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 beta-escin. The Ca2+ sensitivity assessed by the EC50 value for the Ca2+ (0.1-100 mu M)-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 mu M); no significant difference was found in the Ca2+ sensitivity observed with the Ist Ca2+ application between the control and GTP-treated strips. Guanosine-5'-(2-O-thio) diphosphate (GDP beta S, 100 mu M) significantly decreased the Ca2+ sensitivity with the Ist Ca2+ application and eliminated the run-down of Ca2+ sensitivity. GTP (3-150 mu M), 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 (GTP gamma S) and reversed by GDP beta S (100 mu M). 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.