The kinetics of bombesin-stimulated phospholipase D (PLD) activity were examined in Swiss 3T3 fibroblasts. The stimulated activity was found to rapidly desensitize, being completely absent after 40 s. This activity then quickly, but incompletely, resensitized, with PLD being detectable after a 4.5 min wash of the desensitized cells and 75-80% of the activity being recovered after 10 min. The desensitization was dose-dependent; however, the half-maximal stimulatory concentration of bombesin was an order of magnitude lower than that required for bombesin-stimulated second messenger generation and the KD for bombesin receptor binding. This suggested that desensitization was stimulated by a ‘downstream’ effect, but experiments have ruled out changes in protein kinase C activity and Ca2+ concentration. Binding experiments suggested that part of the desensitization is due to receptor internalization, and the requirement for an extracellular agonist for resensitization implies that receptor recycling plays a role. Over an extended time course, cycles of desensitization and resensitization of bombesin-stimulated PLD activity were apparent which may be relevant to mitogenic signalling. These studies add further evidence for a second messenger pathway of PLD activation, and the disparity between the kinetics of diacylglycerol generation and PLD activation supports the possibility that phosphatidic acid may have a messenger role in stimulated cells.

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