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.