Continuous 24-h nicotinic acid infusion in rats causes FFA rebound and insulin resistance by altering gene expression and basal lipolysis in adipose tissue

Abstract

Nicotinic acid (NA) has been used as a lipid drug for five decades. The lipid-lowering effects of NA are attributed to its ability to suppress lipolysis in adipocytes and lower plasma FFA levels. However, plasma FFA levels often rebound during NA treatment, offsetting some of the lipid-lowering effects of NA and/or causing insulin resistance, but the underlying mechanisms are unclear. The present study was designed to determine whether a prolonged, continuous NA infusion in rats produces a FFA rebound and/or insulin resistance. NA infusion rapidly lowered plasma FFA levels (>60%, P < 0.01), and this effect was maintained for ≥5 h. However, when this infusion was extended to 24 h, plasma FFA levels rebounded to the levels of saline-infused control rats. This was not due to a downregulation of NA action, because when the NA infusion was stopped, plasma FFA levels rapidly increased more than twofold (P < 0.01), indicating that basal lipolysis was increased. Microarray analysis revealed many changes in gene expression in adipose tissue, which would contribute to the increase in basal lipolysis. In particular, phosphodiesterase-3B gene expression decreased significantly, which would increase cAMP levels and thus lipolysis. Hyperinsulinemic glucose clamps showed that insulin's action on glucose metabolism was improved during 24-h NA infusion but became impaired with increased plasma FFA levels after cessation of NA infusion. In conclusion, a 24-h continuous NA infusion in rats resulted in an FFA rebound, which appeared to be due to altered gene expression and increased basal lipolysis in adipose tissue. In addition, our data support a previous suggestion that insulin resistance develops as a result of FFA rebound during NA treatment. Thus, the present study provides an animal model and potential molecular mechanisms of FFA rebound and insulin resistance, observed in clinical studies with chronic NA treatment.

Fig. 1.

Plasma FFA levels during saline (open bars) or nicotinic acid (NA) (30 μmol/h; gray bars) infusion in overnight-fasted rats. Effects of short (≤5 h) vs. 24-h infusion were determined in different rats such that plasma FFA levels were measured in similar fasting (18–24 h) states. Data are means ± SE (n = 5 for short and n = 7 or 9 for 24-h infusions). *P < 0.05 vs. saline control.

Fig. 2.

Plasma FFA (A) and glycerol (B) levels during a saline (○) or NA (●) infusion (0–180 min) following 24-h NA infusion. Data are means ± SE (n = 5). *P < 0.05 vs. control.

Fig. 3.

Representative Western blots and quantitative data showing hormone-sensitive lipase (HSL), perilipin (PLIN1), and diacylglycerol acyltransferase-2 (DGAT2) protein contents (A) and semiquantitative RT-PCR showing phosphodiesterase (PDE)-3B mRNA levels (B) in adipose tissue in 24-h saline- or NA-infused rats (open and gray bars, respectively, in A). Data are means ± SE (n = 7). *P < 0.05 vs. saline control.

Fig. 4.

Representative Western blots for total and phosphorylated HSL and quantitation of the ratio of phosphorylated to total HSL. Data are means ± SE (n = 5 for 1.5-h and 7 for 24-h infusions). *P < 0.05 vs. saline control; #P < 0.05 vs. 24-h infusion.

Fig. 5.

Plasma insulin (A), glucose (B), and FFA (C) levels and glucose infusion rate (G_INF; D) during hyperinsulinemic euglycemic clamp (insulin infusion = 1 mU·kg⁻¹·min⁻¹) following 24-h saline (○) or NA (●) infusion. In the NA group, NA infusion was continued during the clamp. Data are means ± SE (n = 5).

Fig. 6.

Plasma insulin (A), glucose (B), and FFA (C) levels and G_INF (D) during hyperinsulinemic euglycemic clamp (insulin infusion=5 mU·kg⁻¹·min⁻¹) following 24-h saline (○) or NA (●) infusion. During the clamps, NA infusion was stopped. Data are means ± SE (n = 7 or 9). *P < 0.05, saline vs. NA.

Fig. 7.

Plasma corticosterone (A), epinephrine (B), and norepinephrine (C) levels in 24-h saline- (open bars) or NA (gray bars)-infused rats. Data are means ± SE (n = 6 or 8).