Overexpression of junctin causes adaptive changes in cardiac myocyte Ca(2+) signaling.
In cardiac muscle, junctin forms a quaternary protein complex with the ryanodine receptor (RyR), calsequestrin, and triadin 1 at the luminal face of the junctional sarcoplasmic reticulum (jSR). By binding directly the RyR and calsequestrin, junctin may mediate the Ca(2+)-dependent regulatory interactions between both proteins. To gain more insight into the underlying mechanisms of impaired contractile relaxation in transgenic mice with cardiac-specific overexpression of junctin (TG), we studied cellular Ca(2+) handling in these mice. We found that the SR Ca(2+) load was reduced by 22% in cardiomyocytes from TG mice. Consistent with this, the frequency of Ca(2+) sparks was diminished by 32%. The decay of spontaneous Ca(2+) sparks was prolonged by 117% in TG. This finding was associated with a lower Na(+)-Ca(2+) exchanger (NCX) protein expression (by 67%) and a higher basal RyR phosphorylation at Ser(2809) (by 64%) in TG. The shortening- and Delta[Ca](i)-frequency relationships (0.5-4 Hz) were flat in TG compared to wild-type (WT) which exhibited a positive staircase for both parameters. Furthermore, increasing stimulation frequencies hastened the time of relaxation and the decay of [Ca](i) by a higher percentage in TG. We conclude that the impaired relaxation in TG may result from a reduced NCX expression and/or a higher SR Ca(2+) leak. The altered shortening-frequency relationship in TG seems to be a consequence of an impaired excitation-contraction coupling with depressed SR Ca(2+) release at higher rates of stimulation. Our data suggest that the more prominent frequency-dependent hastening of relaxation in TG results from a stimulation of SR Ca(2+) transport reflected by corresponding changes of [Ca](i).
Kirchhefer
Uwe
U
Institut für Pharmakologie und Toxikologie, Westfälische Wilhelms-Universität, Domagkstr. 12, 48149 Münster, Germany.
Hanske
Gabriela
G
Jones
Larry R
LR
Justus
Isabel
I
Kaestner
Lars
L
Lipp
Peter
P
Schmitz
Wilhelm
W
Neumann
Joachim
J
D000222:Adaptation, Physiological; D000818:Animals; D020013:Calcium Signaling / Q000502:physiology*; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002155:Calsequestrin / Q000378:metabolism; D004285:Dogs; D015870:Gene Expression; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008822:Mice, Transgenic; D006899:Mixed Function Oxygenases / Q000235:genetics / Q000378:metabolism*; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D009126:Muscle Relaxation / Q000502:physiology; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D010766:Phosphorylation; D019837:Ryanodine Receptor Calcium Release Channel / Q000737:chemistry / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019831:Sodium-Calcium Exchanger / Q000378:metabolism; D000099211:Phospholamban
D002135:Calcium-Binding Proteins; D000252:Calcium-Transporting ATPases; D002155:Calsequestrin; D007545:Isoproterenol; D008565:Membrane Proteins; D006899:Mixed Function Oxygenases; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019831:Sodium-Calcium Exchanger; D000099211:Phospholamban; C498841:Asph protein, mouse