Ca efllux in dialyzed squid axons was measured with ~Ca as a function of internal ionized Ca in the range 0.005-I0/~M. Internal Ca stores were depleted by treatment with CN and dialysis with media free of high energy compounds. The [Ca] ~ was stabilized with millimolar concentrations of EDTA, EGTA, or DTPA. Nonspecific leak of chelated Ca was measured with [14C]-EDTA and found to be 0.02 pmol/cm~s/mM EDTA. Correction of the measured Ca effiux for this leak of chelated calcium was made when appropriate. Ca efflux was roughly linear with internal free Ca in the range 0.005-0. I #M. Above 0. l/~M, efflux was less than proportional to concentration but did not saturate at the highest concentration studied. Ca efflux was reduced about 50% by replacement of external Na with Li at Ca~ ~ I/~M, but was insensitive to such replacement for Ca < 0.1 /~M. Ca efl]ux was insensitive to internal Mg in the range 0-4 mM, indicating that the Ca pump favors Ca over Mg by a factor of about 10 s. Ca effiux was reduced about 60 % by increasing internal Na from I to 80 mM. This effect could represent weak interference of a Ca carrier by Na or a loss of driving force because of a reduction in EN, --Em occasioned by an increase in Nal. A few measurements were made of Ca influx in intact and in dialyzed fibers. In both cases, Ca influx increased when external Na was replaced by Li.

The efflux of Mg ++ from squid axons subject to internal solute control by dialysis is a function of ionized [Mg]l, [Nail, [ATP]I, and [Na]o. The efflux of Mg ++ from an axon with physiological concentrations of ATP, Na, and Mg inside into seawater is of the order of 2-4 pmol/cm2s but this efflux is strongly inhibited by increases in [Na~, by decreases in [ATP]t, or by decreases in [Na]o. The efflux of Mg ++ is largely independent of [Mg]i when ATP is at physiological levels, but in the absence of ATP reaches half the value of Mg efflux in the presence of ATP when [Mg]l is about 4 mM and [Na]t 40 raM. Half-maximum responses to ATP occur at about 350/~M ATP into seawater with Na either present or absent. The Mg efflux mechanism has many similarities to the Ca efflux system in squid axons especially with respect to the effects of ATP, Nao, and Nat on the flux. The concentrations of free Mg and Ca in axoplasm differ, however, by a factor of 105 while the observed fluxes differ by a factor of 102.

With an in vitro chamber method experiments were performed to determine the relative ionic conductances of the nutrient membrane (membrane facing muscularis mucosa). The concentration of a given ion in the nutrient bathing solution was changed, and the ensuing time course of the change in transmucosal potential difference (PD) was recorded. Changing K(+) from 4 to 79 mM produced a response in PD which occurred markedly faster than the response for the reverse change, and similar results were obtained by changing the Cl(-) concentration. It was found that these differences were predicted by the analysis of an idealized model consisting of a membrane in series with a diffusion barrier. When both the K(+) and Cl(-) were changed, such that the product of their concentrations remained constant, the time courses of the responses were again similiar to those predicted on the basis of the model. From the magnitudes of the total PD responses it is shown that in the presence of a 4 mM K(+) nutrient solution, the conductivity of the nutrient membrane appears to be entirely due to the K(+) and Cl(-) conductances, the K(+) conductance being about twice that of the Cl(-). It is also shown that with a 79 mM K(+) nutrient solution the parameters of the membrane were changed such that the conductances of the two ions were approximately equal. The time constant for diffusion of KCl or NaCl across the barrier consisting of the submucosa, muscularis mucosa, and lamina propria is about 1 min.

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