Effects of insulin on the cardiovascular integrating mechanisms of brain stem in cats

The Impact of Insulin Resistance on Cardiovascular Control During Exercise in Diabetes

Patients with diabetes display heightened blood pressure response to exercise, but the underlying mechanism remains to be elucidated. There is no direct evidence that insulin resistance (hyperinsulinemia or hyperglycemia) impacts neural cardiovascular control during exercise. We propose a novel paradigm in which hyperinsulinemia or hyperglycemia significantly influences neural regulatory pathways controlling the circulation during exercise in diabetes.

Exaggerated pressor and sympathetic responses to stimulation of the mesencephalic locomotor region and exercise pressor reflex in type 2 diabetic rats

The cardiovascular responses to exercise are potentiated in patients with type 2 diabetes mellitus (T2DM). However, the underlying mechanisms causing this abnormality remain unknown. Central command (CC) and the exercise pressor reflex (EPR) are known to contribute significantly to cardiovascular control during exercise. Thus these neural signals are viable candidates for the generation of the abnormal circulatory regulation in this disease. We hypothesized that augmentations in CC as well as EPR function contribute to the heightened cardiovascular responses during exercise in T2DM. To test this hypothesis, changes in mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) in response to electrical stimulation of mesencephalic locomotor region (MLR), a putative component of the central command pathway, and activation of the EPR, evoked by electrically induced hindlimb muscle contraction, were examined in decerebrate animals. Sprague-Dawley rats were given either a normal diet (control) or a high-fat diet (14-16 wk) in combination with two low doses (35 mg/kg week 1, 25 mg/kg week 2) of streptozotocin (T2DM). The changes in MAP and RSNA responses to MLR stimulation were significantly greater in T2DM compared with control (2,739 ± 123 vs. 1,298 ± 371 mmHg/s, 6,326 ± 1,621 vs. 1,390 ± 277%/s, respectively, P < 0.05). Similarly, pressor and sympathetic responses to activation of the EPR in diabetic animals were significantly augmented compared with control animals (436 ± 74 vs. 134 ± 44 mmHg/s, 645 ± 135 vs. 139 ± 65%/s, respectively, P < 0.05). These findings provide the first evidence that CC and the EPR may generate the exaggerated rise in sympathetic activity and blood pressure during exercise in T2DM.

Mechanisms mediating insulin-induced hypotension in rats. A role for nitric oxide and autonomic mediators

The mechanisms associated with insulin-induced cardiovascular inhibitory responses were evaluated in untreated normal rats and in normal rats pretreated with an antagonist of nitric oxide (NO) production (L-NAME), with cholinergic, alpha- and beta-adrenergic antagonists, or after ganglionic blockade. Male Wistar rats were anesthetized with a mixture of urethane and alpha-chloralose and placed on a electric heating pad. The femoral artery and vein were cannulated for measurements of mean arterial pressure (MAP), heart rate, plasma glucose, blood sampling, and intravenous injections. Intravenous injection of insulin (5.0 U/kg) in untreated rats resulted in a significant and sustained decrease in arterial blood pressure (average 24%) and in a slight decrease in heart rate. These cardiovascular responses were blocked by L-NAME and by the cholinergic antagonist atropine, suggesting an involvement of NO and the cholinergic receptors, or an effect of insulin on the central nervous system parasympathetic center. The ganglionic blocker hexamethonium attenuated the insulin-induced response. On the other hand, the hypotensive effect of insulin persisted after sympathetic blockade with the alpha-1 antagonist prazosin and the beta-1 antagonist atenolol. We conclude that the insulin-induced decrease in blood pressure is due to both increased cholinergic outflow and to NO production and that an enhanced sympathetic activity possibly mediated by a reactive release of norepinephrine or epinephrine modulates this response.