Differential effects of neuropeptides on the distal and mid-tubules of the house cricket.

In the Malpighian tubules of Acheta, the distal and middle segments are functionally and morphologically quite distinct (Spring and Kim, Mol Comp Physiol 12:130-145, 1993). Furthermore, they respond quite differently to corpora cardiaca (CC) homogenates, dibutyryl cAMP, and A23187 (Kim and Spring, J Insect Physiol 38:373-381, 1992). In this study we compared secretion by these two regions in response to Acheta and Romalea CC extracts, synthetic Manduca sexta diuretic peptide (Mas-DP1), and the family of synthetic myotropic peptides, the achetakinins, isolated from Acheta. Both Acheta and Romalea CC extracts had opposite effects on the two regions: mid-tubule secretion increased 3-fold whereas secretion by the distal segment declined 75-80%. Mas-DP1 increased secretion by the mid-tubule more than 3-fold and had no effect on the distal segment. All of the achetakinins decreased secretion by the distal tubule, with achetakinin 1 being least effective (55% inhibition) and achetakinin 5 being most effective (75% inhibition). Achetakinins 1 and 2 increased mid-tubule secretion by 3.7- and 3.3-fold, respectively, whereas the others had no effect on this region. Regarding HPLC fractions of CC extracts, in general the more hydrophilic fractions inhibited secretion by both distal and mid-tubules. The more hydrophobic fractions were nearly uniformly stimulatory when applied to the mid-tubule, and either inhibited secretion or had no effect on the distal region. The possible interpretations of these data and the implications towards future research are discussed.