Abstract
Inhibition of luteinizing hormone (LH) exocytosis by guanosine 5′-[ γ-thio]triphosphate (GTPγS) in permeabilized pituitary cells has indicated the involvement of one or more GTP-binding proteins in the exocytotic mechanism distal to second messenger generation. We now report that the two inhibitory sites of action of GTPγS can be distinguished by their dependence on GTPγS concentration and their sensitivity to pertussis toxin. Ca 2+-stimulated exocytosis was half-maximally inhibited by 6.8 μM GTPγS, a six-fold higher concentration than that required for inhibition of exocytosis stimulated by phorbol ester plus cAMP. In addition, GTPγS inhibition of Ca 2+ -stimulated exocytosis was insensitive to pertussis toxin, in contrast to the inhibition of exocytosis stimulated by phorbol ester plus cAMP, which was abolished by pretreatment with pertussis toxin. These results indicate that at least two stimulus-specific GTP-binding proteins are involved in regulating LH exocytosis distal to secod messenger generation.
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