Mechanistic link between lidocaine block and inactivation probed by outer pore mutations in the rat micro1 skeletal muscle sodium channel.

1. Mutations that disrupt Na+ channel fast inactivation attenuate lidocaine (lignocaine)-induced use dependence; however, the pharmacological role of slower inactivation processes remains unclear. In Xenopus oocytes, tryptophan substitution in the outer pore of the rat skeletal muscle channel (micro1-W402) alters partitioning among fast- and slow-inactivated states. We therefore examined the effects of W402 mutations on lidocaine block. 2. Recovery from inactivation exhibited three kinetic components (IF, fast; IM, intermediate; IS, slow). The effects of W402A and W402S on IF and IS differed, but both mutants (with or without beta1 subunit coexpression) decreased the amplitude of IM. In wild-type channels, lidocaine imposed a delayed recovery component with intermediate kinetics, and use-dependent block was attenuated in both W402A and W402S. 3. To examine the pharmacological role of IS relative to IM, drug-exposed beta1-coexpressed channels were subjected to 2 min depolarizations. Lidocaine had no effect on sodium current (INa) after a 1 s hyperpolarization interval that allowed recovery from IM but not IS, suggesting that lidocaine affinity for IS is low. 4. Both W402 mutations reduced occupancy of IM in drug-free conditions, and also induced resistance to use-dependent block. We propose that lidocaine-induced use dependence may involve an allosteric conformational change in the outer pore.

Dehydroepiandrosterone and two structural analogs inhibit 12-O-tetradecanoylphorbol-13-acetate stimulation of prostaglandin E2 content in mouse skin.

Dehydroepiandrosterone, a naturally occurring adrenal steroid, is a highly effective tumor chemopreventive agent in laboratory mice and rats, inhibiting spontaneous breast cancer and chemically induced tumors of the lung, colon, skin, liver and thyroid. Dehydroepiandrosterone blocks three processes that have been implicated in experimental tumorigenesis: (i) carcinogen activation through the mixed-function oxidases, (ii) 12-O-tetradecanoylphorbol-13-acetate stimulation of superoxide anion production in neutrophils, and (iii) 12-O-tetradecanoylphorbol-13-acetate stimulation of [3H]thymidine incorporation in mouse epidermis. All of these effects of dehydroepiandrosterone very likely result from glucose-6-phosphate dehydrogenase inhibition and a lowering of the NADPH cellular pool. It is now reported that oral administration of dehydroepiandrosterone (0.2% in the diet) for two weeks inhibits the stimulation in prostaglandin E2 content in mouse epidermis produced by topical application of 12-O-tetradecanoylphorbol-13-acetate. Two synthetic steroids, 16 alpha-fluoro-5-androsten-17-one and 16 alpha-fluoro-5 alpha-androstan-17-one, which are more potent inhibitors of the above three processes in tumorigenesis and are also more effective than dehydroepiandrosterone in inhibiting skin papilloma development in the mouse, are more active in suppressing prostaglandin E2 induction by 12-O-tetradecanoyl-phorbol-13-acetate. These two structural analogs, which also lack specific side-effects associated with dehydroepiandrosterone treatment, may find application as cancer chemopreventive drugs in humans.