Direct inhibitory action of EGTA-Ca complex on reverse-mode Na/Ca exchange inMyxicola giant axons

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Abstract
Giant axons from the marine annelidMyxicola infundibulum were internally dialyzed with solutions containing22Na ions as tracers of Na efflux. In experiments performed in Li-substituted seawater, Na efflux that is dependent on external Ca ion concentration, [Ca2+] o , was measured using dialysis to maintain [Na+] i at 100mm, which enhances the [Ca2+] o -dependent Na efflux component, (i.e., reverse-mode Na/Ca exchange). When dialysis fluid contained EGTA (1mm) to buffer the internal Ca concentration, [Ca2+] i , to desired levels, Na efflux lost its normal sensitivity to external calcium. The inhibition was not simply due to the Ca-chelating action of EGTA to produce insufficient [Ca2+] i to activate Na/Ca exchange. The addition of EGTA inhibited Ca o -dependent Na efflux even when a large enough excess of [Ca2+] i was present to saturate the EGTA and still produce elevated values of [Ca2+] i . Control experiments showed that these high values of [Ca2+] i resulted in normal Na/Ca exchange in the absence of EGTA. It is concluded that the presence of EGTA itself interferes with the manifestation of reverse-mode Na/Ca exchange inMyxicola giant axons.