Regulation of glycogen utilization, but not glucose utilization, by precontraction glycogen levels in vascular smooth...

These experiments were designed to determine whether glycogenolysis was influenced by the glycogen concentration of vascular smooth muscle. Segments of hog carotid artery smooth muscle were allowed to synthesize variable amounts of 1-[13C]glucosyl units of glycogen. Artery segments were then isometrically contracted in the presence of 2-[13C]glucose. Prior to and after isometric contraction, measurements were made of tissue glycogen content and superfusate glucose and lactate concentrations. 2-[13C]Lactate and 3-[13C]lactate peak intensities in the superfusate were measured using 13C-NMR spectroscopy. The tissue glycogen content decreased exponentially during the 4.5 h of isometric contraction (R2 = 0.990), despite more than a 3-fold range of glycogen concentration prior to contraction. The extent of glycogen utilization during a 3 h isometric contraction varied linearly with the precontraction glycogen concentration (R2 = 0.727). Lactate production specifically from glycogen breakdown increased with an increase in precontraction glycogen concentration (R2 = 0.620). During a 3 h isometric contraction neither the glucose utilization (R2 = 0.007) nor lactate production specifically produced from glucose (R2 = 0.00002) varied with the precontraction glycogen concentration. It is concluded that the rate of glycogenolysis is determined by the content of glycogen during prolonged contractions. In addition, precontraction glycogen levels influence the pathway for glycogen utilization but not the pathway for glucose utilization. Therefore, glycolysis and glycogenolysis behave independently in vascular smooth muscle.