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Biomedical subjects

A K Chan

Publications and source records attributed to A K Chan.

At least 73 records · Page 4Linked to original sources
PubMed1986

Characterization of a mouse cell line selected for hydroxyurea resistance by a stepwise procedure: drug-dependent overproduction of ribonucleotide reductase activity.

Hydroxyurea was used as a selective agent in culture, to isolate by a stepwise procedure, a unique mouse L cell line called LHF which exhibited a stable resistance to high concentrations of drug (5 mM). LHF cells contained an elevation in ribonucleotide reductase activity which depended upon whether cells were previously cultured in the presence or absence of hydroxyurea. M1 immunoprecipitation and M2 titration experiments indicated that both ribonucleotide reductase subunits were elevated in drug-resistant cells. Interestingly, a very large drug-dependent change in the M2 activity (about a 100-fold) was observed. Studies on enzyme activity with cycloheximide and actinomycin D indicated that the hydroxyurea-dependent increase in activity required de novo protein synthesis and transcriptional activity. These results are different from other ribonucleotide reductase overproducing cell lines previously described, and indicate that hydroxyurea modulates enzyme activity by an interesting mechanism.

Animals↗
PubMed1986

Hydroxyurea-induced conversion of mammalian ribonucleotide reductase to a form hypersensitive to bleomycin.

Bleomycin is a commonly used chemotherapeutic agent known to cause extensive DNA damage. In this paper we show that bleomycin inhibits ribonucleotide reductase activity in mouse L-cells. The effectiveness of the drug is a result of its metal-chelating properties which enable it to inactivate the iron containing M2 subunit of the enzyme. A hydroxyurea-resistant mouse L-cell line was used to show that the degree of inhibition caused by bleomycin can be greatly enhanced if ribonucleotide reductase has been previously exposed in vivo or in vitro to agents, such as hydroxyurea, which destroy the tyrosine free radical of subunit M2. The increased effectiveness of bleomycin appears to result from a decrease in the stability of the iron center of protein M2 following exposure to hydroxyurea. These findings have important implications in terms of the use of bleomycin as an anticancer agent, especially in combination chemotherapy where it can be used with other drugs that act at ribonucleotide reductase.

Adenosine Diphosphate↗
PubMed1985

Ribonucleotide reductase: an intracellular target for the male antifertility agent, gossypol.

Gossypol is a yellow phenolic compound which reversibly inhibits spermatogenesis making it one of the few effective male antifertility drugs. The cytotoxic effects of gossypol have been associated with its ability to irreversibly inhibit DNA synthesis by a previously unknown mechanism. The results of this study indicate that gossypol is a potent inhibitor of ribonucleotide reductase the rate limiting enzyme activity in DNA synthesis. Furthermore, in agreement with these enzyme studies, DNA synthesis in a hydroxyurea resistant cell line with high levels of ribonucleotide reductase activity showed increased resistance to gossypol when compared to wild type cells with normal levels of reductase activity. Ribonucleotide reductase is the first specific site of action documented for gossypol which can explain its recently described antiproliferative, cell cycle and toxic effects.

Animals↗
PubMed1984

Verapamil cardioplegia: improved myocardial preservation during global ischemia.

To determine whether the calcium antagonist verapamil can produce satisfactory myocardial preservation during global ischemia, we studied three groups of eight dogs. Serial left ventricular biopsy specimens were taken for adenosine triphosphate and creatine phosphate content. Arterial and coronary sinus blood samples were obtained for lactate and oxygen content determination prior to ischemia, immediately after the ischemic interval, and after a 30 minute reperfusion period. Starling and isovolumetric ventricular function curves were determined prior to ischemic arrest and after 45 minutes of reperfusion. All animals were systemically cooled to 25 degrees C, and the aorta was clamped for 120 minutes. Group I had a potassium cardioplegic solution (30 mEq/L) chilled to 4 degrees C and injected into the aortic root. The initial dose was 200 ml and an additional 100 ml was infused at 20 minute intervals. Group II had a solution containing verapamil (0.15 mg/kg/L), diluted in Ringer's solution (4 degrees C), injected into the aortic root. The initial and subsequent doses were as in Group I. Group III received the same solution as Group II, but at room temperature. Alterations in lactate metabolism were not significantly different in any of the three treatment groups. A reduction in oxygen consumption was seen in Group III, but was not found to be statistically significant. However, the reduction in coronary flow at the end of reperfusion was statistically significant in Group III (p less than 0.05). Verapamil given at room temperature resulted in poor preservation of left ventricular function and high-energy stores. Verapamil combined with extreme hypothermia (Group II) provided excellent preservation of left ventricular compliance and contractility. Cold verapamil cardioplegia was superior to potassium cardioplegia for the preservation of adenosine triphosphate.

Adenosine Triphosphate↗
PubMed1983

Hormonal interactions in the control of granulosa cell differentiation.

The main emphasis of this paper is on the changes in function of granulosa cells as they undergo cytodifferentiation in follicles developing from the preantral to the antral stage, and on the hormones present in the milieu of gonadotrophins and steroids which are essential for these events to proceed normally. We found that FSH alone could induce aromatase activity in cultures of immature granulosa cells and that this effect could be duplicated by dibutyryl cyclic AMP. Incubation of cell sonicates under optimal conditions indicated that FSH acted on granulosa cells to increase the cellular concentration of active aromatase. Prior treatment with androgens augmented the FSH effect. Progesterone synthesis is another differentiated function which can be induced in culture by FSH alone and augmented in the presence of androgens. In assessing the enzymes involved in progesterone synthesis we found that cholesterol side-chain cleavage activity had similar hormonal requirements whereas 3 beta-hydroxysteroid dehydrogenase activity was stimulated by FSH alone. FSH also stimulates cyclic AMP binding activity in cultured granulosa cells during cytodifferentiation. These proteins represent another class of intracellular proteins, quite distinct from the steroidogenic enzymes, which increase as the granulosa cells mature. The ability of FSH to induce the appearance of LH and prolactin receptors, and stimulate the secretion of plasminogen activator and proteoglycans is reviewed. It is concluded that the appearance of steroidogenic enzymes and other intracellular proteins, cell-surface and secreted proteins as well as morphological maturation of granulosa cells require the presence of FSH. In the "turning-on" of some of these differentiated functions androgens play a permissive role. Having established events which occur during normal development of the follicle, we considered ways by which this overall process could be interrupted and fertility controlled. Here we describe the ways by which prolactin and LHRH interfere with the normal process of granulosa cell cytodifferentiation.

Animals↗
PubMed1983

Glucocorticoid binding properties of mouse thymic lymphosarcoma cell populations selected for resistance to the cytolytic effects of glucocorticoids in vivo.

Thirteen independently derived murine thymic lymphosarcoma lines were assayed for various aspects of sensitivity to glucocorticoid-induced cytolysis. All tumor lines were sensitive to cytolysis, as evidenced by profound tumor regression after pharmacologic doses of cortisol. All tumor lines contained about 20,000 high-affinity, dexamethasone binding sites/cell. Between 55 and 88% of these presumptive receptor sites underwent nuclear translocation during a 30-minute incubation at 37 degrees C. Dissociation constants (Kd) for the dexamethasone-receptor complex were between 1.5 and 3.6 nM in all cases. Kd for the triamcinolone acetonide-receptor complex were determined for a few tumor lines and were between 0.5 and 0.9 nM. Cytolysis-resistant subpopulations were selected by prolonged glucocorticoid treatment of BALB/c pi mice bearing tumors from seven of the lymphosarcoma lines. All seven resistant subpopulations contained about 20,000 high-affinity, dexamethasone binding sites/cell. Between 57 and 80% of these presumptive receptor sites underwent nuclear translocation under standard assay conditions. No resistant variants exhibited significantly reduced dexamethasone binding or nuclear translocation properties.

Animals↗
PubMed1982

Histochemical and physiological properties of cat motor units after self-and cross-reinnervation.

1. This report describes selected histochemical and physiological properties of the motor units of adult cat soleus muscle approximately one year after self- and cross-reinnervation with the nerve of the heterogenous flexor hallucis longus (f.h.l.). Self-reinnervated f.h.l. motor units are also considered. Whole muscles were tested for fibre reaction to alkaline pre-incubated ATPase, alpha-glycerophosphate dehydrogenase (alpha-GPD) and reduced nicotinamide adenine dinucleotide diaphorase (NADH-D). Motor units were isolated and studied by splitting the ventral root in acute preparations.2. The histochemical fibre type profile in the self-reinnervated muscle was comparable to normal muscle as was mean twitch contraction time, twitch-tetanus ratio and fatigue index. The mean tetanic tension of the soleus self- and cross-reinnervated motor units appeared close to a normal soleus whereas the mean tetanic tension of the f.h.l. self-reinnervated units was significantly less than a normal f.h.l.3. An average of 14% of the fibres of the soleus cross-reinnervated muscles had high ATPase and a alpha-GPD staining intensity in contrast to normal and self-reinnervated soleus in which such fibres are absent. Thus alkaline lability of myofibrillar ATPase increased in some fibres of what was originally a homogeneous population. The small increase in the number of densely staining fibres for ATPase at an alkaline pH (14%) was associated with a 73% decrease in (mean) contraction time (41 +/- 11 ms) of the thirty-three cross-reinnervated muscle units studied, with no unit's contraction time greater than 60 ms. Mean contraction times for the self-reinnervated soleus and f.h.l. muscles were 78 +/- 31 ms and 27 +/- 8 ms respectively.4. All fibres of the soleus cross-reinnervated muscles showed intense reaction to NADH-D, as was true of self-reinnervated soleus. This staining pattern is typical of normal soleus. In concordance, these motor units consistently demonstrated a high resistance to fatigue when stimulated for a four-minute period.5. These results suggest that in the adult self-and cross-reinnervated soleus muscle, there is some active mechanism which regulates the eventual size of motor units as reflected by tetanic tension.6. Change in contraction time from that typical for a soleus unit to that similar to an f.h.l. unit remains incomplete one year after cross-reinnervation. Within this time this partial change in single motor units reflects incomplete neural control of this property rather than a mixture of self- and foreign-innervation.7. A greater degree of independence from neural control to conversion of the histochemically demonstrated myofibrillar ATPase activity exists than is the case for contraction time.

Animals↗
PubMed1980

Hormonal effects upon the rate of proliferation of the murine plasmacytoma MOPC 315.

The growth rate of the transplantable murine plasmacytoma MOPC 315 is decreased by daily injections of 2 mg cortisol. Estradiol injections (0.01 mg/day) have a slight stimulatory effect while testosterone has no detectable effect. Growth rate is unaffected by propylthiouracil, triiodothyronine, and growth hormone. MOPC 315 contains glucocorticoid binding proteins which bind 3H-dexamethasone with high affinity. In intact cells there are 3-4 x 10(4) dexamethasone binding sites per cell as judged by competition experiments in which cells were incubated with a saturating concentration of 3H-dexamethasone in the presence and absence of a 1,000-fold molar excess of unlabelled dexamethasone. Scatchard analysis of binding data reveals the presence of a class of sites which binds about 2 x 10(4) molecules of dexamethasone/cell with an equilibrium dissociation constant (Kd) of about 3 x 10(-8)M. In intact cells, only half of the saturable dexamethasone binding sites have stereochemical specificity for dexamethasone. This was deduced from the observation that dexamethasone binding to about half of these sites was displaced by the addition of a 1,000-fold molar excess of estradiol. Nuclear binding sites for dexamethasone were detected at a level of about 1 x 10(4)/cell. Nuclear dexamethasone binding sites were not displaced by the addition of a 1,000-fold molar excess of estradiol.

Animals↗
PubMed1980

Nonmutational alteration in glucocorticoid sensitivity of lymphosarcoma P1798.

The sensitivity of lymphosarcoma P1798 to glucocorticoids varied as a function of growth conditions. Cells grown in the ascitic fluid were very sensitive to cortisol inhibition of tritiated thymidine ([3H]dThd) incorporation. When ascites cells were inoculated sc into BALB/c mice receiving daily injections of 2 mg cortisol, tumors did not form. However, as tumors grew subcutaneously in control mice, glucocorticoid sensitivity decreased to the arrested by cortisol injection. Cortisol also did not inhibit incorporation of [3H]dThd in cells prepared from large subcutaneous tumors. Measurement of cytoplasmic receptors in cell-free extracts revealed that both ascites and subcutaneous tumor cells contained about 11-12 x 10(4) glucocorticoid binding sites per cell. Receptors in ascites cells had a higher affinity for glucocorticoids than did receptors in subcutaneous cells, which indicated that sensitive and resistant cells contain chemically different classes of receptors. Loss of sensitivity occurred as tumors attained a diameter greater than approximately 1.5 cm. The reproducibility of this transition did not appear to be consistent with a random mechanism for loss of glucocorticoid sensitivity in vivo. Thus the acquisition of resistance by lymphosarcoma P1798 in vivo was concluded to be nonmutational and probably resulted from differentiational alteration in gene expression.

Animals↗
PubMed1978

Ipsilateral limb variation in cats during overground locomotion.

The timing interval between the onset of knee extensor EMG (vastus lateralis) and the onset of the ipsilateral elbow flexor EMG (brachialis) was studied in adult cats during overground walking, trotting and galloping. Concurrent finding of the animals in locomotion was used to relate the electrical activity of the muscles to the physical events in a step cycle. The observed variability in the interval between the onset of knee extensor EMG and the onset of elbow flexor EMG lead to the conclusion that it is not appropriate to postulate a rigid neural coupling mechanism for the control and coordination of ipsilateral limbs in locomotion.

Animals↗
PubMedDate not supplied

Laparoscopic appendicectomy--a successful operation in adults and children.

Laparoscopic appendicectomy has been the subject of several encouraging reports, but has not as yet gained widespread acceptance. We present a series of 159 consecutive laparoscopic appendicectomies performed, over a 4 yr period, in both adults and children. We find the procedure as safe as its open counterpart, with patients fit to leave hospital within the same time period. Perforated appendices were amenable to this procedure, and the location of the appendix did not alter the outcome. Children responded as well as adults post-operatively. Obesity may be an indication for this form of treatment. Removal of displaced faecoliths associated with perforated appendicitis is a difficult technical problem in less than 5 per cent of patients.

Adolescent↗
PubMedDate not supplied

Weekly telephone contact does not enhance the compliance of home apnea monitoring.

OBJECTIVE: To evaluate the effect of weekly telephone contact with families in enhancing the use of home apnea monitors. STUDY DESIGN: This was a prospective, randomized, single-blinded study of 65 infants who were prescribed home apnea monitoring at the time of initial discharge from the hospital. Exclusion criteria included participation in any other study involving home monitoring or nonavailability of home telephone. Infants were randomized either to the "standard" or "telephone" group by a stratified balanced block technique. All families were instructed to use the monitor during the first 4-week period at all times except during bathing and during the second 4-week period at all unattended times and at night. The families in the telephone group were contacted weekly for 8 weeks. The telephone interview reviewed the events of the previous week but did not include specific encouragement to use the monitor. Both groups received routine pediatric care and follow-up at our high-risk premature clinic. The primary outcome measure was compliance measured as the percentage of time as well as the hours per day that the infant spent on the monitor as recorded by the documented monitor. RESULTS: The telephone (n = 30) and standard (n = 32) groups were similar (p > 0.10) with respect to birth weight (1567 +/- 778 versus 1710 +/- 777 gm), gestational age (30.9 +/- 4.2 versus 31.1 +/- 4.6 weeks), maternal age (24.9 +/- 6.0 versus 25.3 +/- 5.4 years), and commercial insurance (46.7% versus 46.9%), a marker of higher socioeconomic status. Compliance of the telephone versus the standard group was similar during the first 4-week period (74.7 +/- 24.9 versus 75 +/- 27.8%, p = 0.85) (17.9 +/- 5.9 versus 18.2 +/- 6.6 hours/day), the second 4 week period (63.4 +/- 29.1 versus 58.9 +/- 30.9%, p = 0.59) (15.2 +/- 7.0 versus 14.1 +/- 7.4 hours/day) and the entire 8-week period (69.3 +/- 24.7 versus 67.7 +/- 26.2%, p = 0.82, Mann-Whitney U-test) (16.7 +/- 6.0 versus 16.1 +/- 6.5 hours/day), respectively. An abnormal pneumocardiogram at the time of discharge was the only identified factor that improved the compliance for the entire 8-week period (73.1 +/- 22 versus 52.1 +/- 28.5%, p = 0.02) (17.5 +/- 5.2 versus 12.5 +/- 6.8 hours/day) and the first 4-week period of monitoring (81.7 +/- 22.9 versus 59.5 +/- 31.3%, p = 0.01) (19.6 +/- 5.5 versus 14.2 +/- 7.5 hours/day). CONCLUSION: Weekly telephone contact, without specific encouragement to use the monitor, did not improve compliance. Compliance was greater in subjects who had abnormal pneumocardiogram results at the time of discharge from hospital regardless of their telephone/standard group assignment. We speculate that in this already compliant population, more targeted advice is necessary to increase compliance.

Apnea↗