DIFFERENTIAL UP-REGULATION OF NA-CA2+ EXCHANGE BY PHOSPHOARGININE ANDATP IN DIALYZED SQUID AXONS()

1. The aim of this study was to characterize further the two main meta bolic pathways of regulation of the Na+-Ca2+ exchanger in squid axons induced by its two naturally ocurring high-energy compounds: ATP and p hosphoarginine (Pal. [Na+](o)-dependent Ca2+ efflux (forward Na-o(+)-C a-1(2+) exchange) and [Ca2+](o)-dependent Ca2+ efflux (Ca-o(2+)-Ca-1(2 +) exchange) were measured in internally dialysed squid axons at 16-17 degrees C. 2. Measurements of changes in the apparent affinity of the Na+-Ca2+ exchanger for transporting (Na-o(+), Na-1(+), Ca-o(2+), Ca-1 (2+)) and regulatory (Ca-1(2+)) ions induced by ATP and Pa show marked differences for the two substrates: (i) ATP strongly alters the affin ity for Na-o(+) and Na-1(+), while Pa does not, and (ii) in the absenc e of Na-1(+), ATP has no stimulatiory effect; on the other hand, Pa ca uses a dramatic increase in Na-o(+)-Ca-1(2+) exchange with little acti vation of Ca-o(2+)-Ca-1(2+) exchange. 3. The MgATP analogue chromium-A TP (CrATP) completely inhibits MgATP stimulation of the Na+-Ca2+ excha nger. Nevertheless, even with the effects of the nucleotide blocked, P a exhibits its usual activation of the [Na+](o)-dependent Ca2+ efflux. 4. None of the classical serine-threonine-tyrosine kinase inhibitors, nor the PP1 and PP2 phosphatase inhibitors, affects either the ATP or the Pa effect. However, intracellular microinjections of an exogenous phosphatase (alkaline phosphatase) completely reverses the stimulatio n of the Na+-Ca2+ exchange induced by ATP and Pa. 5. Prolonged intrace llular dialysis with highly permeable porous capillaries (18 kDa molec ular weight cut-off), which normally induces a complete run-down of th e MgATP effect, does not alter the Pa stimulation of the exchanger, ev en after 6 h of continuous dialysis. 6. We conclude that the ATP and P a modulation of Na+-Ca2+ exchange in an invertebrate nerve fibre are t wo genuinely different mechanisms, which affect the carrier properties in very different ways. An interesting similarity between ATP and Pa is that a phosphorylation-dephosphorylation process seems to be a comm on feature of these two regulation modes.