Abstract

Introduction

Ca²⁺-Dependent Regulation

PIP2-Dependent Regulation

Questions and Speculative Answers

References

Discussion
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Measurements of exchange currents in excised patches have revealed important roles for cytosolic Ca and PIP2 in the regulation of exchange activity. Here we examine the impact of these modes of regulation on Na/Ca exchange activity in transfected Chinese hamster ovary (CHO) cells. The half-maximal concentration for activation of exchange-mediated Ba influx by cytosolic Ca was 44 nM, a value far below the 300-600 nM found with excised patches. Moreover, Ca efflux via Na/Ca exchange was shown to be active at Ca concentrations below 50 nM. These results imply that the exchanger is nearly fully activated by Ca under resting conditions, leaving little room for regulatory modulation of exchange activity by higher concentrations of Ca. Cellular PIP2 levels were reduced 2-fold when cells were plated on polylysine rather than fibronectin; rates of Na/Ca exchange were the same for the two treatments, however. The exchanger's XIP region is thought to interact with PIP2, but mutating the positively charged residues in this region to alanines had no discernible effect on exchange activity or its regulation by Ca. Our results raise questions about the physiological relevance of Ca and PIP2 as regulators of exchange activity in transfected CHO cells.

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Presentation Number SAreeves0755
Keywords: XIP, CHO cells, PIP2, barium influx, calcium efflux