| Abstract: | A study has been made of the increased sensitivity to injected GTP of the sodium efflux in barnacle muscle fibers pre-exposed to aldosterone and of the problem whether or not aldosterone acts by raising the internal ATP level. The results indicate that increased sensitivity to injected GTP develops fully some 8 hr following external application of 10(-6) M aldosterone. Neither actinomycin D nor cycloheximide abolishes this extrasensitivity. This is also true of colchicine and cytochalasin B. The magnitude of the sustained response to injected "dialyzed" cholera toxin or cAMP-protein kinase catalytic subunit is practically the same as that of unexposed fibers. Internal ATP levels in pre-exposed fibers are higher than in unexposed fibers, even in the presence of cycloheximide. Injection of ADP (0.1 M) raises the ATP levels and reduces the ArP levels, more so in unexposed fibers. The suggestion is made that extrasensitivity of pre-exposed fibers to injected guanine nucleotides represents a post-translational phenomenon which might involve delay in the reassociation of R2 with C (of cAMP-PK). |
