Role of hyperglycaemia in the pathogenesis of hypotension observed in type-1 diabetic rats.

Diabetes is a disease characterized by chronic hyperglycaemia secondary to a reduction in the functional efficacy and/or a deficiency of insulin. In fact, patients with diabetes have a shorter life span and a decreased quality of life, mainly as a result of macrovascular and/or microvascular complications (Sobel & Schneider 2005). Although chronic complications are late-occurring, alterations in autonomic nervous tone as an early development of these complications are a hallmark during the course of diabetes (Boulton et al. 2005). Studies of type-1 diabetic patients have shown that autonomic nervous system abnormalities can be reversed when better glycemic control is obtained (Genuth 2006). The earliest detectable feature of cardiac neuropathy is defective parasympathetic control in diabetic patients (Boulton et al. 2005), although cardiac dysfunction in diabetes may involve alterations in β-adrenoceptor-mediated cardiac responses (Gando et al. 1997). The parasympathetic branch of the autonomic nervous system plays an important role in the regulation of cardiac function, which occurs in response to activation of muscarinic cholinergic receptors (mAChR) following release of the neurotransmitter, acetylcholine (Dhein et al. 2001; Harvey & Belevych 2003). The M2 receptor (M2-mAChR) is believed to be the most common muscarinic receptor subtype expressed in cardiac muscle; stimulation of the M2-mAChR inhibits adenylyl cyclase activity to result in a decrease of heart rate and/or ventricular contractility (Dhein et al. 2001; Harvey & Belevych 2003). An impairment of cardiovascular function in streptozotocin (STZ)-diabetic rats has been described within 5 days-to-3 months of induction (De Angelis et al. 2002). In our previous study, hyperglycaemia-induced polyol was shown to be responsible for the increase in cardiac M2-mAChR gene expression of rats with diabetes for 10 days (Liu et al. 2005). Also, it has been documented that an increase in the number of atrial muscarinic receptors, evaluated by a radioligand-binding assay, was associated with the cardiac dysfunction in 30-day diabetic rats (Dall’ago et al. 2007). Taken into together, investigation of the gene expression of cardiac M2-mAChR may clarify the parasympathetic dysfunction during development of diabetes. Characteristic signs of diabetes in type 1-like diabetic rats, such as an increase in feeding behaviour, a decrease in body weight and an increase in blood glucose levels, reach a peak 4 weeks after STZ injection and approach a steady state within 8 weeks of the onset of diabetes; while animals with diabetes for 12-to-16 weeks were considered a late stage of diabetes (Wei et al. 2003). In the clinical setting tight glycemic control at the early stage of diabetes has been shown to be important in reducing diabetic complications (Selwitz & Pihlstrom 2003). In order to specifically determine the role of cardiac M2-mAChR in the early stage of diabetes, whole hearts of diabetic rats induced by STZ injection for 8 weeks were used in the present study. Alterations in the protein and mRNA levels of cardiac M2-mAChR were followed as indicators. Also, the direct effect of hyperglycaemia on changes in M2-mAChR gene expression was further characterized in the cultured H9c2 cardiac cell line incubated with glucose at a high concentration (Yu et al. 1999).