PubMed1980
1. Ca fluxes were studied in a pure cholinergic preparation of nerve endings from Torpedo electric organ. In standard physiological saline solution (280 mM Na+, 3 mM K+) the calcium uptake was at the rate of 8 nmole per mg of protein per minute, equivalent to a calcium influx of 0.6 pmole/cm2 x sec. 2. The rate of calcium uptake was enhanced when synaptosomes were depolarized, increasing potassium concentration in the external medium. Maximum stimulation was reached when the potassium concentration was over 50 mM and averaged 20 nmole Ca2+ per mg protein per minute, equivalent to calcium influx of about 1.5 pmole/cm2 x sec. 3. Protoveratrine and tytiustoxin also stimulated calcium uptake into the nerve terminals and their effect was blocked by tetrodotoxin. Tetrodotoxin didn't block calcium uptake in K+-stimulated synaptosomes. Some inhibitors of transmitter release such as Dantrolene, Verapamil, BetaBungarotoxin and adenine nucleotides prevented the stimulatory effect on calcium uptake by depolarizing agents. 4. The calcium entry increased roughly linearly with a slope proportional to external calcium concentrations up to 20 mM. This effect was greatly increased when either 100 mM K+ or protoveratrine were added to the external medium. 5. Most of the loss of isotope from 45Ca-loaded synaptosomes occurred by a sodium-dependent calcium efflux mechanism with half-activation at 13 mM Na+. This may be equivalent to 1.65 pmole/cm2 x sec. 6. In sodium-free and calcium-free solutions, a residual efflux (5 nmole per mg protein per min) is observed. Metabolic inhibitors such as CN- enhanced this residual efflux. 7. Morphological studies using sodium-free fixation solutions and radioautographic detection of 45Ca are consistent with calcium sequestration by intracellular organelles by a similar mechanism to the calcium buffering system described in disrupted synaptosomes. Synaptic vesicles may play an important role in this calcium sequestration.