Combinative ligand-receptor interactions: effects of cAMP, epinephrine, and met-enkephalin on RAW264 macrophage morphology, spreading, adherence, and microfilaments

Cell surface ligand-receptor interactions play a central role in the regulation and expression of macrophage function. Included among these macrophage membrane receptors are the beta-adrenergic and opioid receptors. We studied the abilities of epinephrine, met-enkephalin, forskolin, and adenosine 3':5' cyclic monophosphate (cAMP) analogues to affect macrophage morphology, spreading, and adherence. Cell spreading was quantitated by measuring the perimeters of adherent cell images recorded by videomicroscopy. Epinephrine induced a dose-dependent decrease in macrophage spreading; at 10(-5) M epinephrine the mean perimeter was 10.4 +/- 0.3 microns in comparison to 15.0 +/- 1.0 microns for controls. The inhibition of spreading can be blocked by the antagonist propranolol. On the other hand, met-enkephalin induced a dose-dependent increase in macrophage spreading, with a perimeter of 18.5 +/- 1.0 microns at 10(-8) M. Since catecholamines and opioids are simultaneously released from chromaffin cells of the adrenal, we examined the combinative effects due to treatment with both ligands. When macrophages were exposed to 10(-5) M epinephrine and 10(-8) M met-enkephalin, cell morphology and spreading were indistinguishable from that due to 10(-5) M epinephrine alone. The epinephrine dose-response curve in the presence of 10(-8) M met-enkephalin was similar to that of epinephrine alone. The beta-adrenergic receptor is apparently capable of diminishing or abrogating the opioid receptor signal(s). These combinative and epinephrine-mediated effects may be at least partially accounted for by the action of cAMP. Forskolin and the cAMP analogues N6-2'-O-dibutyryladenosine 3':5' cyclic monophosphate (dbcAMP) and 8-bromoadenosine 3':5' cyclic monophosphate (Br-cAMP) affected cell morphology and spreading in the same fashion as epinephrine. These differences in morphology and spreading behavior were accompanied by changes in the distribution of F-actin, as judged by phalladicin staining and fluorescence microscopy. We suggest that cAMP and microfilaments play important roles in receptor-mediated neuroregulation of macrophage function.