Uptake and release of calcium in rabbit vagus nerve.

Abstract

1. The uptake and the release of calcium were measured in desheathed rabbit vagus nerves by using45Ca. 2. The calcium uptake reached a plateau after 2 h of incubation; the total content of exchangeable calcium was 0.85±0.12 mM/kg wet nerve. The calcium influx was reduced by La+++ or Pr+++. 3. The Ca efflux followed a double exponential curve. After an initial period of extracellular calcium loss, the time constants of the efflux were 10.7 and 170 min. About 15% of the radiocalcium was strongly bound and could not be removed by homogeneization. 4. Lowering the temperature reduced both components of the efflux with aQ10 of 1.4. 5. The following substances, which interfere with metabolism or active transport, had no effect on the calcium efflux; Cyanide (2 mM), 2.4-dinitrophenol (0.4 mM), monoiodoacetic acid (4 mM), ethacrynic acid (0.5 mM), p-chloromercuribenzoate (0.1 mM), ouabain (0.14 mM). 6. The calcium efflux was reduced by about 30% when the extracellular Na+ was replaced by choline and increased upon re-introduction of Na+. Replacement of the Na+ by Li+ had only a slight effect. 7. The following substances, which interfere with Na+ movements, had no effect on the calcium efflux: Tetracaine (1 mM), tetrodotoxine (0.003 mM), guanethidine (1 mM), bretylium (2 mM), acetylcholine (1.7 mM). 8. It is concluded that calcium efflux is largely passive and mediated partly through a Na/Ca exchange system. This system appears to be different from the Na carrying system responsible for excitation. 9. The experiments suggest that the calcium content of the nerve can be divided into several fractions: Out of a total of 1.85 mM intracellular Ca/kg wet nerve known from other experiments 1 mM is not exchangeable, 0.12 mM tightly bound, two fractions of 0.35 mM each loosely bound and released with different time constants, and finally some Ca is probably ionized.