Elevation of ghrelin by B-adrenergic activation is independent of glucose variations and feeding regimen in the rat

Abstract

Ghrelin is a signal involved in the initiation of meals in rodents and humans. Circulating ghrelin levels are elevated before mealwes and reduced after food intake. Several factors have been identified as effective modulators of ghrelin levels. Vagal activation reduced ghrelin in rats, as well as oral carbohydrate and lipid administration in rats and humans. Some hormones, such as incretins, also reduce ghrelin: GLP-1 reduced ghrelin in humans, and Ex4, a GLP-1 receptor agonist, potently inhibited ghrelin in rodents. On the other hand, fasting promotes increases in ghrelin that anticipate the start of meals. We report that beta-adrenergic activation with isoproterenol promotes large acute elevations of circulating ghrelin levels, both in anesthetized and conscious freely-moving rats, either on "ad libitum" feeding or on a fasting regimen.These effects are dose-dependent, caused by intravenous, intraperitoneal, and oral administration, and independent of variations in glucose levels. Pharmacological modulation of β1 and β2 adrenergic receptors with specific agonists and antagonists showed that ghrelin increases are stimulated by β1-adrenergic activation, but also partially by β2-adrenergic activation, suggesting that activation of both is necessary to elicit complete ghrelin elevations. Meanwhile, glucose increases dependent on adrenergic activation appear to be mediated only by β2-adrenergic receptors. In addition, the effects of isoproterenol on increasing ghrelin levels are potent enough to overcome the marked inhibition exerted by exendin-4 that we have previously demonstrated. We also found that administration of isoproterenol in drinking water increases basal ghrelin levels and simultaneous food intake in animals eating ad libitum. Beta-adrenergic activation promotes increases in ghrelin levels in vivo prior to food intake, both in rats eating ad libitum and in fasting rats that already have elevated ghrelin levels, in a time- and dose-dependent manner. In addition, the effects of isoproterenol on increasing ghrelin levels are potent enough to overcome the marked inhibition exerted by exendin-4 that we have previously demonstrated. We also found that administration of isoproterenol in drinking water increases basal ghrelin levels and simultaneous food intake in animals eating ad libitum. Beta-adrenergic activation promotes increases in ghrelin levels in vivo prior to food intake, both in eating ad libitum and in fasting rats that already have elevated ghrelin levels, in a time- and dose-dependent manner.

Keywords: B-adrenergic; Exendin-4; Ghrelin; Glucose levels; Isoproterenol.

Fig. 1

Dose-Response of intravenous administration of Isoproterenol on ghrelin and glucose levels in anaesthetized ad libitum fed rats. Intravenous administration. Data are means ± SEM, by Kruskal-Wallis test for multiple comparison, *P < 0.05, **P < 0.01, ***P < 0.001, treatment vs. saline. Groups: Saline (n = 7), Isoproterenol 1 mg/kg (n = 7), Isoproterenol 5 mg/kg (n = 7). A. Ghrelin levels (pg/mL) dose-response to isoproterenol. B. Area under the curve (AUC, pg/mlx1000) of ghrelin levels. C. Glucose levels (mg/dL) dose response to isoproterenol. D. Area under the curve (AUC, mg/dLx1000) of glucose levels

Fig. 2

Effects of B-blockers in ghrelin and glucose responses to isoproterenol in anaesthetized ad libitum fed rats. Intravenous administration. Data are means ± SEM, by Kruskal-Wallis test for multiple comparison *P < 0.05, **P < 0.01, ***P < 0.001, treatment vs. Saline; #P < 0.05, ##P < 0.01 vs. Isoproterenol (ISO). Groups: Saline (n = 5), Isoproterenol 0,5 mg/kg (n = 5), Bisoprolol (10 mg/kg) + Isoproterenol 0.5 mg/kg (n = 7), ICI-118551 (10 mg/kg) +Isoproterenol 0.5 mg/kg (n = 7). A. Ghrelin levels (pg/mL) profile and effects of b-blockers in responses to isoproterenol. B. Area under the curve (AUC, pg/mlx1000) of ghrelin levels. C. Glucose levels (mg/dL) profile and effects of b-blockers in responses to isoproterenol. D. Area under the curve (AUC, mg/dLx1000) of glucose levels

Fig. 3

Effects of B-adrenergic agonists on ghrelin and glucose responses in anaesthetized ad libitum fed rats. Intravenous administration. Data are means ± SEM, by Kruskal-Wallis test for multiple comparison *P < 0.05, **P < 0.01, ***P < 0.001, treatment vs. Saline; #P < 0.05, ##P < 0.01 vs. Isoproterenol (ISO). Groups: Saline (n = 5), Isoproterenol 0.5 mg/kg (n = 5), Salbutamol 0.5 mg/kg (n = 7), Xamoterol 0.5 mg/kg (n = 7). A. Ghrelin levels (pg/mL) profile in response to B-adrenergic agonists. B. Area under the curve (AUC, pg/mlx1000) of ghrelin levels. C. Glucose levels (mg/dL) profile in responses B-adrenergic agonists. D. Area under the curve (AUC, mg/dLx1000) of glucose levels

Fig. 4

Dose-Response of intraperitoneal administration of Isoproterenol on ghrelin and glucose levels in freely-moving rats eating ad libitum up to the experiment. Intraperitoneal administration. Data are means ± SEM, by Kruskal-Wallis test for multiple comparison, * <0.05, **P < 0.01, ***P < 0.001, treatment vs. Saline T15. Groups: Saline T0 (n = 5), Saline T15 (n = 5), Isoproterenol 0.5 mg/kg (n = 5), Isoproterenol 1 mg/kg (n = 5), Isoproterenol 5 mg/kg (n = 5). A. Ghrelin levels (pg/mL) dose-response to isoproterenol. B. Glucose levels (mg/dL) dose-response to isoproterenol

Fig. 5

Effects of B-blockers in ghrelin and glucose responses to isoproterenol in freely-moving rats eating ad libitum up to the experiment. Intraperitoneal administration. Data are means ± SEM, by Kruskal-Wallis test for multiple comparison *P < 0.05, **P < 0.01, ***P < 0.001, treatment vs. Saline; #P < 0.05, ###P < 0.001 vs. Isoproterenol (ISO). Groups: Saline (n = 5), Isoproterenol 0.5 mg/kg (n = 5), Bisoprolol 10 mg/kg (n = 5), Bisoprolol (10 mg/kg) + Isoproterenol 0.5 mg/kg (n = 5), ICI-11855110 mg/kg (n = 5), ICI-118551 (10 mg/kg) + Isoproterenol 0.5 mg/kg (n = 7). A. Ghrelin levels (pg/mL) b-blockers effect in responses to isoproterenol. B. Glucose levels (mg/dL) effects of b-blockers in responses to isoproterenol

Fig. 6

Interaction of Exendin-4 and Isoproterenol on ghrelin and glucose responses in freely-moving rats eating ad libitum up to the experiment. Intraperitoneal administration. Data are means ± SEM, by Kruskal-Wallis test for multiple comparison *P < 0.05, **P < 0.01, ***P < 0.001, treatment vs. Saline; #P < 0.05, ###P < 0.001 vs. Isoproterenol (ISO). Groups: Saline (n = 5), Isoproterenol 0.5 mg/kg (n = 5), Ex-4 5 μg/kg (n = 5), Isoproterenol 0.5 mg/kg + Ex-4 5 μg/kg (n = 5). A. Ghrelin levels (pg/mL) at 30 and 60 min after administration of drugs. B. Glucose levels (mg/dL) at 30 and 60 min after administration of drugs

Fig. 7

Interaction of Exendin-4 and Isoproterenol on ghrelin and glucose responses in freely-moving rats fasting 24 h before the experiment. Intraperitoneal administration. Data are means ± SEM, by Kruskal-Wallis test for multiple comparison *P < 0.05, **P < 0.01 treatment vs. Saline; #P < 0.05, ##P < 0.01 vs. Isoproterenol (ISO). Groups: Saline (n = 5), Isoproterenol 0.5 mg/kg (n = 5), Ex-4 5 μg/kg (n = 5), Isoproterenol 0.5 mg/kg + Ex-4 5 μg/kg (n = 5). A. Ghrelin levels (pg/mL) at 30 and 60 min after administration of drugs. B. Glucose levels (mg/dL) at 30 and 60 min after administration of drug

Fig. 8

Effects of Isoproterenol (1 mg/mL) orally in drinking water available 60 min before time 0, on food intake and ghrelin levels in rats eating ad libitum. Data are means ± SEM, by Kruskal-Wallis test for multiple comparison *P < 0.05, **P < 0.01 treatment vs. Saline. A. Accumulated amount of food intake (n = 7). B: Accumulated amount of food intake per 100 gr of body weight (BW) of each rat (n = 7). C: Ghrelin levels at beginning of the experiment (n = 3). D: Food intake per 100 g of body weight (BW) per interval. Data are normalized to 15 min period for relative comparison of different length intervals (n = 7)