Cmn. Terracciano et al., NA-CA2+ EXCHANGE AND SARCOPLASMIC RETICULAR CA2+ REGULATION IN VENTRICULAR MYOCYTES FROM TRANSGENIC MICE OVEREXPRESSING THE NA+-CA2+ EXCHANGER(), Journal of physiology, 512(3), 1998, pp. 651-667
1. The contribution of the sarcoplasmic reticulum (SR) and Na+-Ca2+ ex
changer to intracellular Ca2+ regulation in mouse cardiac myocytes was
investigated by measuring contraction after variable rest intervals,
rapid cooling contractures (RCCs) and fast application of caffeine. Th
e results obtained showed differences from other species in the roles
played by the SR and the Na+-Ca2+ exchanger. They suggest that in mous
e ventricular myocytes there is significant Ca2+ entry via the exchang
er during rest and during the latter part of the Ca2+ transient. 2. In
cardiac myocytes isolated from transgenic mice overexpressing the car
diac Na+-Ca2+ exchanger the time to peak and relaxation of twitches an
d RCCs were faster than in control littermates. The decline of Ca2+, a
ssessed by indo-1, fluorescence, was faster in transgenic myocytes eve
n in the absence of Na+ and Ca2+ in the superfusing solution. This sug
gests that SR Ca2+ uptake is faster in these myocytes. However, no dif
ference in the expression of SERCA2a, phospholamban or calsequestrin m
easured with Western blotting could be found in the two groups. 3. We
measured SR Ca2+ content by integrating the caffeine-induced transient
inward current. The amount of Ca2+ stored in the SR of transgenic mou
se myocytes was 69% greater than in non-transgenic littermates. The in
creased SR Ca2+ content may be responsible for the faster rate of SR C
a2+ release and uptake in cells from transgenic mice. 4. We performed
experiments to assess whether the reversal potential of the Na+-Ca2+ e
xchanger (ENa-Ca) was different in transgenic cardiac cells. We measur
ed a Ni2+-sensitive current elicited by voltage ramps in non-dialysed
myocytes. The current-voltage relationship showed no difference in the
reversal potential of the Na+-Ca2+ exchanger in transgenic and contro
l myocytes. This suggests that the effects on the SR Ca2+ content in t
ransgenic cardiac myocytes can be ascribed to the overexpression of th
e exchanger and are not secondary to changes in intracellular diastoli
c Ca2+ and Na+.