Tri-iodothyronine increases intracellular calcium levels in single rat myocytes

We have studied the effects of acute administration of tri-iodothyronine (T3) on cytosolic free calciumlevels [Ca2+]i in single rat myocytes microinjected with aequorin. Ventricular myocytes were isolated by perfusing rat hearts with collagenase, and healthy, rod-shaped cells were injected to <1 % of their volume with aequorin. The photons emitted from single cells were measured and a conversion to [Ca2+]i made on thebasis of an in vitro calibration after the remaining aequorin had been discharged by cell lysis. Only cells that depolarized reversibly (showing elevated[Ca2+]i levels) when superfused with 80mM KC1, and which gave a substantial signal on lysis with distilled water were used. The [Ca2+]i rose from a resting value of 150\m=+-\56nM(mean \m=+-\SD, n=14) by 127\m=+-\47nMon depolarization with 80mM KC1. Application of T3 (1-100nM) led to an increase (P<0.05) in [Ca2+]i(mean amplitude of 152\m=+-\35nM)before returning to baseline. The median duration of these events was 10 min (range = 1.4-34.4 min). The time to response was shorter when lOOnM T3 was applied (median and range;6.8, 0-14 min) than when 1nM T3 was used (16, 7.0\x=req-\ 56.1 min) (P<0.05). To conclude, physiological concentrations of thyroid hormones caused rapid but transient stimulation of [Ca2+]i in single rat myocytes