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

Abstract

Phosphate efflux was measured as the fractional rate of loss of radioactivity from desheathed rabbit vagus nerves after loading with radiophosphate . The effects of strategies designed to increase intracellular calcium were investigated. At the same time, the exchangeable calcium content was measured using 45Ca. Application of calcium ionophore A23187 increased phosphate efflux in the presence of external calcium in parallel with an increase in calcium content. In the absence of external calcium, there was only a late, small increase in phosphate efflux. For nerves already treated with the calcium ionophore, the phosphate efflux was sensitive to small changes in external calcium, in the range 0.2 to 2 mM calcium, whereas similar increases in calcium in absence of ionophore gave much smaller increases in phosphate efflux. Removal of external sodium (choline substitution) produced an initial increase in phosphate efflux followed by a fall. The initial increase in phosphate efflux was much larger in the presence of calcium, than in its absence. The difference was again paralleled by an increase in calcium content of the preparation, thought to be due to inhibition of Na/Ca exchange by removal of external sodium. Measurements of ATP content and ATP, ADP, phosphate and creatine phosphate ratios did not indicate significant metabolic changes when the calcium content was increased. Stimulation of phosphate efflux by an increase in intracellular calcium may be due to stimulation of phospholipid metabolism. Alternatively, it is suggested that stimulation of phosphate efflux is associated with the stimulation of calcium efflux, possibly by cotransport of calcium and phosphate.

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

“…It has been shown in squid axon that lack of external Na produces a large increase in Ca inflow through the Cao/Nai exchange, which is very dependent on the Na content of the axon (Baker & DiPolo, 1984). Furthermore, in a previous paper we have shown that omission of Nao produces an increase in phosphate efflux, which probably also reflects an increase in the rate of the Cao/ Nai exchange Jirounek et al, 1984).…”

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.…”

Unique uptake and efflux systems of inorganic phosphate in osteoclast-like cells

“…The characteristics and molecular identity of the P i efflux system in various cells have not been well established. A few reports have examined the P i efflux system in opossum kidney (OK) cells (1), pancreatic islets (11,12), and vagus nerves (19). In the proximal tubule, the way in which P i exits from the basolateral side is not well defined.…”