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Actions of the novel antidiabetic agent englitazone in rat hepatocytes
P F Blackmore1, R K McPherson, R W Stevenson
1Department of Pharmacology, Eastern Virginia Medical School, Norfolk.
Abstract:
We examined effects of a novel antidiabetic agent, racemic englitazone (CP 68,722, Pfizer), on normal rat hepatocytes in vitro. For optimal effects, CP 68,722 must be preincubated for approximately 20 minutes. CP 68,722 inhibited the actions of glucagon on glycogenolysis (measured by monitoring cyclic adenosine monophosphate [cAMP] levels, phosphorylase activation, and glucose output) and gluconeogenesis (from 14C-lactate). Since CP 68,722 was able to attenuate the ability of glucagon to increase cAMP levels, this may account for part of its inhibitory actions on glycogenolysis and gluconeogenesis. The observation that CP 68,722 also inhibits the ability of the cAMP analog, 8-(4-chlorophenylthio)-adenosine 3':5'-cyclic monophosphate (8 CPT cAMP), to stimulate phosphorylase a is consistent with an effect of CP 68,722 to activate cAMP-dependent phosphodiesterase. The ability of vasopressin (an agonist known to stimulate glycogenolysis via a Ca(2+)-dependent mechanism) to stimulate phosphorylase a was slightly inhibited by CP 68,722. Another site of action of CP 68,722 was to inhibit hormonal-mediated Ca2+ influx, an effect that would decrease intracellular free calcium ([Ca2+]i), thereby inhibiting the actions of the Ca(2+)-dependent hormones such as alpha 1-adrenergic agonists and vasopressin, agents known to promote glucose output from the liver. In summary, CP 68,722 inhibits glucagon-stimulated glycogenolysis and gluconeogenesis in hepatocytes by a mechanism that may include activation of cAMP phosphodiesterase and inhibition of Ca2+ influx.
Insights
Racemic englitazone (CP 68,722) inhibits key liver processes for glucose production. This novel antidiabetic agent impacts glycogenolysis and gluconeogenesis by affecting cyclic adenosine monophosphate (cAMP) and calcium signaling pathways.
Area of Science:
- Hepatology
- Endocrinology
- Pharmacology
Background:
- Hepatocytes play a crucial role in glucose homeostasis.
- Understanding the mechanisms of antidiabetic agents is vital for managing diabetes.
- Glucagon and calcium signaling pathways regulate hepatic glucose production.
Purpose of the Study:
- To investigate the in vitro effects of racemic englitazone (CP 68,722) on normal rat hepatocytes.
- To elucidate the molecular mechanisms by which CP 68,722 affects glucose metabolism.
- To determine the potential of CP 68,722 as a novel antidiabetic agent.
Main Methods:
- Primary rat hepatocytes were used for in vitro experiments.
- Measurement of cyclic adenosine monophosphate (cAMP) levels to assess glucagon signaling.
- Assays for glycogenolysis (glucose output, phosphorylase activation) and gluconeogenesis from lactate.
- Evaluation of CP 68,722's effect on cAMP analogs and calcium influx.
Main Results:
- CP 68,722 demonstrated optimal effects after a 20-minute preincubation period.
- The agent inhibited glucagon-stimulated glycogenolysis and gluconeogenesis.
- CP 68,722 attenuated glucagon-induced increases in cAMP levels.
- Evidence suggests CP 68,722 activates cAMP-dependent phosphodiesterase and inhibits Ca2+ influx.
- Slight inhibition of vasopressin-stimulated glycogenolysis was observed.
Conclusions:
- Racemic englitazone (CP 68,722) effectively inhibits glucagon-mediated glycogenolysis and gluconeogenesis in rat hepatocytes.
- The antidiabetic effects may stem from activating cAMP phosphodiesterase and inhibiting calcium influx.
- CP 68,722 presents a potential therapeutic strategy for diabetes by modulating hepatic glucose production.