Myoplasmic free calcium concentration reached during the twitch of an intact isolated cardiac cell and during calcium-induced release of calcium from the sarcoplasmic reticulum of a skinned cardiac cell from the adult rat or rabbit ventricle.

Intact cardiac cells from the adult rat or rabbit ventricle were isolated by enzymatic digestion with a progressive increase of the free Ca2+ in the solution. These cells were electrically stimulated in the presence of 2.50 mM free Ca2+, and a twitch of maximum amplitude was elicited by the positive inotropic interventions that were found to be optimum. Then the cells were chemically skinned, and the maximum tension induced by a saturating free Ca2+ was used as a reference to express the tension developed during the twitch of the intact cells. The myoplasmic free Ca2+ reached during the twitch was inferred from the tension-pCa curve. In mechanically skinned cells of the same animal species, the myoplasmic free Ca2+ reached during Ca2+-induced release of Ca2+ from the sarcoplasmic reticulum (SR) was inferred by two methods using (a) the tension-pCa curve and (b) a direct calibration of the transients of aequorin bioluminescence. The induction of a maximum Ca2+ release from the SR required a larger Ca2+ preload of the SR and a higher free Ca2+ trigger in the rabbit than in the rat skinned cells. However, the results obtained with the two methods of inference of the myoplasmic free Ca2+ suggest that in both animal species a maximum myoplasmic free Ca2+ of pCa approximately 5.40 was reached during both the optimum Ca2+-induced release of Ca2+ from the SR of the skinned cells and the optimum twitch of the intact cells. This was much lower than the free Ca2+ necessary for the full activation of the myofilaments (pCa approximately 4.90).