Preheat oven to 400°.
Combine first 5 ingredients in a large bowl. Make a well in center of mixture, and set aside. Combine buttermilk and next 3 ingredients, stirring well with a wire whisk. Stir in bran; let stand 5 minutes.
Add bran mixture to flour mixture, stirring just until dry ingredients are moistened.
Spoon batter evenly into 24 muffin cups coated with cooking spray, filling three-fourths full. Bake at 400° for 22 to 24 minutes or until golden. Remove muffins from pans immediately, and cool on wire racks.