Takashi Akata (1,2,#) and Walter A. Boyle III (1)
(1) Departments of Anesthesiology, Molecular Biology and Pharmacology,
Washington University School of Medicine, St. Louis, MO 63110-1093, U.S.A.
(2) Department of Anesthesiology and Critical Care Medicine, Faculty of
Medicine, Kyushu University, Fukuoka 812-82, Japan
(#) Present address for correspondence: Department of Anesthesiology and
Critical Care Medicine, Kyushu University Hospital, Fukuoka 812-82, Japan
Abstract: Isometric tension was measured to investigate the effects
of guanosine-5'-triphosphate (GTP) on the run-down of myofilament Ca2+ sensitivity
in isolated rat mesenteric arteries permeabilized with beta-escin. The Ca2+
sensitivity assessed by the EC50 value for the Ca2+ (0.1 - 100 microM)-tension
relationship progressively runs down in the control strips, while it was
well-preserved for 5-successive Ca2+ applications in the presence of GTP
(50 microM); no significant difference was found in the Ca2+ sensitivity
observed with the 1st Ca2+ application between the control and GTP-treated
strips. Guanosine-5'-(2-O-thio) diphosphate (GDPbetaS, 100 microM) significantly
decreased the Ca2+ sensitivity with the 1st Ca2+ application and eliminated
the run-down of Ca2+ sensitivity. GTP (3 - 150 microM), applied to the strips
submaximally precontracted with Ca2+, had a little effect on the Ca2+ contractions
in the early stage of experiments, but dramatically enhanced the Ca2+ contractions
in their later stage; its latter effect was mimicked by guanosine-5'-(3-O-thio)
triphosphate (GTPgammaS) and reversed by GDPbetaS (100 microM). The results
suggest: 1) loss of endogenous GTP following permeabilization is involved
in the run-down of Ca2+ sensitivity; and 2) activation of G-proteins is
involved in Ca2+-activation of contractile proteins.
Keywords: Guanosine-5'-triphosphate (GTP), GTP-binding protein (G-protein),
Myofilament Ca2+ sensitivity, Membrane-permeabilization, Vascular smooth
muscle