Analysis of the mechanisms responsible for the generation and decoding of intracellular calcium signals

• Main Author: Bradley, J.
• Published: University of Cambridge 1997
• Subjects: 572
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596849

In many cells, including hepatocytes, receptors coupled to phosphoinositide hydrolysis stimulate oscillatory changes in cytosolic [Ca<SUP>2+</SUP>] ([Ca<SUP>2+</SUP>]<SUB>i</SUB>). I have used two methods to examine the means whereby such [Ca<SUP>2+</SUP>]<SUB>i</SUB> transients are linked to the stimulation of glycogenolysis. By perifusing rat hepatocytes prelabelled with [<SUP>3</SUP>H]-glucose, I showed that phenylephrine increases the rate of [<SUP>3</SUP>H]-glucose release by 2-3 fold, and confirmed this effect using a colorimetric assay for glucose. When cultured hepatocytes were loaded with Fura-2 and incubated with thapsigargin in a Ca<SUP>2+</SUP>-free medium, restoration of external Ca<SUP>2+</SUP> caused Ca<SUP>2+</SUP> entry. Brief pulses (10s or 30s) of extracellular medium containing Ca<SUP>2+</SUP> (5mM), produced [Ca<SUP>2+</SUP>]<SUB>i</SUB> transients comparable to those observed in single hepatocytes treated with Areg8-vasopressin (duration = 1.0 -1.5min). Using a similar protocol to manipulate [Ca<SUP>2+</SUP>]<SUB>i</SUB> in hepatocytes prelabelled with [<SUP>3</SUP>H]-glucose, I demonstrated that single [Ca<SUP>2+</SUP>]<SUB>i</SUB> transients stimulate a 2-fold increase in [<SUP>3</SUP>H]-glucose release (n=4). Moreover, stimulated glucose release outlasted the transient elevation of [Ca<SUP>2+</SUP>]<SUB>i</SUB> having a duration of 7.4 ± 0.6min (n=4). In conclusion, stimulation of glucose release by imposed transients of [Ca<SUP>2+</SUP>]<SUB>i</SUB> in a population of hepatocytes indicates that the discontinuous signal of [Ca<SUP>2+</SUP>]<SUB>i</SUB> transients can be decoded to produce a continuous response. Further characterisation of this decoding process requires single cell measurements of glucose and [Ca<SUP>2+</SUP>]<SUB>i</SUB>. Such measurements are not yet possible with the presently available techniques for enzyme immobilisation on glucose microbiosensors. Overexpression of type I and type III Ins<I>P</I><SUB>3</SUB> receptors results in cell death, the mechanism of which requires further study. It is possible, however, to induce the expression of the type I Ins<I>P</I><SUB>3</SUB> receptor and the effects of increasing Ins<I>P</I><SUB>3</SUB> receptor level on elementary Ca<SUP>2+</SUP> release events should provide insight into the mechanism by which [Ca<SUP>2+</SUP>]<SUB>i</SUB> waves are generated.