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When the Malpighian tubules (Mt) of the house cricket (Acheta domesticus) are treated with dibutyryl adenosine 3', 5'-cyclic monophosphate (db-cAMP; 1 mM), which causes a doubling in secretion rate, more than 50% of the cell volume is occupied by vesicles within 420 sec of exposure. In view of the fact that the increase in vesiculation occurs concomitantly with stimulated fluid transport, we set out to determine whether the vesicles are formed as a result of fluid-phase endocytosis (pinocytosis) and subsequently used to transport fluid to the lumen as one means of increasing transport rate. We used fluorescent fluid-phase markers (Lucifer Yellow Carbohydrazide [LYCH] and Alexa 488 hydrazide) and an electron dense marker (cationized ferritin) to elucidate the degree of endocytosis that occurred with db-cAMP stimulation. We found that, although some fluid is taken into the cells of the mid-tubule via endocytosis, it does not coincide with the level of vacuolation present in stimulated tubules. The amount of LYCH transported into the primary urine by the db-cAMP-stimulated Mt decreased by 40% as compared to the unstimulated transport, and the rate of transport of LYCH was only 30% of the unstimulated tubules. In summary, our findings do not support the theory that the majority of the vesicles or vacuoles comprise intracellular, endocytotic compartments formed via a basolateral endocytotic pathway. We also found no evidence to support the functioning of vesicles or vacuoles as transcellular "shuttling" mechanisms to move fluid from the basal region to the apical membrane and into the lumen.

Relatively little is known about the morphology and ultrastructure of the Malpighian tubules of spiders (Arachnida: Araneae). Our study represents the first investigation of the Malpighian tubules of a theraphosid spider and is the only study to examine the living Malpighian tubules using confocal laser scanning microscopy. In theraphosid spiders, the Malpighian tubules originate from the stercoral pocket in the posterior portion of the opisthosoma and extend forward toward the prosoma in a dendritic pattern. There are three distinct segments (initial, main, and terminal), all dark brown in appearance. Each segment has distinctive ultrastructural features. Both the terminal and the main segment appear to be composed of at least two cell types with finger-like cytoplasmic protrusions associated with one of these types. The terminal segment, which is most proximal to the stercoral pocket, is the largest in diameter. It is composed of large, cuboidal cells containing many mitochondria and lipid inclusions. The main segment is intermediate in diameter with many mitochondria and secretory vesicles present. The initial segment is relatively thin in comparison to the other segments and is intimately associated with the digestive gland. The cells of the initial segment contain very little cytoplasm, fewer mitochondria, secretory vesicles, and prominent inclusions. J. Morphol. 251:73–82, 2002. © 2002 Wiley-Liss, Inc.

The Malpighian tubules (Mt) of insects are responsible for maintaining osmotic homeostasis and eliminating waste from the hemolymph. When stimulated by diuretic factors the tubule cells are able to transport extraordinary volumes of fluid over short periods of time. We have been studying the changes that occur within the cells that accompany and facilitate this phenomenon. We present the ultrastructural changes that occur in the mid-tubule of the house cricket, Acheta domesticus, following exposure to the second messenger analog, dibutyryl cAMP, over the period from 15-420 sec. Vacuolation of the cytoplasm begins as early as 30 sec poststimulation with a significant increase in vacuolation occurring after 120 sec. As expected, there is an increase in the surface area of the basolateral membrane to facilitate the rapid movement of fluid into the cells. Other ultrastructural changes noted to accompany the onset of diuresis include the movement of mitochondria into areas adjacent to transport membranes, the vesiculation of Golgi, mobilization of CaPO(4) spherites, and a direct interaction of these spherites with active mitochondria. We discuss several possible roles for these changes in terms of rapid fluid transport.