Estimation of intracellular pH in muscle of ®shes from dierent thermal environments S.E. Taylor a , S. Egginton b, *, E.W. Taylor a , C.E. Franklin c, 1 , I.A. Johnston c a School of Biological Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK b Department of Physiology, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK c Gatty Marine Laboratory, University of St. Andrews, Scotland, UK Received 29 June 1998; received in revised form 12 October 1998; accepted 13 February 1999 Abstract A technique based on homogenisation of rapidly frozen tissue was used to investigate the regulation of intracellular pH (pH i ) in freshwater and marine ®sh from diverse environmental temperatures. The following species were held at ambient temperatures of ca. 18C (Notothenia coriiceps; Antarctica), 58C (Pleuronectes platessa, Myoxocephalus scorpius; North Sea), and 268C(Oreochromis niloticus; African lakes). The eects of seasonal acclimatisation to 4, 11 and 188C were also examined in rainbow trout in the winter, autumn and summer, respectively. Extracellular (whole blood) pH (pH e ) did not follow the constant relative alkalinity relationship, where pH + =pOH À for any particular temperature, over a range of 1±268C (overall dpH e /dT=0.00920.002 U 8C À1 ; P < 0.001), apparently being regulated by ionic ¯uxes and ventilation. Intracellular pH (pH i ) was also regulated independently of pN(=0.5 pK water) in all species of ®sh examined. The inverse relationship between pH i and environmental temperature gave an overall dpH i /dT of À0.01020.001 U 8C À1 (for both white and red muscle) and À0.004 2 0.003 U 8C À1 (cardiac muscle). However, between 1 and 118C dpH i /dT was much higher (P < 0.001), À0.02220.003 U 8C À1 (white muscle) and À0.02220.004 U 8C À1 (red muscle). The possible adaptive roles for these dierent acid±base responses to environmental temperature variation among tissues and species, and the potential diculties of estimating pH i , are discussed. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Homogenising technique; pH e ; pH i ; Cardiac muscle; Skeletal muscle; Temperature 1. Introduction Most ®sh species are ectothermic, with cell tempera- ture closely re¯ecting that of the environment. Temperature directly in¯uences the rate of enzyme reactions, diusion, and binding of ligands to proteins and proteins to membrane receptors (Somero, 1995). This results in Q 10 values of 2±3 for the rise in meta- bolic rate with acute temperature change. Strategies for the thermal compensation of biological processes are widespread in the animal kingdom, requiring a period of acclimation to the new condition (Cossins and Bowler, 1987). An important modulator of cellular metabolism is pH which, due to the inherent thermal sensitivity of physico±chemical processes, usually varies inversely with temperature; the relationship of extra- cellular pH (pH e ) versus temperature has been typi®ed Journal of Thermal Biology 24 (1999) 199±208 0306-4565/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0306-4565(99)00013-3 * Corresponding author. Tel.: +44-121-414-6902; fax: +44- 121-414-6919. E-mail address: s.egginton@bham.ac.uk (S. Egginton) 1 Current address: Department of Zoology, University of Queensland, Brisbane 4072, Australia