Effects of Naja haje (Egyptian cobra), Naja naja (hooded cobra), Naja nigricollis (spitting cobra) and Naja mossambica mossambica (Mozambique spitting cobra) venoms on the isolated guinea-pig tracheal muscle.

Venoms from N. haje, N. naja, N. nigricollis and N. mossambica were tested on the isolated guinea-pig trachea. The four venoms (1-30 micrograms/ml) contracted the tracheal smooth muscle after a delay of 40-60 sec. A second challenge with the venoms caused either no or a much reduced contraction or a relaxant effect. The contraction could be prevented by pretreatment with antihistaminics, but not by atropine, methysergide or indomethacin, indicating that it is due to histamine release by the venoms. This release requires extracellular Ca2+, as it could be prevented by pretreatment with verapamil. Under conditions which prevented histamine release or its effect, each of the four venoms resulted in a reproducible relaxant effect which was not blocked by propranolol. It is concluded that the venoms have one or more component(s) causing histamine release which masks the relaxation caused by another component(s) of the venoms.