Influence of insulin in the ventromedial hypothalamus on pancreatic glucagon secretion in vivo

Abstract

Objective: Insulin released by the beta-cell is thought to act locally to regulate glucagon secretion. The possibility that insulin might also act centrally to modulate islet glucagon secretion has received little attention.

Research design and methods: Initially the counterregulatory response to identical hypoglycemia was compared during intravenous insulin and phloridzin infusion in awake chronically catheterized nondiabetic rats. To explore whether the disparate glucagon responses seen were in part due to changes in ventromedial hypothalamus (VMH) exposure to insulin, bilateral guide cannulas were inserted to the level of the VMH and 8 days later rats received a VMH microinjection of either 1) anti-insulin affibody, 2) control affibody, 3) artificial extracellular fluid, 4) insulin (50 microU), 5) insulin receptor antagonist (S961), or 6) anti-insulin affibody plus a gamma-aminobutyric acid A (GABA(A)) receptor agonist muscimol, prior to a hypoglycemic clamp or under baseline conditions.

Results: As expected, insulin-induced hypoglycemia produced a threefold increase in plasma glucagon. However, the glucagon response was fourfold to fivefold greater when circulating insulin did not increase, despite equivalent hypoglycemia and C-peptide suppression. In contrast, epinephrine responses were not altered. The phloridzin-hypoglycemia induced glucagon increase was attenuated (40%) by VMH insulin microinjection. Conversely, local VMH blockade of insulin amplified glucagon twofold to threefold during insulin-induced hypoglycemia. Furthermore, local blockade of basal insulin levels or insulin receptors within the VMH caused an immediate twofold increase in fasting glucagon levels that was prevented by coinjection to the VMH of a GABA(A) receptor agonist.

Conclusions: Together, these data suggest that insulin's inhibitory effect on alpha-cell glucagon release is in part mediated at the level of the VMH under both normoglycemic and hypoglycemic conditions.

FIG. 1.

Insulin-induced phosphorylation of Akt is prevented by delivery of anti-insulin affibodies to differentiated adipocytes in vitro (A) and into the ventromedial hypothalamus in vivo during systemic insulin infusion. Relative change in Akt phosphorylation was normalized to β-actin (B). GAPDH, glyceraldehyde-3-phosphate dehydrogenase.

FIG. 2.

Effect of circulating insulin on plasma glucagon, epinephrine, and C-peptide responses to an equivalent fall in plasma glucose produced by infusion of insulin or phloridzin. The absence of an elevation in circulating insulin magnified the glucagon response, despite identical suppression of C-peptide increments of epinephrine. *P < 0.05.

FIG. 3.

Effect of local blockade of VMH insulin during insulin-induced hypoglycemia (above) and VMH delivery of insulin during phloridzin-induced hypoglycemia (below) on GIR, peak glucagon, C-peptide, and epinephrine levels. Blockade of VMH insulin action reduced GIRs and amplified glucagon responses during systemic insulin infusion, whereas VMH insulin microinjection during phloridzin infusion increased GIR and suppressed glucagon responses. (All values ± SEM, *P < 0.05.)

FIG. 4.

Blockade of VMH insulin using either an anti-insulin affibody or a specific insulin receptor antagonist, S961, increases plasma glucagon and glucose in rats that were fasted overnight. This effect was abolished by simultaneous coinjection of a GABA agonist, muscimol. (All values ± SEM, *P < 0.05.)