Continuous measurement of calcium influx in mammalian nonmyelinated nerve fibers: Effects of Na o , Ca o , and electrical activity

Abstract

A new technique for continuous monitoring of the cellular calcium was developed and used for studying the effects of external and internal Na (Nao and Nai), external Ca (Cao), Ca ionophore A23187, and electrical activity on membrane-bound and intracellular Ca in mammalian nonmyelinated nerve fibers. Increasing Cao increased both the membrane-bound and the intracellular Ca. Lowering Nao increased the membrane-bound fraction of Ca indicating that lack of Nao enhanced the capacity of the plasma membrane to bind Ca, and produced an increase of the internal Ca pool. Increasing Nai by treatment with ouabain enhanced the Ca inflow in both, the presence and absence of Nao, presumably by stimulating the Cao/Nai exchange. The Ca ionophore A23187 produced a large and irreversible increase in the intracellular Ca without affecting the membrane-bound fraction. On the other hand, electrical activity, which is known to produce a large increase of the total Ca in squid axon, had no measurable effect on the total calcium content in our preparation. It is concluded that in mammalian nerve fibers a Ca load by exposition to Na-free solution or to A23187 produces an accumulation of Ca into the intracellular Ca stores, whereas during electrical activity the membrane-associated extrusion mechanisms are able to maintain the intracellular Ca2+ below the threshold for intracellular sequestration. Furthermore, the results indicate that the intracellular sequestration mechanisms are dependent on the internal concentration of Na.

Calcium efflux and intracellular exchangeable calcium in mammalian nonmyelinated nerve fibers

“…We have also reported an interdependence between Ca z+ and phosphate fluxes (Jirounek et al, 1982), and suggested that the phosphate efflux may be linked to the operation of the Na+-Ca 2+ exchange (Jirounek et al, 1984b). Furthermore, we have shown that whenever the external Na § was decreased, there was a large increase in the labeling of the intracellular calcium pools (Jirounek et al, 1986). A similar effect was observed when the external phosphate concentration was increased (Jirounek et al, 1984a).…”

“…After 150-min incubation, which time corresponds to the labeling conditions used in the experiments described in the next sections, the amounts of labeled Ca 2+ in pools A 1 and A2 are 0.170 and 0.102 mmol/kg wet weight (w.w.), respectively. The total intracellular Ca 2+ is thus 0.272 mmol/kg w.w., which can be compared to 0.355 mmol/kg w.w. found at isotopic equilibrium in the same preparation by a different method (Jirounek et al, 1986). Fig.…”

“…8 we observe that there, in contrast to the effects of phosphate, a decrease in the intracellular buffering produces a decrease of the efflux. A possible explanation of this discrepancy is that the large influx of 4~ which occurs in Na+-free solution (Jirounek et al, 1986), dilutes the internal 45Ca2+, producing thus an apparent decrease of the 45Ca2+ efflux. We can, however, not exclude that at least a part of the effect is due to the inhibition of the + 2+ Nao-Cain exchange.…”

“…3). In our previous paper (Jirounek et al, 1986) we have shown that during incubation of rabbit vagus nerve in low Na + media there was an important gain in the total 45Ca2+ which was due to the reversal of the Na+-Ca > exchange. Brinley et al (1977) have shown that during condition of net Ca > gain, the great majority of the Ca > entering axons is buffered and only 1/1000 or less is detectable as free calcium.…”

“…In our previous papers we have studied, in rabbit vagus nerve, the membrane-associated Ca 2+-transport systems and have shown that part of the Ca 2+ efflux is mediated by a Na+-Ca 2+ exchange mechanism (Jirounek et al, 1986). We have also reported an interdependence between Ca z+ and phosphate fluxes (Jirounek et al, 1982), and suggested that the phosphate efflux may be linked to the operation of the Na+-Ca 2+ exchange (Jirounek et al, 1984b).…”