A cyclic guanosine monophosphate-dependent pathway can regulate net hepatic glucose uptake in vivo

Abstract

We previously showed that hepatic nitric oxide regulates net hepatic glucose uptake (NHGU), an effect that can be eliminated by inhibiting hepatic soluble guanylate cyclase (sGC), suggesting that the sGC pathway is involved in the regulation of NHGU. The aim of the current study was to determine whether hepatic cyclic guanosine monophosphate (cGMP) reduces NHGU. Studies were performed on conscious dogs with transhepatic catheters. A hyperglycemic-hyperinsulinemic clamp was established in the presence of portal vein glucose infusion. 8-Br-cGMP (50 µg/kg/min) was delivered intraportally, and either the glucose load to the liver (CGMP/GLC; n = 5) or the glucose concentration entering the liver (CGMP/GCC; n = 5) was clamped at 2× basal. In the control group, saline was given intraportally (SAL; n = 10), and the hepatic glucose concentration and load were doubled. 8-Br-cGMP increased portal blood flow, necessitating the two approaches to glucose clamping in the cGMP groups. NHGU (mg/kg/min) was 5.8 ± 0.5, 2.7 ± 0.5, and 4.8 ± 0.3, whereas the fractional extraction of glucose was 11.0 ± 1, 5.5 ± 1, and 8.5 ± 1% during the last hour of the study in SAL, CGMP/GLC, and CGMP/GCC, respectively. The reduction of NHGU in response to 8-Br-cGMP was associated with increased AMP-activated protein kinase phosphorylation. These data indicate that changes in liver cGMP can regulate NHGU under postprandial conditions.

FIG. 1.

Schematic representation of the study. The protocol comprises the basal (−30–0 min) and experimental periods (P1, 0–90 min; P2, 90–180 min; P3, 180–240 min). Somatostatin was infused peripherally, and insulin (fourfold basal) and glucagon (basal) were given intraportally, whereas glucose was delivered intraportally (4 mg/kg/min) and peripherally at a variable rate to increase the HGL twofold basal or arterial blood glucose twofold basal during P1, P2, and P3. The SAL group (n = 10) received intraportal saline during P2 and P3. Ten subjects received intraportal 8-Br-cGMP at 50 µg/kg/min during P2 and P3, five had lowered arterial glucose to match the HGL to that in P1 (CGMP/GLC, n = 5), and five had arterial glucose clamped to that in P1 (CGMP/GCC, n = 5).

FIG. 2.

Systolic (A) and diastolic (B) blood pressure and heart rate (C) during the basal and experimental periods. See Fig. 1 for description of study conditions. Data are means ± SEM; n = 8 in the SAL group, n = 5 in the group that received 8-Br-cGMP in the portal vein in which arterial glucose was maintained at ∼130 mg/dL in P3 (CGMP/GLC), and n = 5 in the group that received 8-Br-cGMP in the portal vein and arterial glucose was maintained at ∼175 mg/dL in P3 (CGMP/GCC). *Significant statistical difference (P < 0.05) from basal period within the group; †significant statistical difference (P < 0.05) from SAL group and from basal period within the group.

FIG. 3.

Arterial blood glucose (A), HGL (B), NHGU (C), net hepatic fractional extraction (NHFE) of glucose (D), total glucose infusion rate (E), and nonhepatic glucose uptake (F) in 42-h–fasted conscious dogs during the basal and experimental periods. See Fig. 1 for description of study conditions. Data are means ± SEM. Average NHGU in P3 data expressed as histogram represent averaged values for the P3 (last hour) in the SAL group (n = 10) and the 8-Br-cGMP–treated group (n = 10). ∆NHFE in P3 from P1 data expressed as histogram represent the change of averaged values for P3 from P1 in the saline group (n = 10) and the 8-Br-cGMP–treated group (n = 10). *P < 0.05 compared with SAL group; †P < 0.05 compared with CGMP/GCC group; ‡P < 0.05 compared with CGMP/GLC group.

FIG. 4.

Phosphorylation of AMPK at Thr172 and ACC at Ser79 in the liver biopsies at the end of the experiments. See Fig.1 for description of study conditions. Data are means ± SEM. The blots shown are representative of three to five blots obtained from independent experiments. *P < 0.05 compared with the SAL group.