Pharmacological studies of jumper ant ( Myrmecia pilosula) venom: Evidence for the presence of histamine, and haemolytic and eicosanoid-releasing factors

Abstract

Jumper ant venom was prepared by extraction of venom sacs in distilled water and centrifugation to remove insoluble material. Jumper ant venom (2 μg/ml) produced a biphasic response on isolated guinea-pig ileum, i.e. an initial rapid contraction followed by a slower prolonged contraction. The histamine antagonist mepyramine (0.1 μM) inhibited the first phase of this response by > 90%. In the isolated rat stomach fundus strip (which is insensitive to histamine), jumper ant venom (6 μg/ml) produced only a single contraction. No tachyphylaxis was observed to repeated doses of jumper ant venom in guinea-pig ileum or rat fundus strip. Responses to jumper ant venom of the egg-albumin-sensitised guinea-pig ileum were not significantly different before and after an in vitro anaphylactic response induced by egg albumin (0.5 mg/ml). Fluorometric assay revealed a mean value of 0.9 ± 0.2% of the dry weight as histamine in jumper ant venom. Both the lipoxygenase/cyclo-oxygenase inhibitor BW755C and the cyclo-oxygenase inhibitor indomethacin significantly inhibited the second phase response to jumper ant venom of the guinea-pig ileum, and the response of the rat fundus strip. The muscarinic receptor antagonist atropine (0.1 μM), the bradykinin antagonist [Thi 5,8,D-Phe 7]-bradykinin (10 μM) and the angiotensin converting enzyme inhibitor captopril (20 μM) did not affect either phase of the venom response in guinea-pig ileum. Jumper ant venom caused haemolysis of guinea-pig blood. The degree of haemolysis was significantly reduced when boiled venom was used. These results suggest that jumper ant venom contains histamine and may cause the release of cyclo-oxygenase products. It also contains a heat-sensitive haemolytic factor.