Evolutionary and physiological variation in cardiac troponin C in relationto thermal strategies of fish

Striated muscle contraction is initiated when troponin C (TnC) binds Ca2+, which activates actinomyosin ATPase. We investigated (i) the variation betw een cardiac TnC (cTnC) primary structure within teleost fish and (ii) the p attern of TnC expression in response to temperature acclimation. There were few differences between rainbow trout (Oncorhynchus mykiss), yellowfin tun a (Thunnus albacares), yellow perch (Perca flavescens), goldfish (Carassius auratus), white sucker (Catostomus commersoni), and icefish (Chaenocephalu s aceratus) in cTnC amino acid sequence. No variation existed in the regula tory Ca2+-binding site (site 2). The site 3 and 4 substitutions were limite d to residues not directly involved in Ca2+ coordination. Fish cTnC primary structure was highly conserved between species (93%-98%) and collectively divergent from the highly conserved sequence seen in birds and mammals. Nor thern blots and polymerase chain reaction showed that thermal acclimation o f trout (3 degrees, 18 degreesC) did not alter the TnC isoform pattern. Whi le cardiac and white muscle had the expected isoforms-cTnC and fast troponi n C (fTnC), respectively-red muscle unexpectedly expressed primarily ftnC. Cold acclimation did not alter myofibrillar ATPase Ca2+ sensitivity, but ma ximal velocity increased by 60%. We found no evidence that TnC variants, ar ising between species or in response to thermal acclimation, play a major r ole in mitigating the effects of temperature on contractility of the adult fish heart.