DEPENDENCE OF ATP CONSUMPTION ON CROSS-BRIDGE PHOSPHORYLATION IN SWINE CAROTID SMOOTH-MUSCLE

1. Ca2+-dependent phosphorylation of the myosin regulatory light chain (MRLC) initiates cross-bridge cycling and contraction in smooth muscl e. A four-state cross-bridge model, in which Ca2+-dependent phosphoryl ation is the only proposed regulatory mechanism, can predict the mecha nical output of the swine carotid media. Our aims were to determine wh ether ATP consumption rates and the economy of force maintenance are r egulated functions of MRLC phosphorylation as predicted by the model. 2. Steady-state force and oxygen consumption were measured in medial r ings of swine carotid arteries activated with depolarizing solutions a nd agents capable of maintaining a wide range of steady-state myoplasm ic Ca2+ and MRLC phosphorylation levels. 3. Suprabasal ATP consumption increased almost linearly with MRLC phosphorylation and exhibited a h yperbolic increase with active stress, as predicted. 4. The economy of stress maintenance fell with increases in suprabasal phosphorylation. 5. In absolute terms the energetic cost of covalent regulation by cro ss-bridge phosphorylation was small, although it may be a significant fraction of the ATP consumption associated with contraction.