Lack of coupling of muscarinic receptors to phosphoinositide metabolism and adenylyl cyclase in human lymphocytes and polymorphonuclear leukocytes: studies in healthy subjects and allergic asthmatic patients.

A change in muscarinic receptor-induced phosphoinositide (PI) metabolism or inhibition of adenylyl cyclase activity could play a role in cholinergic hyperresponsiveness in patients with asthma. In the present study, we considered the possibility of using peripheral blood lymphocytes or polymorphonuclear leukocytes (PMN) as easily accessible models to investigate this hypothesis. Previous studies have indicated the presence of muscarinic binding sites on these cells; however, their transduction mechanisms are still unclear. We found that neither lymphocytes nor PMN from healthy donors, nor lymphocytes from allergic asthmatic patients accumulated inositol phosphates after stimulation with methacholine, whereas positive responses were found for phytohemagglutinin (PHA) in the lymphocytes and for formyl-methionyl-leucyl-phenyl-alanine in the PMN. Normal basal and PHA-stimulated inositol phosphates levels were found in the lymphocytes of the patients. Both levels were significantly enhanced after allergen challenge, indicating that activation of PI metabolism could play a role in the immune response of these cells. Since methacholine was also unable to inhibit adenylyl cyclase activity in both lymphocytes and PMN, we reevaluated the presence of muscarinic binding sites on these cells. Radioligand binding studies with the hydrophilic ligand [3H]-N-methyl-scopolamine surprisingly showed only a very small population of high affinity binding sites on lymphocytes (Bmax 57-127 sites/cell; Kd 45-133 pM), while no high affinity sites could be detected on PMN. The results indicate that leukocytes are not a suitable model to investigate the muscarinic receptor function in relation to bronchial asthma.