Resting and end‐diastolic Ca2+i measurements in the langendorff‐perfused ferret heart loaded with a 19F NMR indicator

Intracellular free calcium concentration (Ca2+i) was measured in Langendorff-perfused ferret hearts (30 degrees C, pH 7.4) by loading paced hearts with the 19F NMR calcium indicator, the 5,5'-difluoro derivative of 1,2-bis(o-aminophenoxy)ethane-N, N,N',N'-tetraacetic acid (5FBAPTA), to an initial cytosolic concentration of approximately 120 microM. Increasing the pacing frequency raised the end-diastolic Ca2+i from 299 +/- 44 nM (mean +/- SEM) at 0.2 Hz to 522 +/- 54 nM at 1.0 Hz and 691 +/- 166 nM at 2.0 Hz. Raising Cao from 1.8 to 7.0 mM at a pacing frequency of 1.0 Hz increased end-diastolic Ca2+i to 625 +/- 39 nM. In unpaced hearts perfused with diltiazem (100 microM), Ca2+i fell rapidly to a steady-state value of < 100 nM after 60 min. Raising Cao from 1.8 to 7.0 mM had no detectable effect on resting Ca2+i. The time course of the Ca2+i transient was measured in hearts paced at 1.1 Hz and perfused with 1.8 mM Cao. The peak Ca2+i was approximately 2 microM at approximately 150 msec after the pacing pulse, and peak developed LVP occurred at 550 msec compared with 280 msec in control hearts not loaded with 5FBAPTA. Comparisons with data obtained by other techniques, including fluorescent Ca2+i indicators, imply that although the end-diastolic Ca2+i values obtained with 5FBAPTA in beating hearts are elevated by the concentrations of intracellular 5FBAPTA required for signal detection, the changes in Ca2+i observed in response to experimental interventions are qualitatively consistent with previous data.