Ca2+ channel blockers distinguish between G protein-coupled pharmacomechanical Ca2+ release and Ca2+ sensitization.
The effects of Ca2+ channel blockers on two modes of G protein-mediated pharmacomechanical coupling, Ca2+ release and modulation of Ca2+ sensitivity of the contractile apparatus, were investigated. Smooth muscles were permeabilized with Staphylococcal alpha-toxin or with beta-escin to avoid effects due to block of sarcolemmal Ca2+ channels, while retaining receptor/G protein coupling. In permeabilized portal vein smooth muscle, verapamil and nifedipine inhibited Ca2+ release induced by an alpha 1-adrenergic agonist (phenylephrine) and by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), but not that induced by inositol 1,4,5-trisphosphate (InsP3). These Ca2+ channel blockers also did not block the phenylephrine- or GTP gamma S-induced force development at constant cytoplasmic Ca2+ ("Ca2+ sensitization"). An alpha 1-blocker (prazosin) inhibited both the Ca2(+)-releasing and Ca2(+)-sensitizing effects of phenylephrine, but not those of GTP gamma S, nor did it block InsP3-induced Ca2+ release. We conclude that Ca2+ channel blockers selectively uncouple the Ca2(+)-releasing, but not the Ca2(+)-sensitizing, component of pharmacomechanical coupling. These findings raise the possibility that pharmacomechanical Ca2+ release may be modulated by dihydropyridine binding proteins at the level of G proteins/phospholipase C and also indicate a divergence of the Ca2(+)-releasing and Ca2(+)-sensitizing effects at some step prior to phospholipase C.
Kobayashi
S
S
Department of Physiology, University of Virginia School of Medicine, Charlottesville 22908.
Gong
M C
MC
Somlyo
A V
AV
Somlyo
A P
AP
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D004533:Egtazic Acid / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000502:physiology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007700:Kinetics; D009119:Muscle Contraction / Q000187:drug effects*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011169:Portal Vein / Q000187:drug effects / Q000502:physiology; D011224:Prazosin / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology*
D002121:Calcium Channel Blockers; D010656:Phenylephrine; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000001:Calcimycin; D004533:Egtazic Acid; D006160:Guanosine Triphosphate; D014700:Verapamil; D019204:GTP-Binding Proteins; D009543:Nifedipine; D002118:Calcium; D011224:Prazosin