CYTOSOLIC AND MITOCHONDRIAL [CA2-1 ACETOXYMETHYL ESTER - EFFECTS OF HIGH EXTRACELLULAR CA2+(] IN WHOLE HEARTS USING INDO)

Assessment of free cytosolic [Ca2+] ([Ca2+](c)) using the acetoxymethy l ester (AM) form of indo-1 may be compromised by loading of indo-1 in to noncytosolic compartments, primarily mitochondria. To determine the fraction of noncytosolic fluorescence in whole hearts loaded with ind o-1 AM, Mn2+ was used to quench cytosolic fluorescence. Residual (i.e. , noncytosolic) fluorescence was subtracted from the total fluorescenc e before calculating [Ca2+](c). Noncytosolic fluorescence was used to estimate mitochondrial [Ca2+]. In hearts paced at 5 Hz (N = 17), noncy tosolic fluorescence was 0.61 +/- 0.06 and 0.56 +/- 0.07 of total fluo rescence at lambda(385), and lambda(456), respectively. After taking i nto account noncytosolic fluorescence, systolic and diastolic [Ca2+](c ) was 673 +/- 72 and 132 +/- 9 nM, respectively, noncytosolic [Ca2+] w as 183 +/- 36 nM and increased to 272 +/- 12 when extracellular Ca2+ w as increased from 2 to 6 mM. This increase in noncytosolic [Ca2+] was inhibited by ruthenium red, a blocker of Ca2+ uptake by mitochondria, We conclude that cytosolic and mitochondrial [Ca2+] can be determined in whole hearts loaded with indo-1 AM by using Mn2+ to quench cytosoli c fluorescence.