title: Regulation of fluid-phase endocytosis in alveolar macrophages.
abstract: We investigated whether fluid-phase endocytosis in rabbit alveolar macrophages (AM) was regulated by alterations in intracellular adenosine 3',5'-cyclic monophosphate (cAMP). Suspensions of freshly isolated AM were incubated with anionic dextrans (mol mass = 10 kDa), coupled to fluorescein isothiocyanate (FITC), at either 37 or 4 degrees C. There was a rapid increase in AM-associated fluorescence, quantified by laser flow-cytometry and video microscopy during the first hour of incubation at 37 degrees C, which was directly proportional to the amount of tracer present in the medium. In contrast, at 4 degrees C, AM fluorescence was similar to autofluorescence. Incubation of AM with forskolin (50 microM) or 3-isobutyl-1-methyl xanthine (IBMX; 0.1 mM) increased their cAMP content by 67 +/- 2 and 52 +/- 5% (mean +/- SE; n = 4) and decreased FITC-dextran uptake by 29 +/- 4 and 31 +/- 4% (n = 3). On the other hand, incubation of AM with 0.5 mM IBMX inhibited FITC-dextran uptake by 62 +/- 4% (n = 3), without any further increase in cAMP. Incubation 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 showed that the rate of FITC-dextran exocytosis was not affected by cAMP. We concluded that fluid-phase endocytosis in rabbit AM is regulated by cAMP and by an additional, cAMP-independent mechanism of IBMX.
mesh_terms: D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D003911:Dextrans; D004705:Endocytosis; D016650:Fluorescein-5-isothiocyanate / Q000031:analogs & derivatives; D007425:Intracellular Membranes / Q000378:metabolism; D016676:Macrophages, Alveolar / Q000187:drug effects / Q000502:physiology*; D008297:Male; D011817:Rabbits
chemical_list: D003911:Dextrans; C015219:fluorescein isothiocyanate dextran; D005576:Colforsin; D000242:Cyclic AMP; D016650:Fluorescein-5-isothiocyanate; D015056:1-Methyl-3-isobutylxanthine