AJP - Regulatory, Integrative and Comparative Physiology, Vol 246, Issue 5 705-R715, Copyright © 1984 by American Physiological Society
Excretion of ouabain by Malpighian tubules of Oncopeltus fasciatus
J. Meredith, L. Moore and G. G. Scudder
An in vitro preparation of Malpighian tubules was used to investigate the
excretion of the polar cardiac glycoside, ouabain, in Oncopeltus fasciatus.
Tubules were found to consist of at least two morphologically and
physiologically distinct segments, both of which metabolized ouabain. The
distal segment (segment II) secreted primary urine and ouabain. Secretion
of ouabain by segment II was not observed to occur against a concentration
gradient and increased with increasing fluid secretion. The proximal
segment (segment I) reabsorbed fluid and ouabain but not metabolites.
Ouabain was reabsorbed against a strong concentration gradient (23-fold),
was independent of fluid reabsorption, and increased with increasing fluid
secretion by segment II. In rapidly secreting Malpighian tubules (a
situation of high cardiac glycoside secretion by segment II), the presence
of segment I reduced the excretion of ouabain by 84-93%, mainly by reducing
ouabain concentration. It appears excretory loss of cardiac glycosides can
be reduced in O fasciatus and thus may be a factor in the sequestration of
cardiac glycosides in this insect.
