Objective: 

In the retinal pigment epithelium (RPE) the activation of L-type Ca²⁺ channels regulates important cell functions such as phagocytosis of shed photoreceptor outer segments. Recently a direct coupling of large-conductance Ca²⁺-activated K⁺ channel (BK channel) activity to L-type Ca²⁺ channel activation via a negative feedback control has been described (Wimmers et al. 2008, Molecular Vision). Aim of this study was to investigate the relevance of these interactions for phagocytosis.

Methods: 

An in vitro approach for phagocytosis was established where purified photoreceptor outer segment (OS) were applied to cultured RPE cells. For in vivo measurements of circadian rhythm of OS phagocytosis, mice with deficiencies for either BK or Cav1.3 were used. Rhodopsin was labeled in retinal cross-sections of mice and rhodopsin-positiv phagosomes in the RPE were quantified at different time points during the day. The retinal function of BK-/- mice was examined by electroretinogram (ERG).

Results: 

Phagocytosis of RPE cells in vitro was decreased by paxilline (5mM) and BayK8644(+) (10mM), blockers specific for BK channel and L-type Ca²⁺ channel. Wild-type mice showed a morning peak of phagocytic activity which decreased to a base level. In the Cav1.3-/- mice we observed a decreased number of phagosomes at activity maximum and during the afternoon an increased number of phagosomes compared with the wild-type control. In the BK-/- mice we observed an increase of the phagocytic activity, a decrease in the peak activity, and a return to lower base level. Both, the peak phagocytic activity in the morning and during the afternoon were significantly different (P< 0.001). Additionally we observed in 16 months old BK-/- mice a reduced OS length and 20% decreased content of rhodopsin. In ERG examinations reduced a-wave indicated reduced photoreceptor activity.

Conclusion: 

In conclusion, the absence of BK channels and Cav1.3 channels lead to altered circadian regulation of phagocytic function of the RPE in vivo. Inhibition of both channels modulate phagocytic activity in vitro. These data indicate for the first time a contribution of ion channels in the regulation of phagocytosis by the RPE.