A STUDY ON THE STRUCTURE-ACTIVITY RELATIONSHIP OF THE CARDIOTONIC STEROIDS.
According to the prevailing opinion of today about the structure-activity relationship of cardenolides and bufadienolides, the structural requirements for the characteristic cardiotonic action of these compounds can be summarized as follows: cis-fusion of the C and D rings, hydroxyl groups in positions 3β and 14β, and a five- or six-membered unsaturated lactone ring having β-configuration at C14 (1). Most of the experimental data referred to as evidences supporting this opinion have come, however, from the determination of toxic doses in the whole animal, mainly in the cat, rather than from observations of the action of these compounds in isolated hearts. It follows as a natural consequence that the compounds with a rather weak toxicity have been condemned to be inactive, even if they have a strong cardiotonic action, in spite of the fact that such a kind of compounds may actually be more important from the clinical point of view. For example, the compound like dihydrodigitoxin of which the lactone ring is saturated, and the compound like resibufogenin, which have an epoxide ring between C14 and C15 instead of C14-OH, generally believed to be inactive based on the toxicity data in the cat, have recently been found to be fairly active when tested in the isolated heart (2-3). The authors also found that, different from the above-cited opinion, the presence of 3β-OH is not indispensable for the cardiotonic action of the cardenolides ; 3-epi-digitoxigenin was found to have a definite cardiotonic action upon the isolated frog's heart (See Fig. 8). These findings seem to warrant reexamination of the problem, on the basis of the action of these compounds in the isolated heart. In recent years several adrenocortical steroids and aldosterone antagonists have been reported to be cardiotonic (4-6), despite the fact that their stereochemical structures are essentially different from those of the cardiotonic steroids. In view of these, the authors took up the problem of the cis-fusion of the C and D rings out of the several important points in the structure of cardiotonic steroids as discussed above and reexamined its importance for the specific cardiotonic action using the isolated frog's heart and the heart-lung preparation of the dog.
Target-site sensitivity in a specialized herbivore towards major toxic compounds of its host plant: the Na+K+-ATPase of the oleander hawk moth (Daphnisnerii) is highly susceptible to cardenolides
The caterpillars of the oleander hawk moth, Daphnis nerii (Linnaeus, 1758) (Lepidoptera: Sphingidae) feed primarily on oleander (Nerium oleander). This plant is rich in cardenolides, which specifically inhibit the Na+K+-ATPase. Since some insects feeding on cardenolide plants possess cardenolide-resistant Na+K+-ATPases, we tested whether D. nerii also possesses this strategy for circumventing cardenolide toxicity. To do so, we established a physiological assay, which allowed direct measurement of Na+K+-ATPase cardenolide sensitivity. Using Schistocerca gregaria, as a cardenolide-sensitive reference species, we showed that D. nerii Na+K+-ATPase was extremely sensitive to the cardenolide ouabain. Surprisingly, its sensitivity is even higher than that of the cardenolide-sensitive generalist, S. gregaria. The presence or absence of cardenolides in the diet of D. nerii did not influence the enzyme’s cardenolide sensitivity, indicating that target-site insensitivity is not inducible in this species. However, despite the sensitivity of their Na+K+-ATPase, caterpillars of D. nerii quickly recovered from an injection of an excessive amount of ouabain into their haemocoel. We conclude that D. nerii possesses adaptations, which enable it to feed on a cardenolide-rich diet other than that previously described in cardenolide specialized insects, and discuss other potential resistance mechanisms.
On a New Subclass of p-Valent Close-to-Convex Mappings Defined by Two-Sided Inequality
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