Dissociation of phosphorylase a activation and contractile activity in rat portal vein

Abstract

Isometric force, glycogen phosphorylase activity and lactate production were measured under conditions known to alter intracellular Ca²⁺ and cAMP to assess the role of these messengers in the coordination of metabolism with contractility in rat portal vein. Total phosphorylase (a+b) activity, was independent of treatment. The activity ratio phosphorylase activity ratio in the presence of isoproterenol and papaverine was dependent on or high‐K⁺ medium, and 0.57 after 20 min treatment with 10–⁵ M isoproterenol+10^‐4 M papaverine. Under both of these conditions the muscle was totally relaxed. The phosphorylase activity ratio in the presence of isoproterenol and papaverine was dependent on extracellular Ca²⁺, both in normal and depolarizing medium. This suggests a lower Ca²⁺ sensitivity of the contractile than the phosphorylase system under these conditions, known to be associated with raised intracellular cAMP. During spontaneous activity and high‐K⁺ induced contractures phosphorylase activity was increased compared to the relaxed state in Ca²⁺‐free medium. A high level of phosphorylase activity (0.48) was elicited by the addition of 300 mM sucrose, which induces a contracture in Ca²⁺‐free medium. Lactate production was in general parallel to phosphorylase activity, except for a relative increase in anoxia. The results suggest that in the intact cell the Ca²⁺‐mediated linkage of contraction and phosphorylase may be modified by cAMP changing the Ca²⁺ sensitivities of the two systems in opposite directions.