Effect of the immunosupressant FK506 on excitation-contraction coupling and outward K+ currents in rat ventricular myocytes.
1. We examined the effects of the immunosupressant drug FK506 on excitation-contraction coupling in isolated rat ventricular myocytes. [Ca2+]i transients were recorded using the intracellular Ca2+ indicators fluo-3 and indo-1 while action potentials (APs) or membrane currents were recorded using patch-type microelectrodes in the whole cell mode. 2. FK506 (25 microM) rapidly and reversibly increased the magnitude of the [Ca2+]i transient in intact cells without changing resting [Ca2+]i or the kinetics of the [Ca2+]i transient, a finding consistent with previous reports that investigated the actions of FK506 on the sarcoplasmic reticulum Ca2+ release channel. 3. The 36% increase in the [Ca2+]i transient produced by FK506 was accompanied by a 293% increase in AP duration (by 293%). Importantly, the addition of FK506 had no effect on the [Ca2+]i transient when the depolarizing duration was controlled in voltage clamp experiments. The increased AP duration could be explained by a marked inward shift in the net membrane current that was observed in these experiments. 4. The net inward current change was not directly responsible for a change in Ca2+ influx, since no change in L-type Ca2+ current (ICa) was observed. Instead, FK506 inhibited both the transient outward K+ current (Ito) and the delayed rectifier K+ current (IK). 5. We conclude that FK506 increases the [Ca2+]i transient during normal contractions by an indirect action: it prolongs the action potential. This action does not appear to depend on the established action of FK506 on the ryanodine receptor. Instead, the inhibition of outward K+ currents prolongs the AP which secondarily increases Ca2+ influx and/or decreases Ca2+ efflux.
duBell
W H
WH
Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore 21201, USA.
Wright
P A
PA
Lederer
W J
WJ
Rogers
T B
TB
D000200:Action Potentials / Q000187:drug effects; D000814:Aniline Compounds; D000818:Animals; D002118:Calcium / Q000378:metabolism; D005456:Fluorescent Dyes; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects; D007166:Immunosuppressive Agents / Q000494:pharmacology*; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008856:Microscopy, Fluorescence; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016559:Tacrolimus / Q000494:pharmacology*; D016276:Ventricular Function; D014966:Xanthenes
D000814:Aniline Compounds; D005456:Fluorescent Dyes; D007166:Immunosuppressive Agents; D014966:Xanthenes; C059715:Fluo-3; D011188:Potassium; D002118:Calcium; D016559:Tacrolimus