title: Rapamycin and FK506 reduce skeletal muscle voltage sensor expression and function.
abstract: FK506 and rapamycin are immunosuppressant drugs that disrupt the interaction of FK506-binding proteins (FKBPs) with ryanodine receptors (RyR1), which form homotetrameric Ca2+ release channels in the sarcoplasmic reticulum (SR) of skeletal muscle. Here, we characterized the effects of short-term treatment (2 h) of skeletal myotubes with either 20 microM FK506 or 20 microM rapamycin on excitation-contraction (EC) coupling, sarcolemmal dihydropyridine receptor (DHPR) function, resting intracellular Ca2+, and levels of SR Ca2+ content. Both rapamycin and FK506 produced remarkably similar effects. Specifically, both drugs reduced the maximal amplitude of voltage-gated SR Ca2+ release ((DeltaF/F)max) by 70-75% in parallel with a 50% reduction in both maximal immobilization resistant charge movement (Qmax) and L-type Ca2+ channel conductance (Gmax). Neither immunosupressant significantly altered steady-state levels of either resting myoplasmic Ca2+ or SR Ca2+ content. Thus, store depletion does not account for the observed reduction in Ca2+ release during EC coupling. Instead, the inhibitory effect on voltage-gated SR Ca2+ release is explained by significant reductions in both the number of functional sarcolemmal voltage sensors and the intrinsic gain of voltage-gated Ca2+ release (i.e. the maximal rate of Ca2+ release per unit gating charge).
authors.affiliation: Department of Biochemistry, Cinvestav-IPN, AP 14-740. Mexico City, DF 07000, Mexico.
authors.forename: Guillermo
authors.initials: G
authors.lastname: Avila
authors.forename: Robert T
authors.initials: RT
authors.lastname: Dirksen
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D004594:Electrophysiology; D007166:Immunosuppressive Agents / Q000494:pharmacology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D019837:Ryanodine Receptor Calcium Release Channel / Q000737:chemistry / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D020123:Sirolimus / Q000494:pharmacology*; D016559:Tacrolimus / Q000494:pharmacology*
chemical_list: D020746:Calcium Channels, L-Type; D007166:Immunosuppressive Agents; D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium; D020123:Sirolimus; D016559:Tacrolimus