Calibration of indo-1 and resting intracellular [Ca]i in intact rabbit cardiac myocytes.
Fluorescent Ca indicators have been extremely valuable in understanding intracellular [Ca] ([Ca]i) regulation in many cell types. The calibration of these indicators in the intracellular environment, however, has been a continuous challenge. We performed in vivo calibrations of indo-1 in isolated rabbit ventricular myocytes loaded with the acetoxymethylester form of indo-1 and used the perforated patch variation of whole cell voltage clamp. Voltage, [Na], and [K] gradients were eliminated to approach equilibrium. We also took advantage of the powerful Na/Ca exchange in cardiac myocytes so that [Ca]i would be equilibrated with [Ca]o (because there was no [Na] or voltage gradient). The equilibration of [Na] and [Ca] across the membrane was tested by measuring the reversal potential of Na current and poking the cell to test for changes in [Ca]i-dependent fluorescence ratio. The apparent dissociation constant, Kd for indo-1 in the cellular environment was 844 nM, which is approximately 2-3 times higher than that in aqueous solutions. In a separate series of experiments, a null point approach was used to determine the [Ca]i in intact cells at rest for very long periods (82 +/- 6 nM). This is lower than that measured 15 s after a train of steady-state twitches ([Ca]i = 294 +/- 53 nM). These experiments also allowed the direct assessment of the shortening versus [Ca]i relationship in intact cells.
Bassani
J W
JW
Department of Physiology, Loyola University School of Medicine, Maywood, Illinois 60153, USA.
Bassani
R A
RA
Bers
D M
DM
D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism*; D005456:Fluorescent Dyes; D066298:In Vitro Techniques; D007211:Indoles; D007424:Intracellular Fluid / Q000378:metabolism; D007700:Kinetics; D008297:Male; D009200:Myocardial Contraction / Q000502:physiology; D009206:Myocardium / Q000378:metabolism*; D011817:Rabbits
D005456:Fluorescent Dyes; D007211:Indoles; C048960:indo-1; D002118:Calcium