Use of the Ca-shortening curve to estimate the myofilament responsiveness to Ca2+ in tetanized rat ventricular myocytes.
We previously estimated the myofilament responsiveness to Ca(2+) in isolated intact ventricular myocytes, using the steady-state relationship between cytosolic Ca(2+) concentration ([Ca(2+)](i)) and cell-shortening during tetanus (Ca-L trajectory). This method was useful and easy; however, it could not be used for a high dose of Ca sensitizer because the instantaneous plots after the application of Ca sensitizer did not make a fixed point of shortening (we used 5% shortening). Therefore we must produce another method to investigate Ca(2+) responsiveness. For an estimation of a wider range of the Ca-L trajectory, we fitted the Ca-L trajectory data with the Hill equation to construct the Ca-shortening curve. To fit this curve, we measured the maximal shortening, which was on average 31.6%. The value of [Ca(2+)](i) to produce the half-maximal shortening (Ca(50)) was dose-dependently decreased by EMD57033 (sensitization). Either isoproterenol or 3-isobutyl-1-methylxanthine increased Ca(50) (desensitization) with a concomitant increase in intracellular c-AMP. EMD57439, a selective PDE-III inhibitor, did not significantly increase the c-AMP concentration and produced little change in Ca(50). These results are in agreement with previous reports with skinned or intact multicellular preparations. The Ca-shortening curve constructed in intact cardiac myocytes can be used to estimate the myofibrillar responsiveness to Ca(2+) in a wide range of [Ca(2+)](i).
Kusakari
Yoichiro
Y
Department of Physiology (II), The Jikei University School of Medicine, 3-25-8 Nishishinbashi, Minato-ku, Tokyo 105-8461, Japan. kusakari@jikei.ac.jp
Hongo
Kenichi
K
Kawai
Makoto
M
Konishi
Masato
M
Kurihara
Satoshi
S
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D008841:Actin Cytoskeleton / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000494:pharmacology* / Q000502:physiology*; D002316:Cardiotonic Agents / Q000494:pharmacology; D048429:Cell Size / Q000187:drug effects; D000242:Cyclic AMP / Q000378:metabolism; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology; D032383:Myocytes, Cardiac / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D011804:Quinolines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D013746:Tetany; D013829:Thiadiazines / Q000494:pharmacology
D002316:Cardiotonic Agents; D010726:Phosphodiesterase Inhibitors; D011804:Quinolines; D013829:Thiadiazines; C070609:EMD 53998; D000242:Cyclic AMP; D007545:Isoproterenol; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine