PHOSPHOARGININE STIMULATION OF NA-CA2+ EXCHANGE IN SQUID AXONS - A NEW PATHWAY FOR METABOLIC-REGULATION()

1. [Na+](o)-dependent Ca2+ efflux (forward Na+-Ca2+ exchange), [P-32]A TP wash-out curves and [ATP] were measured in internally dialysed squi d giant axons at 17-18 degrees C. 2. We found that dialysing squid axo ns without ATP and with [Ca2+](i) around 1 mu M the basal levels of th e [Na+](o)-dependent Ca2+ efflux were significantly higher in the pres ence of N-omega-phosphoarginine (PA). Phosphocreatine, a related phosp hagen, is without effect. 3. PA stimulation of the Na+-Ca2+ exchange o ccurs in the complete absence of ATP (<1 mu M), being independent of, and additive to, the ATP-stimulated [Na+](o)-dependent Ca2+ efflux. PA stimulation of [Na+](o)-dependent Ca2+ efflux is fully and rapidly re versible with a K-m around 7.7 mM. Activation by saturating [PA] is eq uivalent in magnitude to that of ATP. 4. PA stimulation of Na+-Ca2+ ex change is markedly dependent on intracellular Ca2+ and Mg2+ ions. Belo w 0.5 mu M Ca-i(2+) PA effect is negligible, becoming noticeable betwe en 0.8 and 2 mu M. In addition, Ca-i(2+) considerably increases the ra te at which PA activates the Na+-Ca2+ exchange. Although there is no a bsolute requirement of the PA effect for Mg2+ ions, this divalent cati on largely stimulates the PA effect. 5. This work demonstrates, for th e first time, the presence in squid axons of a new form of metabolic r egulation of the Na+-Ca2+ exchange directly and solely related to PA a nd different from that of MgATP. This novel mechanism is likely to pla y a physiological role in Ca2+ extrusion through the Na+-Ca2+ exchange r, particularly at micromolar [Ca2+](i).