Effects of antibiotics on uptake of calcium into isolated nerve terminals
Atchison, W.D.; Adgate, L.; Beaman, C.M.
Journal of Pharmacology and Experimental Therapeutics 245(2): 394-401
1988
ISSN/ISBN: 0022-3565 PMID: 3367298 Document Number: 324368
The goal of the present study was to determine whether several antibiotics which are known to block neuromuscular transmission would impair depolarization-dependent and/or- independent uptake of calcium into isolated nerve terminals prepared form forebrain synaptosomes of rats by conventional methods. Antibiotics tested for potential block of Ca2+ uptake included the aminoglycosides neomycin and streptomycin, the lincosamide clindamycin, oxytetracycline and polymyxin B. Drugs were applied in concentrations ranging from 1 to 1000 .mu.M. Uptake of 45Ca was determined during depolarization induced by an elevated K+ concentration (77.5 mM). Influxes of 45Ca during 1 and 10 sec of depolarization were used to assess Ca2+ uptake via "fast, inactivating path" and total uptake, respectively. Uptake of 45Ca during 10 sec of depolarization into synaptosomes which were previously depolarized for 10 sec in the presence of 77.5 mM K+ but in the absence of external Ca2+ was used to measure uptake during the uptake of 45Ca was depressed significantly by all antibiotics tested except oxytetracycline; however, the various agents differed with respect to their efficacy and potency as blockers of Ca influx. The fast component of uptake, which is thought to be associated with neurotransmitter release, was decreased significantly by all antibiotics. Neomycin and polymyxin were the most potent and most effective at lowering fast phase 45Ca influx; streptomycin, was intermediate in effectiveness at concentrations .gtoreq. 100 .mu.M. Only clindamydin, streptomycin and polymyxin B caused significant reductions in the "slow" phase of 45Ca uptake. At high concentrations, all of the antibiotics except streptomycin caused small, yet significant, alterations in depolarization-independent uptake of 45Ca as well, but these effects were subtracted from uptake after K+ depolarization, and thus had no bearing on those values. Increasing the extracellular Ca2+ concentration was able to overcome in a competitive manner the reduction of the fast component of 45Ca utpake by all the drugs except clindamycin. The rsults of the present studyare consistent with the hypothesis that antagonism of Ca2+ entry through voltage-regulated Ca channels in the nerve terminal membrane contributes to the mechanism by which cetain antibiotics block nehromuscular transmission.