K-DEPENDENT AND HCO3--DEPENDENT ACID-BASE TRANSPORT IN SQUID GIANT-AXONS .1. BASE EFFLUX()

We used microelectrodes to monitor the recovery (i.e., decrease) of in tracellular pH (pH(i)) after using internal dialysis to load squid gia nt axons with alkali to pH(i) values of 7.7, 8.0, or 8.3. The dialysis fluid (DF) contained 400 mM K+ but was free of Na+ and Cl- The artifi cial seawater (ASMT) lacked Na+, K+, and Cl-, thereby eliminating effe cts of known acid-base transporters on pH(i). Under these conditions, halting dialysis unmasked a slow pH(i) decrease caused at least in par t by acid-base transport we refer to as ''base efflux.'' Replacing Kin the DF with either NMDG(+) or TEA(+) significantly reduced base eff lux and made membrane voltage (V-m) more positive. Base efflux in K+-d ialyzed axons was stimulated by decreasing the pH of the ASW (pH(o)) f rom 8 to 7, implicating transport of acid or base. Although postdialys is acidifications also occurred in axons in which we replaced the K+ i n the DF with Li+, Na+, Rb+, or Cs+, only with Rb+ was base efflux sti mulated by low pH(o). Thus, the base effluxes supported by K+ and Rbappear to be unrelated mechanistically to those observed with Lit, Na, or Cs+. The combination of 437 mM K+ and 12 mM HCO3- in the ASW, whi ch eliminates the gradient favoring a hypothetical K+/HCO3- efflux, bl ocked pH(i) recovery in K+-dialyzed axons. However, the pHi recovery w as not blocked by the combination of 37 mM Na+, veratridine, and CO2/H CO3- in the ASMT, a treatment that inverts electrochemical gradients f or H+ and HCO3- and would favor passive H+ and HCO3- fluxes that would have alkalinized the axon. Similarly, the recovery was not blocked by K+ alone or HCO3- alone in the ASW nor was it inhibited by the K-W pu mp blocker Sch28080 nor by the Na-H exchange inhibitors amiloride and hexamethyleneamiloride. Our data suggest that a major component of bas e efflux in alkali-loaded axons cannot be explained by metabolism, a H + or HCO3- conductance, or by a K-H exchanger. However, this component could be mediated by a novel K/HCO3- cotransporter.