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

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 huma n platelets. With the use of RT-PCR techniques and mRNA, the exchanger expr essed in CHRF-288 cells was found to be identical to that expressed in huma n retinal rods. Northern blot analysis of the mRNA for the human retinal ro d exchanger in CHRF-288 cells revealed a major transcript at 5.8 kb with tw o minor bands at 4.9 and 6.8 kb. mRNA for the retinal rod exchanger was als o identified in human platelets. Using Ba2+ influx as a measure of Na+/Ca2 exchange activity in human platelets, we have demonstrated that exchange a ctivity 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 c ould make platelets especially sensitive to daily fluctuations in salt inta ke.