THE EFFECTS OF ACHETA DIURETIC PEPTIDE ON ISOLATED MALPIGHIAN TUBULES FROM THE HOUSE CRICKET ACHETA DOMESTICUS
Acheta diuretic peptide (Acheta-DP) is a corticotropin-releasing factor (CRF)-related peptide found in head extracts of the house cricket Acheta domesticus. The peptide causes a dose-dependent increase in fluid secretion by cricket Malpighian tubules isolated in vitro, and the apparent EC50 is 1.3 nmol l-1, which is within the physiological range for a peptide hormone. The CRF antagonist alpha-helical CRF(9-41) blocks the action of Acheta-DP in a dose-dependent manner, and the IC50 is estimated to be in the micromolar range. Addition of Acheta-DP to isolated Malpighian tubules is followed by a rapid and marked increase in the level of intracellular cyclic AMP. This precedes any change in voltage or fluid secretion, which strongly suggests that cyclic AMP is the intracellular mediator of Acheta-DP activity. Consistent with this, diuretic activity is potentiated by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, and there is a close relationship between the dose&shy;response curves for cyclic AMP production and for fluid secretion. However, exogenous 8-bromo-cyclic AMP does not mimic all the effects of Acheta-DP, and the peptide may have a dual action on isolated tubules. Fluid secretion by tubules dosed repeatedly with Acheta-DP returns to near basal levels after 3&shy;5 h. This cannot be explained by degradation of the peptide, but might be due in part to oxygen and/or metabolite deficiency. However, tubules that are refractory to Acheta-DP can be stimulated by forskolin, 8-bromo-cyclic AMP and extracts of corpora cardiaca, which is indicative of a homologous desensitization of membrane receptors for the diuretic peptide. Differences in the rate of secretion by morphologically distinct regions of cricket Malpighian tubules have been assessed. In unstimulated tubules, the rate of secretion per unit length by the short distal segment is about twice that of the main tubule. However, diuretic peptides (Acheta-DP and achetakinin-I) have little effect on distal tubule secretion, but evoke a two- to fourfold increase in fluid secretion by the main tubule segment.
Coast
G
Kay
I