Possible involvement of actomyosin ADP complex in regulation of Ca2+ sensitivity in alpha-toxin permeabilized smooth muscle

The effects of substrate condition and ADP beta S on the pCa2+-tension relationships were investigated, using alpha-toxin permeabilized rabbit mesenteric artery at 37 degrees C. The contraction induced by 10 microM Ca2+ solution after permeabilization was as large as that induced by 145 mM K+ PSS solution containing 10 microM NE in the intact tissue, indicating that the majority of the cells were permeabilized. The Ca2+ sensitivity was greatly affected by the substrate condition and increasing the ratio of ATP/CP induced a leftward shift of the pCa2+-tension curve. Addition of 100 microM ADP beta S had a similar effect. When the ATP/CP ratio was high, the 0.1 microM Ca2+ solution relaxed the tissue precontracted by 10 microM Ca2+ solution more slowly showing hysteresis. One mM vanadate, which is reported to relax muscle by forming actomyosin-ADP-Vi (AM-ADP-Vi), completely inhibited both contractions induced by 0.18 microM Ca2+ solution containing 2 mM MgADP and 0.3 microM Ca2+ solution containing 0.3 microM PDBu. These results indicated that the population of AM-ADP complex in the crossbridge had increased due to the accumulation of ADP inside the tissue or activation of PKC and that the inhibition of ADP release from AM-ADP complex may be playing a key role in increasing Ca2+ sensitivity of myofilaments.