Hypertonicity increases cAMP in PMN and blocks oxidative burst by PKA-dependent and -independent mechanisms

Hypertonic stress (HS) suppresses neutrophil (PMN) functions. We studied the underlying mechanism and found that HS rapidly (<1 min) increased intracellular cAMP levels by up to sevenfold. cAMP levels correlated with applied hypertonicity and the degree of neutrophil suppression. HS and cAMP-elevating drugs (forskolin and dibutyryl cAMP-acetoxymethyl ester) similarly suppressed extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase activation and superoxide formation in response to N-formylmethionyl-leucyl-phenylalanine (fMLP) stimulation. Inhibition of cAMP-dependent protein kinase A (PKA) with H-89 abrogated the suppressive effects of HS, restoring fMLP-induced ERK and p38 activation and superoxide formation. Inhibition of phosphodiesterase with 3-isobutyl-1-methylxanthine augmented cAMP accumulation and the suppressive effects of HS, while inhibition of adenylyl cyclase with MDL-12330A abolished these effects. These findings suggest that HS-activated cAMP/PKA signaling inhibits superoxide formation by intercepting fMLP-induced activation steps upstream of ERK and p38. In contrast to its effects in the presence of moderate hypertonicity levels (40 mM), H-89 was unable to rescue neutrophil functions from suppression by higher hypertonicity levels (100 mM), indicating that more severe HS suppresses neutrophils via secondary PKA-independent mechanisms.