ORIGINAL PAPER C. Caputo á C. Gerday á J. R. Lopez S. R. Taylor á P. BolanÄos Opposite effects of cooling on twitch contractions of skeletal muscle isolated from tropical toads (Leptodactylidae) and northern frogs (Ranidae) Accepted: 27 August 1998 Abstract Cooling increases the twitch force of frog skeletal muscle (Rana temporaria; Rana pipiens), but decreases the twitch force of tropical toad muscle (Le- ptodactylus insularis). Action potentials and intramem- branous charge movement in frog and toad ®bers were slowed identically by cooling. Cooling increased the in- tegral of twitch Ca 2+ detected by aequorin in frog ®bers (1.4-fold), while also decreasing the peak and slowing the rate of decay. Conversely, cooling decreased the in- tegral (0.6-fold) and the peak of twitch Ca 2+ in toad ®bers, without aecting the rate of decay. The dierence in entire Ca 2+ transients may account for cold-induced twitch potentiation in frogs and twitch paralysis in toads. In sustained contractions of toad ®bers, cooling markedly decreased maximum force caused by: (i) te- tanic stimulation, (ii) two-microelectrode voltage clamp steps, (iii) high [K + ], or (iv) caeine. Maximum force in sustained contractions was decreased moderately by cooling frog ®bers. Rapid rewarming and simultaneous removal of high [K + ] or caeine during a sustained contraction, caused toad muscle force to rise towards the value corresponding to the warm temperature. This did not occur after removing high [K + ] or caeine from toad ®bers kept in the cold. Transmission electron mic- rographs showed no relevant structural dierences. Parvalbumins are thought to promote relaxation of frog muscle in the cold. The unique parvalbumin isoforms in toad muscle apparently lack this property. Key words Skeletal muscle relaxation á Temperature- dependence á Parvalbumins á Tropical toad Leptodactylus insularis á Northern frogs Rana temporaria, Rana pipiens Abbreviations EC excitation-contraction á k maximum steepness of the function á PA parvalbumins á Q max maximum change moved at positve internal potentials á SR sarcoplasmic reticulum TnC troponin C á T-system transverse tubular system á TTX tetrodotoxin á V transition potential Introduction Cooling moderately decreases the force developed in an isometric tetanus of Ranidae skeletal muscle. But twitch force increases as the temperature is lowered (Hill 1951). Hill's explanation was that twitch force depends on a balance between early and late processes in Ranidae skeletal muscle, and cooling has less eect on the early processes than it does on the latter. Subsequent inves- tigations have associated speci®c chemical and physical events with the mechanisms of excitation-contraction (EC) coupling that may achieve this balance (Horowicz 1994). These include the release of Ca 2+ from the sarcoplasmic reticulum (SR), rapid Ca 2+ binding to the low-anity regulatory Ca 2+ -binding sites on troponin C (TnC), and slower binding to complementary high-af- ®nity metal-binding sites on parvalbumins (PA), from which Ca 2+ is slowly retrieved by the energy-dependent pumps of the SR (Gillis 1985; Rall 1996). Our interest in cold-induced twitch potentiation and paralysis, grew from the coincidence of a few articles more than 15 years ago. One article characterized the eects of temperature on the tension developed by skinned ®bers from northern frogs, and revealed a qualitative dierence in apparent Ca 2+ binding to TnC at low and high [Ca 2+ ] (Godt and Lindley 1982). An- other study described a long-lasting change in dye-re- lated absorbance following a single action potential in J Comp Physiol B (1998) 168: 600±610 Ó Springer-Verlag 1998 C. Caputo á J.R. Lopez á P. BolanÄos IVIC, Centro de BiofõÂsica y BioquõÂmica, Caracas 102 A, Venezuela C. Gerday Laboratoire de Biochimie Musculaire (B6), University of Liege, 4000, Liege, Belgium S.R. Taylor (&) Mayo Foundation, 711 Guggenheim Building, Rochester MN 55905 USA e-mail: taylor@mayo.edu, Fax: +1-507-284-9111