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Biomedical subjects

T W White

Publications and source records attributed to T W White.

At least 55 records · Page 3Linked to original sources
PubMed1994

Null mutations of connexin32 in patients with X-linked Charcot-Marie-Tooth disease.

The X-linked form of Charcot-Marie-Tooth disease (CMTX) is associated with mutations in the gene encoding connexin32, a member of the family of proteins forming intercellular channels. We have compared the functional properties of three mutant connexin32 genes with those of the wild-type gene by testing their ability to form intercellular channels in the paired oocyte expression system. Whereas wild-type connexin32 induced the development of large junctional conductance between paired oocytes, no functional channels were detected between pairs expressing CMTX mutants. Furthermore, CMTX mutants selectively acted as dominant inhibitors of intercellular communication by interfering with the channel-forming ability of connexin26 but not with that of connexin40. These results demonstrate a functional loss in the product of a candidate gene for a demyelinating form of CMT.

Animals↗
PubMed1994

Selective interactions among the multiple connexin proteins expressed in the vertebrate lens: the second extracellular domain is a determinant of compatibility between connexins.

Gap junctions are collections of intercellular channels composed of structural proteins called connexins (Cx). We have examined the functional interactions of the three rodent connexins present in the lens, Cx43, Cx46, and Cx50, by expressing them in paired Xenopus oocytes. Homotypic channels containing Cx43, Cx46, or Cx50 all developed high conductance. heterotypic channels composed of Cx46 paired with either Cx43 or Cx50 were also well coupled, whereas Cx50 did not form functional channels with Cx43. We also examined the functional response of homotypic and heterotypic channels to transjunctional voltage and cytoplasmic acidification. We show that all lens connexins exhibited sensitivity to cytoplasmic acidification as well as to voltage, and that voltage-dependent closure of heterotypic channels for a given connexin was dramatically influenced by its partner connexins in the adjacent cell. Based on the observation that Cx43 can discriminate between Cx46 and Cx50, we investigated the molecular determinants that specify compatibility by constructing chimeric connexins from portions of Cx46 and Cx50 and testing them for their ability to form channels with Cx43. When the second extracellular (E2) domain in Cx46 was replaced with the E2 of Cx50, the resulting chimera could no longer form heterotypic channels with Cx43. A reciprocal chimera, where the E2 of Cx46 was inserted into Cx50, acquired the ability to functionally interact with Cx43. Together, these results demonstrate that formation of intercellular channels is a selective process dependent on the identity of the connexins expressed in adjacent cells, and that the second extracellular domain is a determinant of heterotypic compatibility between connexins.

Amino Acid Sequence↗
PubMed1994

Molecular cloning and functional characterization of chick lens fiber connexin 45.6.

The avian lens is an ideal system to study gap junctional intercellular communication in development and homeostasis. The lens is experimentally more accessible in the developing chick embryo than in other organisms, and chick lens cells differentiate well in primary cultures. However, only two members of the connexin gene family have been identified in the avian lens, whereas three are known in the mammalian system. We report here the molecular cloning and characterization of the third lens connexin, chick connexin45.6 (ChCx45.6), a protein with a predicted molecular mass of 45.6 kDa. ChCx45.6 was encoded by a single copy gene and was expressed specifically in the lens. There were two mRNA species of 6.4 kilobase (kb) and 9.4 kb in length. ChCx45.6 was a functional connexin protein, because expression in Xenopus oocyte pairs resulted in the development of high levels of conductance with a characteristic voltage sensitivity. Antisera were raised against ChCx45.6 and chick connexin56 (ChCx56), another avian lens-specific connexin, permitting the examination of the distribution of both proteins. Immunofluorescence localization showed that both ChCx45.6 and ChCx56 were abundant in lens fibers. Treatment of lens membranes with alkaline phosphatase resulted in electrophoretic mobility shifts, demonstrating that both ChCx45.6 and ChCx56 were phosphoproteins in vivo.

Amino Acid Sequence↗
PubMed1994

Expression of chimeric connexins reveals new properties of the formation and gating behavior of gap junction channels.

Direct intercellular communication occurs through specialized channels, which are formed by the interaction of two half-channels, or connexons, contributed by each of the two participating cells. The ability to establish intercellular communication is specified, in part, by the expression of different structural proteins, termed connexins. Connexins can control the establishment of intercellular communication by selectively pairing with some but not other family members. To characterize the protein domains that allow connexins to recognize and discriminate between alternative partners, we have created chimeras composed of selected regions of rat connexin43, which forms channels with Xenopus connexin38, and rat connexin32, which cannot. Pairs of Xenopus oocytes were used to test the ability of the chimeras to form homotypic channels with themselves, and heterotypic channels with the parent connexins or with endogenous Xenopus connexin38. While all hybrid molecules tested were efficiently expressed by oocytes, most were devoid of functional activity. A chimera consisting of connexin32 from the N terminus to the second transmembrane domain, fused to connexin43 from the middle cytoplasmic loop to the C terminus, designated as 3243H4, was able to pair functionally with Xenopus connexin38 and one of its parent connexins, connexin43. Voltage-dependent closure of heterotypic channels containing 3243H4 was asymmetric, exhibited novel characteristics that were not predicted by the behavior of the parent connexins and was dependent on the type of connexin with which 3243H4 was paired. In contrast, 3243H4 was unable to form functional channels with either itself or the other parent, connexin32. Together, these results suggest that these connexins are not composed of functionally exchangeable regions and that multiple domains, namely the middle cytoplasmic portion and the second extracellular domain, can influence the interactions between connexins present in adjacent cells. Furthermore, they indicate that voltage gating is not strictly intrinsic behavior for a given connexin, but can be modulated by the partner connexins to which they are paired. Finally, the finding that 3243H4 is functional only in heterotypic configurations, and cannot form homotypic channels, suggests the existence of a novel form of selectivity: self-discrimination. The latter property may represent another mechanism that operates to control the extent of communication between cells.

Animals↗
PubMed1994

Prepartum body condition and weight influences on reproductive performance of first-calf beef cows.

Data were collected on 476 spring-calving, primiparous cows at six locations in Louisiana over a 2-yr period. Objectives of the study were to evaluate changes in prepartum body condition score (BCS) and BW of heifers when fed varying energy levels and to determine the influences of prepartum BCS and BW changes and BCS at calving on postpartum traits. Heifers were allocated to one of three energy levels from approximately 90 d prepartum to parturition. The diets were formulated to provide low, recommended, and high energy levels based on TDN requirements. After calving, cows were placed on annual ryegrass pasture and managed similarly at each location. Weight and BCS at calving were greater (P < .05) for females with higher BCS at 90 d prepartum (IBCS) and for those assigned to higher energy levels. Calving and calf growth traits were not affected (P > .05) by IBCS, energy level, prepartum changes in BCS and BW, or BCS at calving. Prepartum changes in BCS and BW regulated by varying energy levels had no effect (P > .05) on postpartum reproduction; however, BCS at calving influenced (P < .03) pregnancy rate and days to pregnancy. Cows with BCS 6 and 7 at calving had higher (P < .05) pregnancy rates (87.0 and 90.7%) than those with BCS 4 and 5 (64.9 and 71.4%). Interval to pregnancy for cows with BCS 4 at calving was 10 to 18 d longer (P < .05) than for those with BCS > or = 5.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗
PubMed1992

Mouse Cx50, a functional member of the connexin family of gap junction proteins, is the lens fiber protein MP70.

The crystalline lens is an attractive system to study the biology of intercellular communication; however, the identity of the structural components of gap junctions in the lens has been controversial. We have cloned a novel member of the connexin family of gap junction proteins, Cx50, and have shown that it is likely to correspond to the previously described lens fiber protein MP70. The N-terminal amino acid sequence of MP70 closely matches the sequence predicted by the clone. Cx50 mRNA is detected only in the lens, among the 12 organs tested, and this distribution is indistinguishable from that of MP70 protein. A monoclonal antibody directed against MP70 and an anti-Cx50 antibody produced against a synthetic peptide identify the same proteins on western blots and produce identical patterns of immunofluorescence on frozen sections of rodent lens. We also show that expression of Cx50 in paired Xenopus oocytes induces high levels of voltage-dependent conductance. This indicates that Cx50 is a functional member of the connexin family with unique physiological properties. With the cloning of Cx50, all known participants in gap junction formation between various cell types in the lens are available for study and reconstitution in experimental systems.

Animals↗
PubMed1992

Influence of fish meal and supplemental fat on performance of finishing steers exposed to moderate or high ambient temperatures.

Ninety-six Hereford x Angus steers (mean initial BW = 295 kg) were used in two growth experiments conducted at moderate and high ambient temperatures (AT), 48 steers per AT. Within each AT, calves were assigned to six dietary treatments consisting of a basal diet (approximately 60% corn and 20% grass hay) supplemented with either 0, 2.5, or 5% fat and with either soybean meal (SBM) or Menhaden fish meal (FM) included at levels such that a ratio of 16.3 kcal of NEm per kilogram of CP was maintained. Blood and ruminal fluid were collected 40 d before slaughter. During the final 28 d of the moderate AT experiment, apparent digestibility of dietary components was measured in four individually fed steers from each dietary treatment. Steer ADG was not affected by fat, and DMI and efficiency of gain were not affected (P > .10) by treatment. Average daily gain was lower for steers fed FM than for those fed SBM at moderate AT but higher at high AT (CP source x AT interaction; P < .05). Ruminal ratio of acetate to propionate declined linearly with increasing fat at moderate AT but was not affected by fat at high AT (fat x AT interaction trend; P = .08). Plasma urea N concentration increased linearly (P < .05) with increasing fat and was higher (P < .05) in steers kept at high than in those kept at moderate AT. Although apparent digestibility was not altered in steers fed FM, DM and NDF (P < .05) and ADF (P = .07) digestibility decreased with increasing fat in steers fed SBM (CP source x fat interaction).(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗
PubMed1990

The flow of undigested corn residues through the gastrointestinal tract of cattle.

Two experiments were conducted to measure effects of source and level of roughage on the flow of corn residues through the gastrointestinal tract of cattle. In Exp. 1, steers (195 kg) were fed diets of ground corn with 0, 30 or 60% of ground Coastal bermudagrass hay (Cynodon dactylon) [L.] Pers.) at intakes of 1, 1.5 or 2% of BW in a 9 x 9 Latin square. Experiment 2 consisted of two 4 x 4 Latin squares with either rice hulls (square 1) or ground Coastal bermudagrass hay (square 2) providing 0, 7.5, 15 or 30% of the total diet fed at 1.5% of BW. After a 28-d adjustment period, a portion of the corn in one meal was replaced with cracked corn stained with brilliant green. The concentrations of stained corn residues appearing in the feces subsequent to dosing were fitted to a one-compartment, age-dependent model and compartmental mean residence time (CMRT) and time delay (tau) were estimated. In Exp. 1, increasing the level of intake of the ration from 1% to 1.5 or 2.0% of BW increased (P less than .05) CMRT by 52% and reduced (P less than .05) tau by 41%. In Exp. 2, source of roughage had no effect (P = .95) on CMRT or tau. Combined results of the two experiments indicated that increasing proportion (P) of either roughage was associated with an exponential decline in CMRT of stained corn residues (CMRT = 1211 * e-.0315P) from rations consumed at 1.5 and 2.0% of BW. No consistent effect of roughage type or proportion was noted on time delay in the two experiments collectively. These results indicate that increasing the proportion of roughage in the diet exponentially reduces residence time of corn residues in the ruminoreticulum (CMRT) without affecting residence time in the postgastric segments (tau).

Analysis of Variance↗
PubMed1989

The major maturation glycoprotein found on rat cauda epididymal sperm surface is linked to the membrane via phosphatidylinositol.

The experiments reported here further characterize a approximately 26[3H] kD cell surface glycoprotein that can be detected on rat cauda epididymal sperm using the galactose oxidase/NaB[3H]4 technique (1). When labeled sperm are treated with PI-PLC the 26[3H] kD is completely released from the cell. The released molecule can be recovered undegraded from incubation supernatant. Release by PI-PLC converts the hydrophobic, membrane-anchored form into a hydrophilic molecule as assessed by partition studies using Triton X114. Isoelectric focusing studies using both untreated (control) and PI-PLC treated samples shows that there is charge heterogeneity with two major peaks at pls of approximately 5.0 and approximately 4.5. We also show for the first time that the molecule persists on ejaculated cells.

Animals↗
PubMed1982

Simulated public speaking as a model of clinical anxiety.

Normal male volunteers took single acute doses of either diazepam or placebo under double-blind conditions in three simulated public speaking experiments. Measures of palmar sweating and subjective anxiety showed that anticipation of speaking in public increased anxiety relative to baseline and prestress conditions, and performance of public speaking further increased anxiety. A dose-related anxiolytic effect of diazepam on subjective anxiety supported the model's clinical relevance. Moreover, the intensity of the subject's public speaking phobia predicted both degree of prestress anxiety relief from 10 mg diazepam and overall anxiety level, regardless of medication, throughout the experimental session. A measure of traditionalism predicted placebo and 5 mg diazepam response during prestress: As in previous clinical trials, high traditionalism scorers reported more relief from placebo, whereas low scorers showed more relief from diazepam.

Adolescent↗
PubMed1977

Effects of instructions, biofeedback, and cognitive activities on heart rate control.

In a factorial experiment, 90 male and 90 female subjects were given (a) either instructions to increase heart rate (HR), decrease HR, or no instructions to change their HR; (b) either true biofeedback, false biofeedback, or no biofeedback; and (c) either instructions concerning cognitions to help them change HR or no instructions concerning cognitions. The results indicated that (a) for increasing HR, instructions to increase HR were as effective as the combination of instructions and true biofeedback; (b) for decreasing HR, neither instructions nor biofeedback nor the combination of instructions and biofeedback was more effective than simply sitting quietly; (c) although subjects instructed to increase HR showed higher HRs at the end of training than subjects instructed to decrease, the increase subjects showed a decline in HR from the initial period; (d) almost all subjects reported using cognitions to influence HR, and instructions concerning the use of cognitions did not facilitate changes in HR; and (e) women showed higher HRs, which declined more slowly than those of men, but sex did not interact with any treatment variable. It was concluded that increases (or retarded decreases) in HR were due to instructions, and decreases were due to simple adaptation, thus raising serious questions concerning the effectiveness of the biofeedback component of "biofeedback training" for altering HR.

Biofeedback, Psychology↗