Tr. Shannon et al., Potentiation of fractional sarcoplasmic reticulum calcium release by totaland free intra-sarcoplasmic reticulum calcium concentration, BIOPHYS J, 78(1), 2000, pp. 334-343
Our aim was to measure the influence of sarcoplasmic reticulum (SR) calcium
content ([Ca](SRT)) and free SR [Ca] ([Ca](SR)) on the fraction of SR calc
ium released during voltage clamp steps in isolated rabbit ventricular myoc
ytes. [Ca](SRT), as measured by caffeine application, was progressively inc
reased by conditioning pulses. Sodium was absent in both the intracellular
and in the extracellular solutions to block sodium/calcium exchange. Total
cytosolic calcium flux during the transient was inferred from I-Ca, [Ca](SR
T), [Ca](i), and cellular buffering characteristics. Fluxes via the calcium
current (I-Ca), the SR calcium pump, and passive leak from the SR were eva
luated to determine SR calcium release flux (J(rel)), Excitation-contractio
n (EC) coupling was characterized with respect to both gain (integral J(rel
)/ integral /(Ca) and fractional SR calcium release. Both parameters were v
irtually zero for a small, but measurable [Ca](SRT). Gain and fractional SR
calcium release increased steeply and nonlinearly with both [Ca](SRT) and
[Ca](SR). We conclude that potentiation of EC coupling can be correlated wi
th both [Ca](SRT) and [Ca](SR). While fractional SR calcium release was not
linearly dependent upon [Ca](SR), intra-SR calcium may play a crucial role
in regulating the SR calcium release process.