Abstract
We have examined the binding of the fluorescent Ca indicator indo-1 to cellular protein in permeabilized ventricular myocytes and also to soluble and particulate myocyte protein. Using either a filtration technique or equilibrium dialysis, and conditions similar to those in a cardiac myocyte patch clamped with 100 microM indo-1 in the patch pipette, we found that 72% of the total indo-1 was bound to myocyte protein at a protein concentration of 100 mg/ml. This corresponds to a binding of 3.8 +/- 0.5 nmol indo-1/mg protein. Separation of the myocyte protein into a soluble and a particulate fraction showed that 63% of the bound indo-1 was bound to soluble protein, corresponding to a binding of 3.22 +/- 0.99 nmol/mg, whereas 37% of the bound indo-1 was bound to particulate protein (0.85 +/- 0.14 nmol/mg) at a low [Ca] (pCa approximately 9). Binding of indo-1 in permeabilized myocytes was approximately 60% higher at a saturating Ca concentration (pCa = 3), than under Ca free conditions (1 mM EGTA). Simultaneous measurements of free [Ca] with a Ca selective electrode and indo-1 fluorescence showed that, the dissociation constant (Kd) for Ca was increased 4-5 fold in the presence of permeabilized myocytes as compared to the value obtained in vitro. In agreement with the binding experiments we estimate that the true Kd and the apparent Kd (using ratiometric measurements) for Ca binding to indo-1 are increased approximately four fold, at a myocyte protein concentration of 100 mg/ml.