G protein modulates thyroid hormone-induced Na+ channel activation in ventricular myocytes

Division of Cardiology, Department of Medicine, The David Geffen School of Medicine, University of California, Los Angeles, California 90095 To evaluate the effects of liothyronine (3,5,3'-triiodo- L -thyronine, T 3 ) on Na + channel current ( I Na ) properties, I Na was recorded in adult guinea pig ventricular myocytes. T 3 (1 nM) acutely increased whole cell I Na and shifted the steady-state I Na inactivation curve dose dependently. When the pipette solution contained 100 µM GTP or GTP S, the effect of T 3 on the whole cell I Na was increased two- to threefold. This effect was almost completely abolished by pertussis toxin preincubation. In the cell-attached patch, T 3 increased the open probability of single I Na by reducing the null probability. In the inside-out patch, T 3 effect was 10 times faster than that in whole cell and cell-attached patches while GTP S was present and could be completely washed out. T 3 alone slightly increased the channel open probability by increasing the closed state to open state rate constant ( k CO ) and reducing the null probability. GTP S exposure only increased the number of functional channels. T 3 and GTP S synergistically enhanced the channel open probability 5.8 ± 0.5-fold by increasing k CO , decreasing the open state to absorbing inactivated state rate constant, and greatly reducing the null probability. These results demonstrate that T 3 acts on the cytosolic side of the membrane and acutely activates I Na . Pertussis toxin-sensitive G protein modulation greatly magnifies the T 3 effects on the channel kinetics and null probability, thereby increasing the channel open probability. T 3 ; pertussis toxin-sensitive G protein; whole cell sodium channel current; single channel current; cardiac myocytes