Physiological and molecular characterization of the Na+/Ca2+exchanger in human platelets

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Abstract
In this report, we have demonstrated that Na+/Ca2+exchanger activity in a human megakaryocytic cell line (CHRF-288 cells) is K+dependent, similar to the properties previously described for Na+/Ca2+exchange activity in human platelets. With the use of RT-PCR techniques and mRNA, the exchanger expressed in CHRF-288 cells was found to be identical to that expressed in human retinal rods. Northern blot analysis of the mRNA for the human retinal rod exchanger in CHRF-288 cells revealed a major transcript at 5.8 kb with two minor bands at 4.9 and 6.8 kb. mRNA for the retinal rod exchanger was also identified in human platelets. Using Ba2+influx as a measure of Na+/Ca2+exchange activity in human platelets, we have demonstrated that exchange activity is driven by the transmembrane gradient for K+as well as that for Na+. We propose that the K+dependence of the platelet Na+/Ca2+exchanger could make platelets especially sensitive to daily fluctuations in salt intake.