select c.name
from sys.indexes i 
    join sys.index_columns ic on i.object_id = ic.object_id and i.index_id = ic.index_id
    join sys.columns c on ic.object_id = c.object_id and ic.column_id = c.column_id
where i.is_primary_key = 1 
    and i.object_id = object_id('u_store_i')