Stimulation of fluid secretion by single isolated Malpighian tubules of the house cricket, Acheta domesticus

. Fluid secretion by single isolated Malpighian tubules of Acheta domesticus (L.) is stimulated by aqueous extracts of nervous and neuroendocrine tissues from the cricket. Diuretic activity, expressed as the increase in rate of secretion per microgram tissue protein (pl/mm/min/ug protein), is highest in the CA and CC. The response to aqueous CC extracts is dose‐dependent. The maximum increase in secretion rate is 300–350 pl/mm/min and the ED max and ED 50 are 0.1–0.2 and 0.02–0.03 gland pairs respectively. The diuretic activity in the CC is retained after 5min at 95C, and is freely soluble in 80% methanol. Diuretic activity is, however, greatly reduced after prolonged heating or after treatment with either pronase or chymotrypsin. It is conluded that the diuretic factor(s) is a low molecular weight peptide. A loss of activity after incubation with pyroglutamate amino peptidase suggests that some active peptides present are N‐terminally blocked. The diuretic activity of crude aqueous CC extracts is rapidly lost on standing at room temperature. This is partly prevented by precipitation of protein enzymes by either heat treatment or extraction in methanol. Synthetic vertebrate and insect neuropeptides are generally low in activity increasing fluid secretion by no more than 60pl/mm/min. However, extracts of neuroendocrine tissues from a wide range of insect species are potent stimulants of tubule secretion. Various biogenic amines were tested and have little effect on fluid secretion; thus they cannot contribute greatly to the response of cricket tubules to aqueous tissue extracts.