Vol. 281, Issue 3, 1047-1058, 1997

Differences in Platelet-Activating Factor Receptor Mediated Ca⁺⁺ Response Between Hamster and Guinea Pig Alveolar Macrophages¹

Jin Chen² and Shri N. Giri

Department of Molecular Biosciences, University of California, Davis, California

The different platelet-activating factor (PAF) receptor subtypes were identified in alveolar macrophages of hamster and guinea pig, based on the distinct characteristics of PAF-induced Ca⁺⁺ responses and PAF antagonist potencies to these responses. PAF, but not lyso-PAF (inactive PAF), induced Ca⁺⁺ release from intracellular Ca⁺⁺ stores and the influx of extracellular Ca⁺⁺ in a dose-dependent manner in both hamster and guinea pig alveolar macrophages. The potency for PAF-stimulated Ca⁺⁺ release, however, was significantly different between the two species with EC₅₀ values being 30- and 50-fold higher in Ca⁺⁺ release and Ca⁺⁺ influx responses in guinea pig than hamster, respectively. In addition, there were distinct differences in Ca⁺⁺ influx characteristics between the two species; guinea pig macrophages exhibiting a rapid Ca⁺⁺ extrusion and high sensitivity to thapsigargin (depletion of intracellular Ca⁺⁺ store). The PAF-induced Ca⁺⁺ response was sensitive to G-protein inhibitor pertussis toxin in hamster but not in guinea pig, suggesting the coupling of different types of G-proteins to PAF receptors. Pretreatment of macrophages with tyrosine kinase inhibitor, herbimycin A, caused a dose-dependent decrease in PAF-induced Ca⁺⁺ response in guinea pig but surprisingly an increased response in hamster. These observations suggest the possibility of a dual mechanism, for G-protein and tyrosine kinase, in PAF-induced phospholipase C activation of macrophages from both species and thus Ca⁺⁺ signaling in response to PAF-mediated receptor signal transduction cascade. The PAF-induced Ca⁺⁺ response was desensitized by repetitive stimulation with PAF or pretreatment with protein kinase C activator, mitogen-activated protein kinase, which had a slightly greater potency in guinea pig than hamster. Importantly, three structurally distinct PAF antagonists, WEB2086, L659,989 and CL184005, blocked PAF-induced Ca⁺⁺ responses in a dose-dependent manner with a markedly different potencies between the two species. The IC₅₀ values for inhibiting PAF-induced Ca⁺⁺ release were 2.5- (WEB2086), 650- (L659,989) and 120- (CL184005) fold less in hamster than in guinea pig. The relative potencies of these PAF antagonists in hamster macrophages were L659,989 > CL184005 > WEB2086. However, in guinea pig these three antagonists showed roughly the same potency. Interestingly, the opposite inhibitory effects of these antagonists on PAF-induced Ca⁺⁺ influx were found in the two species, in which the IC₅₀ were 15- (WEB2086) and 5- (CL184005) fold greater in hamster than in guinea pig but no difference in the IC₅₀ value of L659,989 between the two species. Pretreatment of macrophages from both species with these antagonists had no effect on ATP-induced Ca⁺⁺ response, suggesting that the antagonism is specific to PAF receptors. Based on our data, it was concluded that the alveolar macrophages isolated from the bronchoalveolar lavage of hamsters contain a distinct subtype PAF receptor that differs from that of guinea pigs in modulating a different signal transduction pathway.