Kinetic studies of protein kinase A in rat liver during late sepsis

The covalent modification of receptor proteins via phosphorylation and dephosphorylation is one of the principal mechanisms controlling carbohydrate metabolism and is known to be regulated by various protein kinases. Recent studies indicated that many hormones may exert their effects on cellular metabolism by regulating intracellular c-AMP levels and by activating a c-AMP dependent protein kinase, i.e., protein kinase A. The metabolic disturbances during sepsis are characterized by an initial hyperglycemia followed by a progressive hypoglycemia and a depletion of hepatic glycogen content. The latter is coupled with a slowdown in glycogenesis, an accelerated glycogenolysis, and a depression in gluconeogenesis in the liver. Since the liver is the major organ that regulates the homeostatic level of blood glucose, it is conceivable that the sepsis-induced glucose dyshomeostasis might be mediated by changes in protein kinase activity and the kinetic characteristics of enzymes. The present experiment was designed to study the correlation between protein kinase A and the pathophysiology of hepatic glucose dyshomeostasis during sepsis. Sepsis was induced in rats by cecal ligation and puncture (CLP). Late sepsis occurred 18 hours after CLP. Protein kinase A was extracted from the rat livers by acid precipitation and ammonium sulfate fractionation, and then partially purified by DEAE-cellulose. The results show that in the late sepsis, type-I protein kinase A (eluted at low ionic strength) activity was significantly decreased by 34-52% (P < 0.01). The kinetic parameters such as Vmax's for ATP, histone, and c-AMP were also significantly decreased from the control values of 6.1 +/- 0.9, 5.4 +/- 0.8, and 5.1 +/- 1.9 nmoles/mg.min. to 3.6 +/- 0.5, 2.8 +/- 0.3, and 2.5 +/- 0.5 nmoles/mg.min., respectively. Analysis using Hill's equation indicates that the S0.5 and n (Hill coefficient) values of the various substrates and activators for type-I protein kinase A remained unchanged. In the case of type-II protein kinase A (eluted at high ionic strength), the Vmax, S0.5, and n values for ATP, histone, and c-AMP were unchanged during late sepsis. The results of the present study indicate that the activities and kinetic characteristics of type I protein kinase A in rat liver are modified during late sepsis. Since protein kinase A is known to regulate glucose metabolism through adrenergic receptor mediation, these findings may have a pathophysiological significance in the understanding of hepatic glucose dyshomeostasis during sepsis.(ABSTRACT TRUNCATED AT 400 WORDS)