DISCUSSION
Fat body in insects may be considered functionally, analogous to both vertebrate liver and adipose tissue, serving not only for the storage of reserves, but also as a general center for intermediary metabolism (Beenakkers et al. 1985). The hemolymph is the medium through which nutrients are transported from the sites of absorption directly to the sites of tissue respiration or to storage organs and, subsequently, to sites of utilization. Also, it transports waste products to excretory organs and all chemical exchanges between the organs affected (Wigglesworth 1972, Beenakkers et al. 1985).
The present work reports about biochemical modifications occurring in the fat body reserves and in the hemolymph after a blood meal in the hemipteran D. maximus. They were evaluated in fat body through glycogen, total lipids and wet weight of the organ as well as by hemolymphatic levels of carbohydrates, total lipids, HDLp, proteins and uric acid. This hemimetabolous insect, a unique species of their genera, lives among rocks and feeds principally on blood lizards and wild mammalians (Ryckman & Ryckman 1967, Beltran & Carcavallo 1985). Despite the blood meal results critic for reduviids metamorphosis and that the nutritional reserve after moulting has an important effect on the development and growth (Friend & Smith 1985), their ability to survive long periods without feed is well known (Szumlewicz 1976).
Blood ingested by triatomine bugs is primarily stored in the anterior midgut where water absorption occurs quickly whereas the major digestion and absorption processes are relegated to posterior midgut (Terra & Ferreira 1994). These events are influenced by amount of consumed blood, physiological status, sex and temperature (Catalá et al. 1992). Our results indicated that after feeding D. maximus performs its digestion process accumulating reserves continuously up to day 20 post-feeding. In this period, the maximum value of the fat body wet weight was reached at day 10. At days 25 and 30 post-feeding, fat body wet weight showed an appreciable decrease, indicating that the mobilization process of reserves prevails over storage process, representing at day 30 less than 50% of the maximum weight (Fig. 1). However, the ratio between total weight of insect and fat body wet weight did not show significant changes through the times analyzed; the fat body being about 5% of the total weight of insect (data not shown).
Deposits of total lipids and glycogen showed similar patterns, differing at times at which they reached the maximum values (Figs 2, 3). An interesting aspect was observed in glycogen content found at day 30, glycogen at this time being higher than the ones found in unfed insects (time 0). This phenomenon associated to lipidic deposits at time 0 and to nutritional reserves after ecdysis as pointed by Friend and Smith (1985) could suggest that these hematophagues employ the carbohydrates preferably at lipidic reserves, generating fuel to moulting as adult and reserving lipids for eventual flights in order to look for food. This speculation is supported by the higher energetic value of lipids compared to carbohydrates and the fact that lipids are generally the major fuel for flight used for insects (Beenakkers et al. 1985).
Unlike glycogen, the lipidic deposits decreased markedly after day 15 post-feeding up to day 25, when they represented only 3.4 mg per fat body (Fig. 2). They showed triacylglycerols as a predominating lipid amongst non-polar components and phosphatidylcholine and phosphatidylethanolamine as the main polar components (Canavoso et al. 1996).
Influence of starvation upon fat body weight and carbohydrates metabolism has been reported in the phytophagous hemimetabolous Carausius morosus. It was observed that glycogen content decreased after 15 hr of starvation and virtually disappears after 24 hr whereas fat body weight decreased during a 4-day period of starvation by about 30% (Lohr & Gade 1983). Influence of starvation has also been analyzed in the holometabolous Manduca sexta by Ziegler (1991). Thus, it was observed after 5 days of starving a decrease in both, the fat body dry weight and in the lipidic content while glycogen reserves were exhausted. A similar pattern was demonstrated in adult locusts (Jutsum et al. 1975). It is possible therefore that the striking differences in carbohydrate deposits before described may be attributed to different processes of digestion between these hematophagous insects and phytophagues.
On the other hand, levels of hemolymphatic carbohydrates extremely elevated or absent have been reported in insects and some arthropods (Bedford 1977). In this respect, our results obtained for D. maximus were lower compared to other members of the Insecta class: Periplaneta americana (Steele 1961), Locusta migratoria (Goldsworthy 1969), Acheta domesticus (Nowosielski & Patton 1964) and C. morosus (Lohr & Gade 1983).
Since carbohydrates are used as fuel by insects in the initial flight period (Beenakkers et al. 1985), we believe that hemolymphatic carbohydrates may also be required by reduviids during the first phase of this activity. Preliminary experiments carried out in our laboratory with triatomine bugs submitted to flight, showed that a few minutes later the hemolymphatic carbohydrates were undetectable (data not shown).
The insects mobilize the lipids from fat body as diacylglycerols (Beenakkers et al. 1985, Van der Horst et al. 1993). In D. maximus, the hemolym-phatic lipids showed a permanent fluctuation , possibly due to their energetic contribution to the vital functions of tissues (Fig. 2). Ward et al. (1982) have suggested that lipids in flight muscle increase probably at the expense of the fat body lipids.
A similar pattern of changes between HDLp levels and hemolymphatic proteins was observed during post-feeding time (Figs 2, 4), reaching both the maximum values at day 20 post-feeding. Moreover, HDLp represented all time evaluated 17-24% of the hemolymphatic proteins. Our result differs from those found by González et al. (1991) in
Triatoma infestans in both, the levels HDLp and the proteins probably due to a different experimental design and the analytical procedure applied for the quantification of HDLp. On the other hand, it has been pointed out that in starved insects, the hemolymphatic proteins play an important role as reserve (Wyatt 1961). D. maximus showed increasing levels of circulating proteins with fasting up to day 20, probably as a consequence of the predominance of the mechanisms of synthesis and transport of absorbed proteins up to deposit organs upon their catabolism (Fig. 4).Uric acid is the principal nitrogenous excretory product of insects (Wyatt 1961), and more than 90% of the nitrogen excreted by fed Rhodnius prolixus is represented by this metabolite (Wigglesworth 1931). The levels of uric acid in hemolymph of D. maximus increased from day 10 (0.06 mg/ml) up to 0.16 mg/ml on day 30 (Fig. 4), in agreement with those reported by Barret and Friend (1966) employing a non-enzymatic method and starving fifth instar of R. prolixus.
Since hemolymph volumes decline with time (Barret & Friend 1966) it is controversial if the uric acid increase observed in D. maximus is due to an increase of proteic catabolism or results merely from a decrease of the hemolymph volume. In case this last assumption were correct, either uric acid would not be produced during starvation or if the uric acid is formed, the amount would be equal to their clearance. Since Wigglesworth (1972) has shown that stored proteins in fat body of R. prolixus disappear during starvation, the former suggestion appears more probable.
During the period analyzed, hemolymphatic urea was not detected in D. maximus. For production of urea, arginase has been suggested to be involved in many species of insects (Cochran 1985). Apparently, D. maximus would lack arginase although Friend and Smith (1985) have reported the presence of urea in R. prolixus urine 2-3 hr post-feed, pointing to their probable exogen origin.
We conclude that physiological events such as fasting affect the main metabolic processes in these insects. Future research, directed towards the qualitative and quantitative changes of the lipidic components from fat body induced by starvation, may lead to a better physiological and biochemical knowledge of these obligately hematophagous insects and could contribute to understanding the need of one poor nutritional status to begin the flight. In triatomine bugs, flight can take place when food sources are absent or inaccesible or when vectors colonize other geographic areas. Moreover, flight has been pointed out as responsible for the active dispersion of these insects, with implication on their control and on the American trypanosomiasis epidemiology.
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