Effect of helicokinins and ACE inhibitors on water balance and development of Heliothis virescens larvae

The diuretic activity of the helicokinins I (YFSPWG-amide), II (VRFSPWG-amide) and III (KVKFSAWG-amide) was tested on Heliothis virescens larvae. All three kinins increased fluid secretion in isolated Malpighian tubules in a dose-dependent manner. Injections into the haemolymph caused a significant reduction in weight gain after 24 h and, in the case of helicokinin I, led to an increased mortality of 43% within 6 days. When truncated analogues of helicokinin I were tested in vitro, only the pentapeptide (FSPWG-amide) stimulated fluid secretion. Tested in vivo the pentapeptide did not influence normal development of the larvae. An alanine scan of helicokinin I showed that the substitution of phenylalanine, tryptophan and glycine led to a massive decrease or even loss of diuretic activity. The substitution of the other amino acids had no effect in vitro. The ACE inhibitors captopril, enalapril-maleate and lisinopril were tested for their influence on the development of the larvae. In combination with one of the helicokinins the in vivo injection of the ACE inhibitors led to increased rates of mortality and/or reductions in pupal weight.