Effects of calcium and lanthanum on phosphate efflux from nonmyelinated nerve fibers

Abstract

Phosphate efflux was measured as the fractional rate of loss of radioactivity from rabbit vagus loaded with radiophosphate. The effects of changes in extracellular calcium and of lanthanum have been investigated. In Locke solution with normal, 0.9 mM, calcium and without phosphate, the fractional rate of loss was 1.62 X 10(-3) min-1 at 120 min after the beginning of the washing period and fell slowly (9% hr-1) during washing from 2 to 6 hr. Addition of calcium to the Locke solution produced a transient increase followed by a reversible maintained increase in phosphate efflux. The latter was 40 and 75% above efflux in normal calcium for 20 and 50 mM calcium, respectively. Removal of calcium, with or without addition of EGTA, produced only a transient increase in phosphate efflux, with no subsequent maintained change. Addition of low concentrations of lanthanum produced a reversible inhibition of phosphate efflux. Half-maximal inhibition was at 3.5 micro M lanthanum and appeared to be due to binding of lanthanum to more than one, probably two, sites. Measurements of inhibition by lanthanum at different calcium concentrations did not indicate any competition between calcium and lanthanum. It is suggested that a least a part of phosphate efflux depends on internal calcium and that lanthanum acts by preventing release of phosphate from the phosphate transport mechanism.

Phosphate transport processes in eukaryotic cells

“…This type of Pi transport has been studied most extensively in kidney and intestine, where it is responsible for the rate-limiting step in transepithelial Pi transport. Studies have now been initiated on P~ transport processes in heart and skeletal muscle (Nuutinen & Hassinen, 1981;Medina & Illingsworth, 1980), nerve fibers (Jirounek et al, 1982(Jirounek et al, , 1984, across the placenta (Stulc & Stulcova, 1984;Brunette & Allard, 1985), and in a variety of cultured cell lines. In addition, uptake of P~ by cancer cells has been studied (Wehrle & Pedersen, 1982;Bowen & Levinson, 1983), partly to determine whether Pi uptake plays a role in rapid growth.…”

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).…”

Involvement of intracellular calcium in the phosphate efflux from mammalian nonmyelinated nerve fibers

“…In efflux experiments with nerves loaded with 45Ca the mean rate constant of calcium efflux (at 0.9 mM of extracellular calcium and in absence of phosphate) was about 1.25 • 10 -4 sec -1, and the steady value of the exchangeable calcium pool, calculated from our influx experiments was, for the same experimental conditions, 1.2 mM/kg wet weight (unpublished results). A further argument for this possibility is that low concentrations of lanthanum, which block the Na-independent part of the phosphate efflux [16], in squid giant axon inhibit the residual calcium efflux that persists in absence of external sodium and calcium almost completely [5]. The corresponding mean rate constant of phosphate elflux and the exchangeable phosphate pool are 1.7 x 10 -5 sec -I (this paper) and 6.9 mmol/kg wet weight [3], respectively.…”

“…We have observed recently that changes in extracellular calcium concentration modify the phosphate efflux in rabbit vagus nerve [16]. An increase in extracellular calcium to 20 mg produced a rapid increase of efflux of phosphate to a value about 40% higher than the efflux in 0.9 mM external calcium.…”