The portions of the 45Ca influx and 22Na efflux that were activated by physiological concentrations of intracellular free Ca2+, [Ca2+]i, were studied in internally perfused single giant barnacle muscle cells. Since both fluxes were activated by intracellular Ca2+ (Cai) and the Ca influx was dependent on internal Na+ (Nai), the fluxes appear to be coupled (Na/Ca exchange). Tracer Ca/Ca and Na/Na exchanges were eliminated by employing tris(hydroxymethyl)aminomethane (Tris) as the predominant external cation. Under these circumstances, the ratio of the external Ca2+ (Cao)-dependent, Cai-activated Na+ efflux to the Nai-dependent, Cai-activated Ca influx was 3.1-3.2 Na+/1 Ca2+, when the intracellular Na+ concentration, [Na+]i was either 30 or 46 mM. This is the first direct measurement of the Na/Ca exchange stoichiometry. In many types of cells, the Na/Ca exchange system appears to operate in parallel with a plasma membrane ATP-driven Ca pump that has a lower capacity (maximum velocity), but higher affinity for Ca2+ than the Na/Ca exchanger. The data on the stoichiometry and activation by internal Ca2+ imply that the turnover of the Na/Ca exchanger is modulated during periods of cell activity. When the cells are depolarized, the Na/Ca exchange system is activated by the rising [Ca2+]i, and Ca2+ entry via the exchanger is promoted. Then, at repolarization, Ca2+ exits rapidly, primarily via the exchanger. However, in resting cells, with a low [Ca2+]i, much (but not all) of the Ca2+ efflux is probably mediated by the ATP-driven Ca pump.

To gain insight into the mechanism(s) by which cells sense volume changes, specific predictions of the macromolecular crowding theory (A. P. Minton. In: Cellular and Molecular Physiology of Cell Volume Regulation, edited by K. Strange. Boca Raton, FL: CRC, 1994, p. 181-190. A. P. Minton, C. C. Colclasure, and J. C. Parker. Proc. Natl. Acad. Sci. USA 89: 10504-10506, 1992) were tested on the volume of internally perfused barnacle muscle cells. This preparation was chosen because it allows assessment of the effect on cell volume of changes in the intracellular macromolecular concentration and size while maintaining constant the ionic strength, membrane stretch, and osmolality. The predictions tested were that isotonic replacement of large macromolecules by smaller ones should induce volume decreases proportional to the initial macromolecular concentration and size as well as to the magnitude of the concentration reduction. The experimental results were consistent with these predictions: isotonic replacement of proteins or polymers with sucrose induced volume reductions, but this effect was only observed when the replacement was > or = 25% and the particular macromolecule had an average molecular mass of < or = 20 kDa and a concentration of at least 18 mg/ml. Volume reduction was effected by a mechanism identical with that of hypotonicity-induced regulatory volume decrease, namely, activation of verapamil-sensitive Ca2+ channels.

Coupled Na § exit/Ca 2+ entry (Na/Ca exchange operating in the Ca 2+ influx mode) was studied in giant barnacle muscle cells by measuring ~Na + efflux and 4~Ca2+ influx in internally perfused, ATP-fueled cells in which the Na + pump was poisoned by 0.1 mM ouabain. Internal free Ca ~+, [Ca ~+ ]i, was controlled with a Ca-EGTA buffering system containing 8 mM EGTA and varying amounts of Ca ~+. Ca ~+ sequestration in internal stores was inhibited with caffeine and a mitochondrial uncoupler (FCCP). To maximize conditions for Ca 2+ influx mode Na/Ca exchange, and to eliminate tracer Na/Na exchange, all of the external Na § in the standard Na+sea water (NaSW) was replaced by Tris or Li + (Tris-SW or LiSW, respectively). In both Na-free solutions an external Ca 2+ (Cao)-dependent Na + efflux was observed when A Nacdependent Ca ~+ influx was also observed in Tris-SW. This Ca ~+ influx also required [Ca2+]i > 10 -s M. Internal Ca 2+ activated a Na~-independent Ca 2+ influx from LiSW (tracer Ca/Ca exchange), but in Tris-SW virtually all of the Cai-actirated Ca ~+ influx was Nal-dependent (Na/Ca exchange). Half-maximal activation was observed with [Na+]i = 30 raM. The fact that internal Ca 2+ activates both a Cao-dependent Na + effiux and a Nai-dependent Ca 2+ influx in Tris-SW implies that these two fluxes are coupled; the activating (intracellular) Ca ~+ does not appear to be transported by the exchanger. The maximal (calculated) Nardependent Ca ~+ influx was -25 pmol/cm~.s. At various [Na+]i between 6 and 106 mM. the ratio of the Cao-dependent Na + effiux to the Nai-dependent Ca 2+ influx was 2.8-3.2:1 (mean = 3.1:1); this directly demonstrates that the stoichiometry (coupling ratio) of the Na/Ca exchange is 3:1. These observations on the coupling ratio and kinetics of the Na/Ca exchanger imply that in resting cells the exchanger turns over at a low rate because of the low [Ca~+]i; much of the Ca ~+ extrusion at rest Address reprint requests to Dr.

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, has a short half-life and is difficult to obtain; ii) squid giant axons, the ideal preparation to carry out transport studies under "zero-trans" conditions, are only available during the summer months; and iii) the ionic fluxes mediated by the Mg 2+ transporter are very small and difficult to measure. The purpose of this manuscript is to review how these limitations have been recently overcame and to propose a novel hypothesis for the plasmalemmal Mg 2+ transporter in squid axons and barnacle muscle cells. Overcoming the limitations for studying the plasmalemmal Mg 2+ transporter has been possible as a result of the following findings: i) the Mg 2+ exchanger can operate in "reverse", thus extracellular Mg 2+ -dependent ionic fluxes (e.g., Na + efflux) can be utilized to measure its activity; ii) internally perfused, voltage-clamped barnacle muscle cells which are available all year long can be used in addition to squid axons; and iii) phosphoinositides (e.g., PIP 2 ) produce an 8-fold increase in the ionic fluxes mediated by the Mg 2+ exchanger. The hypothesis that we postulate is that, in squid giant axons and barnacle muscle cells, a 2Na+2K+2Cl:1Mg exchanger is responsible for transporting Mg 2+ across the