REDUCTION OF THE FERTILIZING-CAPACITY OF SEA-URCHIN SPERM BY CANNABINOIDS DERIVED FROM MARIJUANA .3. ACTIVATION OF PHOSPHOLIPASE-A2 IN SPERM HOMOGENATE BY DELTA-9-TETRAHYDROCANNABINOL

Inhibition of the egg jelly induced acrosome reaction by DELTA-9-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-stearoyl-2-[1-C-14]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-mu-M THC produced a significant (P < 0.001) increase in phospholipase A2 activity. THC activated phospholipase A2 in a concentration (1-100-mu-M) 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(beta-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.