New Insights into How PAF, a Mediator of Inflammation and Allergic Responses, Triggers Bladder Contraction

Research Background and Findings 

The urinary bladder stores urine and expels it during urination through contraction of the smooth muscle in the bladder wall. Ca²⁺ influx from outside the cell plays an important role in urinary bladder smooth muscle contraction. Ca²⁺ can enter smooth muscle cells through several pathways, with voltage-dependent Ca²⁺ channels (VDCCs) representing one of the major routes. In urinary bladder smooth muscle, Ca²⁺ influx through L-type VDCCs plays a particularly important role in contraction.

Platelet-activating factor (PAF) is a bioactive lipid originally identified for its ability to activate platelets. It is now recognized as an important lipid mediator produced by various cell types and involved in inflammation and allergic responses. PAF has been implicated in anaphylaxis and a range of inflammatory and allergic conditions and exerts diverse biological effects, including smooth muscle contraction.

The research team previously reported that PAF strongly contracts guinea pig urinary bladder smooth muscle and enhances spontaneous contractile activity. They also demonstrated that extracellular Ca²⁺ influx is essential for this response and that L-type VDCCs constitute a major Ca²⁺ entry pathway. However, because part of the PAF-induced response persisted even after inhibition of L-type VDCCs, the researchers hypothesized that additional Ca²⁺ entry pathways might also contribute to PAF-induced bladder contraction.

In the present study, the researchers focused on store-operated Ca²⁺ entry (SOCE) (Note 5), a mechanism by which extracellular Ca²⁺ enters cells when Ca²⁺ stores in the endoplasmic reticulum are depleted. Orai1 is one of the principal Ca²⁺ channels involved in this process. Using Synta66, an Orai1 inhibitor, the team investigated the contribution of Orai1 to PAF-induced contraction of urinary bladder smooth muscle.

Application of PAF to isolated guinea pig urinary bladder smooth muscle produced a strong contraction, which was markedly suppressed by Synta66. PAF-induced contraction was also strongly inhibited by verapamil, an L-type VDCC inhibitor (Figure 1). These findings suggest that Orai1-mediated Ca²⁺ entry contributes to PAF-induced urinary bladder smooth muscle contraction in association with L-type VDCC-dependent contractile mechanisms.

The researchers also examined contractions induced by the re-addition of extracellular Ca²⁺ after depletion of intracellular Ca²⁺ stores, without directly stimulating PAF receptors. These contractions were strongly suppressed by both Synta66 and verapamil. Thus, the functional association between Orai1-mediated Ca²⁺ entry and VDCC-dependent contractile mechanisms may not be limited specifically to PAF receptor stimulation.

A particularly notable finding was that the contribution of Orai1 differed depending on the stimulus. Synta66 only partially suppressed contractions induced by ACh and ATP, major neurotransmitters involved in normal bladder function. In contrast, PAF-induced contraction was markedly suppressed by Synta66 (Figure 1). Moreover, Synta66 had no effect on contraction induced by a high concentration of potassium, which directly depolarizes the cell membrane and activates VDCCs.

These findings suggest that Orai1 makes a relatively limited contribution to contractions evoked by neurotransmitters involved in normal bladder function, whereas it may play a greater role in PAF-induced contraction associated with inflammatory or other pathological conditions. Thus, the contribution of Orai1 to urinary bladder smooth muscle contraction may differ between normal and disease-associated states (Figure 2).

The researchers further examined several candidate molecules that might link Orai1-mediated Ca²⁺ entry to VDCC-dependent contraction, including the Na⁺/Ca²⁺ exchanger NCX1, TRPC1/4/5 channels, and the Ca²⁺-activated Cl⁻ channel ANO1. However, pharmacological inhibition of these molecules did not significantly alter PAF-induced contraction. These results suggest that, under the present experimental conditions, these molecules are unlikely to constitute the major pathway linking Orai1 to VDCC-dependent mechanisms.

The present study does not directly establish how Orai1 and VDCC-dependent mechanisms are functionally linked. In addition, because the study did not use disease models of inflammatory bladder disorders, it does not directly demonstrate that the contribution of Orai1 increases under pathological conditions. Future studies directly examining membrane potential, channel activity, and intracellular Ca²⁺ levels, together with experiments using disease models, will be needed to clarify these mechanisms.

These findings provide new fundamental insight into the relationship between inflammatory lipid mediators and bladder function. A therapeutic approach that suppresses abnormal bladder contraction under pathological conditions while minimizing effects on normal bladder function would be desirable. If future studies using disease models confirm that Orai1 preferentially contributes to pathological bladder contraction, these findings could help clarify the mechanisms underlying bladder dysfunction associated with abnormal contraction, including overactive bladder, and support the development of new therapeutic approaches.

Figure 1. PAF-Induced Bladder Contraction Is Strongly Suppressed by Orai1 Inhibition
Representative contractile traces of guinea pig urinary bladder smooth muscle are shown. Contractile responses were evaluated using the area under the curve (AUC) over a defined period after stimulation rather than the peak amplitude of transient contraction. The analysis periods were 60 min for PAF, 10 min for ACh, and 3 min for the ATP analog α,β-methylene ATP (α,β-mATP). PAF-induced contraction was strongly suppressed by both the Orai1 inhibitor Synta66 and the L-type VDCC inhibitor verapamil. In contrast, Synta66 only partially suppressed contractions induced by ACh and α,β-mATP, whereas verapamil strongly suppressed these responses. In the traces, “w” indicates washout. For experiments with PAF, an inhibitor cocktail was used to minimize the influence of endogenous neurotransmitters and allow the direct response to PAF to be evaluated. These findings suggest that although VDCCs are important for all of these contractile responses, Orai1 may play a greater role in PAF-induced contraction.