The rates of Ca2+ dissociation and cross-bridge detachment from ventricular myofibrils as reported by a fluorescent cardiac troponin C.
The rate-limiting step of cardiac muscle relaxation has been proposed to reside in the myofilament. Both the rates of cross-bridge detachment and Ca(2+) dissociation from troponin C (TnC) have been hypothesized to rate-limit myofilament inactivation. In this study we used a fluorescent TnC to measure both the rate of Ca(2+) dissociation from TnC and the rate of cross-bridge detachment from several different species of ventricular myofibrils. The fluorescently labeled TnC was sensitive to both Ca(2+) dissociation and cross-bridge detachment at low Ca(2+) (presence of EGTA), allowing for a direct comparison between the two proposed rates of myofilament inactivation. Unlike Ca(2+) dissociation from TnC, cross-bridge detachment varied in myofibrils from different species and was rate-limited by ADP release. At subphysiological temperatures (<20 °C), the rate of Ca(2+) dissociation from TnC was faster than the rate of cross-bridge detachment in the presence of ADP. These results support the hypothesis that cross-bridge detachment rate-limits relaxation. However, Ca(2+) dissociation from TnC was not as temperature-sensitive as cross-bridge detachment. At a near physiological temperature (35 °C) and ADP, the rate of cross-bridge detachment may actually be faster than the rate of Ca(2+) dissociation. This provides evidence that there may not be a simple, single rate-limiting step of myofilament inactivation.
Department of Physiology and Cell Biology, College of Medicine, The Ohio State University, Columbus, Ohio 43210, USA.
Sean C
SC
Little
Brandon J
BJ
Biesiadecki
Ahmet
A
Kilic
Robert S D
RS
Higgins
Paul M L
PM
Janssen
Jonathan P
JP
Davis
D000244:Adenosine Diphosphate / Q000737:chemistry / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D004533:Egtazic Acid / Q000737:chemistry; D005456:Fluorescent Dyes / Q000737:chemistry* / Q000378:metabolism; D006352:Heart Ventricles / Q000737:chemistry* / Q000378:metabolism; D006358:Hot Temperature; D009206:Myocardium / Q000737:chemistry* / Q000378:metabolism; D009210:Myofibrils / Q000737:chemistry* / Q000378:metabolism; D011817:Rabbits; D019209:Troponin C / Q000737:chemistry* / Q000378:metabolism
D005456:Fluorescent Dyes; D019209:Troponin C; D004533:Egtazic Acid; D000244:Adenosine Diphosphate; D002118:Calcium