Preheat the oven to 350 degrees F. Prepare a 9x5 inch loaf pan with nonstick cooking spray. In a bowl, sift together the flour, baking soda, and salt. In another bowl, beat together the butter and sugar until fluffy, about 5 minutes, scraping down the sides as needed.
To the butter mixture, add the eggs one at a time, beating between additions. Stir in the sour cream, poppy seeds, grapefruit zest, vanilla extract, and grapefruit juice until blended.
Add the flour mixture slowly and beat just until mixed, scraping down the sides as needed. Transfer the mixture to the loaf pan.
Bake for 55 to 65 minutes or until a toothpick inserted in the center returns mostly clean with crumbs attached. Allow the bread to cool in the pan for 10 minutes prior to flipping over and cooling completely on a wire rack.
In a small bowl, combine the powdered sugar and grapefruit juice until smooth. Drizzle over the bread and allow it to harden, then slice and serve.