CLARK, T.M.; Indiana Univ. South Bend: Actions of Serotonin on the Posterior Stomach of Larval Aedes aegypti.

Serotonin produces a long-lasting lumen-positive shift in the transepithelial potential of isolated, perfused posterior stomach. This effect is more pronounced at the anterior end than at the posterior end of this region demonstrating heterogenous distribution of transport mechanisms within this region. The space constant, a measure of the distance over which an electrical event degrades along a tubular structure, decreases by 30% in response to serotonin indicating a decrease in the transepithelial resistance. Prolonged hyperpolarization together with decreased resistance demonstrates that serotonin does not act by closing channels, but is instead consistent with stimulation of ATPase activity. Na/K ATPase activity is not stimulated by serotonin, as perfusion with ouabain, a specific Na/K ATPase inhibitor, does not block the response to serotonin, and serotonin causes luminal Na⁺ activity to decreased in stopped-flow conditions. Na/K ATPase is present in the apical membrane, however, as perfusion with ouabain hyperpolarizes the TEP when amino acids are present in the saline. This response is abolished in amino acid-free saline. Na/K ATPase thus appears to drive apical Na⁺ -amino acid symport in posterior stomach, by maintaining a Na⁺ gradient across the apical membrane. Inhibitors of the vacuolar-type proton ATPase (V-ATPase) fail to alter the unstimulated TEP or block stimulation by serotonin after 15 minutes of exposure. While bafilomycin A₁ has no discernable effects at any time, preliminary data suggest that concanamycin A may decrease the response of the TEP to serotonin, although this effect has not reached the level of statistical significance.