Enhancement Effect under High-Glucose Conditions on U46619-Induced Spontaneous Phasic Contraction in Mouse Portal Vein

Abstract

The effect of the thromboxane A 2 analog 9,11-dideoxy-11␣, 9␣-epoxymethanoprostaglandin F 2␣ (U46619) on spontaneous phasic contractions in the mouse portal vein was studied. U46619 induced concentration-dependent (1-100 nM) increases in amplitude, frequency, and contractile period (ONtime) of the contraction. Both amplitude and ON-time were enhanced significantly under high-glucose (HG; 4-fold greater than normal) conditions. This hyperactivation may be associated with portal vein dysfunction in diabetes. However, the mechanisms remain unclear. HG enhanced the U46619-induced accumulation of endogenous diacylglycerol (DG). Phospholipase C inhibition suppressed accumulation under normal conditions; however, this suppression was not observed under HG conditions. The HG-induced enhancement of U46619-induced contraction was inhibited by protein kinase C (PKC)inhibition. This finding indicated that accumulated DG might increase PKC activity. Activated PKC stimulated DG kinase activation as a feedback mechanism. DG kinase inhibition also suppressed the HG-induced enhancement of contraction. Increased myo-inositol incorporation was detected under HG conditions, indicating an acceleration of phosphatidylinositol (PI) turnover. This acceleration was inhibited by PKC and DG kinase inhibitors. These findings indicated that HG treatments increased DG synthesis derived from incorporated glucose, PKC, and DG kinase activation. These responses induce hyperactivation of the amplitude and contractile period of contraction mediated by acceleration of PI turnover. This series of responses may be involved in the dysfunction of the portal vein under the HG conditions occurring with diabetes.The portal vein functions as the main transfer vessel from the digestive organs to the liver. To effect highly efficient transfer, the portal vein exhibits spontaneous, intermittent rhythmic contractions (Miwa et al., 1997). Dysfunction of this tissue leads not only to reduction in blood supply to the liver but also to serious diseases such as varix mediated by reflux of blood (MacMathuna, 1992). An understanding of the basic mechanisms modulating these spontaneous contractions is a prerequisite for investigation of potential pathophysiological processes. Calcium and potassium channel activity is associated with spontaneous contraction (Loirand et al., 1990;Helliwell and Large, 1997). Regulation of calcium channels is believed to occur via plasma membrane potential and/or intracellular signaling systems. However, the mechanisms involved in increased mechanical activity mediated by receptor stimulation and their related intracellular factors are poorly understood.We have previously reported that receptor-mediated contractile responses in vascular smooth muscle are altered in diabetes and the mechanisms involved change in phosphatidylinositol (PI) turnover (Nobe et al., 2002). Phosphatidylinositols, diacylglycerol (DG), protein kinase C (PKC), and DG kinase are key elements of PI turnover. Increased contractility in many smooth muscle cell ...

Glucose-Dependent Enhancement of Spontaneous Phasic Contraction Is Suppressed in Diabetic Mouse Portal Vein: Association with Diacylglycerol-Protein Kinase C Pathway

“…Under high glucose conditions, 100 nM U46619 induced a maximal peak value of 4.38 Ϯ 0.34 mN (n ϭ 7), and the EC 50 value was 3.3 nM (n ϭ 7). Responses induced by U46619 under normal and high glucose conditions in C57Bl mice were similar to those described previously in the ddY mouse PV (Nobe et al, 2003).…”

“…Significant enhancement of U46619-induced contractions under high glucose conditions in C57Bl mouse PV (Fig. 1A) was similar to that observed in our previous study (Nobe et al, 2003). The patterns of increases in amplitude and ON-time and lack of effect on frequency were well correlated with the response in ddY mouse (Nobe et al, 2003).…”

“…PVs were mounted on a hook attached to an isometric force transducer (NEC San-ei Instruments Ltd., Tokyo, Japan). Optimal tension was established by adjustment of the length of the vessels to a point where maximum peak-to-peak oscillations of spontaneous isometric contractions were observed (Nobe et al, 2003). This passive tension was maintained throughout the experiment.…”

“…Mice were anesthetized with ether. PVs were dissected and prepared for analysis as described previously (Nobe et al, 2003). Briefly, vessels were rinsed in cold bicarbonatebuffered physiological salt solution (PSS), and loose fat and connective tissue were removed.…”

“…Dysfunction of this tissue leads not only to reduction in blood supply to the liver This work was supported in part by a Showa University Grant-in-Aid forbut also to serious diseases such as varix mediated by reflux of blood (MacMathuna, 1992). We previously reported that the magnitude of the spontaneous phasic contractions in mouse PV was increased in the presence of the thromboxane A 2 analog U46619 (Nobe et al, 2003). We also reported that the U46619-induced contraction was significantly enhanced under high glucose conditions (twice the level of the normal conditions; 22.2 mM).…”