Ca transients in cardiac myocytes measured with a low affinity fluorescent indicator, furaptra.

Intracellular calcium ion (Ca2+]i) transients were measured in single rat ventricular myocytes with the fluorescent indicator furaptra. Cells were voltage clamped with a single patch electrode containing the K+ salt of furaptra and fluorescence at 500 nm was measured during illumination with 350 and 370 nm light. Depolarizing voltage-clamp pulses elicited Ca2+]-dependent fluorescent transients in 30 of 33 cells tested. The peak change in Ca2+]i elicited by 50-ms depolarizations from -70 to +10 mV was 1.52 +/- 0.25 microM (mean +/- SEM, n = 7). The size of the Ca2+]i transient increased in response to 10 microM isoproterenol, prolongation of the depolarization, and increasing pipette Na+]. Because furaptra is sensitive to Ca2+ and Mg2+, changes in Mg2+]i during the Ca2+]i transient could not be measured. Instead, a single-compartment model was developed to simulate changes in Mg2+] during Ca2+] transients. The simulations predicted that a 2 microM Ca2+] transient was accompanied by a slow increase in Mg2+] (14-29 microM), which became larger as basal Mg2+] increased (0.5-2.0 mM). The Mg2+] transient reached a peak approximately 1 s after the peak of the Ca2+] transient with the slow changes in Mg2+] dominated by competition at the Ca2+/Mg2+ sites of Troponin. These changes in Mg2+], however, were so small and slow that they were unlikely to affect the furaptra fluorescence signal at the peak of the Ca2+]i transient. The Ca2+]i transient reported by furaptra appears to be larger than that reported by other Ca2+ indicators; however, we conclude this larger transient is at least as accurate as Ca2+]i transients reported by the other indicators.