Surfactant comes in a big hydrophobic glob, so it is perhaps not surprising that it takes some time to unspool it through a fusion pore. But post-fusion regulation might be relevant to other exocytic cargos, such as those that are only slowly released from proteoglycan tethers or other matrices, or even untethered cargos present in vesicles that flicker open and closed only briefly.
Haller et al. point to calcium as a major regulator of post-fusion pore behavior. Calcium is released by both of the two stimuli that are known to potentiate surfactant release from alveolar cells—ATP and mechanical stretch from breathing—and in the current study, an increase in intracellular free calcium dilates the fusion pores. The calcium could be acting on the cytoskeleton to reduce cortical tension around fusion pores, thus allowing their expansion, but the details of this regulation remain unknown. ▪