1. Department of Biological Sciences, Southern Methodist University, 6501 Airline Road, Dallas, Texas 75275, MHSC, MMRI, and Department of Physiology, Univerity of Debrecen, H-4012 Debrecen, P.O. Box 22, Hungary, and Cell Physiological Research Group of the Hungarian Academy of Sciences, H-4012 Debrecen, P.O. Box 22, Hungary