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Biomedical subjects

P B Simpson

Publications and source records attributed to P B Simpson.

8 recordsLinked to original sources
PubMed1995

Divalent cation entry in cultured rat cerebellar granule cells measured using Mn2+ quench of fura 2 fluorescence.

In this study the rate of Mn2+ quench of fura-2 fluorescence evoked by glutamatergic and cholinergic agonists, depolarization and Ca2+ store modulators was measured in cultured cerebellar granule cells, in order to study their effects on Ca2+ entry in isolation from effects on Ca2+ store release. The rate of fluorescence quench by 0.1 mM Mn2+ was markedly increased by 25 mM K(+)-evoked depolarization or by 200 microM N-methyl-D-aspartate (NMDA), with a significantly greater increase occurring during the rapid-onset peak phase compared to the plateau phase of the K(+)- or NMDA-evoked [Ca2+]i response. The stimulatory effect of NMDA on Mn2+ quench was abolished by dizocilpine (10 microM), but nitrendipine (2 microM), while decreasing the rate of basal quench, did not affect NMDA-stimulated Mn2+ entry. This suggests that nitrendipine may not act on NMDA channels in granule cells, at least under these conditions, and that voltage-operated Ca2+ channels are involved in control quench whereas the NMDA-evoked quench is dependent on entry through the receptor channel. The t1/2 of quench was unaffected by alpha-amino-hydroxyisoxazole propionic acid (200 microM) and carbamyl choline (1 mM). Neither thapsigargin (10 microM) nor dantrolene (30 microM) significantly affected the rate of quench under control or NMDA- or K(+)-stimulated conditions, which confirms that the previously reported inhibitory effects on [Ca2+]i elevations evoked by these agents are due to actions on Ca2+ stores. However, thapsigargin elevated [Ca2+]i in the presence of normal [Ca2+]o but not in nominally Ca(2+)-free medium, indicating that it evokes Ca2+ entry in cerebellar granule cells, probably subsequent to store depletion, which appears to be either too small to be detected by Mn2+ quench or to occur via Mn(2+)-impermeant channels.

Animals↗
PubMed1995

Neuronal Ca2+ stores: activation and function.

The intracellular concentration of free Ca2+ ([Ca2+]i) displays complex fluctuations in response to a variety of stimuli, and acts as a pluripotent signal for many neuronal functions. It is well established that various 'metabotropic' neurotransmitter receptors can mediate the mobilization of Ca2+ stores via actions of inositol-polyphosphate second messengers, and more recent evidence suggests that 'ionotropic' receptor-mediated Ca2+ signals in neurones might also involve release of Ca2+ from intracellular stores. These two mechanisms of release of Ca2+ enable considerable temporal and spatial complexity of increases in the [Ca2+]i via multiple interactions at the level of intracellular-receptor activation. The complexity of Ca2+ signalling that is elicited via these interconnecting pathways might underlie mechanisms that are central to information transfer and integration within neuronal compartments.

Animals↗
PubMed1994

Potentiation of N-methyl-D-aspartate-evoked elevation of intracellular Ca2+ concentrations by exogenous glycine in cerebellar granule cells.

The effect of glycine on the intracellular free Ca2+ concentration ([Ca2+])i response to N-methyl-D-aspartate (NMDA) was examined in small groups of cerebellar granule cells loaded with fura 2. NMDA alone evoked a long-lasting monophasic [Ca2+]i plateau, which was abolished by removal of extracellular Ca2+, or addition of the NMDA channel antagonist dizocilpine or the glycine site antagonist 5,7-dichlorokynurenic acid, virtually unaffected by the L-type Ca2+ channel antagonist (-)-PN 202 791, and greatly, though variably, potentiated by addition of glycine. In the presence of glycine the response to NMDA was clearly biphasic. However, there was no consistent relationship between the magnitudes of the peak and plateau phases of the response, and their temporal relationship was also highly variable. The potentiation seen with exogenous glycine was highly dependent on plating density, which may be the result of higher levels of endogenous glycine in more dense cultures. Our results provide an explanation of the inconsistent findings previously reported by different groups on the potentiation of the [Ca2+]i response to NMDA by exogenous glycine.

Animals↗
PubMed1994

Chronic activation of muscarinic and metabotropic glutamate receptors down-regulates type I inositol 1,4,5-trisphosphate receptor expression in cerebellar granule cells.

The ability of receptors coupled to phosphoinositide turnover to evoke accumulation of inositol 1,4,5-trisphosphate (InsP3) over extended incubation periods, and consequently to affect the level of InsP3 receptor expression, was studied in cultured cerebellar granule cells. The cholinergic agonist carbachol (CCh; 1 mM) evoked a biphasic accumulation of InsP3, a rapid three- to fourfold peak increase over control levels at approximately 10 s, decreasing within 1 min to a long-lasting plateau elevation. Using an antibody against the type I InsP3 receptor, it was demonstrated that > 50% down-regulation of type I InsP3 receptor expression in cerebellar granule cells occurred within 1 h of incubation with 1 mM CCh. Over 24 h, 1 mM CCh caused an approximately 85% decrease in type I InsP3 receptor levels, and significant decreases in immunoreactivity were evident at much lower concentrations of CCh. Direct assessment of total InsP3 receptor expression using a radioligand binding method also detected down-regulation, but to an apparently lesser extent. 1-Aminocyclopentane-1S,3R-dicarboxylic acid (200 microM), an agonist of metabotropic glutamate receptors, evoked a marked decrease in type I InsP3 receptors after 24 h of incubation. These findings demonstrate that a functional consequence of maintained InsP3 production in cerebellar granule cells is the down-regulation of InsP3 receptor expression and that this down-regulation may be a common mechanism of action of phosphoinositide-linked receptors during prolonged stimulation.

Animals↗
PubMed1993

Involvement of intracellular stores in the Ca2+ responses to N-Methyl-D-aspartate and depolarization in cerebellar granule cells.

The [Ca2+]i of cerebellar granule cells can be increased in a biphasic manner by addition of NMDA or by depolarization (induced by elevating the extracellular K+ level), which both activate Ca2+ influx. The possibility that these stimuli activate Ca2(+)-induced Ca2+ release was investigated using granule cells loaded with fura 2-AM. Dantrolene, perfused onto groups of cells during the sustained plateau phase of the [Ca2+]i response to K+ or NMDA, was found to reduce the response to both agents in a concentration-dependent manner. Preincubation with thapsigargin (10 microM) substantially reduced the plateau phase of the [Ca2+]i response to K+ and both the peak and plateau phases of the NMDA response. Preincubation with ryanodine (10 microM) also reduced both the K(+)-evoked plateau response and both phases of the NMDA response. Neither had a consistent effect on the peak response to K+. The effects of thapsigargin and ryanodine on the NMDA response were partially additive. These results demonstrate that in cerebellar granule cells a major component of both K(+)- and NMDA-induced elevation of [Ca2+]i appears to be due to release from intracellular stores. The partial additivity of the effects of thapsigargin and ryanodine suggests that these agents affect two overlapping but nonidentical Ca2+ pools.

Animals↗
PubMed1977

Phospholipid monolayers and systemic lupus erythematosus.

A hypothesis is presented of the fundamental cause of the systemic lupus erythematosus. From a study of the molecular interaction of procainamide with phospholipid monolayers, it is proposed that the drug causes phospholipid loss from the cell membrane. Chronic phospholipid loss would lead to cell lysis and antinuclear antibody production.

Anesthetics, Local↗