Dm. Bers et Jr. Berlin, KINETICS OF [CA](I) DECLINE IN CARDIAC MYOCYTES DEPEND ON PEAK [CA](I), American journal of physiology. Cell physiology, 37(1), 1995, pp. 271-277
The rate of decline of free intracellular Ca concentration ([Ca](i)) i
s a potentially useful index of the function of Ca transport systems.
However, interpretations of these results may depend on multiple Ca tr
ansport systems and interaction with intracellular Ca binding sites. W
e measured [Ca](i) in voltage-clamped ventricular myocytes isolated fr
om rat hearts using indo 1 fluorescence. Conditions were chosen where
[Ca](i) decline was expected to depend almost exclusively on the sarco
plasmic reticulum Ca pump. The half time of [Ca](i) decline (t(1/2)) d
ecreased as the amplitude of the intracellular Ca (Ca-i) transient inc
reased. This is not the result that would be expected from a transport
system where the transport rate is a linear function of free [Ca](i).
In this case the time constant of [Ca](i) decline (tau) should be ind
ependent of the peak value of [Ca](i). This is also true if linear buf
fering of Ca-i is included. We develop a simple but more realistic the
oretical framework where Ca transport rate and Ca binding both depend
on free [Ca](i) with Michaelis-Menten type functions. We demonstrate t
hat the observed decline in apparent tau with increasing peak [Ca](i)
is entirely expected on theoretical grounds and over a wide range of c
haracteristics for Ca transport and binding. We conclude that one cann
ot draw inferences about the intrinsic Ca transport function based on
tau values unless the Cai transient has a comparable size.