INVESTIGATION INTO VANADATE-INDUCED POTENTIATION OF SMOOTH-MUSCLE CONTRACTILITY IN THE RABBIT ISOLATED ILEUM

The action of sodium orthovanadate (Na3VO4) on spontaneous mechanical activity of the longitudinal muscle was investigated in isolated segme nts of rabbit distal ileum. Vanadate (0.3-1000 muM) concentration-depe ndently enhanced the amplitude of phasic contractions (pendular moveme nts) and caused the muscle tone to slightly increase at the highest co ncentrations. Both these effects were mimicked by the Ca2+ channel act ivator BAY K 8644 (10-1000 nM). Vanadate- and BAY K 8644-induced poten tiation of mechanical activity was antagonized by the Ca2+ entry block er nifedipine (3 nM). In Ca2+-free, K+-depolarized preparations, vanad ate (100 muM) failed to contract the musculature, but potentiated the contractile response to applied calcium (CaCl2: 30-300 muM). The actio n of vanadate was similar to that of BAY K 8644 (3 nM) and was antagon ized by nifedipine (0.1 nM). These results suggest that extracellular calcium is required for vanadate-induced smooth muscle excitation whic h, at least in part, appears to arise from facilitation of calcium inf lux through voltage-dependent Ca2+ channels.