GTP-GAMMA-S REMOVAL OF D-600 BLOCK OF SKELETAL-MUSCLE EXCITATION-CONTRACTION COUPLING

G proteins interacting with dihydropyridinee receptors (DHPR) in trans verse tubules (TT) of skeletal muscle may have a role in skeletal exci tation-contraction (EC) coupling. The aim of this study was to determi ne the effects of G protein-specific nucleotides [guanosine 5'-O-(3-th iotriphosphate) (GDP beta S) and guanosine 5'-O-(2-thiodiphosphate) (G DP beta S)] on the EC coupling mechanism in the presence of D-600, an agent that blocks EC coupling by immobilizing the voltage-sensing subu nit of the DHPR in its inactivated state. By use of the mechanically p eeled single-fiber preparation from rabbit adductor magnus skeletal mu scle, 50 mu M GTP gamma S and 500 mu M GDP beta S were applied with th e fiber in a D-600-induced state of blocked EC coupling. Neither nucle otide served as an independent stimulus for sarcoplasmic reticulum (SR ) Ca2+ release when added to the TT polarizing bath under conditions o f D-600 block. The presence of GTP gamma S or GDP beta S during a comp lete EC coupling cycle removed the D-600 block of EC coupling, despite continuous bath D-600. After the nucleotides were washed out, in the continued presence of D-600, the D-600 block of EC coupling was reesta blished. In contrast, GTP gamma S added only during the period of TT d epolarization under D-600 block did not remove the D-600 block of EC c oupling, even though GTP gamma S did stimulate SR Ca2+ release. GTP ga mma S had no effect on submaximum (0.5-1.0 mM) caffeine contractures a nd thus is unlikely to be acting through the Ca2+-induced Ca2+ release mechanism of tile SR. These data suggest that the molecular binding s ite for GTP gamma S and GDP beta S is likely to be in the TT near the DHPR, perhaps on a G protein.