Reduction of the fertilizing capacity of sea urchin sperm by cannabinoids derived from marihuana. III. Activation of phospholipase a₂ in sperm homogenate by Δ⁹-tetrahydrocannabinol

Inhibition of the egg jelly induced acrosome reaction by Δ⁹-tetrahydrocannabinol (THC) is associated with the localized disruption of the nuclear envelope and the formation of lipid deposits in sea urchin sperm. This suggests that THC may activate phospholipase(s) within the sperm. We now report effects of THC on phospholipase A₂ activity in homogenates of sea urchin sperm using 1-stearoyl2-[1-¹⁴C] arachidonyl phosphatidylcholine as substrate. The release of radioactive arachidonic acid was measured after a 30-min incubation with the enzyme. In the absence of exogenous Ca²⁺, 100 μM THC produced a significant (P < 0.001) increase in phospholipase A₂ activity. THC activated phospholipase A₂ in a concentration (1-100 μM) and time-dependent (0-30 min) manner. Exogenous calcium (10 mM) significantly augmented basal (P < 0.001) and THC-stimulated (P < 0.005) phospholipase A₂ activity. Calcium chelators [ethylene glycol bis(β-aminoethyl ether) N,N,N′,N′,-tetraacetic acid (EGTA) and 1,2-bis(O-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA)] inhibited the basal level of phospholipase A₂ activity in the sperm homogenate, and prevented the activation of phospholipase A₂ by THC. Submicromolar levels of free calcium ions were required for THC stimulation of phospholipase A₂. Cannabinol which mimics the effects of THC on the acrosome reaction also activated phospholipase A₂ in sperm homogenate. These results suggest that THC may alter lipid metabolism in sperm by activating calcium-dependent phospholipase A₂. Putative metabolites derived from this process may inhibit the acrosome reaction and thereby reduce the fertilizing capacity of sea urchin sperm.