Feeding responses of the horsefly, Tabanus nigrovittatus (Diptera: Tabanidae) to purinergic phagostimulants

Each year in late June, 1982-1986, the phagostimulatory potencies of purinergic compounds (ATP and derivatives of ATP) were tested on wild-caught female Tabanus nigrovittatus. In all, 15 compounds were investigated. They were dissolved in 0.15 M sodium chloride solution and presented at 37 degree C in an artificial feeding apparatus in which the flies fed upward through a Parafilm M membrane. Potencies ranked, ATP gt ADP gt 2’dATP gt ATP-gamma-S = 2’dADP gt AMP-PCP = AtetraP gt AMP-PNP = 2’3’dd-ATP = AMP = APS. 3’dATP, Ara-ATP, and Ado were very weak phagostimulants and c-AMP induced no gorging at l mM. Previous work that reported ADP gt ATP was an error. The average ED-50 for ATP is 21.2 mu-M and ADP is 39.3 mu-M. Any changes in the phosphate chain of ATP caused a significant reduction in potency. Removal of the oxygen at the 3’ position on the ribose reduced potency much more than its removal at the 2’ position. Substitution of solvents other than the sodium chloride solution changed ED-50 values for ATP and ADP. Addition of 5% bovine albumin and 10 mM sodium bicarbonate to the 0.15 M sodium chloride increases the potency of ATP nearly 3-fold and the potency of ADP by more than 7-fold. ADP dissolved in choline chloride was as potent as ADP in sodium chloride solution. Addition of calcium ions to the solvent reduced the potency of ADP, probably because of the activation of apyrase enzymes. The potency of ADP was determined each year as an indicator of seasonal variability. Its ED-50 values ranged from 33.3 mu-M in 1982 to 45.7 mu-M in 1983. June 1985 was unusually cold and the ED-50 value for ADP was 92.7 mu-M. Data from 1985 were segregated from the other data. Seasonal effects must be considered when field-collected insects are used.