C. Lamont et Da. Eisner, THE SARCOLEMMAL MECHANISMS INVOLVED IN THE CONTROL OF DIASTOLIC INTRACELLULAR CALCIUM IN ISOLATED RAT CARDIAC TRABECULAE, Pflugers Archiv, 432(6), 1996, pp. 961-969
We performed experiments using the calcium indicator Indo-1 to determi
ne the relative roles of the sar colemmal mechanisms involved in the r
egulation of diastolic intracellular calcium concentration ([Ca2+](i))
in trabeculae from the rat heart. Ryanodine was used to eliminate sar
coplasmic reticulum (SR) function. In the functional absence of the SR
, 76.8 +/- 3.9% of the calcium was extruded by the Na-Ca exchange carr
ier in the [Ca2+](i) range of diastolic concentration +/- 200-400 nM.
This was assessed by measuring the recovery of [Ca2+](i) from small pe
rturbations in the presence and absence of extracellular sodium. The s
teady-state relationship between [Ca2+](o) and [Ca2+](i) was linear ov
er the range of 1-40 mM, a 20-fold increase of [Ca2+](o) produced a 1.
97 fold +/- 0.13-fold increase in [Ca2+](i) (n = 5). In the absence of
extracellular sodium raising [Ca2+](o) had a variable effect. In some
preparations there was little change of [Ca2+](i) while in others the
response was almost as large as in control conditions. We conclude th
at the Na-Ca exchanger contributes approximate to 77% of sarcolemmal c
alcium extrusion following small perturbations in [Ca2+](i) and that t
his fraction does not diminish as the [Ca2+](i) declines. In addition
we have shown a sodium-independent entry of calcium into quiescent car
diac muscle under resting conditions.