Investigation into a possible insulin-independent autoregulatory intrahepatic mechanism of hepatic ketogenesis

To study an insulin-independent autoregulatory mechanism of hepatic ketogenesis, catheters were placed in hepatic (H), portal (P), mesenteric (M), and femoral (V) veins, and femoral artery (A) of 5 ewes made diabetic with alloxan (.05g/kg bw, iv). Euglycemia was maintained with daily insulin injections until 3 d prior to experiment, when injections were withheld to induce ketosis. Experiments consisted of 4 infusion periods: (1) p-amminohippurate (PAH, 1.5% @ .764 ml/min) infusion into M (Control); (2) PAH as in (1) and octanoate (C8, 3.2 mmol carbon/min) into M; (3) PAH and C8 as in (2) and B-hydroxybutyrate (BOHB, .4 mg/kg³/⁴) into V; and (4) PAH, C8 and BOHB as in (3) and carnitine (C, 1.18 mM/min) into V. For each period, 3 blood samples were taken simultaneously from H, P, and A at 15 minute intervals and analyzed for ketones, non-esterified fatty acids (NEFA), glucose and PAH. Relative to control, C8 decreased P and H blood flows (3.1 and 3.7 vs. 1.9 and 2.4 1/min, P<.05) and increased net hepatic NEFA uptake (416 vs 629 μmol/min, P<.05). Relative to C8, BOHB decreased hepatic NEFA uptake (629 vs 424 μmol/min, P<.05) and increased plasma free carnitine (13.0 vs 15.5 μM, P<.1). Relative to BOHB, carnitine increased hepatic glucagon uptake (1.95 vs 4.08 μg/hr, P<.1) and extraction percent (4 vs 11 %, P<.05). Hepatic ketogenesis did not change with treatments. This data suggests that BOHB does not have an inhibitory effect on hepatic ketogenesis and that carnitine is not limiting for hepatic fatty acid oxidation when C8 is the primary fatty acid metabolized. A possible insulin-independent regulatory mechanism of hepatic ketogenesis was studied in ewes surgically catheterized in hepatic (H), portal (P), mesenteric (M), and femoral (V) veins, and femoral artery (A). Six experiments were conducted on 3 ewes made diabetic with alloxan (.05g/kg bw, iv). Euglycemia was maintained with daily insulin injections (Iletin-100, ~30 lU/d) until 3 d prior to experiments, when injections were withheld to induce ketosis. Experiments consisted of 3 infusion periods: (1) p-aminohippurate (PAH, 1.5% @ .764 ml/min) infusion into M (Control); (2) PAH as in (1) and β-hydroxybutyrate (BOHB, .4 mg/kg³/⁴) into V; and (3) PAH and BOHB as in (2) and L-carnitine (C, 1.18 mM/min) into V. For each period, each infusate was allowed to equilibrate for 1 h and then 3 blood samples were taken simultaneously from H, P, and A at 15 minute intervals and analyzed for BOHB, acetoacetate (ACAC), non-esterified fatty acids (NEFA), glucose and PAH. P blood flow was 2.1 1/min and 84% that of H, and was not affected (P>.1) by infusate. Relative to control, BOHB decreased (P<.01) NEFA arterial concentrations (1223 to 572 /μmol) and hepatic (238 to 66 μmol/min) and total splanchnic (216 to 71 μmol/min) net uptakes. BOHB increased ACAC arterial concentrations (223 to 508 μmol, P<.01) and decreased (P<.1) hepatic (-3 to -81 μmol/min, P<.1) (i.e., decreased ketogenesis) and total splanchnic (33 to -29 μmol/min) release. BOHB had no effects on hepatic BOHB flux. Carnitine had no effect on any parameters measured. According to this data, BOHB may regulate ketogenesis as observed by decreased hepatic NEFA uptake and ketogenesis to ACAC, a major ketone product, but paradoxically had no effect on BOHB production. Carnitine in contrast had no net effect on hepatic ketogenesis nor NEFA uptake, suggesting that it may not be extracted by the liver in quantities sufficient to counteract observed effects of BOHB.