Md. Zhao et al., PROPERTIES OF TRICARBOXYLATE AND TETRACARBOXYLATE CA2-MUSCLE FIBERS( INDICATORS IN FROG SKELETAL), Biophysical journal, 70(2), 1996, pp. 896-916
Recently a number of lower-affinity fluorescent Ca2+ indicators have b
ecome available with principal absorbance bands at visible wavelengths
. This article evaluates these indicators, as well as two shelter wave
length indicators, mag-fura-5 and mag-indo-1, for their suitability as
rapid Ca2+ indicators in frog skeletal muscle fibers. With three lowe
r-affinity tricarboxylate indicators (mag-fura-5, mag-indo-1, and magn
esium orange), the change in fluorescence in response to an action pot
ential (Delta F) appeared to track the myoplasmic Ca2+ transient (Delt
a[Ca2+]) without delay. With three lower-affinity tetracarboxylate ind
icators (ETC, calcium-orange-5N, and calcium-green-5N) and one tricarb
oxylate indicator (magnesium green), Delta F responded to Delta[Ca2+]
with a small delay. Unfortunately, with the tetracarboxylate indicator
s, other problems were detected that appear to limit their usefulness
as reliable Ca2+ indicators. Surprisingly, Delta F from mag-fura-red,
another tricarboxylate indicator, was biphasic (with 480 nm excitation
), a feature that also greatly limits its usefulness. With several of
the indicators, estimates were obtained for the myoplasmic value of K-
D,(Ca) (the indicator's dissociation constant for Ca2+) and found to b
e elevated severalfold in comparison with the value measured in a simp
le salt solution. These and other problems related to the quantitative
use of Ca2+ indicators in the intracellular environment are evaluated
and discussed.