The cardiac sarcolemmal Na-Ca exchanger (NCX) is allosterically regulated
by Ca(i) such that when Ca(i) is low, NCX current (I(NCX)) deactivates.
In this study, we used membrane potential (E(m)) and I(NCX) to control
Ca entry into and Ca efflux from intact cardiac myocytes to investigate
whether this allosteric regulation (Ca activation) occurs with Ca(i)
in the physiological range. In the absence of Ca activation, the
electrochemical effect of increasing Ca(i) would be to increase
inward I(NCX) (Ca efflux) and to decrease outward I(NCX). On the
other hand, Ca activation would increase I(NCX) in both directions.
Thus, we attributed Ca(i)-dependent increases in outward I(NCX)
to allosteric regulation. Ca activation of I(NCX) was observed in
ferret myocytes but not in wild-type mouse myocytes, suggesting that
Ca regulation of NCX may be species dependent. We also studied transgenic
mouse myocytes overexpressing either normal canine NCX or this same
canine NCX lacking Ca regulation (Delta680-685). Animals with the
normal canine NCX transgene showed Ca activation, whereas animals
with the mutant transgene did not, confirming the role of this region
in the process. In native ferret cells and in mice with expressed
canine NCX, allosteric regulation by Ca occurs under physiological
conditions (K(mCaAct) = 125 +/- 16 nM SEM approximately resting Ca(i)).
This, along with the observation that no delay was observed between
measured Ca(i) and activation of I(NCX) under our conditions, suggests
that beat to beat changes in NCX function can occur in vivo. These
changes in the I(NCX) activation state may influence SR Ca load and
resting Ca(i), helping to fine tune Ca influx and efflux from cells
under both normal and pathophysiological conditions. Our failure
to observe Ca activation in mouse myocytes may be due to either the
extent of Ca regulation or to a difference in K(mCaAct) from other
species. Model predictions for Ca activation, on which our estimates
of K(mCaAct) are based, confirm that Ca activation strongly influences
outward I(NCX), explaining why it increases rather than declines
with increasing Ca(i).