The Ca2+:H+ coupling ratio of the plasma membrane calcium ATPase in neurones is little sensitive to changes in external or internal pH

Roger C Thomas

doi:10.1016/j.ceca.2011.03.004

Abstract

To explore the effects of both external and internal pH (pHo and pHi) on the coupling between Ca2+ extrusion and H+ uptake by the PMCA activity in snail neurones H+ uptake was assessed by measuring surface pH changes (ΔpHs) with pH-sensitive microelectrodes while Ba2+ or Ca2+ loads were extruded. Ru360 or ruthenium red injection showed that injected Ca2+ was partly taken up by mitochondria, but Ca2+ entering through channels was not. External pH was changed using a mixture of three buffers to minimise changes in buffering power. With depolarisation-induced Ca2+ or Ba2+ loads the ΔpHs were not changed significantly over the pH range 6.5–8.5. With Ca2+ injections into cells with mitochondrial uptake blocked the ΔpHs were significantly smaller at pH 8.5 than at 7.5, but this could be explained in part by the slower rate of activity of the PMCA. Low intracellular pH also changed the ΔpHs responses to Ca2+ injection, but not significantly. Again this may have been due to reduced pump activity at low pHi. I conclude that in snail neurones the PMCA coupling ratio is either insensitive or much less sensitive to pH than in red blood cells or barnacle muscle.

Highlights

Cell calcium must be kept very low to allow fast and efficient intracellular Ca2+ signalling
Allowing for variation in the pump rate with pH, these findings suggest that the snail neurone plasma-membrane calcium ATPase (PMCA) coupling ratio is little changed by external or internal pH
3.2 The effect of external pH on surface pH changes caused by activation of the PMCA by Ca2+ or Ba2+ influx

Summary

Introduction

Cell calcium must be kept very low to allow fast and efficient intracellular Ca2+ signalling. In nerve cells two ATPase pumps are involved in removing Ca2+ from the cytoplasm. These are the sarco-endoplasmic reticulum calcium ATPase (SERCA) which drives uptake into the endoplasmic reticulum and the plasma-membrane calcium ATPase (PMCA) which extrudes Ca2+ across the plasma membrane. The PMCA is inhibited by extracellular alkalinisation and intracellular acidification [4, 5, 6, 7, 8]. In some cells there is a second plasma membrane Ca2+ pump driven by Na+ influx rather than by ATP, the Na+/Ca2+ exchanger, but in many nerve cells the PMCA is the principle Ca2+ extrusion mechanism at low Ca2+ loads.[4,7,9 10]

Methods

Results

Conclusion