Abstract
The cytosolic free Ca2+ concentration of isolated rat myocytes that were resistant to addition of external Ca2+ (Ca2+-tolerant) has been measured by two independent methods, the null point titration technique and by use of Quin 2 as an intracellular Ca2+ probe. Values obtained for quiescent cells were in the range of 170 to 270 nM. Using Quin 2-Ca2+ fluorescence to monitor changes of cytosolic free Ca2+ ([Ca2+]i) in the presence of 0.65 mM external Ca2+, separate additions of the Ca2+ ionophore ionomycin, the mitochondrial uncoupling agent 1799 or the respiratory inhibitor KCN each caused an increase of [Ca2+]i of about 3-fold. The Quin 2 loaded myocytes responded to electrical stimulation by a transient increase of [Ca2+]i, which peaked about 75% above the resting level. The rise of [Ca2+]i was complete within 50 ms and declined gradually to the resting level. The β-agonist isoproterenol caused up to a 100% increase in the amplitude of the Quin 2-Ca2+ fluorescence change, with a half maximal effect at 130 µM. The stimulation-induced [Ca2+]i transient was abolished by addition of 100 µM propanolol after 10 µM isoproterenol.
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