CHARACTERIZATION OF EXCHANGE INHIBITORY PEPTIDE EFFECTS ON NA+ CA2+ EXCHANGE IN RAT AND HUMAN BRAIN PLASMA-MEMBRANE VESICLES/

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 me mbrane vesicles. XIP had very high potency with respect to the inhibit ion of the initial velocity of intravesicular Na+-dependent Ca2+ uptak e in both rat brain [IC50 = 3.05 +/- 0.69 mu M (mean +/- SE)] and huma n brain (IC50 = 3.58 +/- 0.58 mu M). The maximal inhibition seen in ra t brain vesicles was similar to 80%, whereas human brain vesicles were inhibited 100%. XIP also inhibited extravesicular Na+-dependent Ca2release, and the inhibitory effect was enhanced by increasing the extr avesicular Na+ concentration. In contrast, the inhibitory effect of be pridil was competitive with respect to extravesicular Na+. When XIP wa s added at steady state (5 min after the initiation of intravesicular Na+-dependent Ca2+ uptake), it was found that the intravesicular Ca2content declined with time. Analysis of unidirectional fluxes for Ca2 at steady state showed that 50 mu M XIP inhibited Ca2+ influx and eff lux similar to 85 and 70%, respectively. This result suggested that XI P inhibited both Na+/Ca2+ exchange and Ca2+/Ca2+ exchange but had no e ffect on the passive release pathway for Ca2+. The results suggest str uctural homology among cardiac, rat, and human brain exchangers in the XIP binding domain and that the binding of Na+ or other monovalent ca tions, e.g., K+, is required for XIP to have its inhibitory effect on Ca2+ transport.