Arthropod membranes

Pyrethroids, such as p,p -DDT, are toxic because they interact with Na+ channels of the axonal membrane, thereby disturbing the transmission of nerve action potential (Eldefrawi and Eldefrawi 1990, and Chapter 5, Section 5.2.4 of this book). In both cases, marked hydrophobicity leads to bioconcentration of the insecticides in the axonal membrane and reversible association with the Na+ channel. Consequently, both DDT and pyrethroids show negative temperature coefficients in arthropods increasing temperature brings decreasing toxicity because it favors desorption of insecticide from the site of action. 

Close modulation of the physical states of lipids is important in a variety of other contexts. Lipids play major roles as buoyancy-regulating devices in which temperature-induced alterations in lipid density play important functions in setting the overall buoyancy of the organism (e.g., the spermaceti organ of sperm whales see Clarke, 1979). Cuticular lipids function as important barriers to water movement in terrestrial arthropods (Gibbs, 1998 Hadley, 1981), and the chemical compositions and physical states of these lipids manifest temperature responses comparable in many ways to those observed for membrane lipids. 

Avermectin Bia has been shown to stimulate chloride ion uptake into cockroach muscle ( 140 and block transmission at the arthropod neuromuscular junction by increasing GABA-mediated chloride ion permeability (15), (16). To extend our understanding of ion channel types present in the surface membrane of the synaptosome we examined the effects of ivermectin (22,23-dihydroavermectin B.) and some closely related milbemycins. Both ivermectin and the milbemycins examined, with the exception of milbemycin a9, were... 

In addition to Its structural role In membranes, discussed In Chapter 5, cholesterol Is the precursor for several Important bloactive molecules. They Include bile acids (see Figure 18-6), which are made In the liver and help emulsify dietary fats for digestion and absorption In the Intestines, steroid hormones produced by endocrine cells (e.g., adrenal gland, ovary, testes), and vitamin D produced In the skin and kidneys. Arthropods need cholesterol or other sterols to produce membranes and ecdysterold hormones, which control development however, they cannot make the precursor sterols themselves and must obtain these compounds In their diet. Another critical function of cholesterol Is Its covalent addition to Hedgehog protein, a key signaling molecule In embryonic development (Chapter 15). 

The membranes of diatoms, and the chitin-impregnated membranes of arthropods, are Type 4. 

Arterivirus (equine arteritis virus and simian hemorrhagic fever virus). Enveloped, polyhedral, positive-sense, and ssRNA. Synthesis occurs in the host cell cytoplasm maturation involves budding of nucleocapsids through the host cell plasma membrane. Viruses are released via cell lysis (Arterivirus). Many replicates in arthropods and vertebrates. 

It is remarkable that the photosensitivity of the insects, Crustacea, arthropods, cephalopods, together with the vertebrates and even the halophilic bacteria, is based on a common chemistry. In each organism, the pigment responsible for absorption of light consists of a retinaldehyde (Cl) prosthetic group coupled by a protonated aldimine bond to a membrane protein. The reactions following light absorption always involve cis-trans isomerization, a process that sometimes leads to deprotonation of the aldimine bond. 

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