Characterization of exchange inhibitory peptide effects on Na+/Ca2+ exchange in rat and human brain plasma membrane...

The inhibitory effects of Na+/Ca2+ exchange inhibitory peptide (XIP), which corresponds to residues 219-238 of the Na+/Ca2+ exchange protein from canine heart, were studied in both rat and human brain plasma membrane vesicles. XIP had very high potency with respect to the inhibition of the initial velocity of intravesicular Na(+)-dependent Ca2+ uptake in both rat brain [IC50 = 3.05 +/- 0.69 microM (mean +/- SE)] and human brain (IC50 = 3.58 +/- 0.58 microM). The maximal inhibition seen in rat brain vesicles was approximately 80%, whereas human brain vesicles were inhibited 100%. XIP also inhibited extravesicular Na(+)-dependent Ca2+ release, and the inhibitory effect was enhanced by increasing the extravesicular Na+ concentration. In contrast, the inhibitory effect of bepridil was competitive with respect to extravesicular Na+. When XIP was added at steady state (5 min after the initiation of intravesicular Na(+)-dependent Ca2+ uptake), it was found that the intravesicular Ca2+ content declined with time. Analysis of unidirectional fluxes for Ca2+ at steady state showed that 50 microM XIP inhibited Ca2+ influx and efflux approximately 85 and 70%, respectively. This result suggested that XIP inhibited both Na+/Ca2+ exchange and Ca2+/Ca2+ exchange but had no effect on the passive release pathway for Ca2+. The results suggest structural homology among cardiac, rat, and human brain exchangers in the XIP binding domain and that the binding of Na+ or other monovalent cations, e.g., K+, is required for XIP to have its inhibitory effect on Ca2+ transport.