Non-insulin-dependent diabetes mellitus (NIDDM) is often associated with hypertension leading to a specifically high cardiovascular risk in these patients. However, there is evidence that insulin resistance and hyperinsulinaemia are not only characteristic for diabetic patients but also for some non-diabetic populations in which a cluster of cardiovascular risk factors is observed (hypertension, hyper- triglyceridaemia, obesity). Therefore, hyperinsulinaemia and insulin resistance have been suggested to be of major pathophysiological importance for the development of this syndrome (syndrome X). Since imidazoline receptors are currently considered to be a specific pharmacological target for blood pressure reduction, it is important to know whether and in which way these compounds affect the glucose homoeostasis and insulin release.
The influence of moxonidine on glucose tolerance in vivo was determined in healthy control rats, in rats receiving a high fructose diet for 6 weeks to induce insulin resistance, hyperinsulinaemia and hypertension, and in rats receiving in addition to a high fructose diet moxonidine (1.5 mg/kg body weight daily). In vitro, using isolated pancreatic islets of mice, long-lasting effects (chronic) and immediate (acute) effects of moxonidine on β -cell function were determined by basal and glucose stimulated insulin release in two different experimental systems: (1) islets were exposed for 24 h (37°C) to various concentrations of moxonidine ranging from 1 nmol/l to 1 mmol/l, followed by a washing procedure to remove excess of moxonidine and then used for the β-cell function test; (2) islet cultures were incubated again with moxonidine for 24 h (37°C) with either 1 nmol/l or 1 µmol/l. In contrast to the first experiments, however, after the washing procedure moxonidine was added at the same concentration as used for preincubation to test its direct effect on β-cell function.
In healthy control rats acute administration of moxonidine in vivo impaired the glucose tolerance in high dosages, which effectively reduced the blood pressure (>1 mg/kg body weight). This effect was, however, smaller that that observed by clonidine. In fructose-fed rats, moxonidine completely prevented the development of insulin resistance, hyperinsulinaemia and hypertension. In vitro, pancreatic islets preincubated with moxonidine exhibited dose-dependently both stimulatory and inhibitory chronic effects on β -cell function compared with that in controls. Preincubation of islet cultures with moxonidine at concen- trations between 1 nmol/l and 1 mmol/l resulted in a reduction of basal insulin release which was very pronounced at concentrations higher than 100 nmol/l. The results ob- tained for glucose-stimulated insulin release opposed in part those for basal insulin release, since the preincubation with moxonidine up to 10 µmol/l gave rise to an increased insu- lin release. An additional direct effect of moxonidine with a marked reduction of glucose-stimulated insulin release was observed, however, when moxonidine was present during the preincubation (24 h) and the functional test at a concen- tration of 1 nmol/l or 1 µmol/l.
Our data suggest that a causal linkage exist between the development of hypertension and insulin re- sistance/hyperinsulinaemia in the high fructose diet rat model. Since central activation of imidazoline receptors by moxonidine can prevent this syndrome, it follows that an overactivity of the sympathetic nervous system is of major importance. Suppression of this sympathetic overactivity might be an effective approach to reduce hypertension and the concomitant metabolic defect. Therefore, such an inter- ventional strategy could contribute to reduce the cardiovas- cular risk of NIDDM patients and patients with other forms of insulin resistance/hyperinsulinaemia such as metabolic cardiovascular syndrome.