Effects of melittin on a model renal epithelium, the toad urinary bladder

The bee venom melittin, 10(-6) M, on the mucosal (urinary) side of the toad urinary bladder (in vitro), markedly decreased transepithelial potential difference, short-circuit current (Isc, sodium-dependent) and resistance. However, these effects were not seen when the toxin was placed on the opposite (serosal) side of the membrane preparation. The electrical effects were accompanied by a large increase in the transepithelial permeability to 22Na. The response was not changed by meclofenamic acid (which blocks formation of prostaglandins) but it was inhibited by La3+. In the presence of amiloride, which usually inhibits active Na transport and Isc, melittin, on the mucosal side, increased the Isc. The action of melittin appears to involve an interaction with anionic sites, which mediate its effects. Such sites appear to be present on the apical plasma membranes of the toad bladder epithelial cells, but they are not as abundant or they are inaccessible on the basal plasma membrane.