Glenn C. Rodrigo

1. The K(ATP) channel opener diazoxide has been proposed to protect cardiac muscle against ischaemia by opening mitochondrial K(ATP) channels to depolarize the mitochondrial membrane potential, DeltaPsi(m). We have used the fluorescent dye TMRE to measure DeltaPsi(m) in adult rat freshly isolated cardiac myocytes exposed to diazoxide and metabolic inhibition. 2. Diazoxide, at concentrations that are highly cardioprotective (100 or 200 microM), caused no detectable increase in TMRE fluorescence (n=27 cells). However, subsequent application of the protonophore FCCP, which should collapse DeltaPsi(m), led to large increases in TMRE fluorescence (>300%). 3. Metabolic inhibition (MI: 2 mM NaCN+1 mM iodoacetic acid (IAA) led to an immediate partial depolarization of DeltaPsi(m), followed after a few minutes delay by complete depolarization which was correlated with rigor contracture. Removal of metabolic inhibition led to abrupt mitochondrial repolarization followed in many cells by hypercontracture, indicated by cell rounding and loss of striated appearance. 4. Prior application of diazoxide (100 microM) reduced the number of cells that hypercontracted after metabolic inhibition from 63.7+/-4.7% to 24.2+/-1.8% (P< 0.0001). 5-hydroxydeanoate (100 microM) reduced the protection of diazoxide (46.8+/-2.7% cells hypercontracted, P< 0.0001 vs diazoxide alone). 5. Diazoxide caused no detectable change in flavoprotein autofluorescence (n=26 cells). 6. Our results suggest that mitochondrial depolarization and flavoprotein oxidation are not inevitable consequences of diazoxide application in intact cardiac myocytes, and that they are also not essential components of the mechanism by which it causes protection.

19F NMR indicators have been used to measure the free cytosolic cation concentrations ([Mn+]i, where M is the atomic symbol and n is the value of the charge) of Ca2+, H+, and Mg2+ in perfused ferret hearts. The [Ca2+]i transient, cytosolic pH (pHi), and [Mg2+]i have also been followed at 16 phases in the cardiac cycle in hearts paced at 1.25 Hz at 30 degrees C. The initial [Ca2+]i rose rapidly after a 50-ms delay, was maximal at greater than 1.5 microM after 150 ms, and declined thereafter to the initial concentration. In contrast, no significant changes in pHi (pH 7.03 +/- 0.08) or [Mg2+]i (1.2 +/- 0.1 mM) were detected in the cycle. A decrease in developed pressure when the [Ca2+]i indicator (but not the pHi or [Mg2+]i indicator) was loaded into hearts was substantially reversed by the addition of 50 microM ZnCl2 to the perfusion medium. The Zn2+ was taken up into the myoplasm and displaced Ca2+ bound to the indicator, a symmetrically substituted difluoro derivative of 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA), as evidenced by the appearance of the Zn-5FBAPTA resonance. The decrease in developed pressure caused by 5FBAPTA, therefore, may be due to its Ca2+ buffering effect on the myoplasm. By coloading hearts with the [Ca2+]i and pHi indicators, simultaneous measurement of several [Mn+]i was demonstrated, which should provide a useful addition to the methods available to monitor cardiac function and pharmacology.