Phosphoinositide metabolism and metabolism-contraction coupling in rabbit aorta.
We tested a hypothesis that metabolism-contraction coupling in vascular smooth muscle is controlled by the rate of delivery of energy to ATP-dependent reactions in the inositol phospholipid transduction system that generate second messengers exerting control on smooth muscle force. Rabbit aorta was contracted by norepinephrine (NOR) under conditions of normoxia and hypoxia (bath PO2 less than 40 mmHg), and changes in inositol phospholipid pool sizes and metabolic flux rates (JF) were determined. JF was determined by labeling free cytosolic myo-inositol by incubation of unstimulated muscle with myo-[3H]inositol and then measuring rates of incorporation of this isotope into inositol phospholipids and inositol phosphates when the muscle was activated by NOR. JF measured during maintenance of NOR-induced force was markedly inhibited during hypoxia to 40-50% of that determined during normoxia; rates of increases in inositol phosphate radioactivities were similarly depressed during NOR activation under hypoxia. The hypoxia-induced decrease in JF was associated with four- to fivefold increase in phosphatidylinositol 4-phosphate (PIP) total pool size, suggesting PIP kinase was inhibited and rate limiting. Total pool sizes of phosphatidylinositol, phosphatidylinositol bisphosphate, and phosphatidic acid were unchanged from values seen during activation under normoxia. These data suggest that activation of inositol phospholipid metabolism, which generates inositol 1,4,5-trisphosphate (IP3) and diacylglycerol, is blunted under conditions where aerobic energy production is inhibited. Data are consistent with "rate-limiting" effects of decreased ATP delivery, or decreased phosphate potential, on PIP kinase and reactions that control resynthesis of phosphatidylinositol.