Beating the cold: the functional evolution of troponin C in teleost fish.
The sensitivity of the cardiac myocyte contractile element for Ca(2+) decreases with temperature. As myocyte contractility is regulated by changes in cytosolic [Ca(2+)], this desensitizing effect represents a challenge for temperate fish such as the rainbow trout, Oncorhynchus mykiss, living in environments where temperatures are low and variable. To allow cardiac function in a temperate environment it is thought that the comparatively high Ca(2+) sensitivity of trout cardiac myocytes compensates for the effects of low temperature on myocyte contractility. The high Ca(2+) sensitivity of the trout myocyte is due, at least in part, to changes in the amino acid sequence of the thin filament protein, cardiac troponin C (cTnC). cTnC is the Ca(2+)-activated switch that triggers myocyte contraction. The isoform of cTnC cloned from trout ventricle (ScTnC) is 92% identical to mammalian cTnC (McTnC) and is significantly more sensitive to Ca(2+). This result suggests that ScTnC has evolved in trout to allow cardiac function at low temperatures. cTnC also appears to play a role in maintaining cardiac function when temperatures change. Increasing myofibrillar pH according to alpha-stat regulation, as would occur when temperature decreases, increases Ca(2+) sensitivity. A similar increase in pH also sensitizes cTnC to Ca(2+). ScTnC therefore appears critical in maintaining cardiac function in trout at low temperatures as well as during changes in temperature.
Gillis
Todd E
TE
Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6. tegillis@sfu.ca
Tibbits
Glen F
GF
D000064:Acclimatization / Q000502:physiology; D000595:Amino Acid Sequence; D000818:Animals; D003081:Cold Climate; D019143:Evolution, Molecular; D005399:Fishes / Q000235:genetics* / Q000502:physiology*; D008322:Mammals / Q000235:genetics / Q000502:physiology; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009200:Myocardial Contraction; D012487:Salmonidae / Q000235:genetics / Q000502:physiology; D017386:Sequence Homology, Amino Acid; D019209:Troponin C / Q000737:chemistry / Q000235:genetics* / Q000502:physiology*
D019209:Troponin C