Inorganic phosphate regulates the contraction‐relaxation cycle in skinned muscles of the rabbit mesenteric artery.

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Abstract
1. The effects were investigated of inorganic phosphate (Pi) on the Ca2+-dependent and Ca2+-independent contractions evoked in chemically skinned smooth muscles of the rabbit mesenteric artery. 2. The relation between the concentration of Ca2+ and tension showed a sigmoidal curve in the range of pCa 7-5.5. Pi (over 1 mM) inhibited the Ca2+-induced contraction, shifted the pCa-tension curve to the right and increased the Hill number from 2 to 3. Calmodulin did not change the Hill number and attenuated the inhibitory action of Pi as estimated from the shift of the curve, but this agent did not modify the increased Hill number in the presence of Pi. 3. Pi consistently inhibited the Ca2+-independent contractions provoked by application of trypsin-treated myosin light chain kinase, of MgATP following adenosine-5''-o-(3-thiotriphosphate) (ATP.gamma.S) and Ca2+, and of a solution containing high Mg2+. These inhibitory actions of Pi were inversely proportional to the amplitude of the contraction. When Pi was applied simultaneously with ATP.gamma.S and Ca2+, there was no change in the amplitude of Ca2+-independent contractions provoked by the application of MgATP. 4. The amplitude of the rigor contraction evoked by ATP-free solution was less than 7% of that of the 10 .mu.M-Ca2+-induced contraction. When ATP was removed from the solution during the Ca2+ contraction, the rigor contraction was also generated. Pi did not inhibit either type of contraction. 5. With a decrease in the concentration of Ca2+ from 10 .mu.M to below 1 nM, the tissue relaxed at a slower rate than the rate of rise of the Ca2+-induced contraction. The slow relaxation was not modified by a change in the concentration of EGTA or addition of 1 .mu.M-calmodulin. 6. Pi reduced, and high Mg2+ prolonged the time required for the relaxation. This action of Pi was not prevented in the presence of calmodulin or of high Mg2+. 7. The rates of rise and fall of the Ca2+-induced contraction depended on the concentration of MgATP, and Pi consistently inhibited the Ca2+-induced contraction in the presence of any given concentration of MgATP. 8. We conclude that Pi may accelerate the detachment of cross-bridges between the contractile proteins. Thus, the amplitude of Ca2+-induced contraction is slightly inhibited and the relaxation is markedly facilitated. However, the site of action of Pi may differ from that of MgATP. The action of Pi is also discussed in relation to the latch mechanism, i.e. Pi may prevent the latch due to acceleration of the detachment of cross-bridges, but it may not directly contribute to the latch mechanism, itself.