SPRING, J.H.*; HAZELTON, S.R.: Excretion in the house cricket, Acheta domesticus: Effects of cAMP on membrane dynamics, cell ultrastructure and secretion.

In Acheta domesticus, the Malpighian tubules (Mt) are composed of three regions, each consisting of a single cell type. The cells of the mid-tubule display the classic architecture of transport epithelia, possessing extensive basolateral infoldings and a dense brush border. The mid-tubule is secretory in function and responds to endocrine stimulation. Our research centers on correlating the physiological response of the mid-tubule with the structural changes within the cells following the addition of various secretagogues. One of the most striking ultrastructural changes occurs when the Mt are stimulated with the second messenger analogue, dibutyryl cAMP. The increase in secretion rate (approximately doubling) is accompanied by notable changes in ultrastructure, the most prominent being membrane re-organization to increase the surface area of the basolateral infolds and vesiculation. The extensive pattern of endomembrane distribution noted in unstimulated tissue disappears following stimulation and the concretions (spherites) begin to dissolve. The area of the cell given over to vesicles/vacuoles more than doubles over 420 s post-stimulation. We followed the membrane dynamics of the mid-tubule using TEM, SEM coupled with freeze-cleaving followed by extraction of the cytosol, and various fluorescent tracers. Using this SEM technique (developed in our laboratory) we were able to observe the increase in basolateral infoldings along with the presence of dilated areas of membrane suggestive of the vesiculation pattern of stimulated tissue previously observed with TEM. Our findings suggest that there is an intricate endomembrane network that vesiculates in response to rapid fluid transport possibly to compartmentalize fluid to prevent dilution of the cytoplasm. Supported by NSF grant IBN-9807948 to JHS.