Influence of a constitutive increase in myofilament Ca(2+)-sensitivity on Ca(2+)-fluxes and contraction of mouse heart ventricular myocytes.
Chronic increases in myofilament Ca(2+)-sensitivity in the heart are known to alter gene expression potentially modifying Ca(2+)-homeostasis and inducing arrhythmias. We tested age-dependent effects of a chronic increase in myofilament Ca(2+)-sensitivity on induction of altered alter gene expression and activity of Ca(2+) transport systems in cardiac myocytes. Our approach was to determine the relative contributions of the major mechanisms responsible for restoring Ca(2+) to basal levels in field stimulated ventricular myocytes. Comparisons were made from ventricular myocytes isolated from non-transgenic (NTG) controls and transgenic mice expressing the fetal, slow skeletal troponin I (TG-ssTnI) in place of cardiac TnI (cTnI). Replacement of cTnI by ssTnI induces an increase in myofilament Ca(2+)-sensitivity. Comparisons included myocytes from relatively young (5-7months) and older mice (11-13months). Employing application of caffeine in normal Tyrode and in 0Na(+) 0Ca(2+) solution, we were able to dissect the contribution of the sarcoplasmic reticulum Ca(2+) pump (SR Ca(2+)-ATPase), the Na(+)/Ca(2+) exchanger (NCX), and "slow mechanisms" representing the activity of the sarcolemmal Ca(2+) pump and the mitochondrial Ca(2+) uniporter. The relative contribution of the SR Ca(2+)-ATPase to restoration of basal Ca(2+) levels in younger TG-ssTnI myocytes was lower than in NTG (81.12±2.8% vs 92.70±1.02%), but the same in the older myocytes. Younger and older NTG myocytes demonstrated similar contributions from the SR Ca(2+)-ATPase and NCX to restoration of basal Ca(2+). However, the slow mechanisms for Ca(2+) removal were increased in the older NTG (3.4±0.3%) vs the younger NTG myocytes (1.4±0.1%). Compared to NTG, younger TG-ssTnI myocytes demonstrated a significantly bigger contribution of the NCX (16±2.7% in TG vs 6.9±0.9% in NTG) and slow mechanisms (3.3±0.4% in TG vs 1.4±0.1% in NTG). In older TG-ssTnI myocytes the contributions were not significantly different from NTG (NCX: 4.9±0.6% in TG vs 5.5±0.7% in NTG; slow mechanisms: 2.5±0.3% in TG vs 3.4±0.3% in NTG). Our data indicate that constitutive increases in myofilament Ca(2+)-sensitivity alter the relative significance of the NCX transport system involved in Ca(2+)-homeostasis only in a younger group of mice. This modification may be of significance in early changes in altered gene expression and electrical stability hearts with increased myofilament Ca-sensitivity.
Department of Pharmacology, University of California Davis, Davis, CA 95616, United States.
Jose L
JL
Puglisi
Department of Physiology and Biophysics, College of Medicine, University of Illinois at Chicago, Chicago, IL 60612, United States.
Paul H
PH
Goldspink
Department of Neurobiology, Physiology, and Behavior, University of California Davis, Davis, CA 95616, United States.
Aldrin V
AV
Gomes
Department of Physiology and Biophysics, College of Medicine, University of Illinois at Chicago, Chicago, IL 60612, United States.
Megan S
MS
Utter
Department of Pharmacology, University of California Davis, Davis, CA 95616, United States.
Donald M
DM
Bers
Department of Physiology and Biophysics, College of Medicine, University of Illinois at Chicago, Chicago, IL 60612, United States. Electronic address: solarorj@uic.edu.
R John
RJ
Solaro
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects; D002478:Cells, Cultured; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D005260:Female; D015870:Gene Expression; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D032383:Myocytes, Cardiac / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D009210:Myofibrils / Q000378:metabolism*; D019210:Troponin I / Q000235:genetics / Q000378:metabolism
D000697:Central Nervous System Stimulants; D019210:Troponin I; D002110:Caffeine; D002118:Calcium
Arrhythmia; Ca-sensitizer; Hypertrophic cardiomyopathy; Troponin I