Effects of FK-506 on Contraction and Ca 2+ Transients in Rat Cardiac Myocytes

Abstract

FK-506 binding protein (FKBP) has been reported to be closely associated with the ryanodine receptor in skeletal and cardiac muscle and to modulate sarcoplasmic reticulum (SR) Ca 2+ release channel gating in isolated channels. FK-506 can inhibit the activity of FKBP, thereby reversing its effects on SR Ca 2+ release. We investigated the function of FKBP during normal contractions and Ca 2+ transients in intact rat ventricular myocytes loaded with fluorescent Ca 2+ indicators. FK-506 significantly increased steady state twitch Ca 2+ transients and contraction amplitudes even under conditions in which the SR Ca 2+ load and Ca 2+ current were unaltered, suggesting that FK-506 increases the fraction of SR Ca 2+ released during excitation-contraction (E-C) coupling. Action potentials were somewhat prolonged, consistent with the larger Ca 2+ transients causing greater inward Na + -Ca 2+ exchange current. FK-506 did not affect SR Ca 2+ uptake but modestly decreased Ca 2+ extrusion via Na + -Ca 2+ exchange in intact cells (although no effect on Na + -Ca 2+ exchange was seen in sarcolemmal vesicles). In most cells, FK-506 caused an increase in SR Ca 2+ content during steady state stimulation, as assessed by caffeine-induced contractures. This was probably due to the inhibition of Ca 2+ efflux via Na + -Ca 2+ exchange. FK-506 also accelerated the rest decay of SR Ca 2+ content and increased the frequency of resting Ca 2+ sparks about fourfold. The increase in frequency of these basic Ca 2+ release events was not associated with changes in the amplitude or duration of the Ca 2+ sparks. We conclude that FK-506 increases the fraction of SR Ca 2+ released during normal twitches and enhances the rate of SR Ca 2+ release during rest. FK-506 also inhibits Na + -Ca 2+ exchange, although this effect may be indirect. These effects are consistent with an important SR-stabilizing effect of FKBP in intact rat ventricular myocytes.