Is Guanosine-5'-triphosphate involved in calcium-activation of contractile proteins in vascular smooth muscle?

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 β-escin. The Ca²⁺ sensitivity assessed by the EC₅₀ value for the Ca²⁺ (0.1-100 μM)-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 μM); 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 (GDPβS, 100 μM) significantly decreased the Ca²⁺ sensitivity with the 1st Ca²⁺ application and eliminated the run-down of Ca²⁺ sensitivity. GTP (3-150 μM), 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 (GTPγS) and reversed by GDPβS (100 μM). 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.