Platelet-Activating Factor Evokes Ca2+ Transients After the Blockade of Ryanodine Receptor by Dantrolene in RAW 264.7 Macrophages

In the present study we studied platelet-activating factor (PAF)-, and ATP-induced increases in intracellular Ca²⁺ concentration (Ca²⁺]_i) using RAW 264.7 macrophages filled with fura-2/AM and imaged with fluorescence video microscopy. We found that the prevalence of detectable Ca²⁺]_i responses to PAF application was significantly higher in the presence of dantrolene. Dantrolene itself significantly decreased basal Ca²⁺]_i of macrophages compared to control cases after a 20-min incubation period. In the dantrolene-treated cells even the peak Ca²⁺]_i in response to PAF (as an average of all cells) was below the baseline of control suggesting that decreased Ca²⁺]_i plays a permissive role in the Ca²⁺ rise induced by PAF in macrophages. In contrast to the effect of PAF, neither the amplitude of response to ATP nor the frequency of responding cells changed significantly during dantrolene treatment in our experiments. These cells were able to respond to a standard immune stimulus as well: lipopolysaccharide (LPS) was able to increase Ca²⁺]_i. Our data indicate that the effectiveness of PAF to increase Ca²⁺]_i in RAW 264.7 macrophages depends on the resting Ca²⁺]_i. It has also been shown in this study that PAF and ATP differently regulate Ca²⁺ homeostasis in macrophages during inflammatory response and therefore they possibly differently modulate cytokine production by macrophages.