REGULATION OF FLUID-PHASE ENDOCYTOSIS IN ALVEOLAR MACROPHAGES

We investigated whether fluid-phase endocytosis in rabbit alveolar mac rophages (AM) was regulated by alterations in intracellular adenosine 3',5'-cyclic monophosphate (cAMP). Suspensions of freshly isolated AM were incubated with anionic dextrans (mel mass = 10 kDa), coupled to f luorescein isothiocyanate (FITC), at either 37 or 4 degrees C. There w as a rapid increase in AM-associated fluorescence, quantified by laser flow-cytometry and video microscopy during the first hour of incubati on at 37 degrees C, which was directly proportional to the amount of t racer present in the medium. In contrast, at 4 degrees C, AM fluoresce nce was similar to autofluorescence. Incubation of AM with forskolin ( 50 mu M) or 3-isobutyl-1-methyl xanthine (IBMX; 0.1 mM) increased thei r cAMP content by 67 +/- 2 and 52 +/- 5% (mean +/- SE; n = 4) and decr eased FITC-dextran uptake by 29 +/- 4 and 31 +/- 4% (n = 3). On the ot her hand, incubation of AM with 0.5 mM IBMX inhibited FITC-dextran upt ake by 62 +/- 4%; (n = 3), without any further increase in cAMP. Incub ation of AM with 0.4 mM 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (CPT-cAMP), a cell-permeable analogue of cAMP, decreased FITC-dextran uptake by 48 +/- 5% (n = 6). Pulse-chase experiments sho wed that the rate of FITC-dextran exocytosis was not affected by cAMP. We concluded that fluid-phase endocytosis in rabbit AM is regulated b y cAMP and by an additional, cAMP-independent mechanism of IBMX.