2 – Hormone Action at the Cellular Level

Publisher Summary
This chapter reviews the influence that endocrine hormones have on the growth and development of insects. The mechanism of action of none of the insect hormones is known with certainty, and it is postulated that they may act at several of the many possible control points. As several orders of insects are separated widely in evolutionary time, it might be that growth hormones do not act uniformly in all insects. The classical scheme of endocrine control of molting and metamorphosis in holometabolous insects has been plagued by questions raised as a result of contemporary investigations in the functioning of insect hormones. An exception to the classical scheme is the finding that renewed secretion by the brain is not always needed for each molt. Insect neurosecretory cells appear to play important roles in many other physiological processes. One important question that needs answering relates to the regulation and mechanism of synthesis and release of neurosecretion in insect cells. The activity of α-ecdysone—arthropod hormones that are produced by the prothoracic gland in insects and that trigger molting and metamorphosis and are synthesized from chiolestrol—and various side chain deoxyecdysones is partially because of their metabolism to β-ecdysone.

Physiology of growth and development: endocrine aspects

Publisher Summary The postembryonic development of insects is characterized by a series of synchronized events, controlled by the moulting and juvenile hormones. In the adult insect, tissue maintenance, homeostasis, and reproduction are also under hormonal control. The needs of these processes are met by anabolic or catabolic metabolism. As the requirement for amino acids, carbohydrate, and lipid used in the synthesis of structural elements and the energy required for endothermic reactions varies during different stages of development, and under different environmental conditions, a mechanism of metabolic control seems imperative. There is no evidence to support the notion that any of the known hormones occurring in insects functions as a “metabolic” hormone. The idea that such a hormone exists, and may be one of those already described, has been extant since the metabolic effects of hormones were first described and originated in the observation that allatectomy decreased the oxygen consumption of whole insects. Insects provide excellent model systems for the study of a number of metabolic problems, the importance of which extends far beyond the insects. More specifically, it seems certain that, because synthetic JH is now available, much attention will be directed toward the metabolic aspects of its action, particularly because of its potential use as an insecticide. The interaction of JH with female fat body to initiate the synthesis of vitellogenin in relatively large quantities may well prove a powerful instrument in elucidating the complex mechanisms by which hormones control the synthesis of proteins. The most pressing need in the immediate future is to isolate the various hormones and factors, which have been shown to be biologically active.

Hormonal Control of Metabolism in Insects

SUMMARY Previous investigations on the effects of monovalent cations on the gene activities (formation of puffs) of polytene chromosomes utilized whole salivary glands or nuclei and suggested that the K + /Na + ratio in the nuclear sap was of primary importance in influencing gene activities. To determine whether these effects of ions were direct or indirect, we have employed isolated polytene chromosomes incubated in salt solutions of varying ionic composition. Our observations reveal differential effects of K + , Na + , Mg 2+ , Ca 2+ and combinations thereof on specific chromosome regions. The band at region I-18-C that is specifically affected by K + is the same one that puffs when salivary gland nuclei are incubated in a K + -rich medium and this is believed to be an ecdysone-specific puff. Bands at region I-19-A are specifically affected by Na + and are the same ones that form puffs when salivary gland nuclei are incubated in a Na + -rich medium. This region is believed to be juvenile hormone-specific. Effects on the chromosome are also described that are the result of incubation in solutions containing Mg 2+ and Ca 2+ . Analogies are drawn between the observed effects of ions on isolated chromosomes and their effects on chromatin isolated from mammalian cells. For example, when chromatin is exposed to solutions of increasing ionic strength, hydration is followed by aggregation and histone dissociation. This may be analogous to what is observed in the isolated chromosome under similar conditions; namely, expansion, contraction and expansion.

Differential Effects of K+ and Na+ on Specific Bands of Isolated Polytene Chromosomes of Chironomus Tentans

Publisher Summary The principle of differential gene activation states that the differences among cells having the same genetic content are the result of different sets of genes being turned on or off. Differential gene activation is the basis of cell differentiation and function. The control of gene activity in higher organisms can take place at more than one level, for example, at the level of DNA, RNA polymerase, or histone. By interacting with DNA, histone determines whether RNA polymerase can ever bind at the respective chromosome region and whether mechanisms operating at other levels can ever come into play. Because the interaction between histone and DNA is ionic, it is primarily influenced by the ionic milieu of the chromosomal environment. Inorganic anions such as chloride or phosphate may affect the histone-DNA interaction as organic ions, such as amino acids, nucleotides, polyamines, or even more complex “ions” such as phosphoprotein, acidic protein, or RNA.

Gilbert Baumann

Papers

Zur Wirkung des Juvenilhormons: Elektrophysiologische Messungen an der Zellmembran der Speicheldrüse von Galleria mellonella