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Biomedical subjects

S Vijayaraghavan

Publications and source records attributed to S Vijayaraghavan.

At least 37 records · Page 2Linked to original sources
PubMed1996

Primate sperm contain protein phosphatase 1, a biochemical mediator of motility.

Sperm motility initiation, capacitation, and hyperactivation are modulated by an interplay of intracellular Ca2+, cAMP, and pH. Mechanisms by which these mediators alter sperm function have not been elucidated but may involve reversible alterations in regulatory protein phosphorylation. Studies were designed 1) to investigate the influence of the protein phosphatase (PP) inhibitor calyculin A (CA) on human sperm motility and 2) to characterize the CA-sensitive PP and its endogenous regulators in human and rhesus monkey sperm. CA (50 nM) treatment of human sperm resulted in an increase in percentage motility and an acceleration in mean path velocity. Inhibition of either protein phosphatase-1 (PP1) or protein phosphatase-2A (PP2A) could be responsible for this motility stimulation, since both of these phosphatases are sensitive to nanomolar quantities of CA. PP activity in human (n = 4) and rhesus monkey (n = 4) sperm sonicates was measured using [32P]-phosphorylase-a, the preferred substrate for PP1 and PP2A, in the absence of divalent cations. Human (6.2 +/- 4.5 x 10(-2) mU/10(6) sperm) and monkey (4.3 +/- 0.8 x 10(-2) mU/10(6) sperm) sonicates contained activity tentatively identified as PP1 on the basis of inhibition profiles in okadaic acid (OA) and CA. Western blot analysis with antibodies against various isoforms of PP1 subsequently documented the presence of PP1 gamma 2 in human and monkey sperm. PP1 activity in most tissues is regulated by the heat-stable inhibitors I1 or I2. Sperm sonicates contained inhibitor activity similar to I2 as well as glycogen synthase kinase-3 (GSK-3) activity, which is involved in the activation of the PP1-I2 complex. These results indicate, for the first time, that human and rhesus monkey sperm contain PP1 and regulators of PP1 and that inhibition of PP1 activity by CA can enhance motility.

Adult↗
PubMed1995

Acetylcholine differentially affects intracellular calcium via nicotinic and muscarinic receptors on the same population of neurons.

Multiple receptor subtypes activated by the same ligand but coupled to different second messengers can produce divergent signaling in a cell, while receptors activated by different ligands but sharing the same second messenger can produce convergent signaling. We show here that chick ciliary ganglion neurons have three classes of receptors activated by the same neurotransmitter, acetylcholine, and that all three regulate the same second messenger, intracellular free calcium. Activation of muscarinic receptors on the neurons stimulates phosphatidylinositol turnover and induces calcium oscillations that are initiated and maintained by calcium release from caffeine/ryanodine-insensitive intracellular stores. Extracellular calcium is required to sustain the oscillations, while cadmium abolishes them. Activation of either of two classes of nicotinic receptors, distinguished both by location on the neurons and by subunit composition, induces a single, rapid elevation in intracellular calcium without inducing phosphatidylinositol turnover. The nicotinic responses are entirely dependent on extracellular calcium, show no dependence on release from internal stores, and do not display oscillations. Low concentrations of the native agonist, acetylcholine, induce repetitive calcium spikes in the neurons characteristic of muscarinic receptors, while higher concentrations induce nonoscillating increases in intracellular calcium that include contributions from nicotinic receptors. The three classes of receptors also differ in the acetylcholine concentration required to elicit a response. These differences, together with differences in receptor location and sources of calcium mobilized, may enable the receptor subtypes to target different sets of calcium-dependent processes for regulation.

Acetylcholine↗
PubMed1995

Carbohydrate metabolism in hepatic tissue of freshwater catfish Clarias batrachus L. during dimethoate exposure.

Clarias batrachus L., a freshwater catfish of commercial importance, was exposed to a sublethal concentration of dimethoate (Rogor). The acute effect of dimethoate on some aspects of carbohydrate metabolism of hepatic tissue was studied after 1, 2, 4 and 8 days. The glycogen content was depleted, whereas the lactate level was elevated throughout the experimental period and an increase in lactate dehydrogenase was observed. The activity levels of glycogen phosphorylase a and ab in the hepatic tissue were increased during dimethoate stress. The results suggest that carbohydrate metabolism was adversely affected in the hepatic tissue by the organophosphate insecticide dimethoate.

Animals↗
PubMed1995

Arachidonic acid as a possible negative feedback inhibitor of nicotinic acetylcholine receptors on neurons.

Neuronal acetylcholine receptors, being highly permeable to calcium, are likely to regulate calcium-dependent events in neurons. Arachidonic acid is a membrane-permeant second messenger that can be released from membrane phospholipids by phospholipases in a calcium-dependent manner. We show here that activation of neuronal acetylcholine receptors triggers release of 3H-arachidonic acid in a calcium-dependent manner from neurons preloaded with the fatty acid. Moreover, low concentrations of arachidonic acid reversibly inhibit the receptors and act most efficiently on receptors likely to have the highest permeability to calcium, namely receptors containing alpha 7 subunits. Low concentrations of arachidonic acid also reversibly inhibit alpha 7-containing receptors expressed in Xenopus oocytes following injection of alpha 7 cRNA. The oocyte results indicate following injection of alpha 7 cRNA. The oocyte results indicate that the inhibition is a feature of the receptors rather than a consequence of neuron-specific machinery. The inhibition is not mediated by specific metabolites of arachidonic acid because the effects can be mimicked by other fatty acids; their effectiveness correlates with their content of double bonds. In contrast to arachidonic effects on calcium currents, inhibition of neuronal nicotinic receptors by the fatty acid cannot be prevented by blocking production of free radicals or by inhibiting protein kinase C. An alternative mechanism is that arachidonic acid binds directly to the receptors or perturbs the local environment in such a manner as to constrain receptor function.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Science for art's sake.

Hollywood's absent-minded professor, always a cliché, is now more unrealistic than ever before. Just how can biological research move forward in the modern era?

Art↗
PubMed1994

Evidence against a functional ATP-dependent calcium extrusion mechanism in bovine epididymal sperm.

Bovine epididymal sperm resuspended in ionic buffers take up relatively large amounts of calcium. This uptake, which is almost entirely mitochondrial, apparently bypasses the sperm cytosol. The direct mitochondrial loading is an unusual aspect of sperm calcium uptake, which suggests that the plasma membrane region surrounding the mitochondria should be highly permeable to calcium, whereas the membrane domains surrounding the head and tail regions of sperm should be impermeable. This study was undertaken to determine the role of a plasma membrane calcium ATPase in sperm calcium homeostasis. Kinetics of calcium (45Ca2+) uptake into intact and permeabilized caudal epididymal sperm confirmed that mitochondrial calcium uptake occurs with virtually no resistance from the surrounding plasma membrane. Cytoplasmic calcium accumulation by sperm depleted of intracellular ATP, measured in the presence of mitochondrial calcium uptake inhibitors, showed no increase upon energy depletion as would be expected if an ATP-dependent calcium extrusion mechanism were present. Furthermore, lowering the incubation temperature to further reduce the activity of the calcium ATPase in these energy-depleted sperm was also without effect on calcium accumulation. The calcium ATPase inhibitor vanadate, even at high concentrations, failed to increase intracellular 45Ca2+ accumulation. However, vanadate was effective in inhibiting motility showing that the compound was accumulated into sperm to inhibit flagellar dyenin ATPase. Therefore, the lack of effect of vanadate on 45Ca2+ accumulation was not due to its inability to enter sperm. Other calcium ATPase inhibitors such as quercetin, thapsigargin, and cyclopiazonic acid, which readily demonstrate ATP-dependent calcium extrusion in other somatic cells, were also without effect on sperm calcium accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1994

Neuronal acetylcholine receptors that bind alpha-bungarotoxin with high affinity function as ligand-gated ion channels.

Neuronal membrane components that bind alpha-bungarotoxin with high affinity can increase intracellular levels of free calcium, demonstrating the components function as nicotinic receptors. Though such receptors often contain the alpha 7 gene product, which by itself can produce ionotropic receptors in Xenopus oocytes, numerous attempts have failed to demonstrate an ion channel function for the native receptors on neurons. Using rapid application of agonist, we show here that the native receptors are ligand-gated ion channels which are cation selective, prefer nicotine over acetylcholine, and rapidly desensitize. Much of the calcium increase caused in neurons by the receptors under physiological conditions appears to result from their depolarizing the membrane sufficiently to trigger calcium influx through voltage-gated channels.

Animals↗
PubMed1994

Apolipoprotein-D polymorphism: a genetic marker for obesity and hyperinsulinemia.

Obesity frequently clusters with hypertension, hyperlipidemia, non-insulin-dependent diabetes mellitus, and ischemic heart disease with hyperinsulinemia as syndrome X. Although central obesity has been recognized to have a strong genetic component, few candidate genes have been studied in this disorder. After a recently described association between the apolipoprotein-D (Apo-D) gene polymorphism and non-insulin-dependent diabetes mellitus by our group, we have now looked at a TaqI polymorphism of the Apo-D gene in two other components of syndrome X, namely obesity and hyperinsulinemia. Apo-D genotype differences were found between obese subjects (n = 57) and slim controls (n = 57; P = 0.006). Furthermore, in the obese group an association was found between the Apo-D genotype and fasting insulin (P < 0.001). Preliminary evidence, therefore, suggests that the TaqI Apo-D polymorphism can be used as a genetic marker for obesity and several components of syndrome X.

Adult↗
PubMed1993

Serotonin modulates nicotinic responses of adrenal chromaffin cells.

5-Hydroxytryptamine (5-HT) specifically and reversibly inhibits nicotine-induced currents and catecholamine release in bovine adrenal chromaffin cells in culture. Pharmacological analysis indicates that the inhibition is not mediated by known 5-HT receptor subtypes. The inhibition is noncompetitive over a range of nicotine concentrations between 1 and 100 microM. Preincubation with either 5-HT or substance P significantly protects the response from nicotine-induced desensitization. It is concluded that 5-HT inhibits nicotinic acetylcholine receptors on bovine adrenal chromaffin cells, probably by binding to a noncompetitive site on the receptor itself. Because both blood and the chromaffin cells contain 5-HT, the inhibition provides an opportunity for negative control of catecholamine secretion from the adrenals.

Adrenal Glands↗
PubMed1993

Does the rapid response to cisplatin-based chemotherapy justify its use as primary treatment for intracranial germ-cell tumours?

We reviewed presentation, diagnostic problems and outcome of eight cases of primary intracranial germ-cell tumour (4 germinoma, 4 teratoma) treated with Cisplatin-based chemotherapy at our centre over the last ten years. Three patients received primary chemotherapy with Cisplatin-based regimens followed by radiotherapy for subsequent relapse, two were treated with a combination of chemotherapy and external radiotherapy, and three received chemotherapy for relapse after radiotherapy. The response to Cisplatin-based chemotherapy was rapid, with some patents exhibiting symptomatic improvement within 24 h. Four patients achieved complete remission within 21 days, and three of these have remained progression-free. Four patients in total have survived for 32 to 128+ months. Six of seven patients tested pre-treatment had central diabetes insipidus and five had partial anterior pituitary failure. The endocrine deficit progressed in two, with no recovery in any patient. It is arguable that chemotherapy should be the primary therapy in all such cases diagnosed on the basis of tumour markers and imaging, with surgery and/or radiotherapy as later options. As these tumours are rare, such questions can only be answered by collaboration studies.

Adolescent↗
PubMed1992

Inhibition of the nicotinic acetylcholine response by serotonergic and muscarinic agents in chick ciliary ganglion neurones.

Chick ciliary ganglion neurones were investigated by whole cell voltage clamp recordings. The ACh- or nicotine-induced inward current was partially inhibited by perfusing the neurones with 5-HT. This effect was rapid (< or = 1 min), dose-dependent (50-1000 microM) and quickly reversible. The selective 5-HT1A agonist 8-OH-DPAT (10 microM) reduced the nicotinic ACh response more potently, irrespective of the absence or presence of propranolol (1 microM), a known 5-HT1A antagonist. Other serotonergic antagonists, like ICS 205-930 (1 microM), mianserin (10 microM) and methysergide (10 microM), also failed to antagonize the 5-HT-mediated decrease in the nicotinic response. Muscarine (50 microM) did not affect the nicotine-induced inward current but the muscarinic agonist oxotremorine (10 microM) also decreased the nicotine-induced inward current. Atropine, at small concentrations failed to block this effect but caused some reduction of the ACh response itself at larger (1-10 microM) concentrations. It is suggested that 5-HT may modulate synaptic transmission in ciliary ganglion neurones in vivo. The site of action of 5-HT, oxotremorine and atropine might be at or close to the ACh receptor complex, because of the fast onset and reversibility of the effects and lack of specificity for structurally different drugs.

Acetylcholine↗
PubMed1992

Nicotinic receptors that bind alpha-bungarotoxin on neurons raise intracellular free Ca2+.

Many populations of vertebrate neurons have a membrane component that binds alpha-bungarotoxin and cholinergic ligands. Despite the abundance of this component and its similarities to nicotinic receptors, its function has remained controversial. Using a fluorescence assay, we show here that activation of the component elevates the intracellular concentration of free Ca2+, demonstrating a receptor function for the toxin-binding component. Whole-cell voltage-clamp and intracellular recordings did not detect a significant current resulting from receptor activation, possibly because the currents were small or the receptors rapidly desensitized. The rise in intracellular free Ca2+ caused by the receptor was prevented by Ca2+ channel blockers. This suggests a signaling cascade likely to have important regulatory consequences for the neuron.

Animals↗
PubMed1990

Changes in the mitochondrial calcium influx and efflux properties are responsible for the decline in sperm calcium during epididymal maturation.

This study was undertaken to determine the role of calcium ion, a key regulator of the intensity and form of motility in mature demembranated sperm, in the development of motility during passage through the bovine epididymis. Cellular calcium levels in bovine caput and cauda epididymal spermatozoa were measured with three different techniques. 45Ca2+ uptake measurements revealed that net calcium uptake and Ca2(+)-Ca2+ exchange in caput spermatozoa were about 2 to 3 times higher than in caudal spermatozoa. Intracellular free calcium determination with the calcium fluorophore Fura 2 showed that the levels were 6 times higher in caput spermatozoa. The values for caput and caudal sperm were 875 +/- 55 nM (n = 15) and 155 +/- 6 nM (n = 24), respectively. Total cellular calcium levels quantitated by atomic absorption were 626 +/- 30 (n = 48) and 304 +/- 19 (n = 46) ng/10(8) sperm in caput and caudal epididymal sperm, respectively. At least one of the reasons for the high calcium content of caput epididymal sperm is the result of a higher rate and extent of mitochondrial calcium accumulation in caput compared to caudal sperm. Mitochondrial calcium uptake rates measured in digitonin permeabilized cells revealed uptake rates 2- to 3-fold higher in caput compared to caudal sperm. However, mitochondrial calcium efflux rates were identical in caput and caudal epididymal sperm. The efflux rates in both cell types were unaffected by external sodium levels but were found to be proportional to pH. Alkalinization or acidification of internal pH of intact sperm resulted in a corresponding lowering or elevation of cytoplasmic free calcium levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Cyclic AMP-dependent phosphorylation of a neuronal acetylcholine receptor alpha-type subunit.

Chick ciliary ganglion neurons have nicotinic acetylcholine receptors (AChRs) that mediate synaptic transmission through the ganglion. A cAMP-dependent process has previously been shown to enhance the ACh response of the neurons 2- to 3-fold without requiring the synthesis of new receptors. We show here that the receptors can be phosphorylated in situ by a cAMP-dependent process. The phosphorylation occurs predominantly on components of 50 and 58 kDa. Both derive from putative ligand-binding alpha 3 subunits, with the smaller phosphorylated species probably representing a degradation product of the larger. The increase in receptor phosphorylation caused by incubating the neurons with a cAMP analog parallels the increase observed in the ACh response, with respect to both time course and relative extent. The phosphorylation of ciliary ganglion AChRs differs from that reported for electric organ AChRs, which occurs primarily on the non-ligand-binding gamma and delta subunits and increases the rate of agonist-induced receptor desensitization.

1-Methyl-3-isobutylxanthine↗
PubMed1989

Calcium uptake by bovine epididymal spermatozoa is regulated by the redox state of the mitochondrial pyridine nucleotides.

Immature caput epididymal sperm accumulate calcium from exogenous sources at a rate 2- to 4-fold greater than mature caudal sperm. Calcium accumulation by these cells, however, is maximal in the presence of lactate as external substrate. This stimulation of calcium uptake by optimum levels of lactate (0.8-1.0 mM) is about 5-fold in caput and 2-fold in caudal sperm compared to values observed with glucose as substrate. Calcium accumulation by intact sperm is almost entirely mitochondrial as evidenced by the inhibition of uptake by rotenone, antimycin, and ruthenium red. The differences in the ability of the various substrates in sustaining calcium uptake appeared to be related to their ability to generate NADH (nicotinamide adenine dinucleotide). Previous reports have documented that mitochondrial calcium accumulation in several somatic cells is regulated by the oxidation state of mitochondrial NADH. A similar situation obtains for bovine epididymal sperm since calcium uptake sustained by site III oxidation of ascorbate in the presence of tetramethyl phenylenediamine and rotenone was also stimulated by NADH-producing substrates, including lactate, and inhibited by substrates generating NAD+ (nicotinamide adenine dinucleotide, oxidized form). Further, calcium uptake by digitonin-permeabilized sperm in the presence of succinate was stimulated when NADH oxidation was inhibited by rotenone. The compounds alpha-keto butyric, valeric, and caproic acids, which generate NAD+, inhibited the maximal calcium uptake observed in the presence of succinate and rotenone, and the hydroxy acids lactate and beta-hydroxybutyrate reversed this inhibition. These results document the regulation of sperm calcium accumulation by the physiological substrate lactate, emphasize the importance of mitochondria in the accumulation of calcium by bovine epididymal sperm, and suggest that the mitochondrial location of the isozyme LDH-X in mammalian sperm may be involved in the regulation of calcium accumulation.

3-Hydroxybutyric Acid↗
PubMed1988

Low molecular weight factor in bovine caudal epididymal fluid that stimulates calcium uptake in caput spermatozoa.

Secretions from the mammalian epididymis contain proteins that bind to developing sperm and are presumed to play a role in sperm maturation. The biochemical functions in sperm of most of these proteins are not known. In this report we describe the presence of a low molecular weight compound in bovine caudal epididymal luminal fluid (CF) that has a potent stimulatory effect on calcium (45Ca2+) uptake in immature caput epididymal spermatozoa. The studies were initially undertaken to characterize the effect of the protein caltrin, present in bovine seminal plasma (BSP), on calcium uptake into caput spermatozoa. Caltrin is known to block calcium influx into mature bovine sperm. Unexpectedly, the kinetics of calcium uptake into caput sperm showed a biphasic response when treated with BSP, namely, a stimulation of uptake at 1 to 5 min and inhibition of uptake after this time. Since caudal sperm do not show this biphasic response, we reasoned that BSP contained a factor derived from CF that must interact with developing sperm before the binding of caltrin to sperm can prevent further calcium uptake. We first demonstrated that preincubation of caput sperm with CF eliminated the biphasic calcium uptake effect induced in caput sperm by BSP and that caudal fluid alone had a potent stimulatory effect on calcium uptake in caput sperm. Half-maximal stimulation (fivefold over control) occurred at a caudal fluid protein concentration of 0.27 mg/ml. Partial purification of the factor indicates that it is of low molecular weight (MW approximately 1,000), but further chemical characterization has not been carried out and its epididymal site of origin is not known. The results indicate that the regulation of intracellular calcium levels in sperm differs in immature and mature bovine sperm in that an epididymal factor promotes calcium uptake during epididymal maturation, and the seminal fluid protein caltrin prevents it at ejaculation.

Animals↗
PubMed1987

Use of sodium nitroprusside induced hypotensive anaesthesia for reducing blood loss in patients undergoing lienorenal shunts for portal hypertension.

A randomized study was performed on 18 patients to determine whether hypotensive anaesthesia, induced by sodium nitroprusside (SNP), would reduce blood loss during elective proximal lienorenal shunt operations for portal hypertension which was due to extrahepatic obstruction or non-cirrhotic portal fibrosis. Eight patients received SNP intraoperatively to reduce the systolic blood pressure to below 95 mmHg, and there were 10 control patients. Blood loss, the number of units of blood transfused, and urine output during surgery were measured together with the postoperative drainage from the abdominal and chest tubes over the first 24 h. There was a significantly lower blood loss (mean +/- s.d. 517 +/- 220 versus 1286 +/- 523 ml; P less than 0.01) and number of units of blood transfused was less (0.9 +/- 0.9 versus 3.0 +/- 1.2; P less than 0.01) in the SNP patients than in the controls. The urine output was greater in the SNP group (606 +/- 211 versus 399 +/- 139 ml, n.s.). Postoperative drainages from the chest and abdomen were similar. Hypotensive anaesthesia with SNP reduces operative blood loss and transfusion requirement in patients with good liver function undergoing proximal lienorenal shunts for portal hypertension.

Adolescent↗