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 [Ca-2+](i) response s 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 dantrole ne-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 macroph ages. In contrast to the effect of PAF, neither the amplitude of response t o ATP nor the frequency of responding cells changed significantly during da ntrolene treatment in our experiments. These cells were able to respond to a standard immune stimulus as well: lipopolysaccharide (LPS) was able to in crease [Ca2+](i). Our data indicate that the effectiveness of PAF to increa se [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 the y possibly differently modulate cytokine production by macrophages.