|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online February 29, 2008
Journal of Experimental Biology 211, 835-836 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.009589
JEB Classics |
ACTIVE TRANSPORT IN INSECT RECTA
University of California at Irvine
tbradley{at}uci.edu
|
) is one of a
trio of articles in Volume 41 of The Journal of Experimental Biology
in which John Phillips examines the issue of active transport in the insect
rectum (Phillips,
1964a,b,c).
The rectum of insects is a highly aerobically active organ. Active transport,
that is the coupling of metabolic energy to the movement of compounds across
the epithelium, is vital for osmoregulation and for the retention of ions,
water and nutrients. The paper, entitled `Rectal absorption in the desert
locust, Schistocerca gregaria Forskal. III. The nature of the
excretory process', pulls together findings from the two previous papers in
the series to show how insects manage osmoregulation and excretion to thrive
in some of the world's most inhospitable environments. By the 1960s, it had long been known that epithelia were capable of solute and solvent transport against an osmotic gradient, through a process known as active transport. Whereas today, transport can be studied using histology, protein localization, gene expression or immunofluorescence, in the 1960s active transport was approached principally as a thermodynamic issue. While the term was reserved for situations in which a compound was found to move across an epithelium against its concentration gradient, an additional criterion was that other sources of energy for the process, e.g. electrical gradients, pressure gradients and solvent drag, needed to be ruled out. Demonstrations of active transport therefore required rigorous, detailed studies designed to eliminate all other possible explanations of the movement of a compound against its concentration gradient.
At the time of this article, active transport had been extensively
investigated in a variety of vertebrate tissues
(Curran and Schwarz, 1960;
Anderson and Ussing, 1960). In
those studies, many examples of active transport of ions and organic nutrients
had been revealed, but no examples of the active transport of water had been
found (Robinson, 1960). Given
the importance of fluid secretion in biological systems, this was a surprising
development. None-the-less, the conclusion had been reached that active
transport of water did not occur in vertebrates.
The situation in insects was, however, less clear. In particular, the
hindguts of terrestrial insects produce very concentrated excreta and in the
process appear to be able to move water from the gut lumen into the hemolymph
against a substantial osmotic gradient. It was this process that Phillips
undertook to investigate in detail. In doing so, he faced two formidable
challenges. The first was to isolate the rectum in order to study the
transport of solutes and water under replicable, controlled conditions. He did
this by surgically isolating the rectum from input from the midgut and
Malpighian tubules by ligating the gut just anterior to the rectum. He then
flushed out the gut contents with saline or distilled water and replaced the
gut contents with solutions of known makeup
(Phillips, 1964a). In this
manner, he produced an in vivo preparation with natural innervation
and tracheation that could be studied in a quantitative, replicated
manner.
The second challenge was to be able to accurately measure ionic and osmotic
parameters in the tiny fluid samples that could be isolated from the rectal
lumen. This problem was solved by cannulating the rectal lumen and removing
samples at intervals using a micrometer burette. Chloride ion and osmotic
concentrations were determined using the microanalytical techniques developed
by Ramsay and co-workers (Ramsay and Brown,
1955; Ramsay et al,
1955). Changes in the volume of fluid in the rectum were the most
critical and difficult measurements to make. Phillips solved this problem by
using radioactively labelled serum albumen, a large solute that could not
cross the rectal epithelium, as a volume marker
(Phillips, 1964a). He was thus
able to simultaneously measure changes in chloride concentration, total
osmotic concentration and fluid flux.
Using these techniques, Phillips demonstrated that active ion transport
occurs in the locust rectum (Phillips,
1964b). Simultaneous microelectrode measurements showed that
chloride transport set up an electrical gradient that facilitated the movement
of sodium and potassium from the lumen into the hemolymph.
The most startling and significant finding was that water could be
transported from the rectum to the hemolymph when the rectum was filled with a
solution of 800 mOsm xylose, even though the hemolymph remained at a lower
concentration of about 400 mOsm (Phillips,
1964a). This result was surprising since the xylose-containing
fluid was devoid of ions and xylose had been shown not to cross the rectal
wall. The movement of water could therefore not be attributed to a known
osmotic gradient in the appropriate direction nor to a coupling to the
movement of solute. In addition, careful measurements of the hydrostatic
pressure in the rectum demonstrated that the pressures generated (on the order
of 0.2 atm) were far lower than those required to offset the osmotic gradient
(11.2 atm). Measurement of ionic concentrations in the rectal fluid during
absorption also demonstrated that ions were not cycled into the lumen and then
resorbed. In short, the locust rectum met fully the criteria for active
transport of water.
In his third paper (Phillips,
1964c), Phillips measured ionic and osmotic concentrations and
rates of fluid flow in the midgut, Malpighian tubules and rectum of intact
animals. By comparing these to the rates of ion and water uptake he had
measured in the ligated preparations, he demonstrated that rectal capacity for
ion and water transport exceeds the rate at which ions and water enter the
rectum in the urine. His results provided a mechanistic explanation for the
observation that the fecal pellets of Schistocerca are in equilibrium
with a very concentrated solution, but essentially devoid of sodium, potassium
and chloride ions. Phillips had simultaneously shown how ions and water were
retained in the locust under desiccating conditions, and clarified the
mechanisms by which highly concentrated feces were produced
(Phillips, 1964c).
These papers demonstrated that water was moving thermodynamically `uphill' in the absence of simultaneous ion uptake or hydrostatic pressure gradients sufficient to explain the rate of water movement or the equilibrium condition. These were the most complete and rigorous studies to date on rectal function in terrestrial insects. They elucidated the physiological mechanisms used by all terrestrial insects for retaining ions and water and concentrating the excreta.
These papers were of great interest to students of fluid transport in
epithelia in the early 1960s. One of the major questions at the time was how
could insects produce a concentrated urine through water extraction using only
one cell type (the rectal pad cell), a process that vertebrates can only
achieve in complex organs involving dozens of cell types (e.g. the mammalian
kidney). The results galvanized insect physiologists due to the possibility
that insects might possess processes for transporting water that were distinct
from those of other animal groups. The papers spawned intense study of insect
recta using both ultrastructural (Wall,
1971) and physiological
(Phillips, 1970) methods. This
resulted in our current understanding that although water does move from a
concentrated to a less concentrated fluid in the insect rectum, the water is
in fact moved passively by osmotic forces, with intracellular clefts providing
the site of hyperosmotic fluid accumulation. These studies in turn contributed
to our understanding of the processes by which insect recta are able to take
up water from subsaturated atmospheres
(Noble-Nesbitt, 1978), a vital
process for insects inhabiting extremely dry habitats. As such, the papers by
Phillips (Phillips,
1964a,b,c)
opened the modern era of rigorous physiological study of insect recta,
providing insights into a major adaptation permitting insects to exploit the
terrestrial realm.
Footnotes
Timothy Bradley discusses John Phillips's 1964 paper: Rectal absorption in the desert locust, Schistocerca gregaria Forskal. III. The nature of the excretory process. A copy of the paper can be obtained from http://jeb.biologists.org/cgi/reprint/41/1/69.
References
Anderson, B. and Ussing, H. H. (1960). Active transport. In Comparative Biochemistry (ed. M. Florkin and H. S. Mason), pp. 371-402. London: Academic Press.
Curran, P. F. and Schwartz, G. G. (1960).
Sodium, chloride and water transport by rat colon. J. Gen.
Physiol. 43,555
-572.
Noble-Nesbitt, J. (1978). Absorption of water vapor by Thermobia domestica and other insects. In: Comparative Physiology: Water, Ions and Fluid Mechanics (ed. K. Schmidt-Nielsen, L. Bolis and S. H. P. Maddrell), pp. 53-66. Cambridge, UK: Cambridge University Press.
Phillips, J. E. (1964a). Rectal absorption in
the desert locust, Schistocerca gregaria Forskal. I. Water.
J. Exp. Biol. 41,15
-38.
Phillips, J. E. (1964b). Rectal absorption in
the desert locust, Schistocerca gregaria Forskal. II. Sodium,
potassium and chloride. J. Exp. Biol.
41, 39-67.
Phillips, J. E. (1964c). Rectal absorption in
the desert locust, Schistocerca gregaria Forskal. III. The nature of
the excretory process. J. Exp. Biol.
41, 69-80.
Phillips, J. E. (1970). Apparent active water transport by insects excretory systems. Amer. Zool. 10,413 -436.[Medline]
Ramsay, J. A. and Brown, R. H. J. (1955). Simplified apparatus and procedure for freezing-point determinations upon small volumes of fluid. J. Sci. Instrum. 32,372 -375.[CrossRef]
Ramsay, J. A., Brown, R. H. J. and Croghan, P. C. (1955). Electrometric titration of chloride in small volumes. J. Exp. Biol. 32,822 -829.[Abstract]
Robinson, J. R. (1960). Metabolism of
intracellular water. Physiol. Rev.
40,112
-149.
Wall, B. J. (1971). Local osmotic gradients in the rectal pads of an insect. Fed. Proc. 30, 42-48.[Medline]
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  H. Dircksen Insect ion transport peptides are derived from alternatively spliced genes and differentially expressed in the central and peripheral nervous system J. Exp. Biol., February 1, 2009; 212(3): 401 - 412. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
