1. From the Department of Physiological Sciences, Facultad de Medicina, Universidad Nacional de Colombia, Bogota, Colombia; and the2Departments of Physiology,
2. From the Department of Physiological Sciences, Facultad de Medicina, Universidad Nacional de Colombia, Bogota, Colombia; and the
3. Departments of Physiology,
4. Ophthalmology and
5. Departments of Physiology,4Medicine, University of Pennsylvania, School of Medicine, Philadelphia, Pennsylvania.