1. Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.
2. Beecham Pharmaceuticals Research Division, Biosciences Research Centre, Great Burgh, Yew Tree Bottom Road, Epsom, Surrey KT18 5XQ, U.K.
3. Zoologisches Institut, Lehrstuhl für Tierphysiologie, Universität Münster, Hindenburgplatz 55, 440 Münster, Federal Republic of Germany