AJP - Regulatory, Integrative and Comparative Physiology, Vol 246, Issue 5 759-R769, Copyright © 1984 by American Physiological Society
Passage of solutes through walls of Malpighian tubules of Rhodnius by paracellular and transcellular routes
M. J. O'Donnell, S. H. Maddrell and B. O. Gardiner
The walls of isolated upper Malpighian tubules of the insect, Rhodnius
prolixus, are much more permeable to small relatively unchanged solutes
(ethanol, xylose, and mannitol) than to larger or more charged solutes
(acetate, glycine, tyrosine, and inulin). The more permeable solutes
rapidly reach concentrations in the tubule cells equivalent to their
concentrations in the bathing medium; the less permeable solutes do not
penetrate into the cells. The time course of accumulation of permeable
solute in the cells matches the time course of the appearance of solute in
the lumen. Substances injected into the hemolymph of fed R. prolixus appear
in the urine at concentrations predictable from the permeability of in
vitro tubules, supporting the idea that the in vitro permeability of the
tubules is representative of their properties in the intact insect. It is
suggested that the rapid transcellular penetration of small solutes through
the Malpighian tubules reflects the large areas of cell membrane. The area
of cell membrane exceeds that of the paracellular clefts by a factor of
10(5).
