Altered intracellular calcium signalling after PAF stimulations in polymorphonuclear leukocytes from asthmatic patients

Platelet activating factor (PAF), a potent lipid mediator, has been implicated in the pathogenesis of airways inflammation in bronchial asthma. Binding of PAF to its receptor (Nakamura et al., 1991) leads to changes of intracellular Ca²⁺ concentration ([Ca²⁺]_i) that is crucial to cell activation. Therefore, the aim of our study was to investigate whether PMNL of asthmatic patients stimulated with PAF(10⁻⁷ M) differ in relation to changes of [Ca²⁺]_i from cells of healthy subjects. PMNL from asthmatic patients revealed attenuated first response to PAF stimulation—increase in [Ca²⁺]_i (A[Ca²⁺]_i) was 1.3-fold lower in cells of asthmatics (P < 0.05) versus PMNL of healthy subjects. As determined in experiments with low extracellular calcium concentration, Ca²⁺ release from internal stores tended to be increased in asthmatics and hence the difference in total Ca²⁺ response was related to decrease in Ca²⁺ influx. Thus the contribution of Ca²⁺ from internal stores to the total first Ca²⁺ response upon PAF stimulation was two-fold higher (58 ± 18 vs. 29 ± 8%, P < 0.001) in PMNL of asthmatics compared with healthy subjects. Two subsequent Ca²⁺ responses evoked by stimulations with the agonist in 1 mM Ca²⁺ buffer did not differ between study groups. In low Ca²⁺ buffer PMNL of 50% of asthmatics responded to the second stimulation while cells of healthy subjects remained unresponsive. The altered Ca²⁺ responses in PMNL of asthmatic subjects may reflect previous contact with mediator(s) that occur in vivo which may be at least partially explained by the phenomenon of down regulation reported for PAF receptor upon cell stimulation.