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Biomedical subjects

R A Mead

Publications and source records attributed to R A Mead.

At least 19 recordsLinked to original sources
PubMed1994

Expression of epidermal growth factor receptor in the preimplantation uterus and blastocyst of the western spotted skunk.

The western spotted skunk is unique in that its blastocysts undergo a 180-220-day period of arrested development before implantation. We investigated the potential role of epidermal growth factor (EGF)-related growth factors in regulating uterine and embryonic development in this species by studying the status of EGF receptor (EGF-R) in these tissues during delayed implantation and resumption of embryonic development. The cell-specific distribution of EGF binding sites and the expression of EGF-R mRNA were assayed by autoradiography and Northern blot analysis, respectively. The size of EGF-R was determined by affinity cross-linking studies, and its bioactivity was examined by determining EGF-dependent subcellular protein tyrosine kinase (PTK) activity. EGF binding sites were localized in the uterine luminal and glandular epithelium, endometrial stroma, myometrium, and blood vessels during both stages of pregnancy. As examined by Northern blot hybridization, a cRNA probe specific to mouse EGF-R hybridized to poly(A)+ RNA of skunk uteri. Transcripts similar to those of mouse uterine EGF-R were identified. [125I]-EGF was cross-linked to a 170-kDa protein both in the uterus and in blastocysts collected during the delayed implantation and periimplantation periods. However, EGF-induced PTK activity was significantly elevated above background levels during the period of renewed embryonic development, but not during arrested embryonic development. The results suggest that EGF-related growth factors may play an important role in regulating embryonic development in this species and that a change in the number and/or functional status of the EGF-R may be a prerequisite for blastocyst activation and implantation in the spotted skunk.

Animals↗
PubMed1993

Effect of melatonin on pituitary secretion of prolactin in vitro during delayed implantation and the periimplantation period in the spotted skunk.

The pars tuberalis of the western spotted skunk is the only region of the brain known to bind 2-[125I]iodomelatonin, thus suggesting that this region of the pituitary might be essential in mediating the effects of melatonin in this species. Female skunks were killed during short- and long-day photoperiods. Hemisections of the adenohypophysis, with and without the pars tuberalis were cultured in the presence and absence of 10 nM melatonin to determine whether the pars tuberalis mediates the previously observed suppressive effects of melatonin on the seasonal increase in plasma prolactin levels in this species. Pituitaries obtained during long-day photoperiods secreted significantly more prolactin than those obtained during the short-day photoperiod. Melatonin had no effect on pituitary response to gonadotropin releasing hormone and failed to inhibit prolactin secretion during the 48 h culture period. However, melatonin significantly reduced the inhibitory effects of dopamine on prolactin secretion. The latter response was observed in both photoperiods and in the presence and absence of the pars tuberalis. These results suggest that the pars tuberalis does not mediate the inhibitory effect of melatonin on prolactin secretion in the absence of the hypothalamus.

Animals↗
PubMed1993

Embryonic diapause in vertebrates.

Embryonic diapause occurs in many species of vertebrates, but the physiological mechanisms which control this fascinating process are exceedingly different in the diverse groups which employ this reproductive strategy. In nonmammalian species and some bats, reduction in rate of embryonic development is temperature dependent, but this is not the case in most mammals. Development becomes arrested at the blastocyst stage of embryogenesis in mammals which exhibit delayed implantation, whereas postimplantation development is continuous but retarded in species exhibiting delayed development. The hormonal control of diapause is remarkably different in the various species. Pituitary secretion of prolactin prevents implantation in the tammar wallaby but hastens renewed development and implantation in the mink and spotted skunk. Ovariectomy results in the eventual death of blastocysts in mustelids but induces renewed development and implantation in the armadillo. Luteal function, as evidenced by elevated progesterone secretion, is essentially constant in the roe deer and armadillo, whereas the luteal cells fail to complete their differentiation and secrete low levels of progesterone in carnivores. Progesterone will induce implantation in the tammar wallaby, but estrogen is required to induce renewed development and implantation in rodents. Neither progesterone and/or estrogens appear to be capable of stimulating implantation in carnivores. The uterus plays an important role in maintaining the embryos in a viable state throughout the period of diapause. In many species the uterus undergoes histological changes and secretes increased amounts of protein, yet we still do not understand the role, if any, these proteins play in initiating renewed embryonic development. Thus the phenomenon of embryonic diapause still holds many mysteries for scientists to solve.

Animals↗
PubMed1993

Role of ovarian steroids in development of uterine binding sites for prolactin in the ferret.

The objectives of this study were to investigate 1) the presence of specific prolactin (PRL) binding sites in the ferret uterus, 2) the uterine location of 125I-labeled ovine PRL (oPRL) binding sites, 3) changes in uterine PRL binding sites during pseudo-pregnancy, and 4) regulation of PRL binding sites by ovarian steroids. Binding was determined through use of 125I-oPRL and 300-800 micrograms of protein from the 50,000 x g particulate fraction. Optimal binding occurred within 6 h at 25 degrees C. Scatchard analysis of saturation data revealed a single set of high-affinity (Kd = 4.99 x 10(-11) +/- 0.88 M), low-capacity (22.76 +/- 1.62 fmol/mg) binding sites. Analysis of hormonal specificity revealed that ovine growth hormone (oGH) cross-reacted with oPRL for the uterine binding sites, displacing 38% of the bound ligand. However, no inhibition of 125I-oPRL binding occurred in the presence of a 500-fold excess of bovine thyroid-stimulating hormone (bTSH), ovine LH (oLH), or ovine FSH (oFSH), suggesting hormonal specificity of the binding sites that are located in the luminal and glandular epithelium. Prolactin binding to ferret uterine membranes increased during the first half of pseudopregnancy, plateaued between Days 21 and 28, and then declined. The concentration of PRL binding sites in uteri of ferrets on Day 1 of pseudopregnancy was 4.91 +/- 0.42 fmol/mg of protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Autoradiographic localization of binding sites for 2-[125I]iodomelatonin in the pars tuberalis of the western spotted skunk (Spilogale putorius latifrons).

Pineal melatonin secretion mediates photoperiodic regulation of implantation of blastocysts in the female western spotted skunk. Autoradiography studies demonstrated that high affinity 2-[125I]iodomelatonin binding sites are present in the pars tuberalis but not in any other hypophyseal region or in the hypothalamus or thalamus of this species. This restricted localization of 2-[125I]iodomelatonin binding, which was characteristic of spotted skunks sacrificed at various times during their annual reproductive cycles, suggests that the pars tuberalis mediates photoperiodic responses in this species.

Animals↗
PubMed1991

Prolactin levels in the western spotted skunk: changes during pre- and periimplantation and effects of melatonin and lesions to the anterior hypothalamus.

Prolactin (PRL) is the primary pituitary hormone responsible for initiating increased function of the corpus luteum and blastocyst implantation in the western spotted skunk. Therefore, we have designed experiments to validate a PRL RIA, characterize the preimplantation profile in PRL secretion, and determine the effects of exogenous melatonin and lesions to the anterior hypothalamic area (AHA) on PRL secretion in the skunk. These objectives were investigated with a heterologous RIA using canine PRL standards and antiserum. Displacement curves of skunk pituitary extract and serum were parallel to the canine PRL standard curve. Growth hormone-releasing hormone injection did not cause a significant change in plasma PRL levels as detected by the assay (p = 0.74). Injection of pimozide increased and bromocriptine decreased plasma PRL levels (p less than 0.05). A seasonal trend in plasma PRL levels was observed during the preimplantation period, with mean concentrations ranging from 5 ng/ml during the period of short day length in January to 17.1 ng/ml during the long day photoperiod in early May. The average date of blastocyst implantation in this study was 2 May (n = 16). Silastic capsules containing melatonin (n = 5) significantly delayed both the seasonal rise in plasma PRL levels and the time of implantation (p less than 0.05) compared to controls with empty capsules (n = 4). Lesions to the AHA (AHAx, n = 5) eliminated these effects of melatonin on both the rise in PRL and time of implantation. PRL levels were highly correlated with progesterone levels (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Testicular response to melatonin or suprachiasmatic nuclei ablation in the spotted skunk.

Testes of the Western spotted skunk enlarge and regress seasonally. The pineal hormone, melatonin, may be important in timing this seasonal reproductive activity. Likewise, the suprachiasmatic nuclei (SCN) have been implicated as possible neural regulators of pineal and reproductive events. These experiments were conducted to determine whether ablation of the SCN or constant administration of melatonin alters timing of the seasonal pattern of testicular regression and recrudescence. Male skunks (n = 24) were treated as follows: six received two empty Silastic capsules, six received two melatonin-filled Silastic capsules, six received sham lesions to the SCN, and six received lesions to the SCN (SCNx). All skunks were exposed to a natural photoperiod and had regressed testes at the onset of the experiment. Four of six males from the SCNx group had an average of 94 +/- 11.3% of these nuclei destroyed. Sham SCNx, animals with less than 40% of the SCN ablated, and males with empty capsules did not have fully enlarged testes until October. SCNx and melatonin-treated skunks exhibited a hastening of testicular recrudescence with maximal testis size being reached in June. Skunks with lesions to the SCN maintained enlarged testes for 5 months while all other groups exhibited rapid regression after attaining maximal testis size. Testicular regression occurred from July through October in animals receiving continuous melatonin, while controls exhibited recrudescence during this same period. Our data suggest that the SCN, melatonin, and thus the pineal gland, play a role in regulating the seasonal testicular cycle of the spotted skunk.

Analysis of Variance↗
PubMed1990

Photomanipulation of sexual maturation and breeding cycle of the steppe polecat (Mustela eversmanni) and other techniques for more rapid propagation of the species.

Twenty steppe polecats were divided into 2 groups, each consisting of 4 males and 6 females, and subjected to either a natural photoperiod (controls) or alternating periods of short (8 h light/16 hr dark for 8-9 weeks) and long days (16 h light/8 h dark for 16-20 weeks). The experimental photoperiod significantly accelerated sexual maturation in both sexes, with males developing maximal testis size within 57 days and females breeding after an average of 52 days exposure to 16L/8D. Males in the experimental group completed 2 1/2 testicular cycles and participated in mating during 3 successive breeding seasons during the 18 month period whereas males in the control group completed a single testicular cycle and only had an opportunity to mate during a single breeding season. Females in the experimental group produced 3 litters whereas females in the control group only gave birth to a single litter. Litter size averaged 6.9 +/- 2.0 (n = 23) and did not significantly differ with age, parity, or treatment. Pseudopregnant females returned to estrus within 12 days after the expected date of parturition, were bred, and gave birth to kits. Polecats which were subjected to the experimental photoperiods completed more molting cycles and underwent more photoperiod-induced changes in body weight than those in the control group. Death or removal of kits within 8 days after birth resulted in 12/12 females returning to estrus within 6-26 days. Eleven of these females were remated and gave birth to kits. Eight domestic ferrets readily accepted neonatal polecat kits and 5 successfully reared kits, although kit survival was quite poor.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Vitamin C disposition in young and elderly men.

The pharmacokinetics of vitamin C after a 500-mg oral tablet dose were compared in 15 healthy young men aged 25.20 +/- 2.54 (means +/- SD) and in 15 healthy elderly men aged 69.31 +/- 2.52 y. The doses were characterized with the subjects in two states of vitamin C nutriture: a depleted state, which was achieved by 4-5 wk of compliance with a vitamin C-restricted diet of less than 10 mg/d and a supplemented state, in which the subjects were given 500 mg vitamin C/d for 3 wk. Plasma and urine samples were collected for 72 h after the dose of vitamin C from depleted subjects and for 24 h from supplemented subjects and analyzed for vitamin C. Several of the pharmacokinetic indices measured were different in depleted vs supplemented subjects but none exhibited any age-related differences. This indicates that vitamin C nutriture affects vitamin C pharmacokinetics but age does not.

Administration, Oral↗
PubMed1990

Retention and metabolic fate of [3H]-melatonin in the spotted skunk.

Two experiments were designed to determine which tissues accumulate [3H]-melatonin and the metabolic fate of this hormone in the spotted skunk. Tritiated melatonin was injected into the jugular vein of 10 anesthetized skunks 1-3 h before the onset of darkness and allowed to circulate for 22 min before the vasculature was flushed with saline to clear radioactivity from the blood. Selected tissues were removed from five skunks and oxidized in a Packard Biological Oxidizer which yielded 95 +/- 5% recovery of radioactivity. Relatively high amounts of radioactivity were found in the pineal (367 +/- 304 dpm/mg tissue), ovary (69 +/- 38 dpm/mg), pituitary (89 +/- 56 dpm/mg), liver (107 +/- 29 dpm/mg), and kidney (63 +/- 15 dpm/mg). Relatively small amounts of [3H] were found in different brain regions (approximately 6-7 dpm/mg). The uterus, pancreas, and temporalis muscle also accumulated radioactivity (approximately 13 dpm/mg). The lung retained the least amount of radioactivity (4 +/- 1.3 dpm/mg). In the second experiment, hypothalami, pituitaries, and ovaries were removed from the remaining five females. Radioactivity from these tissues was extracted and subjected to thin-layer chromatography. Melatonin accounted for approximately 70% of the radioactivity recovered while 6-hydroxymelatonin and unidentified more polar compounds made up the majority of the melatonin metabolites. These data indicate that tissues other than the hypothalamus are able to accumulate [3H]-melatonin.

Analysis of Variance↗
PubMed1990

Reproductive cycle of the steppe polecat (Mustela eversmanni).

In laboratory conditions, in a natural photoperiod, testicular redevelopment began in late December. Maximal testis size was attained by the end of February. Testicular regression began in mid-May and was complete by the end of August. Oestrus was first observed in late March and continued throughout April. Females mated for the first time between 30 March and 8 April. Mating generally coincided with peak concentrations of urinary oestrone conjugates and when vaginal lavages contained greater than 90% cornified epithelial cells. Blastocyst implantation occurred by Day 13 and the post-implantation period was 29 days. Gestation ranged from 39 to 43 days and first parturition occurred in mid-May. Concentrations of urinary oestrone conjugates and free progesterone were elevated during the first half of pregnancy, reaching maximum values at mid-pregnancy, and then gradually declined as parturition neared. Litter size of primiparous females averaged 6.8 young/female. Females that were pseudopregnant or lost their litters shortly after birth, and several with weaned kits, exhibited a second oestrus.

Animals↗
PubMed1989

Psychological differentiation, arousal, and lipreading efficiency in hearing-impaired and normal children.

Psychological differentiation, psychophysiological arousal, hearing capacity, and ability to lipread unrelated sentences presented in a silent technicolor film were investigated in 62 children (mean age 10 years, 4 months), classified into three hearing categories (normal, mild to moderate hearing loss, severe hearing loss) based on their average hearing threshold in the range of speech frequencies for their best ear. All subjects were administered the Rod and Frame Test, Craig Lipreading Inventory, and skin conductance measures under two conditions (pre-task and task). The results showed that the ability to lipread sentences from a film was related significantly to psychological differentiation (cognitive style) and pre-task conductance level, but was not related to hearing capacity or task conductance level. The best predictors of lipreading ability were successful performance on the Rod and Frame Test and high increases in skin conductance from pre-task to task situation.

Arousal↗
PubMed1989

Role of prolactin and luteinizing hormone in regulating timing of implantation in the spotted skunk.

The western spotted skunk exhibits an obligate delay of implantation lasting 200-220 days. The pituitary is essential for luteal activation. The corpora lutea, in turn, secrete the hormones necessary for blastocyst implantation. Two experiments were designed to determine which pituitary hormones are responsible for increasing luteal activity and induction of implantation. Forty-two pregnant skunks with delayed implanting blastocysts were treated as follows: 13 served as untreated controls, 6 received 0.5 mg prolactin (PRL) daily, and 5 received diluent beginning in January. Four received 1.5 mg bromocriptine (CB-154) daily, 3 received both CB-154 and PRL, 3 received diluent, 5 received a gonadotropin-releasing hormone agonist (GnRHa) dispensed from osmotic minipumps, and 3 received diluent dispensed from osmotic minipumps starting in April. The skunks were subjected to a natural photoperiod. Duration of preimplantation and blood levels of progesterone and luteinizing hormone were measured. PRL significantly (p less than 0.05) shortened and CB-154 significantly (p less than 0.05) prolonged the duration of preimplantation when compared to controls (148 +/- 33.6 vs. 251 +/- 3.2 vs. 199 +/- 5.1 days, respectively). PRL was able to reverse the inhibitory effect of CB-154 when both were administered simultaneously (195 +/- 4.0 vs. 251 +/- 3.2 days). GnRHa had no significant (p greater than 0.05) effect on duration of preimplantation (199 +/- 5.1 days) when compared to controls (203 +/- 3.2 days). These results indicate that PRL is the primary pituitary hormone responsible for increased luteal activity and subsequent blastocyst implantation in the spotted skunk.

Animals↗
PubMed1989

Lesions to the anterior hypothalamus prevent the melatonin-induced lengthening of delayed implantation.

The anterior hypothalamic area (AHA) has been postulated as a site of action for melatonin. We tested the hypothesis that lesions to the AHA (AHAx) would counteract the inhibitory effect of exogenous melatonin on blastocyst implantation in the spotted skunk by removing a possible site of action. Forty-seven females were treated as follows during delayed implantation. In Exp 1, five received empty Silastic capsules, five received Silastic capsules containing melatonin, six received sham AHAx plus empty capsules, none received AHAx plus empty capsules, and eight received AHAx plus capsules containing melatonin. In Exp 2, four skunks each received two empty capsules, five skunks each received two capsules containing melatonin, and five skunks received AHAx plus capsules containing melatonin. All capsules were inserted sc in the interscapular region 14-35 days after surgery in Exp 1 and 2 weeks before surgery in Exp 2. Surgery was performed between January 22 and February 12, 1988, in Exp 1 and on March 2-3, 1989, in Exp 2. The skunks were subjected to a natural photoperiod, and the duration of preimplantation was measured. In Exp 1, AHAx plus empty capsules significantly (P less than 0.05) shortened the duration of preimplantation (163 +/- 14.7 days) compared to that in sham AHAx or intact controls (193 +/- 26.1 and 188 +/- 10.6 days, respectively). Melatonin significantly (P less than 0.05) prolonged the duration of preimplantation (289 +/- 2.9 days) in intact skunks, but failed to do so in skunks with AHAx, as the preimplantation period was significantly shortened (159 +/- 6.1 days). In Exp 2, AHAx reversed the inhibitory effect of melatonin on the duration of preimplantation (191 +/- 21.5 days), as intact melatonin-treated skunks had a significantly longer preimplantation period (260 +/- 2.5 days) than skunks receiving empty capsules (191 +/- 16.4 days). The inhibitory effect of melatonin was reversible in all intact skunks, as blastocysts implanted 23 days, on the average, after cessation of treatment with melatonin. These data are consistent with the hypothesis that a portion of the AHA and/or adjacent regions play an essential role in timing blastocyst implantation in the spotted skunk. The lesions may have given this result by ablating a neural pathway controlling PRL secretion and may or may not have involved a site of action for melatonin.

Animals↗
PubMed1989

Hormonal induction of oestrus and pregnancy in anoestrous ferrets (Mustela putorius furo).

Three doses of FSH were tested for their ability to induce oestrus in ferrets. A dose of 0.25 mg, administered twice daily, induced oestrus and breeding in most females within 6-13 days. Addition of 5 i.u. hCG during the final stages of follicular development enhanced the percentage of females with implantation sites (85%), and 23% of the ferrets so treated gave birth to kits, none of which survived for more than 3 days. This may have been due to insufficient prolactin secretion and thus inadequate luteal maintenance and milk production, as these females were maintained on a short-day photoperiod.

Anestrus↗
PubMed1988

Studies on the mechanism of action of prostaglandin F2 alpha induced luteolysis in rats.

The effects of prostaglandin F2 alpha (PGF2 alpha) administration on the utilization of low density lipoprotein (LDL) and progesterone secretion were examined in dispersed luteal cells from rat ovaries. Immature rats were rendered pseudopregnant with administration of pregnant mare serum gonadotropin and human chorionic gonadotropin. Animals were sacrificed at different times after PGF2 alpha (5 mg/kg) or vehicle administration on day-5 of pseudopregnancy. Administration of PGF2 alpha in vivo decreased human chorionic gonadotropin (hCG) binding to luteal cell membranes in vitro but enhanced binding of LDL. Utilization of labelled cholesterol for steroid synthesis from reconstituted LDL [(3H)-CL-LDL] by dispersed luteal cells was enhanced following PGF2 alpha administration. This suggests that the LDL pathway is not suppressed during prostaglandin induced luteolysis. Progesterone and total progestin secretion in response to N6-2'-0-Dibutyryladenosine 3'5'-cyclic monophosphate (cAMP) was decreased at 2, 4 and 24 hours following PGF2 alpha administration demonstrating a post-cAMP defect in steroidogenesis. Addition of the hydroxylated sterols, 20 or 25-OH cholesterol as substrate stimulated progesterone secretion in vehicle treated rats in a dose dependent fashion with 20-OH cholesterol being more potent. Progesterone secretion in response to stimulation with luteinizing hormone (LH) and cAMP from vehicle treated rats was less than that observed with 20 or 25-OH cholesterol, indicating that endogenous substrate may be a limiting factor in steroid synthesis. The maximal capacity of luteal tissue to produce progestins following PGF2 alpha administration was determined with 20-OH cholesterol as the substrate. The results suggest that the post-cAMP defect at 4 hours following PGF2 alpha administration may be due to failure of the cells to mobilize endogenous cholesterol. However at 24 hours following PGF2 alpha administration the decreased ability of luteal cells to convert cholesterol to pregnenolone may contribute to decreased progesterone synthesis.

Animals↗
PubMed1988

Partial characterization of a luteal factor that induces implantation in the ferret.

This study was designed to test the hypothesis that ferret corpora lutea (CL) secrete a compound that acts in conjunction with progesterone to induce blastocyst implantation and to identify the chemical nature of this compound. CL and the residual ovarian tissue, obtained predominantly on the ninth day of pseudopregnancy, were extracted with 0.05 M phosphate-buffered saline. The extracts were injected into pregnant ferrets that had been ovariectomized on Day 6 of pregnancy and had received Silastic implants containing progesterone. Aqueous luteal extracts, but not those of the residual ovarian tissue, induced implantation in test animals. Fractionation of the luteal extracts by passage through a series of filters with molecular weight (MW) cutoffs ranging from 500 to 50,000 consistently revealed that the biologically active fraction was retained on the filter with the highest MW cutoff employed. Moreover, blastocyst implantation failed to occur in ovariectomized, progesterone-treated ferrets after one-half of a luteal preparation (MW greater than 50,000) was incubated with a broad-spectrum protease. These data are consistent with the hypothesis that CL of the ferret secrete a protein during the preimplantation period that is essential for blastocyst implantation.

Animals↗