AJP - Regulatory, Integrative and Comparative Physiology, Vol 257, Issue 5 1096-R1102, Copyright © 1989 by American Physiological Society
Fluid secretion and microvillar ultrastructure in mosquito malpighian tubules
T. J. Bradley and C. Snyder
Department of Developmental and Cell Biology, University of California, Irvine 92717.
The Malpighian tubules of fourth instar larvae, pupae, and female adults of
the mosquito Aedes taeniorhynchus were examined with regard to in vitro
fluid secretion rate and the ultrastructural features of the microvillar
border of the primary cells. In vitro fluid secretion rates were determined
after stimulation with 5-hydroxytryptamine. While larval tubules are
capable of rapid fluid secretion, the tubules of pupae exhibit very low
rates of secretion, indistinguishable from 0 nl/h. The capacity to secrete
fluid returns after the pupal-adult molt and is further enhanced after
blood feeding. Similar results were obtained in tubules stimulated in vitro
with dibutyryl adenosine 3',5'-cyclic monophosphate. Ultrastructural
examination of the microvillar border of the primary cells of the
Malpighian tubules revealed that the period of reduced secretion capacity
in the pupal tubules is correlated with a marked reduction in microvillar
volume, microvillar surface area, and mitochondrial content in the
microvillar border. The results suggest that microvilli of a certain size
and containing extensions of mitochondria are required for rapid fluid
transport. The absence of these conditions in pupal tubules cannot be
overcome by in vitro stimulation with known secretagogues and therefore
represents a physiological limit on transport performance in the pupal
tubules of mosquitoes.
