1. Michael E. DeBakey Institute for Comparative Cardiovascular Sciences and Department of Veterinary Physiology and Pharmacology, College of Veterinary Medicine, Texas A&M University, College Station, Texas 77843;
2. Department of Biology, University of Akron, Akron, Ohio 44325; and
3. Department of Physiology, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio 44272