _i ² 4) the ratio and 5) the ratio _i ² /² where denotes the mean of the u _i and _u ² is the variance of the u _j. According to the dependence of the phenotypic stability on these factors some conclusions can be easily derived from this V-formula. Furthermore, two different approaches for a calculation of necessary or optimal numbers of components using the phenotypic variance V are discussed: A. Determination of optimal numbers in the sense that a continued increase of the number of components brings about no further significant effect according to stability. B. A reduction of b % of the number of components but nevertheless an unchanged stability can be realized by an increase of the mean of the u _i by 1% (with and _u ² assumed to be unchanged). Numerical results on n (from A) and 1 (from B) are given. Computing the coefficient of variation v for the character total yield and solving for the number n of components one obtains an explicit expression for n dependent on v and the factors 2.-5. mentioned above. In the special case of equal variances, _i ² = _o ² for each i, the number n depends on v, x = (₀/)² and y = (_u/)². Detailed numerical results for n = n (v, x, y) are given. For x 1 and y 1 one obtains n = 9, 20 and 79 for v = 0.30, 0.20 and 0.10, respectively while for x 1 and arbitrary y-values the results are n = 11, 24 and 95.This publication is an extended version of a lecture given at the 1984-EUCARPIA meeting (Section Biometrics in Plant Breeding) in Stuttgart-Hohenheim (Federal Republic of Germany)     

The inheritance of host plant resistance and its effect on the relative infection efficiency of Magnaporthe grisea in rice cultivars

The inheritance of host plant resistance and its effect on the relative infection efficiency for leaf blast was studied in the crosses IR36/CO39 (partially resistant × highly susceptible) and IR36/IR64 (both partially resistant). On the natural scale, gene action appeared multiplicative. After log transformation, additive effects described most of the genetic variation in the cross IR36/CO39, while additive and dominance effects were about equal in magnitude in the cross IR36/IR64. Dominance was towards increased resistance. No transgressive segregation occurred in the cross IR36/CO39. The number of genes that reduce lesion number was estimated to be zero in CO39 and five or more in IR36. The cross IR36/IR64 showed transgressive segregation in both directions, and IR36 and IR64 each contain at least one gene that is not present in the other cultivar. The heritabilities (narrow sense) in the F₂ were low (range 0.06–0.16), while narrow sense heritabilities based on F₃ lines were much higher (range 0.41–0.68). Lesion numbers in F₃ lines were reasonably correlated with those in F₅ progenies derived from the same F₂ plant (r was±0.6 in both crosses). Partial resistance can be effectively improved by selecting the most resistant plants from the most resistant F₃ lines.     

Electric field-induced breakdown of lipid bilayers and cell membranes: A thin viscoelastic film model

Summary A simple viscoelastic film model is presented, which predicts a breakdown electric potential having a dependence on the electric pulse length which approximates the available experimental data for the electric breakdown of lipid bilayers and cell membranes (summarized in the reviews of U. Zimmermann and J. Vienken, 1982,J. Membrane Biol. 67:165 and U. Zimmermann, 1982,Biochim. Biophys. Acta 694:227). The basic result is a formula for the time of membrane breakdown (up to the formation of pores): =(/C)/( _m ² ₀ ² U ⁴/24Gh ³+T ²/Gh–1), where is a proportionality coefficient approximately equal to ln(h/2₀),h being the membrane thickness and ₀ the amplitude of the initial membrane surface shape fluctuation ( is usually of the order of unity), represents the membrane shear viscosity,G the membranes shear elasticity modules, _m the membrane relative permittivity, ₀=8.85×10^–12 Fm,U the electric potential across the membrane, the membrane surface tension andT the membrane tension. This formula predicts a critical potentialU _c ;U _c =(24Gh ³/ _m ² ₀ ² )^1/4 (for = andT=0). It is proposed that the time course of the electric field-induced membrane breakdown can be divided into three stages: (i) growth of the membrane surface fluctuations, (ii) molecular rearrangements leading to membrane discontinuities, and (iii) expansion of the pores, resulting in the mechanical breakdown of the membrane.     

An investigation of the role of solutes in the xylem sap and in the xylem parenchyma as the source of root pressure

Summary Solute osmotic potentials (_x) in the vessels of hydroponically grown maize roots were measured to assess the osmotic-xylem-sap mechanism for generating root pressure (indicated by guttation). Solutes in vessels were measured in situ by X-ray microanalysis of plants frozen intact while guttating. Osmotic potentials outside the roots (_o) were changed by adding polyethylene glycol to the nutrient solution. Guttation rate fell when _o was decreased, but recovered towards the control value during 3–5 days when _o was greater than or equal to –0.3 MPa, but not when _o was equal to –0.4 MPa. In roots stressed to _o = –0.3 MPa, _x, was always more positive than _o, and _x changed only slightly (ca. 0.05 MPa). Thus the adjustment in the roots which increased root pressure cannot be ascribed to _x, contradicting the osmotic-xylem-sap mechanism. An alternative driving force was sought in the osmotic potentials of the vacuoles of the living cells (_v), which were analysed by microanalysis and estimated by plasmolysis. _v showed larger responses to osmotic stress (0.1 MPa). Some plants were pretreated with abundant KNO₃ in the nutrient solution. These plants showed very large adjustments in _v (0.4 MPa) but little change in _x (0.08 MPa). They guttated by 4 h after _o was lowered to –0.4 MPa. It is argued that turgor pressure of the living cells is a likely alternative source of root pressure. Published evidence for high solute concentrations in the xylem sap is critically assessed.Abbreviations _o external water potential - _x osmotic potential of xylem sap - _v osmotic potential of vacuolar sap - EDX energy dispersive X-ray microanalysis - CSEM cryo-scanning electron microscope - LN₂ liquid nitrogen - PEG polyethylene glycol     

High crossability of wild barley (Hordeum spontaneum C. Koch) with bread wheat and the differential elimination of barley chromosomes in the hybrids

Four bread wheat (Triticum aestivum L.) cultivars, Aobakomugi, Chinese Spring, Norin 61 and Shinchunaga, were pollinated with five barley lines/cultivars consisting of three cultivated barley (Hordeum vulgare L.) lines, Betzes, Kinai 5 and OHL089, and two wild barley (Hordeum spontaneum C. Koch) lines, OUH602 and OUH324. Crossability, expressed as the percentage of embryo formation, varied from 0 to 55.4% among the cross combinations. The two wild barley lines generally had a higher crossability than the previously reported best pollinator, Betzes, and some Japanese wheat cultivars were better as the female parent than Chinese Spring. Ninety four hybrid plants were obtained from 250 embryos cultured, and their somatic chromosome numbers ranged from 21 to 36. Eighteen plants were mosaic in chromosome number. Twenty one-chromosome plants appeared most frequently (45.7%) followed by 28-chromosome plants (14.9%). C-banding analysis revealed that elimination of barley chromosomes was mainly responsible for the occurrence of aneuploid plants. In hypoploids derived from Betzes-crosses, chromosome 5 was preferentially eliminated as previously reported, while in hypoploids derived from OUH602-crosses, chromosome 4 was preferentially eliminated. The wild barley line OUH602 may be a useful parent for producing a new wheat-barley addition set because of its high crossability with wheat and a different pattern of chromosome elimination.     

Genotype effects on plant regeneration in callus and suspension cultures of Avena

Regenerative potential of the calli of nineteen genotypes of Avena sativa, Avena nuda, Avena byzantina and one interspecific hybrid were compared over three successive cultures. Highly significant genotype and genotype × subculture interactions were observed. Among the highest plant regenerable genotypes were Corbit (first subculture); GAF/Park and 88Ab3073 (second subculture); and GAF/Park and 87Ab5932 (third subculture). These genotypes regenerated on an average 10 to 17 plants each from a 200 mg callus mass after a 30 to 45 proliferation period. GAF/Park, a progeny of an interspecific cross, regenerated plants at a significantly higher level (11.85 plants/rep), followed by the similarly performing A. sativa (6.23 plants) and A. nuda (5.06 plants) genotypes, which were significantly higher than the A. byzantina genotypes (2.07 plants). Four genotypes were tested for their adaptability to suspension culture and plant regeneration potential by separating their cells and cell clusters into two sizes: larger and smaller than 3 mm. Larger clusters yielded plants for three genotypes GAF/Park, 88Ab3073, and Tibor. The smaller clusters only regenerated plants for GAF/Park and 88Ab3073. From one gram of callus used to initiate suspensions of GAF/Park and 88Ab3073, 119.9 and 18.8 plants, respectively, were regenerated. The plants regenerated for various genotypes from agar-solidified or suspension culture experiments had normal growth and seed set. This study confirms high and sustained regenerative capabilities of GAF/Park, a restricted genotype due to the weedy Avena fatua genetic background and identifies alternative genotypes, especially 88Ab3073 for future tissue culture and transformation studies.     

Potential induced in a spherical cell by an intracellular point source and an extracellular point sink

Summary The potential is calculated for all time, inside and outside a spherical cell for a point source of current inside the cell and a point sink located a finite distance outside the cell. The source and sink are step functions in time. An eigenfunction expansion is obtained, valid for arbitrary =_m a/_i , where _i and _m are the conductivities inside the cell and in the membrane, respectively, a is the cell radius and the membrane thickness. For small , the eigenfunction expansion is expanded in powers of . The time dependence of the potential contains transients with two widely differing time constants =C_m a/_i, where C_m is the membrane surface capacitance, and _m=/. Closed-form expressions are obtained for the two leading terms, for small , after the rapid transient is over. The remaining time dependence is only in the potential inside the cell, and is a simple exponential increase, independent of position within the cell. It is found that the transmembrane potential is insensitive to the location of the extracellular sink at long times, but not at short times. The dependence of the potential on location of source, sink, and observer is studied for long times after the quick transients are over. A uniqueness theorem is derived for the solution to Laplace's equation for the membrane boundary condition.This work was supported in part by NSF Grant No. GB-24965. Dr. Peskoff is the recipient of NIH Special Research Fellowship No. 1F03 GM 55849-01. Mr. Ramirez is a Ford Foundation Pre-doctoral Fellow.     

Amber mutations in the structural gene for RNA polymerase sigma factor of Escherichia coli

Summary Amber mutants of Escherichia coli K-12 affected in the structural gene (rpoD) for th subunit of RNA polymerase have been obtained from a strain harboring a temperature-sensitive amber suppressor (supF-Ts6) which is active only at low temperatures. These mutants grow normally at low temperature (30°C) but do not grow at high temperature (42°C) due to the inability to synthesize factor. In one mutant studied in detail (rpoD40), the rate of -factor synthesis at 30°C is about half that of the wild type and is decreased to 10%–15% within 1 h of incubation at 42°C. The synthesis of core polymerase subunits or bulk protein is virtually unaffected at least for 2 h. The defect of the mutant in synthesis and growth at high temperature can be suppressed by any of the amber suppressors tested (supD, supE or supF). RNA-polymerase holoenzymes prepared from the mutant cells carrying each of the suppressors (grown at 42°C) exhibit different thermostabilities attributable to alterations in the factor. The reduced synthesis in the mutant is accompanied by the synthesis of polypeptide tentatively identified as amber fragment. These results as well as the genetic mapping data indicate that the amber mutation (rpoD40) resides within the structural gene for the factor and directly affects synthesis upon inactivation of the suppressor at high temperature.     

Caseins of various origins and biologically active casein peptides and oligosaccharides: Structural and physiological aspects

Summary The first part of the present review is focused on structural aspects concerning the so far studied casein fractions of various origins: they are compared to the four classical major bovine caseins (_sl-, _s2- - and ). The calcium-sensitive casein fractions are always phosphorylated whereas -caseins are glycosylated. The study of the casein genes showed that the calcium-sensitive caseins diverged from a common ancestral gene and during the evolution, intergenic and intragenic duplications occurred. The considerable conservation of the phosphorylation sites emphasizes the importance of phosphorylated residues for the function of caseins, i.e. the formation of micelles and the binding of Ca²⁺. In -caseins all the prosthetic sugar groups are linked by O-glycosidic linkages: their number varies from 0 to 5 in bovine -casein and up to 10 in human -casein. The structures of the known -casein carbohydate moieties are described. Finally the milk clotting process (interaction -casein/chymosin) is compared to the blood clotting process (interaction fibrinogen/thrombin): a large number of similarities could be noted between both clotting phenomena.The second part of the review is devoted to the study of short casein peptides endowed with various biological activities. Some of them behaved as immunomodulators or casomorphins or angiotensin I converting enzyme inhibitors; others demonstrated an effect on platelet functions. A strategic zone containing immunostimulating and opioid peptides could be located in cow and human -caseins. Furthermore bitter peptides, emulsifying peptides, calcium absorption enhancing peptides, chymosin-inhibiting peptides, have also been described and several further properties have been attributed to the -caseinoglycopeptide; two tetrasaccharides isolated from the latter possess blood group activities.In conclusion caseins, the main milk proteins, should not only be considered as a nutriment but as a possible source of biologically active components.If, in the future, some of the discussed active peptides cannot be characterized in vivo, they can all, nevertheless, be synthesized and used either as food additives or in pharmacology.     

Bacterial RNA polymerases: structural and functional relationships

The essential role of DNA-dependent RNA polymerases in gene expression and the fact that the multimeric species are highly conserved throughout nature makes these enzymes a particular fascinating area of study. Here we shall review the conservation of structures and their relationship to function, especially in the multimeric eubacterial RNA polymerases, paying particular attention to the core subunit and to recent studies of -factors of both the ⁷⁰ and ⁵⁴ families. We shall conclude with a brief consideration of phage-encoded RNA polymerases and phage-mediated modification of the host enzyme, and of the evolution of RNA-synthesising enzymes.     

Surface charge density estimation by 9-aminoacridine fluorescence titration: improvements and limitations

A large number of surface charge density () and surface potential (_o) estimations have been based on 1) titrations of the fluorescence of 9-aminoacridine released from the diffuse double layer adjacent to negatively charged membrane surfaces by non-adsorbing monovalent and divalent cations, and 2) calculations using experimental data from the titration curves and the Gouy-Chapman theory of the diffuse double layer. In this paper we discuss the different simplifying approximations employed in the earlier calculations and recommend modified formulas for the calculations. The latter have been derived without any simplifying approximation concerning the ionic (electrolyte) composition of the titration assays. We also show that depends, to some extent, on the concentrations of buffer and vesicles in the assays and present experimental evidence that decamethonium (decane-1,10-bis-trimethylammonium), a bulky organic divalent cation, can be satisfactorily used for the estimation of under well-defined conditions, despite its putative interaction with membranes.Abbreviations 9-AA 9-aminoacridine - (DeM)²⁺ decamethonium - (DiM)²⁺ dimethonium - EDTA ethylenediaminetetraacetic acid - EGTA ethylene glyol-bis(-aminoethyl ether) N,N,N,N-tetraacetic acid - (HeM)²⁺ hexamethonium - MES 2-(N-morpholino) ethanesulfonic acid - MOPS 4-morpholinopropanesulfonic acid - PM plasma membrane - Tris tris(hydroxymethyl)aminomethane - surface charge density - _o surface potential Correspondence to: A. Bérczi     

Genetic architecture of a heterotic cross between two populations of maize

Summary The nature and magnitude of variability in the interpopulation cross of Mezcla Amarillo Selection (MAS), an introduction from CIMMYT, Mexico, and J607, a population developed in India using indigenous, American, and Yugoslavian germplasm, were studied. Interpopulation progenies developed by following the North Carolina Design I were evaluated at two locations. The additive genetic variance component in interpopulation cross, _A(12) ² , and in one population assuming the other population as tester, _A12 ² and _A21 ² were significant for all the traits evaluated, namely ear length, ear girth, kernel rows and days to silk, with one exception. For kernel rows, the dominance variance component, _A(12) ² , was also significant but it was smaller than _A(12) ² . The variance component due to dominance X location interaction, _DL(12) ² , was significant for all traits except kernel rows. In the case of ear length and ear girth, _DL(12) ² was greater than the other components. _AL(12) ² , _AL12 ² and _AL21 ² were not significant for any trait. Expected genetic advance indicated a superiority of half-sib reciprocal recurrent selection over full-sib reciprocal recurrent selection.     

Converging catabolism of 2,4,6-trinitrophenol (picric acid) and 2,4-dinitrophenol by Nocardioides simplex FJ2-1A

Initial F₄₂₀-dependent hydrogenation of 2,4,6-trinitrophenol(picric acid) generated the hydride -complex of picrate and finally the dihydride complex.With 2,4-dinitrophenol the hydride -complex of 2,4-dinitrophenolis generated. The hydride transferring enzyme system showed activity against several substituted2,4-dinitrophenols but not with mononitrophenols. A K_m-value of0.06 mM of the hydride transfer for picrate as substrate was found. The pH optimaof the NADPH-dependent F₄₂₀ reductase and for the hydride transferase were 5.5and 7.5, respectively. An enzymatic activity has been identified catalyzing the releaseof stoichometric amounts of 1 mol nitrite from 1 mol of the dihydride -complexof picrate. This complex was synthesized by chemical reduction of picrate and characterizedby ¹H and ¹³C NMR spectroscopy. The hydride -complex of 2,4-dinitrophenolhas been identified as the denitration product. The nitrite-eliminating activitywas enriched and clearly separated from the hydride transferring enzyme system byFPLC. 2,4-Dinitrophenol has been disproven as a metabolite of picrate (Ebert et al. 1999)and a convergent catabolic pathway for picrate and 2,4-dinitrophenol with thehydride -complex of 2,4-dinitrophenol as the common intermediate has been demonstrated.     

Somatic embryogenesis,plant regeneration and somaclonal variation in barley

In vitro culture of immature embryo and young leaf tissues was carried out with five cultivars of barley, Hordeum vulgare. Two cultivars (Albacete and Porthos) responded poorly from both types of explants, while the three others (Dissa, Golden Promise and Ingrid) produced a high frequency of embryogenic callus from these explants (25–60%). For Dissa and Ingrid, young leaf explants were slightly better than immature embryo explants for embryogenic callus induction, while immature embryo cultures of Golden Promise responded better than young leaf explants. Thus, there appears to be a significant genotype × explant interaction in the initiation of embryogenic callus in barley.Some phenotypic variants were detected among the regenerated plants of Golden Promise and Ingrid, most originating by epigenetic changes. Only in one case was the variant phenotype heritable, probably due to a mutation in the chloroplast DNA. Mitotic alteractions were not detected. Consequently, somaclonal variation did not appear to be a very frequent event in plants regenerated from 1- to 6- month-old cultures of barley.     

Membrane pathways for water and solutes in the toad bladder: II. Reflection coefficients of the water and solute channels

Summary Urea and water transport across the toad bladder can be separately activated by low concentrations of vasopressin or 8 Br-cAMP. Employing this method of selective activation, we have determined the reflection coefficient () of urea and other small molecules under circumstances in which the bladder was transporting urea or water. An osmotic method for the determination of was used, in which the ability of a given solute to retard water efflux from the bladder was compared to that of raffinose (=1.0) or water (=0). When urea transport was activated (low concentration of vasopressin), for urea and other solutes was low, (_urea,0.08–0.39;_acetamide, 0.55; _{ethylene glycol}, 0.60). When water transport was activated (0.1mm 8 Br-cAMP) _urea approached 1.0 _urea also approached 1.0 at high vasopressin concentrations. In a separate series of studies, _urea was determined in the presence of 2×10^–5 m KMnO₄ in the luminal bathing medium. Under these conditions, when urea transport is selectively blocked, _urea rose from a value of 0.12 to 0.89. Thus, permanganate appears to close the urea transport channel. These findings indicate that the luminal membrane channels for water and solutes differ significantly in their dimensions. The solute channels, limited in number, have relatively large radii. They carry a small fraction (approximately 10%) of total water flow. The water transport channels, on the other hand, have small radii, approximately the size of a water molecule, and exclude solutes as small as urea.