Bestrophin protein is involved in ion transport across the basolateral membrane of the retinal pigment epithelium. The mammalian genome encodes 4 members of the bestrophin family. Bestrophins have been proposed to comprise a new family of Ca²⁺-activated Cl^- channels¹. We have recently demonstrated (abstract by Matchkov et. al) that siRNA mediated downregulation of bestrophin-4 is associated with the disappearance of a recently demonstrated² cGMP-dependent Ca²⁺-activated Cl^- current in vascular smooth muscle cells (SMCs). Here we study the distribution of bestrophin-4-and cGMP dependent Cl^- channel activity in the vasculature to further test the possibility that bestrophin-4 is important for SMC Cl^--channel activity.

We combined electrophysiological detection (patch clamp) of Ca²⁺-activated Cl^- currents in SMCs from vascular beds of different origin with the expression pattern of bestrophin-4 using PCR, Western blot and immunohistochemistry.

Using RT-PCR we identified bestrophin-4 mRNA expression in rat mesenteric small arteries, aorta and renal arteries, while no expression was seen in pulmonary arteries. Using a bestrophin-4 specific antibody (verified by Western blotting of the heterologously expressed epitope) Western blot detected a ~65 kDa band in cell lysates from rat mesenteric small arteries and aorta, which was not seen in pulmonary arteries and when preincubated with the immunizing peptide. The distribution of bestrophin-4 mRNA and protein has a pattern similar to the cGMP-dependent Cl^- current in SMCs of different origins. Immunohistochemistry identified bestrophin-4 both in endothelial and SMCs of the vascular tree in the brain, heart, kidney and mesentery, but not in the lungs. We suggest that bestrophin-4 is important for the cGMP dependent, Ca²⁺ activated Cl^- conductance in many vascular beds and may play a role for both SMC and endothelial function.