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Biomedical subjects

A C Meyer

Publications and source records attributed to A C Meyer.

At least 19 recordsLinked to original sources
PubMed2001

Estimation of quantal size and number of functional active zones at the calyx of Held synapse by nonstationary EPSC variance analysis.

At the large excitatory calyx of Held synapse, the quantal size during an evoked EPSC and the number of active zones contributing to transmission are not known. We developed a nonstationary variant of EPSC fluctuation analysis to determine these quantal parameters. AMPA receptor-mediated EPSCs were recorded in slices of young (postnatal 8-10 d) rats after afferent fiber stimulation, delivered in trains to induce synaptic depression. The means and the variances of EPSC amplitudes were calculated across trains for each stimulus number. During 10 Hz trains at 2 mm Ca(2+) concentration ([Ca(2+)]), we found linear EPSC variance-mean relationships, with a slope that was in good agreement with the quantal size obtained from amplitude distributions of spontaneous miniature EPSCs. At high release probability with 10 or 15 mm [Ca(2+)], competitive antagonists were used to partially block EPSCs. Under these conditions, the EPSC variance-mean plots could be fitted with parabolas, giving estimates of quantal size and of the binomial parameter N. With the rapidly dissociating antagonist kynurenic acid, quantal sizes were larger than with a slowly dissociating antagonist, suggesting that the effective glutamate concentration was increased at high release probability. Considering the possibility of multivesicular release and moderate saturation of postsynaptic AMPA receptors, we conclude that the binomial parameter N (637 +/- 117; mean +/- SEM) represents an upper limit estimate of the number of functional active zones. We estimate that during normal synaptic transmission, the probability of vesicle fusion at single active zones is in the range of 0.25-0.4.

Animals↗
PubMed2000

Secondary amenorrhea and infertility caused by an inhibin-B-producing ovarian fibrothecoma.

OBJECTIVE: To report a case of secondary amenorrhea and infertility caused by an inhibin-B-producing ovarian fibrothecoma. DESIGN: Case report. SETTING: Academic medical center. PATIENT: A 37-year-old woman with a 2-year history of secondary amenorrhea and infertility. INTERVENTION(S): Operative removal of a 5-cm ovarian fibrothecoma. MAIN OUTCOME MEASURE(S): Luteinizing hormone, FSH, E2, inhibin-B, TSH, and prolactin measured preoperatively and postoperatively. Immunostaining of tumor cells for inhibin and LH. RESULT(S): Preoperative hormone levels were as follows: FSH, 1.7 mIU/mL; LH, 23.4 mIU/mL; E2, 31 pg/mL; and inhibin B, 1,154 pg/mL. Three weeks postoperatively, the FSH was 1.5 mIU/mL, LH decreased to 7.1 mIU/mL, E2 increased to 276 pg/mL, and inhibin-B decreased to 17 pg/mL. The fibrothecoma did not stain for LH but was strongly positive for inhibin. Regular menstrual cycles resumed 28 days postoperatively. CONCLUSION(S): Inhibin-B produced by an ovarian tumor profoundly suppressed FSH levels and resulted in secondary amenorrhea and infertility. Use of sensitive and specific immunoassays for inhibin-A and -B may aid in the differential diagnosis of hormonally active ovarian tumors.

Adult↗
PubMed1999

Released fraction and total size of a pool of immediately available transmitter quanta at a calyx synapse.

The size of a pool of readily releasable vesicles at a giant brainstem synapse, the calyx of Held, was probed with three independent approaches. Using simultaneous pre- and postsynaptic whole-cell recordings, two forms of presynaptic Ca2+ stimuli were applied in rapid succession: uncaging of Ca2+ by flash photolysis and the opening of voltage-gated Ca2+ channels. The ensuing transmitter release showed a nearly complete cross-inhibition between the two stimuli, indicating the depletion of a limited pool of about 700 transmitter quanta. The pool size was confirmed in experiments using enhanced extracellular Ca2+ concentrations, as well as short, high-frequency stimulus trains. The results reveal a surprisingly large pool of functionally available vesicles, of which a fraction of about 0.2 is released by a single presynaptic action potential under physiological conditions.

Action Potentials↗
PubMed1995

Analysis of clonal CD8+ T cell expansions in normal individuals and patients with rheumatoid arthritis.

In the course of studying the circulating TCR repertoire in humans, we noted several individuals with an increase in the percentage of CD8+ T cells expressing a particular V region. In some cases, these CD8 expansions were dramatic, occupying over 40% of the total CD8 repertoire. Using a panel of mAbs to different TCR V regions, we found that over 30% of healthy adults (> 35 years of age) harbor an expansion that alters the peripheral blood CD8 TCR repertoire. A wide range of V regions were expressed by these expansions. Considering that the mAbs used cover only a portion of the V beta repertoire, the data suggest that over 70% of adults are likely to harbor such expansions. Junctional region sequencing showed that the CD8 subset expansions were clonal, and serial studies as long as 4 years showed that they persisted indefinitely. Expansions were not identified in the CD4 population. Discordant expression of one large V beta 6.7+ clone was found in one identical twin set, suggesting the possibility that an environmental exposure is involved in their generation and/or expansion. In one large family, we found five family members with a large CD8 subset expansion. Remarkably similar usage of J beta regions was noted, and two individuals demonstrated V beta 3-expressing clones with homologous CDR3 regions, differing by only one major substitution. The repertoire data from this family suggest that the T cell clones have arisen in response to a common Ag. Studies of patients with rheumatoid arthritis found a significantly increased frequency of circulating CD8 subset expansions that expressed a different V region repertoire compared with the healthy individuals studied. Overall, our results emphasize a frequent alteration in the human CD8 TCR repertoire, most likely related to an environmental exposure, in both healthy individuals and patients with rheumatoid arthritis. The presence of these expansions will be important to consider in any study of human TCR repertoire, and their implication for health and disease will be important to understand.

Adolescent↗
PubMed1986

Dihydropyridine calcium antagonists depress the amplitude of the plasma melatonin cycle in baboons.

An investigation into the effects of verapamil and some dihydropyridine derivatives on plasma melatonin levels was undertaken in baboons. In a number of separate experiments, groups of young male chacma baboons (mean body weight 13 kg) received intraperitoneal injections of the drugs, under ketamine anaesthesia, roughly 30 minutes prior to the following time points: 1200, 1800, 0000, 0200, 0600 and 1200 h. Lights went off at 1800 h and came on at 0600 h. The drugs used, and their respective dosages (expressed per kg body mass), were verapamil up to 4 mg/kg, nifedipine at 0.2 mg/kg, nitrendipine at 0.5 mg/kg and nisoldipine at 0.1 mg/kg. Blood samples, taken at the said time points, were assayed for melatonin. The nighttime peak of the plasma melatonin cycle was significantly depressed by all three dihydropyridine calcium antagonists (up to 40%), while verapamil, even at the relatively high total dose of 24 mg/kg per day, had no significant effect on the circulating plasma melatonin levels.

Animals↗
PubMed1981

Circadian changes in microtubules, synaptic ribbons and synaptic ribbon fields in the pinealocytes of the baboon (Papio ursinus).

In baboons kept under controlled lighting conditions, microtubules (MT) are readily seen in the perikaryal cytoplasm and in the perivascular processes of pinealocytes. A significant increase in the number of MT, single synaptic ribbons (SR) and the formation of synaptic ribbon fields (RF, i.e. organelles which consist of multiple dense rodlets or plates, and vesicles), occur during the dark phase of a circadian light-dark cycle. MT may act as "tracks" for the oriented flow of vesicles derived from the smooth endoplasmic reticulum, to cytoplasmic sites where RF are being formed. The origin of the dense rodlets of RF remains unknown. Structural differences between SR and RF indicate that the latter organelles are not directly involved in impulse propagation between adjacent baboon pinealocytes. RF may function as storage organelles for some of the pineal secretory products which are formed in large amounts during the dark phase.

Animals↗
PubMed1979

Microfilaments, the smooth endoplasmic reticulum and synaptic ribbon fields in the pinealocytes of the baboon (Papio ursinus).

Microfilaments (MF, 5-8 nm in diameter) are a prominent feature of the pinealocyte cytoplasm of baboons (Papio ursinus) kept under controlled lighting conditions. MF occurred as a filamentous network in these cells during the light phase of the diurnal light-dark cycle, while a close structural association was noted between MF and the membranes of the smooth endoplasmic reticulum (SER). This association was especially evident during the dark period. Increased numbers of single synaptic ribbons (SR, vesicle-crowned rodlets), together with large aggregations of SR, i.e., ribbon fields (RF), were seen in the pinealocyte cytoplasm of baboons killed during the dark phase. It is suggested that the vesicles of RF may arise from those of the SER and that MF may play a role in the movement of SER-vesicles to those areas of the cytoplasm where new RF are being formed.

Animals↗
PubMed1978

[25 Civitas].

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Humans↗
PubMed1978

Carcinoma of the breast. A clinical study.

An analysis of 1,686 surgically treated carcinomas of the breast in one community showed no statistically significant differences in five- and ten-year survival for simple, modified radical, or radical mastectomy. Further confirmation was obtained by computation of relative survival which in addition showed that older women more nearly approach normal life expectancy than younger ones. Bilaterality was found to be a decreasing function of age. Patients with medial and lateral tumors did not have significantly different rates of survival or sites of distant metastases.

Adult↗