Overexpression of the Na(+)/Ca(2+) exchanger and inhibition of the sarcoplasmic reticulum Ca(2+)-ATPase in ventricular myocytes from transgenic mice.
BACKGROUND
Myocytes from failing hearts produce slower and smaller Ca(2+) transients associated with reduction in expression of sarcoplasmic reticulum (SR) Ca(2+) ATPase and an overexpression of Na(+)/Ca(2+) exchanger. Since the physiological role of both these proteins is competing for, and removing, Ca(2+) from the cytoplasm, overexpression of the exchanger may compensate for less effective SR Ca(2+) uptake. This study demonstrates this compensatory effect and provides a quantitative description of the results.


METHODS
Ventricular myocytes from transgenic mice overexpressing the Na(+)/Ca(2+) exchanger (TR) and nontransgenic littermates (NON) were used. Cell shortening, cytoplasmic [Ca] (using indo-1 AM) and electrophysiological parameters were monitored.


RESULTS
TR myocytes displayed faster Ca(2+) transients and twitches compared with NON myocytes. Superfusion with thapsigargin prolonged the time-course of Ca(2+) transients of TR myocytes until these were equal to the ones measured in NON myocytes. The amount of SR Ca(2+)-ATPase (SERCA) inhibition needed to obtain such transients was calculated as a function of V(max) for the Ca(2+) flux via SERCA and found to be 28%. In TR myocytes V(max) for the Ca(2+) flux via Na(+)/Ca(2+)exchange was 240% of NON myocytes. When Ca(2+) transients in TR myocytes were slowed by thapsigargin to similar values to the ones recorded in NON myocytes, SR Ca(2+) content was also correspondingly reduced.


CONCLUSIONS
The results suggest that in pathophysiological conditions where there is a reduction in SERCA function, overexpression of Na(+)/Ca(2+) exchanger can compensate and allow normal Ca(2+) homeostasis to be maintained. In mouse ventricular myocytes a 2.4-fold increase in Na(+)/Ca(2+) exchange activity compensates for a reduction in SERCA function by 28% so maintaining the duration of the Ca(2+) transient.
Terracciano
C M
CM
Imperial College School of Medicine at NHLI, Cardiac Medicine, Dovehouse Street, SW3 6LY, London, UK. c.terracciano@ic.ac.uk
Philipson
K D
KD
MacLeod
K T
KT
D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D018929:Cell Culture Techniques; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D051379:Mice; D008822:Mice, Transgenic; D009206:Myocardium / Q000166:cytology / Q000201:enzymology / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology
D004791:Enzyme Inhibitors; D019831:Sodium-Calcium Exchanger; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D002118:Calcium