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.

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