Phe-Met-Arg-Phe-NH₂ (FMRFa)-related peptides inhibit Na⁺-Ca²⁺ exchange in cardiac sarcolemma vesicles

The molluscan cardioexcitatory tetrapeptide FMRF-amide (Phe-Met-Arg-Phe-NH₂) and related peptides inhibit Na⁺-Ca²⁺ exchange in calf cardiac sarcolemma vesicles. FMRFa itself has a low inhibitory potency (IC₅₀ = 750 λM) which completely resides in its COOH-terminal RFa portion. The physiologically active analog FLRFa is 10-fold more potent (IC₅₀ = 60 μM). Two other substitutions of the Met₂ in FMRFa, by either He or Lys increase inhibitory potency 7- and 50-fold, respectively. The inhibitory potency increases 300-500-fold if the NH₂-terminal Phe¹ in FMRFa is substituted by either Val or His (IC₅₀ = 1-2 μM). The inhibitory activity of W_nLRFa (IC₅₀ = 40 μM) is lost when either the NH₂-terminal amino group is acylated or the NH₂-terminal Trp¹ is deleted. These data suggest that the COOH-terminal portion is essential for the basic low potency inhibition of Na⁺-Ca²⁺ exchange, whereas the NH₂-terminal portion is important for the potentiation of the inhibitory activity. Although the IC₅₀ values of various peptides range widely (10^-6-10^-3 M), all of them induce a complete inhibition. The dose-response pattern of the peptide-induced inhibition is identical for the Na⁺-Ca²⁺ exchange and its partial reaction, the Ca²⁺-Ca²⁺ exchange. The inhibitory effect is reversible and affects both Na_i(or Ca_i)-dependent ⁴⁵Ca uptake and Na_o-dependent ⁴⁵Ca efflux, suggesting that the bidirectional movements of ions are altered. A mild pretreatment of vesicles with trypsin augments the Na⁺-Ca²⁺ exchange 1.5-fold but diminishes the inhibitory potency 3-4-fold, suggesting that the inhibition is mediated by an extravesicular membrane protein. The characteristics of the peptide-induced inhibition resemble the effect of opiates on Na⁺-Ca²⁺ exchange. FLRFa and dextrorphan (a non-opioid stereoisomer of an opiate agonist) are mutually exclusive inhibitors, suggesting that they may bind to the same site. This putative site lacks the pharmacological properties of opiate receptors and may be located either on the Na⁺-Ca²⁺ exchanger or at its vicinity. Endogenous analogs of FMRFa may regulate intracellular calcium via Na⁺-Ca²⁺ exchange.