P@siolodcal Review EDITORIAL BOARD L. Reuss, Editor; M. L. Jennings, Associate Editor Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, TX 77550-2781 I. M. Arias R. F. Coburn B. R. Duling R. E. Godt Z. W. Hall J. D. Neil1 E. R. Per1 M. P. Sheetz AMERICAN SOCIETY AND MOLECULAR European FOR BIOCHEMISTRY BIOLOGY Editorial OF GENERAL INSTITUTE OF NUTRITION J.-M. Besson(France) C. A. R. Boyd (England) J. Bures (Czechoslovakia) S. Grillner (Sweden) B. Kanner (Israel) H. Murer (Switzerland) W. J. Rietveld (Netherlands) W. Singer (Federal Republic of Germany) R. E. Weber (Denmark) N. Kretchmer BIOPHYSICAL SOCIETY P. Hess SOCIETY FOR NEUROSCIENCE D. R. Humphrey AMERICAN SOCIETY FOR CELL of European Committee Department of Physiology The University of Manchester Manchester MI3 9PT, England PHYSIOLOGISTS I. S. Cohen AMERICAN Committee R. Green, Chairman M. P. Czech E. J. Masoro SOCIETY Editorial BIOLOGY Z. Werb Corresponding Members P. W. Gage (Australia/New Zealand) M. Ito (Japan) R. Latorre (Latin America) Publications Committee C. M. Tipton, Chair D. L. Kunze L. B. Rowe11 J. A. Schafer Ex oficio Editorial B. B. Rauner, Publications Stag L. Chambers,Production M. E. Lowy, Copy Editor Manager and Executive Editor S. G. Schultz, President M. Frank, Executive Director Manager Physiologic& Reviews (ISSN-0031-9333) is published quarterly (January, April, July, and October) by The American Physiological Society, 9650 Rockville Pike, Bethesda, MD 20814-3991. Subscription Prices (postpaid). Institutional: $200.00, US; $260.00, Canada & Mexico; $266.00, elsewhere. Non-member individual: $169.00, US; $209.00, Canada & Mexico; $219.00, elsewhere. APS Member, $84.50. Subscriptions from outside the United States are payable in US currency or full equivalent. Japanese yen price is available from our agent: USACO Corporation, 13-12 Shimbashi l-Chrome, Minato-Ku, Tokyo 105, Japan. Subscriptions are accepted on a calendar-year basis only. Single Copies and Buck Issues: when available, $55.00 each postpaid. Second-class postage paid at Bethesda, MD, and at additional mailing offices. POSTMASTER: send address changes to Physiological Reviews, 9650 Rockville Pike, Bethesda, MD 20814-3991. Printed in the United States of America. Copyright 1992 by The American Physiological Society. The code at the bottom of the first page of an article indicates the copyright owner’s consent that copies of that article may be made beyond that permitted by sections 107 and 108 of the US Copyright Law -unless the copies are for general distribution, for advertising, for creating new works, or for resale-provided the per-copy fee is paid through the Copyright Clearance Center, Inc., 21 Congress St., Salem, MA 01970. 0031-9333/92 $2.00. Change of Address: The Journal must be advised of a change of address at least six weeks prior to date of issue, with both the subscriber’s old and new address given. Undelivered copies resulting from an address change will not be replaced. Subscriptions and orders should be sent to: PHYSIOLOGICAL REVIEWS, 9650 Rockville Pike, Bethesda, MD 20814-3991