Deregulation by zinc of the sodium efflux in barnacle muscle fibers

Abstract

Single muscle fibers from the barnacle Balanus nubilus were employed to study the behavior of the resting Na + efflux toward external and internal application of zinc (Zn 2+). This involved both unpoisoned and ouabain-poisoned fibers. The results obtained are as follows: (i) External application of Zn 2+, e.g., 2 mM (a maximal dosage) in 10 mM Hepes-ASW (pH 7.3) causes a fall in the resting Na + efflux which exceeds that caused by 10 −4 M ouabain in companion controls. (ii) The buffer of choice is found to be Hepes, rather than HCO 3 − or imidazole. (iii) The observed fall in the resting Na + efflux caused by external application of Zn 2+ is concentration-dependent, the IC 50 being 10 μM. (iv) The inhibitory effect of Zn 2+ is partially reversible; occasionally, however, reversibility is not seen. (v) The Zn 2+-insensitive component of the Na + efflux is reduced by 10 −4 M ouabain. (vi) The ouabain-insensitive component of the Na + efflux is reduced by external application of Zn 2+. This response is concentration-dependent. (vii) Preinjection of EGTA reduces the sensitivity of the Na + efflux to external application of Zn 2+. This is true of both unpoisoned and ouabain-poisoned fibers. (viii) (a) The resting Na + efflux is reduced by injecting Zn 2+. Ouabain application reduces the remaining Na + efflux. (b) Injection of Zn 2+ reduces the ouabain-insensitive component of the Na + efflux. (c) External application of Zn 2+ following the injection of Zn 2+ reduces the remaining Na + efflux. Ouabain is ineffective when applied after both maneuvers. (d) Injection of Zn 2+ after its external application is without effect. Subsequent application of ouabain is also without effect. (e) Injection or external application of Zn 2+ reduces the ouabain-insensitive Na + efflux. Whereas in the former case subsequent external application of Zn 2+ reduces the remaining Na + efflux, in the latter case Zn 2+ injection after external application of Zn 2+ is ineffective. Collectively, these results provide evidence in support of the hypothesis that Zn 2+ is a potent inhibitor of the ouabain-sensitive and ouabain-insensitive components of the Na + efflux, and that the inhibitory effect is partly due to the entry of Zn 2+ into the myoplasm. They also raise the possibility that the inhibitory effect caused by Zn 2+ injection may be the result of Zn 2+ leakage from the fiber interior.