Biomedical subjects

G B Martin

Publications and source records attributed to G B Martin.

At least 37 records · Page 2Linked to original sources

PubMed2003

Reproduction and plasma concentrations of leptin, insulin and insulin-like growth factor 1 in growth-hormone-transgenic female sheep before and after artificial insemination.

The transgenic sheep used in this study expressed an additional copy of the gene for ovine growth hormone (GH), so they had continuously high plasma concentrations of GH. They were used to test whether the GH transgene affected plasma concentrations of the metabolic hormones leptin, insulin-like growth factor 1 (IGF-1) and insulin, and whether these effects were associated with changes in conception, pregnancy or parturition following artificial insemination. Compared with control animals, the GH-transgenic sheep had higher bodyweight, lower body condition score and less subcutaneous fat (P < 0.05). These sheep also had lower plasma concentrations of leptin, higher plasma concentrations of insulin, and higher plasma concentrations of IGF-1 (P < 0.001). A similar proportion of GH-transgenic and control ewes came into oestrus, but the conception rate to artificial insemination was lower in GH-transgenic ewes than in the controls. Only four live lambs were recovered from 12 GH-transgenic ewes (33%) compared with 38 lambs from 43 controls (88%). This outcome was not associated with any difference in plasma progesterone profile in the period leading up to artificial insemination (Day 0). The GH-transgenic ewes had lower concentrations of FSH at all times measured (Day -19, Day -2 and Day 19). These results indicate that appropriate regulation of GH secretion from pituitary or peripheral tissues is necessary for normal reproduction and normal levels of metabolic hormones. Chronically high concentrations of GH were associated with increased levels of IGF-1 and insulin, and decreased levels of leptin.

Animals↗

PubMed2003

Use of a new drug delivery formulation of the gonadotrophin-releasing hormone analogue Deslorelin for reversible long-term contraception in male dogs.

In the present study, we tested the effect of treatment with a slow-release implant containing the gonadotrophin-releasing hormone agonist Deslorelin(TM) (Peptech Animal Health Australia, North Ryde, NSW, Australia) on pituitary and testicular function in mature male dogs. Four dogs were treated with Deslorelin (6-mg implant) and four were used as controls (blank implant). In control dogs, there were no significant changes over the 12 months of the study in plasma concentrations of luteinising hormone (LH) or testosterone, or in testicular volume, semen output or semen quality. In Deslorelin-treated dogs, plasma concentrations of LH and testosterone were undetectable after 21 and 27 days, testicular volume fell to 35% of pretreatment values after 14 weeks and no ejaculates could be obtained after 6 weeks. Concentrations returned to the detectable range for testosterone after 44 weeks and for LH after 51 weeks and both were within the normal range after 52 weeks. Semen characteristics had recovered completely by 60 weeks after implantation. At this time, the testes and prostate glands were similar histologically to those of control dogs. We conclude that a single slow-release implant containing 6 mg Deslorelin has potential as a long-term, reversible antifertility agent for male dogs.

Animals↗

PubMed2003

Partial resistance of tomato to Phytophthora infestans is not dependent upon ethylene, jasmonic acid, or salicylic acid signaling pathways.

We compared tomato defense responses to Phytophthora infestans in highly compatible and partially compatible interactions. The highly compatible phenotype was achieved with a tomato-specialized isolate of P. infestans, whereas the partially compatible phenotype was achieved with a nonspecialized isolate. As expected, there was induction of the hypersensitive response (HR) earlier during the partially compatible interaction. However, contrary to our expectation, pathogenesis-related (PR) gene expression was not stimulated sooner in the partially compatible interaction. While the level of PR gene expression was quite similar in the two interactions, the LeDES gene (which encodes an enzyme necessary for the production of divinyl ethers) was expressed at a much higher level in the partially compatible interaction at 48 h after inoculation. Host reaction to the different pathogen genotypes was not altered (compared with wild type) in mutant tomatoes that were ethylene-insensitive (Never-ripe) or those with reduced ability to accumulate jasmonic acid (def-1). Similarly, host reaction was not altered in NahG transgenic tomatoes unable to accumulate salicylic acid. These combined data indicate that partial resistance in tomato to P. infestans is independent of ethylene, jasmonic acid, and salicylic acid signaling pathways.

Cyclopentanes↗

PubMed2003

Fertility in male sheep: modulators of the acute effects of nutrition on the reproductive axis of male sheep.

Animals adjust the time of year that they reproduce through their ability to perceive and respond to critical aspects of their environment, such as photoperiod, nutrition or the socio-sexual milieu, and their genotype determines the degree of response to each stimulus. Ultimately, information from environmental cues filters through to the GnRH neurones in the brain which are the primary regulator of fertility. Each of these cues has been studied in isolation and the mechanisms by which they affect GnRH secretion are now better, if not fully, understood. In the field, the brain centres that control GnRH must integrate information from all cues at any given time before 'formulating a reproductive decision'. In this review, the effect of this integration is illustrated by showing how the acute GnRH response to a nutritional signal can be modulated by genotype, photoperiod and social cues, to the point of being completely blocked under some circumstances. Candidate pathways that may mediate these modulatory effects at both the whole body and brain have been proposed, although none of these pathways are confirmed and some have not yet been studied. As a guide for further research, we propose a working model that integrates the inputs and explains the interactions between them.

Animal Nutritional Physiological Phenomena↗

PubMed2002

Folliculogenesis and ovarian expression of mRNA encoding aromatase in anoestrous sheep after 5 days of glucose or glucosamine infusion or supplementary lupin feeding.

Improved nutrition increases ovulation rate in sheep and there is evidence that intra-ovarian pathways mediate responses to nutrition. An experiment was conducted to examine the effect of dietary energy on folliculogenesis. Anoestrous Merino ewes were fed a diet of wheat straw alone (control, n = 5), or wheat straw supplemented with lupins (500 g day(-1), n = 5). Other ewes were fed wheat straw and infused with glucose (50 mmol h(-1), n = 5) or with glucosamine (3.5 mmol h(-1), n = 5). Intravaginal progestagen sponges were inserted for 12 days, and nutritional treatments were started 5 days before sponge removal. At sponge removal, the ewes were injected with a regimen of GnRH pulses (500 ng every 4 h from 0 to 12 h; 250 ng every 2 h from 14 to 24 h; and 200 ng every 1 h from 25 to 36 h) to simulate normal follicular development. Thirty-six hours after sponge removal, the animals were killed and the ovaries were collected and stored at -80 degrees C. The ovaries were sectioned serially every 10 microm. Every 20th section was stained (to estimate number and diameter of follicles) and every 17-19th section was probed by in situ hybridization for P(450) aromatase. Data were analysed using ANOVA and chi-squared tests. There was an effect of treatment (P < 0.05) on the number of follicles 2-3, 3-4 and 6-7 mm in diameter. Aromatase-positive follicles (1.6-7.9 mm) were detected in 31 follicles from 15 ewes across all four groups. In ten animals, the largest follicle was aromatase-positive. The diameters of aromatase-positive follicles were larger (P = 0.004) in lupin fed compared with glucose-infused ewes (4.9 +/- 0.5, 3.6 +/- 0.7, 5.3 +/- 0.5 and 4.2 +/- 0.5 mm for control, glucose-infused, lupin-fed and glucosamine-infused groups, respectively). Treatment did not affect the plasma concentration of FSH when compared with controls, indicating that the energy supplements were modifying recruited (2-3 mm and 3-4 mm) and selected follicles (> 6 mm) directly. In conclusion, dietary energy can directly stimulate folliculogenesis in recruited and selected follicles, and this effect may be mediated by changes in systemic leptin concentrations and the hexosamine energy-sensing pathway in the follicle.

Analysis of Variance↗

PubMed2002

The mature male sheep: a model to study the effects of nutrition on the reproductive axis.

We have been using mature male sheep to study the ways in which nutrition affects reproduction, with a focus on the brain-pituitary axis and, in particular, GnRH activity. The sheep model has four major advantages for such studies. Firstly, sheep are large enough to support long periods of frequent serial sampling of peripheral blood, hypophyseal portal blood and cerebrospinal fluid from the cerebral ventricles. Importantly, this can be done with freely moving animals and, thus, avoids many of the potential complications associated with restraint. The second advantage, particularly relevant to nutrition-reproduction interactions, is the vast history of nutritional research for this species, providing us with techniques (for example, gut cannulation) and an extensive database on the requirements of sheep for energy, protein and specific dietary components such as amino acids, fatty acids and trace elements. Thirdly, subtle manipulations of diet can be used that cover the range seen in the real world, where animals (including humans) rarely encounter the uninterrupted, ad libitum food supplies that are normal for laboratory animals. Within this normal range of conditions, clear reproductive outcomes can be measured without resorting to starvation and both short- and long-term responses can be studied. Finally, the sheep model has an intrinsic economic relevance and findings from sheep can be transposed readily to other agricultural mammals. The sheep model is also relevant to human biology, often directly, but also indirectly because it often leads us to question the significance of findings from studies with rodents. Using mature male sheep to study the reproductive endocrine responses to acute and chronic changes in diet, we have shown that glucose does not appear to be involved directly, but that fatty acids can stimulate GnRH-dependent pathways that initiate changes in testicular function. Our work also indicates that GnRH-independent (perhaps also neuroendocrine) mechanisms may be involved. In the brain, it seems likely that intracerebral insulin and orexin are important mediators of the GnRH response to nutritional changes, but it is unlikely that leptin plays a role, at least in mature animals.

Animal Nutritional Physiological Phenomena↗

PubMed2001

Ancient origin of pathogen recognition specificity conferred by the tomato disease resistance gene Pto.

We have investigated the origin of the Pto disease resistance (R) gene that was previously identified in the wild tomato species Lycopersicon pimpinellifolium and isolated by map-based cloning. Pto encodes a serine-threonine protein kinase that specifically recognizes strains of Pseudomonas syringae pv. tomato (Pst) that express the avirulence gene avrPto. We examined an accession of the distantly related wild species Lycopersicon hirsutum var. glabratum that exhibits avrPto-specific resistance to Pst. The Pst resistance of L. hirsutum was introgressed into a susceptible Lycopersicon esculentum background to create the near-isogenic line 96T133-3. Resistance to Pst(avrPto) in 96T133-3 was inherited as a single dominant locus and cosegregated with a restriction fragment length polymorphism detected by the Pto gene. This observation suggested that a member of the Pto gene family confers Pst(avrPto) resistance in this L. hirsutum line. Here we report the cloning and characterization of four members of the Pto family from 96T133-3. One gene (LhirPto) is 97% identical to Pto and encodes a catalytically active protein kinase that elicits a hypersensitive response when coexpressed with avrPto in leaves of Nicotiana benthamiana. In common with the Pto kinase, the LhirPto protein physically interacts with AvrPto and downstream members of the Pto signaling pathway. Our studies indicate that R genes of the protein kinase class may not evolve rapidly in response to pathogen pressure and rather that their ability to recognize specific Avr proteins can be highly conserved.

Base Sequence↗

PubMed2001

Arabidopsis genome sequence as a tool for functional genomics in tomato.

Tomato is a well-established model organism for studying many biological processes including resistance and susceptibility to pathogens and the development and ripening of fleshy fruits. The availability of the complete Arabidopsis genome sequence will expedite map-based cloning in tomato on the basis of chromosomal synteny between the two species, and will facilitate the functional analysis of tomato genes.

Arabidopsis↗

PubMed2001

Innate immunity in plants.

Studies of receptors and signal-transduction components that play a role in plant disease resistance have revealed remarkable similarities with innate immunity pathways in insects and mammals. In plants, specific receptors encoded by disease-resistance genes interact with products of microbial effector genes to activate defence responses. Resistance proteins have been found to have motifs in common with components of immune response pathways in mammals and invertebrates, and to rely on similar downstream signalling components. In the future, the sharing of ideas among plant and animal biologists is likely to broaden our understanding of defence responses in diverse organisms.

Animals↗

PubMed2001

Photoperiodic control of the concentration of luteinizing hormone, prolactin and testosterone in the male emu (Dromaius novaehollandiae), a bird that breeds on short days.

The objective of this study was to establish, for a short-day breeding bird, the male emu, whether the breeding season is principally controlled by changes in photoperiod, and to investigate the endocrine mechanisms involved. Two groups of adult males were subjected to three alternating periods of 150-185 days of 14 h light/day (LD) and 10 h light/day (SD) terminating in a 360-day period of LD or SD. Transfer from LD to SD led to increases in plasma concentrations of luteinizing hormone (LH) and testosterone, after 82 +/- 8 and 73 +/- 3 (SEM) days, and an increase in prolactin concentrations after 115 +/- 12 days. Concentrations of LH and testosterone began to decrease before transfer back to LD, at a time when prolactin concentrations were approaching peak values. Transfer from LD to 360 days of SD resulted in increases in LH and testosterone concentrations, and these terminated after an increase in prolactin concentrations. After transfer from SD to 360 days of LD, plasma concentrations of LH and testosterone began to increase, after delays of 222 +/- 24 and 225 +/- 13 days, and were high at the end of the study, while prolactin values remained depressed throughout. These observations clearly show that seasonal breeding in the emu is directly controlled by changes in photoperiod. The dynamics of the hormonal responses to change of photoperiod suggest that, despite being short-day breeders, the photoregulation of breeding in emus involves mechanisms that are currently accepted for birds, rather than mechanisms that have been proposed for short-day breeding mammals. The initiation of breeding in emus is due to dissipation of photorefractoriness by short days which leads to an increase in the secretion of gonadotrophins to levels that are sufficient to support full reproductive condition. The termination of breeding, while days are still short, is due to the antigonadotrophic action of prolactin which, unusually for birds, increases while the days are still short. In conclusion, breeding activity in male emus is strongly controlled by photoperiod. Emus are short-day breeders, but the central mechanisms that regulate the secretion of reproductive hormones seem to be similar to those previously proposed for long-day breeding birds. The pattern of prolactin secretion in emus suggests an important role for this hormone in the termination of the breeding cycle.

Animals↗

PubMed2001

Plasma thyroid hormones and growth hormone in embryonic and growing emus (Dromaius novaehollandiae).

Growth hormone (GH), thyroxine (T4) and tri-iodothyronine (T3) are known to be involved in the regulation of growth and development in a variety of avian species. It has been suggested that an absence of GH and thyroid hormones in ostriches is the cause of their neoteny, a phenomenon in which juvenile characteristics are retained into adulthood. Neoteny is typical of all ratites, the single group of flightless birds that includes the ostrich, but similar endocrine studies have not been performed for other members of the group, such as the emu. To test the neoteny hypothesis further, in the present study we measured the plasma concentrations of T4, T3 and GH in emus during embryonic development and from hatching to 1 year of age. Concentrations of T4 and GH increased during the last weeks of incubation, whereas concentrations of T3 were highly variable. After hatching, the concentrations of both thyroid hormones were high during the first 3 days of life and then fell to a constant low level. Plasma concentrations of GH were high at the time of hatching and decreased gradually over the first 22 weeks of age; thereafter, the concentrations of GH were highly variable. No correlation was observed between hormone concentrations and live weight at any time. These results support the hypothesis that thyroid function is abnormally low in ratites, whereas patterns of GH secretion are similar to those observed in other birds. Dysfunction of the thyroid axis could explain, in part, the neotenous physical aspect of adult emus.

Animals↗

PubMed2001

Modelling reproduction in farm animals: a review.

During the last few decades, our knowledge of the biology of the reproductive system has improved significantly and also become more quantitative. In parallel, there has been an increase in the development of mathematical models, as both research tools and management tools, that can integrate our knowledge of reproductive events and then attempt to predict reproductive efficiency. This review considers these modelling approaches according to the level of organization that they address, from representation of the dynamic functions of the reproductive axis to the prediction of reproductive performance of a flock or herd. We emphasize the contribution of modelling to the exploration and understanding of a complex system-a system that responds to a host of internal regulatory mechanisms as well as to an array of external factors such as nutrition, photoperiod and sociosexual signals. To date, models have proven useful for depicting the kinetics of some of the endocrine signals that are implicated in perturbations of the reproductive axis and they have also been successful in the exploration of ovarian dynamics during the oestrous cycle. However, two areas are still largely unexplored by biomathematical models: the dynamic links between the different stages of the reproductive life of an animal, and mechanistic influences of environmental factors on the reproductive axis.

Animals↗

PubMed2001

Effect of nutritional supplementation on quantities of glucose transporters 1 and 4 in sheep granulosa and theca cells.

The stimulatory effect of nutritional supplementation on ovarian activity in sheep has been linked to an increase in glucose availability that, with insulin, directly decreases follicular steroidogenesis. Glucose uptake occurs by glucose transporters, but it is not known which glucose transporters are present in the sheep ovary or whether they are affected by nutritional stimulation. The aim of this study was to determine whether widely distributed glucose transporter 1 (GLUT1) or insulin-responsive GLUT4 are present in the granulosa or theca cells of sheep ovarian follicles, and whether their concentrations are affected by nutritional stimulation. Merino ewes (n = 49-51 per group) were stimulated nutritionally for 5 days before luteolysis with lupin grain or with one of two regimens of a glucogenic mixture, administered orally, which increases blood glucose concentrations towards the upper end of the normal range. Water was used as a control. Ovaries (n = 3 per group) were dissected and the granulosa cells and thecal shell from individual follicles were examined for glucose transporters using western blotting. GLUT1 concentration was 7-18 times higher in the granulosa than in the theca cells. GLUT4 was detected at a similar concentration in both types of cell. Nutritional treatment had no effect on the concentration of GLUT1 or GLUT4 in either tissue, and did not increase ovulation rate, despite increased concentrations of glucose and insulin. Concentrations of glucose transporters were not correlated with follicular concentrations of oestradiol or androstenedione. The presence of GLUT1 and GLUT4 in the granulosa and theca of sheep follicles indicates that the transporters have a role within the ovary in the modulation of follicular function.

Animal Feed↗

PubMed2001

Use of a GnRH analogue implant to produce reversible long-term suppression of reproductive function in male and female domestic dogs.

Continuous low-dose administration of a GnRH analogue postpones oestrus in bitches and suppresses reproductive function in dogs. A new drug delivery formulation that could enhance the practicality of this approach for the control of reproduction has been developed. The objective of the present study was to determine whether this method of delivery could, by sustained release of the GnRH analogue deslorelin, act as a reversible anti-fertility agent in domestic male and female dogs for periods exceeding 1 year. Several long-term studies were performed, which monitored reproductive function in 30 dogs and 52 bitches. Suppression of reproductive function in male dogs was dose-related. Spermatogenesis was suppressed for more than a year in 14 of 16 dogs that received doses of > 0.25 mg deslorelin kg-1. In females, postponement of oestrus for periods of up to 27 months was observed, but there was no relationship between the stage of the oestrous cycle at the start of treatment and the duration of efficacy. Treatment-induced effects on fertility were reversible in both sexes. In summary, sustained release deslorelin implants were shown to elicit reversible long-term reproductive control in male and female domestic dogs.

Animals↗

PubMed2001

Suppression of the oestrous responses of bitches to the GnRH analogue deslorelin by progestin.

Studies were undertaken in Australia and Belgium to determine whether the initial pro-oestrous-oestrous responses of anoestrous bitches to treatment with deslorelin administered in a s.c. implant were inhibited by progestin treatment. Thirty-nine bitches of mixed breeding were treated daily with 2 mg megestrol acetate kg-1 body weight for 21 (group 1, n = 5) or 14 days (group 2, n = 10), or with 1 mg megestrol acetate kg-1 body weight for 14 days (group 3, n = 10). A deslorelin (6 mg) implant was placed s.c. on day 14 (group 1) or day 7 (groups 2 and 3) of treatment. Bitches not treated with progestin also received a deslorelin implant (group 4, n = 9) or were untreated controls (group 5, n = 9). Signs of pro-oestrus-oestrus were not observed in bitches in groups 1, 2 and 5, but were observed in bitches in groups 3 (4/10) and 4 (9/9). Four bitches in group 4 were mated, two of which became pregnant. The pregnancies failed at about day 40 of gestation and were associated with low plasma progesterone concentrations. Treatment with progestin inhibited the pro-oestrous-oestrous responses of bitches to deslorelin.

Animals↗

PubMed2000

AvrPto-dependent Pto-interacting proteins and AvrPto-interacting proteins in tomato.

The plant-intracellular interaction of the avirulence protein AvrPto of Pseudomonas syringae pathovar tomato, the agent of bacterial speck disease, and the corresponding tomato resistance protein Pto triggers responses leading to disease resistance. Pto, a serine/threonine protein kinase, also interacts with a putative downstream kinase, Pto-interactor 1, as well as with members of a family of transcription factors Pto-interactors 4, 5, and 6. These proteins are likely involved, respectively, in a phosphorylation cascade resulting in hypersensitive cell death, and in defense gene activation. The mechanism by which the interaction of AvrPto and Pto initiates defense response signaling is not known. To pursue the hypothesis that tertiary interactions are involved, we modified the yeast two-hybrid protein interaction trap and conducted a search for tomato proteins that interact with Pto only in the presence of AvrPto. Five classes of AvrPto-dependent Pto interactors were isolated, and their interaction specificity confirmed. Also, to shed light on a recently demonstrated virulence activity of AvrPto, we conducted a standard two-hybrid screen for tomato proteins in addition to Pto that interact with AvrPto: i.e., potential virulence targets or modifiers of AvrPto. By constructing an N-terminal rather than a C-terminal fusion of AvrPto to the LexA DNA binding domain, we were able to overcome autoactivation by AvrPto and identify four classes of specific AvrPto-interacting proteins.

Amino Acid Sequence↗

PubMed2000

Thr38 and Ser198 are Pto autophosphorylation sites required for the AvrPto-Pto-mediated hypersensitive response.

The tomato Pto kinase confers resistance to Pseudomonas syringae pv. tomato expressing the AvrPto protein. To elucidate the role of Pto autophosphorylation in disease resistance, eight sites autophosphorylated by Pto in vitro were identified by a combination of HPLC purification of tryptic phosphopeptides, MALDI-TOF/MS analysis and Edman degradation. Mutational analysis of the autophosphorylation sites revealed that Pto residues Thr38 and Ser198 are required for AvrPto-Pto- mediated elicitation of a hypersensitive response in the plant. Thr38, which is the main Pto autophosporylation site and is located outside the kinase catalytic domain, was also required for Pto kinase activity and its physical interaction with AvrPto, the Pti1 kinase and the transcription factor Pti4. Ser198, located in the Pto activation domain, was dispensable for kinase activity and for interaction with AvrPto. However, a mutation at this site resulted in altered Pto interactions with the Pti1 kinase and the Pto interactors of unknown function Pti3 and Pti10. These results suggest that autophosphorylation events at Pto Thr38 and Ser198 are required for signal transduction by Pto and participate in distinct molecular mechanisms.

Amino Acid Sequence↗

PubMed2000

Signal recognition and transduction mediated by the tomato Pto kinase: a paradigm of innate immunity in plants.

Plant disease resistance is the result of an innate host defense mechanism, which relies on the ability of the plant to recognize pathogen invasion and to efficiently mount defense responses. In tomato, resistance to the pathogen Pseudomonas syringae is mediated by the specific interaction between the plant serine/threonine kinase Pto and the bacterial protein AvrPto. This article reviews molecular and biochemical properties that confer to Pto the capability to function as an intracellular receptor and to activate a signaling cascade leading to the induction of defense responses.

Bacterial Proteins↗