Decreased response of plasma catecholamine to stress in diabetic rats

Cardiovascular and adrenaline-releasing effects of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin in streptozotocin diabetic rats

The 5-hydroxytryptamine1A (5-HT1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) has been reported to trigger sympathoinhibition, as evidenced by its cardiovascular effects, and adrenal catecholamine secretion. The purpose of this study was to analyze the cardiovascular and adrenaline-releasing effects of 8-OH-DPAT in 1 week streptozotocin diabetic rats. 8-OH-DPAT-induced changes in mean arterial pressure (MAP) and heart rate (HR) were determined directly in anesthetized rats, whilst changes in plasma adrenaline (and plasma corticosterone and glucose) levels were analyzed in conscious rats. Resting blood pressure and heart rate were diminished in diabetics, when compared with controls. These changes were associated with a decrease in body weight and a marked increase in resting plasma glucose levels. Diabetes did not affect MAP response to 8-OH-DPAT, except for a decrease in the amplitude of MAP maximal fall, which was associated with a diminished bradycardic response to 8-OH-DPAT. Blood pressure response to prazosin (0.5 mg/kg) in 8-OH-DPAT-pretreated rats was also diminished in diabetics. Lastly, diabetes prevented the adrenaline-releasing and hyperglycemic effects of 8-OH-DPAT (250 ug/kg).

Heart rate control in diabetic rabbits: blunted response to isoproterenol

To study the effects of chronic diabetes on heart rate and adrenergic responsiveness we compared unanesthetized diabetic rabbits, 10-13 mo after alloxan monohydrate injection, to age-matched controls. There were no significant differences found between groups for body or heart weight. Both resting and intrinsic heart rate (the latter obtained after atropine sulfate and propranolol HCl) were similar. In addition, serum and left ventricular epinephrine and norepinephrine concentrations as well as left ventricular beta-receptor density and affinity were unchanged in diabetic animals. Heart rate responses to isoproterenol were blunted in diabetics at the three highest doses. Base-line mean blood pressure was modestly lower in diabetic rabbits, and parallel declines in pressure for both groups were observed in response to isoproterenol. The diminished heart rate response to isoproterenol in diabetic rabbits may be due to diminished myocardial sensitivity to catecholamines, possibly combined with altered baroreceptor reflexes. These experiments may provide an explanation for the blunted heart rate response to exercise described in human diabetics.

Adrenergic nerve function and contractile activity of the caudal artery of the streptozotocin diabetic rat

The adrenergic nerve function and contractile responses of the densely innervated caudal artery of the 8-week streptozotocin (SZ) (65 mg/kg i.p.) diabetic rat were investigated. Segments of this artery were removed from diabetic and control rats, placed in Krebs-filled tissue baths (37 degrees C) and isometric tension recorded. Contractile responses to sympathetic nerve activation by electrical stimulation and to cumulative concentrations of norepinephrine (NE) and tyramine were recorded. In order to determine NE content, the NE was extracted from the caudal artery, isolated by adsorption chromatography, and quantified by HPLC with electrochemical detection. NE accumulation and release were also studied by quantifying the amount of tritiated NE [( 3H]NE) and its metabolites in extracts of the tissue or incubation medium. The responses of the caudal artery of SZ diabetic rats to electrical stimulation (4-16 Hz) and to tyramine (1 X 10(-5)-1 X 10(-4) M) were significantly less than those of arteries from control rats and the NE content reduced by 41%, while sensitivity to NE was unchanged. Diabetic arteries also accumulated and released more [3H]NE than did arteries from control rats. These results establish that neurovascular function of the isolated caudal artery of the 8-week SZ diabetic rat is abnormal and suggest that problems in the ability of adrenergic nerves to store and release NE may contribute to this dysfunction. Such changes may play a role in the cardiovascular disturbances associated with diabetic autonomic neuropathy.

Impaired catecholamine secretion as a cause of diabetic autonomic neuropathy

Human and animal studies were performed to investigate the causes of diabetic autonomic neuropathy. Human diabetics, with and without autonomic neuropathy, were measured for plasma catecholamine response to insulin hypoglycemia and for urinary catecholamine excretion. In streptozotocin-diabetic rats, plasma catecholamine response and tissue catecholamine concentrations were measured at various stages of the disease. As the duration of the diabetic state lengthens in rats, there is a time-proportional stepwise decrease in plasma catecholamine response. This is similar to the clinical course observed in human diabetics, which also includes a reduction of catecholamine excretion after the appearance of autonomic neuropathy. After 6 weeks of diabetes, rat tissue is found to have an increased concentration of catecholamines; this may represent a compensatory reaction to the difficulties of secretion. At 13 weeks of diabetes, tissue catecholamine concentrations return to almost normal, when plasma responses have disappeared. These results suggest that the impaired secretion of catecholamines in diabetics may be a cause of diabetic autonomic neuropathy.

Altered norepinephrine turnover and metabolism in diabetic cardiomyopathy

Cardiac norepinephrine turnover and metabolism were examined in rats 8 weeks after the induction of chronic diabetes by an intravenous injection of streptozotocin (65 mg/kg). Cardiac norepinephrine concentration, norepinephrine turnover, and norepinephrine uptake were markedly increased in chronic diabetes in comparison with control values; these changes were reversible by 28-day insulin therapy. When the animals were exposed to cold for 6 hours, norepinephrine turnover rate constant increased in control and decreased in diabetic animals; cold exposure also increased norepinephrine concentration in diabetic hearts. Both cardiac norepinephrine concentration and turnover rate in diabetic rats were restored toward control values by ganglionic blockade with pentolinium. The conversion of [3H]tyrosine to [3H]catecholamine was enhanced and tyrosine hydroxylase as well as dopa decarboxylase activities were increased in diabetic hearts. The higher concentrations of [3H]normetanephrine and deaminated catechols indicated a faster metabolic rate of norepinephrine metabolism in hearts from diabetic rats; both monoamine oxidase and catechol-O-methyltransferase activities were also increased. The increased activities of the enzymes for the synthesis and metabolism of norepinephrine were not evident on treating the diabetic animals with insulin. These data not only support the view that chronic diabetes in rats is associated with increased sympathetic activity but also indicate that the cardiac norepinephrine concentration in diabetic rats may be maintained at a higher than normal level by an increased synthesis and uptake of norepinephrine in the adrenergic nerve terminals.

Abnormalities in plasma catecholamine response and tissue catecholamine accumulation in streptozotocin diabetic rats: a possible role for diabetic autonomic neuropathy

Plasma catecholamine levels, determined by high performance liquid chromatography, were elevated in response to blood withdrawal in normal rats. Such a response was also observed in streptozotocin diabetic rats 2 and 6 weeks after disease onset, but was no longer seen at 13 weeks. Tissue (adrenal, heart, skin, kidney) catecholamine levels in diabetic rats were increased at 6 weeks as well as at 13 weeks. These abnormalities were corrected by insulin treatment in at least a part of diabetic rats. The present data suggest that there might be a catecholamine accumulation, which is later accompanied with an impairment of catecholamine secretion, in diabetic rats, and they gave a basis for an inference that similar changes might play some role in the pathogenesis of diabetic autonomic neuropathy in man.