Antihypertensive Agent Moxonidine Enhances Muscle Glucose Transport in Insulin-Resistant Rats

Methods

Results

Study 1: 10 mg/kg Moxonidine and Ad Libitum Feeding

The initial study involved treatment of ad libitum–fed lean and obese Zucker rats with a dose of moxonidine known to elicit maximal reductions in blood pressure in insulin-resistant rat models.^{10 18} Chronic treatment with 10 mg/kg moxonidine of ad libitum–fed obese Zucker rats resulted in markedly less weight gain and a significantly lower (17%, P<.05) final body weight compared to obese vehicle-treated animals (Table 1⇓), likely due to reduced food consumption at this high dose.¹⁰ In contrast, chronic treatment with this dose of moxonidine did not significantly affect body weight gain in ad libitum-fed lean Zucker rats. Heart weight was significantly reduced by moxonidine treatment in both lean (11%, P<.05) and obese (14%, P<.05) Zucker rats.

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Table 1.

Effects of Chronic Moxonidine Treatment on Body Weight, Heart Wet Weight, and Fasting Plasma Glucose, Insulin, and Free Fatty Acids in Zucker Rats

Compared with obese controls, obese Zucker rats treated with 10 mg/kg moxonidine displayed significantly reduced (P<.05) fasting plasma insulin (19%) and free fatty acids (28%) levels, whereas fasting glucose levels were not altered (Table⇑). Only fasting plasma insulin was greater (23%, P<.05) in the lean animals because of the 10 mg/kg moxonidine treatment.

During the OGTT, the glucose response (incremental area under the curve) was 67% lower (98±27 versus 301±27 mmol/L×minute, P<.05) in the 10 mg/kg moxonidine–treated obese animals compared with the obese control group (Fig 1⇓). Plasma insulin was significantly lower at the 30- and 60-minute time points of the OGTT in the moxonidine-treated obese animals. Associated with this increased glucose tolerance, insulin-mediated glucose transport in the epitrochlearis muscle of the 10 mg/kg moxonidine-treated obese animals was enhanced by 70% (P<.05) (Fig 2⇓).

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Figure 1.

Effects of chronic treatment with 10 mg/kg moxonidine and ad libitum feeding on glucose tolerance in lean and obese Zucker rats. Animals were treated for 21 days with either vehicle or moxonidine and the glucose and insulin responses to a 1 g/kg OGTT were determined. Data are mean±SE for 8 to 10 animals per group. *P<.05 versus vehicle-treated group at same time point.

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Figure 2.

Effect of chronic treatment with 10 mg/kg moxonidine and ad libitum feeding on insulin-mediated skeletal muscle glucose transport activity in lean and obese Zucker rats. Animals were treated for 21 days with either vehicle or moxonidine (MOX). 2-Deoxyglucose uptake in the absence of insulin (open bars) and in the presence of insulin (filled bars), as well as the net increase in 2-deoxyglucose uptake due to insulin (shaded bars), were assessed in the isolated epitrochlearis muscles. Data are mean±SE for 8 to 10 animals per group. *P<.05 versus vehicle-treated group at same time point.

Interestingly, chronic treatment of ad libitum–fed lean Zucker rats with 10 mg/kg moxonidine led to a 40% reduction (106±29 versus 177±36 mmol/L×minute) in the glucose response during the OGTT (see Fig 1⇑). This was associated with a 39% greater insulin response (5436±1497 versus 3899±672 pmol/L×minute). Insulin-mediated skeletal muscle glucose transport activity was not altered (Fig 2⇑).

Study 2: 2 and 6 mg/kg Moxonidine and Pair Feeding

To avoid the confounding influence of altered body weight gain on these metabolic parameters in the obese animals, a subsequent study was conducted in which pair feeding of obese Zucker rats was incorporated and two doses of moxonidine, one with a more pronounced hypotensive effect (6 mg/kg) and another with a marginal hypotensive effect (2 mg/kg),¹⁸ were used. With pair feeding (20 g chow per day, the amount consumed by the 6 mg/kg moxonidine–treated obese group), the final body weights of the 2 and 6 mg/kg moxonidine–treated obese groups were not significantly different from those of the respective vehicle-treated control groups (Table⇑). Heart weights again were significantly reduced in the 2 mg/kg (5%, P<.05) and 6 mg/kg (13%, P<.05) moxonidine–treated obese groups compared with their respective controls.

Compared with controls, obese Zucker rats treated with 6 mg/kg moxonidine displayed significantly (P<.05) reduced fasting plasma insulin (17%) and free fatty acids (36%) (Table⇑). The glucose response during an OGTT was 47% lower (164±25 versus 311±28 mmol/L×minute, P<.05), whereas the insulin response tended to be lower (39830±3114 versus 54096±7379 pmol/L×minute, 26%) (Fig 3⇓). Finally, insulin-mediated glucose transport activity in the isolated epitrochlearis muscle was 39% greater (P<.05) than control after the 6 mg/kg moxonidine treatment period (Fig 4⇓).

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Figure 3.

Effects of chronic treatment with 2 or 6 mg/kg moxonidine on glucose tolerance in pair-fed obese Zucker rats. Animals were treated for 21 days with either vehicle or moxonidine, and the glucose and insulin responses to a 1 g/kg OGTT were determined. All animals consumed 20 g of chow per day. Data are mean±SE for 8 to 10 animals per group. *P<.05 versus vehicle-treated group at same time point.

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Figure 4.

Effect of chronic treatment with 2 or 6 mg/kg moxonidine on insulin-mediated skeletal muscle glucose transport activity in pair-fed obese Zucker rats. Animals were treated for 21 days with either vehicle or moxonidine (MOX). 2-Deoxyglucose uptake in the absence of insulin (open bars) and in the presence of insulin (filled bars), as well as the net increase in 2-deoxyglucose uptake due to insulin (shaded bars), were assessed in the isolated epitrochlearis muscles. All animals consumed 20 g of chow per day. Data are mean±SE for 8 to 10 animals per group. *P<.05 versus vehicle-treated group at same time point.

In contrast, the 2 mg/kg moxonidine treatment did not significantly alter fasting plasma insulin or free fatty acid levels (Table⇑). Although the glucose response during the OGTT of these 2 mg/kg moxonidine–treated obese animals was 41% lower (143±14 versus 244±21 mmol/L per minute, P<.05) (Fig 3⇑), this likely was not due to altered insulin action, because the insulin response (Fig 3⇑) and insulin-mediated muscle glucose transport activity (Fig 4⇑) were not changed significantly after treatment with 2 mg/kg moxonidine.

Discussion

Moxonidine is a centrally acting, selective imidazoline I₁-receptor modulator that reduces sympathetic outflow, thereby lowering blood pressure in a number of experimental animal models and human subjects.^{8 11} Because essential hypertension is often accompanied by insulin resistance,^{1 2 3} it is important to determine the effect of antihypertensive treatments on insulin action. In the present study, we have demonstrated that chronic treatment of the insulin-resistant, hyperinsulinemic, and dyslipidemic obese Zucker rat with moxonidine leads to improved whole-body glucose tolerance (Figs 1⇑ and 3⇑), likely resulting from enhanced insulin-stimulated skeletal muscle glucose transport activity (Figs 2⇑ and 4⇑) and possibly via reduced circulating free fatty acid levels (Table⇑).

The results of this study are essentially in agreement with those of Ernsberger et al,^{10 11} who studied the metabolic effects of moxonidine on the ad libitum–fed obese spontaneously hypertensive Koletsky rat. These investigators reported that chronic (90 days) oral administration of a high dose of moxonidine (8 mg/kg per day) resulted in reductions in fasting plasma insulin, triglycerides, and total cholesterol,¹⁰ as well as in significantly improved oral glucose tolerance.^{10 11} However, these results are confounded by the large body-weight loss experienced by the moxonidine-treated obese animals, likely due to the reduced food intake of this group.¹⁰ We have demonstrated that chronic administration of moxonidine can elicit beneficial metabolic adaptations (Table⇑, Fig 3⇑) in the absence of significant differences in body weight between the drug-treated and control groups (Table⇑). Moreover, we have significantly extended the investigation of Ernsberger et al¹⁰ by demonstrating that enhanced skeletal muscle glucose transport likely underlies the increased glucose tolerance in the moxonidine-treated obese groups (Fig 3⇑).

It is clear from the present results and those of Ernsberger et al^{10 11} that the effects of chronic 10 mg/kg moxonidine treatment are quite different in lean versus obese rats. In the lean, insulin-sensitive, and normolipidemic animals, the primary effect of moxonidine treatment was on improved glucose tolerance during the OGTT, most likely as a result of enhanced insulin levels during the test (Fig 1⇑). This would suggest that moxonidine in these lean animals acts primarily on the β-cells of the pancreas to increase insulin secretion.¹¹ Whereas some previous in vitro studies have indicated that, at high doses, moxonidine can inhibit basal insulin secretion from isolated pancreatic islets,^{19 20} other investigations indicate that lower doses of moxonidine actually can potentiate the effect of glucose to stimulate insulin release from isolated pancreatic islets.²⁰ In the obese, insulin-resistant, and dyslipidemic animals, the enhanced glucose tolerance after moxonidine treatment was associated with a lower plasma level of insulin during the OGTT (Fig 1⇑), enhanced insulin-stimulated muscle glucose transport activity (Fig 2⇑), and lower circulating free fatty acid levels (Table⇑). This indicates that the mode of action of this agent in the obese animal is much more complex and includes central, sympatholytic actions of the drug, as well as actions, directly or indirectly, on skeletal muscle and adipose tissue.

It is of considerable interest that other central sympatholytic agents, such as clonidine, produce a hyperglycemic response and worsen glucose tolerance when administered in vivo in animals and in humans.^{21 22 23} It is believed that these detrimental effects of clonidine on glucose metabolism are mediated by the activation of α₂-adrenergic receptors located in the central nervous system²³ and on the β-cells of the pancreas.²² Because moxonidine selectively activates I₁-imidazoline receptors and has relatively minimal interaction with α₂-adrenergic receptors,¹¹ this highlights the potential importance of the involvement of I₁-imidazoline receptors in the beneficial effects of central sympatholytic agents in improving glucose tolerance in conditions of insulin resistance.

Substantial evidence has accumulated indicating that, under certain conditions, insulin resistance of skeletal muscle glucose disposal is related to elevated circulating free fatty acid levels.²⁴ In the present study, the obese control animals displayed markedly elevated plasma-free fatty acid levels (Table⇑) and insulin resistance of skeletal muscle glucose transport (Fig 2⇑) compared with age-matched lean animals. Moreover, chronic treatment with the higher dose of moxonidine (6 mg/kg) was associated with a substantial reduction in free fatty acid levels (Table⇑) and an increase in insulin-stimulated muscle glucose transport activity (Fig 4⇑), whereas after treatment with a lower dose (2 mg/kg), neither variable was significantly altered compared with the vehicle-treated control. The reduction in free fatty acid levels may have resulted from decreased adipose tissue lipolysis, because moxonidine is known to reduce circulating levels of norepinephrine.¹⁸ It is possible that the improvement in insulin action observed with chronic moxonidine treatment was secondary to the reduction in plasma free fatty acids elicited by this intervention.

Interestingly, Ishizuka et al²⁵ recently reported that the level of IRS-1, an essential component of the intracellular insulin signaling pathway for activation of glucose transport,²⁶ was reduced by 30% in skeletal muscle from the obese spontaneously hypertensive Koletsky rat compared with lean hypertensive littermates. Moreover, the IRS-1 level increased by 24% in the obese animals after chronic treatment with 8 mg/kg per day moxonidine. Although these data are consistent with a putative role of an enhanced IRS-1 level in the metabolic improvements associated with this moxonidine treatment, it is impossible to attribute the increase in IRS-1 solely to the effects of moxonidine, because these ad libitum–fed, moxonidine-treated animals also experienced a substantial reduction in body weight.¹⁰

Several other antihypertensive agents also elicit similar beneficial metabolic adaptations in the animal model of insulin resistance, hyperinsulinemia, and dyslipidemia used in the present study. The angiotensin-converting enzyme inhibitors captopril²⁷ and trandolapril²⁸ and the β-adrenergeric receptor modulator (β₁-antagonist/β₂-agonist) celiprolol²⁹ increase insulin-stimulated skeletal muscle glucose transport and bring about a decline in circulating insulin and free fatty acid levels when administered at doses known to reduce blood pressure. The improvement in insulin action on skeletal muscle glucose transport in this animal model of insulin resistance elicited by moxonidine (70%) is very similar in magnitude to that brought about by the ACE inhibitors (60% to 70%)^{27 28} or celiprolol (68%).²⁹ As with chronic ACE inhibitor treatment,^{27 28} chronic moxonidine also leads to small but significant reductions in cardiac mass, even in lean animals (Table⇑). This effect of moxonidine may be related to a reduced sympathetic drive to the heart, or it may be related to its ability to reduce peripheral vascular resistance.⁸ Based on the results of the present study and previous reports,^{10 11} it appears that moxonidine, a sympatholytic compound, can be considered, with the previously mentioned antihypertensive agents, to be a potentially useful intervention in the overall treatment of individuals with Syndrome X. Thus, with these types of compounds, one could treat not only hypertension but also the other associated pathologies, primarily insulin resistance, hyperinsulinemia, impaired glucose tolerance, and dyslipidemia, thereby reducing multiple cardiovascular disease risk factors.

It should be noted that additional mechanisms other than improved skeletal muscle glucose transport may underlie some of the improvement in glucose tolerance brought about by the lower dose of moxonidine (Fig 3⇑). Although a portion of the enhanced glucose tolerance may be attributed to the small (≈20%), but statistically insignificant, increase in insulin-mediated muscle glucose transport (Fig 4⇑), it is also possible that decreased hepatic glucose production may be caused by moxonidine treatment at this dose. It is clear that further investigations in this area are necessary to address this point.

In conclusion, the sympatholytic antihypertensive agent moxonidine, a centrally acting, selective I₁-imidazoline receptor modulator (putative agonist), effectively and dose-dependently enhances whole-body glucose tolerance in the obese Zucker rat, an animal model of insulin resistance, hyperinsulinemia, glucose intolerance, and dyslipidemia. This effect likely is mediated by an increase in insulin action on skeletal muscle glucose transport and by a reduction in circulating free fatty acid levels. This compound appears to be useful in the treatment of obese hypertensive and insulin-resistant syndromes, such as Syndrome X. Future studies should investigate the potential cellular adaptations that may underlie the improvements of insulin-stimulated glucose transport activity after treatment with the antihypertensive agent moxonidine.

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