Distribution of brevetoxin (PbTx-3) in mouse plasma: association with high-density lipoproteins.

Red tides have been documented on the Gulf Coast of Florida as early as 1530 (Taylor 1917). They occur almost annually and often persist for many months (Woodcock 1948). Red tides are caused by the dinoflagellate Karenia brevis (formerly Gymnodinium breve and Ptychodiscus brevis), which produces a series of polycyclic ethers called brevetoxins (PbTx) (Davis 1948; Lin et al. 1981; Martin and Chatterjee 1969; Poli et al. 1986 ). These events are responsible for fish, waterfowl, and marine mammal mortalities (Landsberg, 2002; Landsberg and Steidinger 1998) as well as for human intoxication. Aerosol forms of the toxin are produced by wind and wave action and move inshore, causing transient respiratory irritation in people that inhale the toxin (Pierce 1990). Humans can also experience the more severe symptoms of neurotoxic shellfish poisoning (NSP) as a result of consuming shellfish that have accumulated brevetoxin (McFarren et al. 1965). The action of brevetoxins on its pharmacologic target, the voltage-sensitive sodium channel, leads to activation at normal resting potential and inhibition of inactivation (Huang et al. 1984; Sheridan and Adler, 1989; Westerfield et al. 1977). However, before reaching its target tissue, brevetoxin must be absorbed into the general blood supply. Administration of [3H]PbTx-3 orally to rats leads to accumulation of toxin in the liver, stomach, and intestines (Cattet and Geraci 1992). Fat-soluble substances, such as brevetoxins, coalesce into fat droplets in the stomach and are emulsified into mixed micelles by the action of bile acids secreted into the intestine. The mixed micelles are likely absorbed via phagocytosis by absorptive cells of the intestinal villi. Toxin then enters lymph vessels that drain to the subclavian vein to enter the liver via the portal circulation. In the liver, fat-soluble toxicants are in part detoxified by a two-step process that conjugates a polar entity that is released back into the intestine, where it may be reabsorbed and eventually eliminated in urine. Several toxicokinetic studies treating rats with [3H]-PbTx3 have shown that blood retains detectable levels of the radioligand (Benson et al. 1999; Cattet and Geraci 1993; Poli et al. 1990b). More recently, studies treating mice, rats, or fish and measuring toxin using receptor assay and radioimmunoassay have quantified blood brevetoxin and determined that substantial levels (20–30 nM) are retained in all three species (Fairey et al. 2001; Radwan et al. 2005; Woofter et al. 2003, 2005). These values are approximately one order of magnitude higher than the concentration of toxin necessary to activate voltage-gated sodium channels in nerve, heart, or muscle (Bottein Dechraoui and Ramsdell 2003). This indicates that a significant fraction of brevetoxin is bound to elements in blood, reducing its availability to be biologically active at the sodium channel. Brevetoxin, like other lipophilic agents, is likely to partition to cellular elements in blood, and brevetoxin immunoreactivity has been reported in tissue lymphocytes and macrophages of manatees poisoned by brevetoxins (Bossart et al. 1998). Additionally, brevetoxins may associate with specialized transport proteins in the plasma. Blood is technically a tissue containing a fluid matrix known as the plasma. Plasma is a protein-rich solution containing a diverse number of proteins that serve a variety of functions, including transport, immune response, and tissue-to-tissue signaling (Anderson and Anderson 2002). A primary function of plasma is the distribution of insoluble substances through the use of carrier proteins. Albumin is the most abundant plasma protein and serves as a low-affinity, high-capacity binding protein for steroid hormones and delivers these insoluble substances to target cells in the peripheral circulation (Kragh-Hansen 1981). Plasma also includes more highly specialized binding proteins. Perhaps best known are the high-affinity binding proteins for sex steroids, thyroid hormones, and corticosteroids. In addition, the plasma contains a unique lipoprotein transport system for the precursor of these steroid hormones, cholesterol. Lipoproteins, protein-coated fatty acid/cholesterol complexes, distribute cholesterol to tissues and remove cholesterol from the plasma. Plasma carrier proteins are believed to have evolved, at least in part, to serve a functional role to bind and transport nonendogenous hydrophobic substances (Baker 2002). We examined the binding of brevetoxin to plasma carrier proteins in mice, looking especially at lipoproteins, and evaluated the role of plasma carrier proteins in the distribution of brevetoxins to target tissues and the elimination of brevetoxins from the organism.