Evidence that binding of Indo-1 to cardiac myocyte protein does not markedly change K d for Ca 2+

Abstract

Quantitative measurement of [Ca 2+] i with the fluorescent Ca 2+-indicators Indo-1 and Fura-2 is complicated by the possibility that the value of the dissociation constant (K d) may be influenced by binding to intracellular proteins. We investigated this question in cultured chick ventricular myocytes by use of two different Indo-1 calibration methods. First, the Indo-1 fluorescence ratio (R) ( 400 500 nm ) was measured in beating myocytes loaded by exposure to Indo-1/AM. Then, cells were exposed to the Ca 2+ ionophore Br A-23187 and fluorescence ratio was measured in the presence of 500 nM Ca 2+ (EGTA-Ca 2+ buffer). Subsequently cells were permeabilized to Ca 2+ by a 1 min exposure to 25 μM digitonin in the presence of ‘zero’ Ca 2+ (10 mM EGTA) and saturating 1 mM Ca 2+ to obtain R min, R max and β. We then calculated [Ca 2+] i from the formula ( [Ca 2+] i = K d [(R − R min) (R max − R)]β ). With K d = 250 nM, calculated systolic [Ca 2+] i was 750 ± 44 nM and diastolic 269 ± 19 nM (means ± SEM, n = 16). The R value calculated for an assumed [Ca 2+] i = 500 nM using the above formula and digitonin derived constants was very similar to the value measured using Br A-23187 (digitonin, 0.67 ± 0.03: Br A-23187, 0.66 ± 0.03, ns). As the Br A-23187 method is independent of the value chosen for K d, we conclude that the K d of 250 nM for Indo-1 measured in free solutions closely approximates the K d for intracellular Indo-1 in these cells, and that therefore the K d of Indo-1 for Ca 2+ does not appear to be markedly affected by binding to proteins or other intracellular molecules.