Insulin reverses hypertension and hypothalamic depression in streptozotocin diabetic rats

Striatal dopamine receptors modulate the expression of insulin receptor, IGF-1 and GLUT-3 in diabetic rats: effect of pyridoxine treatment

The incidence of type 2 diabetes mellitus is rising at alarming proportions. Central nervous system plays an important part in orchestrating glucose metabolism, with accumulating evidence linking dysregulated central nervous system circuits to the failure of normal glucoregulatory mechanisms. Pyridoxine is a water soluble vitamin and it has important role in brain function. This study aims to evaluate the role of pyridoxine in striatal glucose regulation through dopaminergic receptor expressions in streptozotocin induced diabetic rats. Radio receptor binding assays for dopamine D(1), D(2) receptors were done using [(3)H] 7-chloro-3-methyl-1-phenyl-1,2,4,5-tetrahydro-3-benzazepin-8-ol and [(3)H] 5-chloro-2-methoxy-4-methylamino-N-[-2-methyl-1-(phenylmethyl)pyrrolidin-3-yl]benzamide. Gene expressions were done using fluorescently labeled Taqman probes of dopamine D(1), D(2) receptor, Insulin receptor, Insulin like growth factor-1(IGF-1) and Glucose transporter-3 (GLUT-3). Bmax of dopamine D(1) receptor is decreased and B(max) of dopamine D(2) was increased in diabetic rats compared to control. Gene expression of dopamine D(1) receptor was down regulated and dopamine D(2) receptor was up regulated in diabetic rats. Our results showed decreased gene expression of Insulin receptor, IGF-1 and increased gene expression of GLUT-3 in diabetic rats compared to control. Pyridoxine treatment restored diabetes induced alterations in dopamine D(1), D(2) receptors, Insulin receptor, IGF-1, GLUT-3 gene expressions in striatum compared to diabetic rats. Insulin treatment reversed dopamine D(1), D(2) receptor, GLUT-3 mRNA expression, D(2) receptor binding parameters in the striatum compared to diabetic group. Our results suggest the potential role of pyridoxine supplementation in ameliorating diabetes mediated dysfunctions in striatal dopaminergic receptor expressions and insulin signaling. Thus pyridoxine has therapeutic significance in diabetes management.

Muscarinic M₁, M₃ receptor modulation in the corpus striatum of streptozotocin induced diabetic rats as a function of age

OBJECTIVES

In this study we have investigated muscarinic M₁, M₃ receptor kinetics and the functional role of IP3 and cGMP in the corpus striatum of both young and old diabetic and insulin-treated diabetic rats.

METHODS

Radioreceptor binding assays was done in the corpus striatum using specific antagonists QNB and DAMP. IP3 and cGMP assay using [3H]IP3 and [3H]cGMP Biotrak assay system kits.

KEY FINDINGS

M₁ receptor increased and M₃ receptor decreased in control old rats when compared with young control rats. In young diabetic groups M₁ receptor increased and M₃ receptor decreased. Old diabetic groups showed reversed M₁ and M₃ receptors compared with their controls. IP3 and cGMP content increased in old control rats compared with young control rats. IP3 content increased in young diabetic rats and decreased in old diabetic rats. cGMP content was increased significantly in both young and old diabetic groups. Insulin treatment reversed these altered parameters near to control.

CONCLUSIONS

Our studies showed that M₁ and M₃ receptors, IP3 and cGMP were functionally regulated during diabetes as function of age, which will have immense clinical significance.

Glutamate (mGluR-5) gene expression in brain regions of streptozotocin induced diabetic rats as a function of age: role in regulation of calcium release from the pancreatic islets in vitro

Metabotrophic glutamate receptors (mGluRs) modulate cellular activities involved in the processes of differentiation and degeneration. In this study, we have analysed the expression pattern of group-I metabotropic glutamate receptor (mGlu-5) in cerebral cortex, corpus striatum, brainstem and hippocampus of streptozotocin induced and insulin treated diabetic rats (D+I) as a function of age. Also, the functional role of glutamate receptors in intra cellular calcium release from the pancreatic islets was studied in vitro. The gene expression studies showed that mGlu-5 mRNA in the cerebral cortex increased siginficantly in 7 weeks old diabetic rats whereas decreased expression was observed in brainstem, corpus striatum and hippocampus when compared to control. 90 weeks old diabetic rats showed decreased expression in cerebral cortex, corpus striatum and hippocampus whereas in brainstem the expression increased significantly compared to their respective controls. In 7 weeks old D+I group, mGlu-5 mRNA expression was significantly decreased in cerebral cortex and corpus striatum whereas the expression increased significantly in brainstem and hippocampus. 90 weeks old D+I group showed an increased expression in cerebral cortex, while it was decreased significantly in corpus striatum, brainstem and hippocampus compared to their respective controls. In vitro studies showed that glutamate at lower concentration (10(-7) M) stimulated calcium release from the pancreatic islets. Our results suggest that mGlu-5 receptors have differential expression in brain regions of diabetes and D+I groups as a function of age. This will have clinical significance in management of degeneration in brain function and memory enhancement through glutamate receptors. Also, the regulatory role of glutamate receptors in calcium release has immense therapeutic application in insulin secretion and function.

Antihyperglycemic and insulin secretory activity of Costus pictus leaf extract in streptozotocin induced diabetic rats and in in vitro pancreatic islet culture

AIM OF THE STUDY

The leaves of Costus pictus D. Don were used extensively for its antihyperglycemic activity by the people in Kerala, India. In the present study, the antihyperglycemic and insulin secretory activity of an aqueous extract of Costus pictus leaf extract was investigated in streptozotocin induced diabetic rats.

MATERIALS AND METHODS

Oral Glucose Tolerance Test was done to determine the effective dose of Costus pictus extract. Aqueous extract of Costus pictus leaves was given orally to the diabetic rats for 14 days. The insulin secretory action of the leaf extract was investigated using isolated pancreatic islets from rat. Liver glucose uptake activity was measured using D-[14C] glucose.

RESULTS

The oral administration of an aqueous extract of Costus pictus at a dose of 250 mg/kg body weight significantly decreased the blood glucose with significant increase in plasma insulin level in diabetic rats at the end of 14 days treatment. The Costus pictus leaf extract significantly increased glucose induced insulin secretion at both 4 mM and 20 mM glucose concentrations which represents normal physiological and diabetic condition respectively. The decreased glucose uptake activity of the liver of diabetic rats was reverted to near normal levels after the treatment with Costus pictus leaf extract.

CONCLUSION

Our results suggest the glucose lowering effect of Costus pictus to be associated with the potentiation of insulin release from pancreatic islets and enhancement of peripheral utilization of glucose.

Muscarinic M(1), M(3) receptors function in the brainstem of streptozotocin induced diabetic rats: their role in insulin secretion from the pancreatic islets as a function of age

In the present study, we have investigated acetylcholine esterase (AChE) activity and muscarinic M(1), M(3) receptors kinetics in the brainstem of both young and old streptozotocin induced and insulin treated diabetic rats (D + I). Also, the functional role of acetylcholine and muscarinic receptors in insulin secretion from the pancreatic islets was studied in vitro. 90 week old control rats showed decreased V(max) (P < 0.001) for AChE compared to 7 week old control rats. V(max) was decreased (P < 0.001) in 7 week diabetic groups whereas 90 week old diabetic groups showed increased (P < 0.001) V(max) when compared to their respective controls. Binding studies using [(3)H]QNB and [(3)H]DAMP of 90 week old control showed significant increase in the B(max) (P < 0.001) and K(d) (P < 0.01) of muscarinic M(1) receptors whereas M(3) receptor number was decreased significantly (P < 0.001) with no change in affinity when compared to 7 week old control respectively. M(1) receptor number was decreased significantly (P < 0.001) whereas M(3) receptor number was increased significantly (P < 0.001) in both 7 week and 90 week old diabetic rat groups compared to their respective controls. The competition curve for [(3)H]QNB fitted for two sited model in 7 week old groups whereas fitted for one sited model in 90 week old groups. [(3)H]DAMP was fitted for two sited model in both 7 week and 90 week old groups. Insulin treatment significantly reversed (P < 0.001) the binding parameters to near control level. In vitro studies showed that acetylcholine through muscarinic M(1) and M(3) receptors stimulated insulin secretion from the pancreatic islets. Thus our studies suggest that both brainstem and pancreatic muscarinic M(1), M(3) receptors differentially regulate the cholinergic activity and insulin secretion which will have clinical significance in the management of diabetes and insulin treatment as a function of age.

Effects of a high-glucose environment on the pituitary growth hormone-releasing hormone receptor: type 1 diabetes compared with in vitro glucotoxicity

The present study investigated the effects of diabetes and high glucose on GHRH receptor (GHRH-R) mRNA and protein levels in the pituitary of diabetic rats 2, 21, and 60 days post-streptozotocin (post-STZ) administration. Two days post-STZ, the 2.5-kb GHRH-R mRNA transcript was increased. Twenty-one days post-STZ, both the 2.5- and 4-kb transcripts and a 72-kDa (125)I-GHRH-GHRH-R complex were elevated. Sixty days post-STZ, the 4-kb transcript remained increased and the 45-kDa (125)I-GHRH-GHRH-R complex (functional receptor) was decreased. Hypothalamic GHRH mRNA and serum total IGF-I levels were reduced at all three time points. To better understand the role of high glucose on GHRH-R regulation, time-course effects of 33 compared with 6 mM d-glucose (DG) were examined in cultured anterior pituitary cells from 2-mo-old healthy rats. Membrane lipoperoxidation was present in 33 mM DG, and GHRH-R mRNA levels were diminished after 24 h, Fluo-GHRH internalization was marginal after 16-24 h, and GHRH-induced cAMP levels were decreased after 24 and 48 h. Altogether, these results indicate that the increase of the 2.5-kb GHRH-R mRNA transcript in vivo could be a consequence of a decrease of hypothalamic GHRH mRNA levels in STZ rats. Since it does not affect primarily functional GHRH-R levels, the initial diminution of circulating IGF-I levels could result from a decreased GHRH-R stimulation by GHRH. Thus, the effect of glucotoxicity would be related to a decrease of functional GHRH-R protein, as observed in rats 60 days post-STZ and in cultured pituitary cells from healthy rats exposed to a high-glucose environment.

Decreased [3H] YM-09151-2 binding to dopamine D2 receptors in the hypothalamus, brainstem and pancreatic islets of streptozotocin-induced diabetic rats

In the present study dopamine was measured in the hypothalamus, brainstem, pancreatic islets and plasma, using HPLC. Dopamine D2 receptor changes in the hypothalamus, brainstem and pancreatic islets were studied using [3H] YM-09151-2 in streptozotocin-induced diabetic and insulin-treated diabetic rats. There was a significant decrease in dopamine content in the hypothalamus (P<0.001), brainstem (P<0.001), pancreatic islets (P<0.001) and plasma (P<0.001) in diabetic rats when compared to control. Scatchard analysis of [3H] YM-09151-2 in the hypothalamus of diabetic rats showed a significant decrease in Bmax (P<0.001) and Kd, showing an increased affinity of D2 receptors when compared to control. Insulin treatment did not completely reverse the changes that occurred during diabetes. There was a significant decrease in Bmax (P<0.01) with decreased affinity in the brainstem of diabetic rats. The islet membrane preparation of diabetic rats showed a significant decrease (P<0.001) in the binding of [3H] YM-09151-2 with decreased Kd (P<0.001) compared to control. The increase in affinity of D2 receptors in hypothalamus and pancreatic islets and the decreased affinity in brainstem were confirmed by competition analysis. Thus our results suggest that the decreased dopamine D2 receptor function in the hypothalamus, brainstem and pancreas affects insulin secretion in diabetic rats, which has immense clinical relevance to the management of diabetes.

Aldosterone biosynthesis, regulation, and classical mechanism of action

Circulating aldosterone is principally made in the glomerulosa zone of the adrenal cortex by a series of enzyme steps leading to the conversion of cholesterol to aldosterone. Uniquely, aldosterone's production is regulated at two critical enzyme steps: (1) early in its biosynthetic pathway (the conversion of cholesterol to pregnenolone cholesterol side chain cleavage enzyme) and (2) late (the conversion of corticosterone to aldosterone by aldosterone synthase). A variety of factors modify aldosterone secretion--the most important are angiotensin II (AngII), the end-product of the renin-angiotensin system (RAS), and potassium. However ACTH, neural mediators and natriuretic factors also contribute at least over the short run. Aldosterone's classical epithelial effect is to increase the transport of sodium across the cell in exchange for potassium and hydrogen ions. Although still controversial, there is an increasing body of data that supports the hypothesis that aldosterone can be synthesized in tissues outside of the adrenal cortex, specifically in the heart and the vasculature. Aldosterone's biosynthesis appears to be regulated in these tissues similar to what occurs in the adrenal cortex. The role of this extra adrenal aldosterone production in health and disease is as of yet undetermined.

Effects of spironolactone on systolic blood pressure in experimental diabetic rats

BACKGROUND

Mineralocorticoid hormones, which maintain electrolyte balance and blood pressure, are thought to be associated not only with the expression of renal 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2), but also with that of intracellular mineralocorticoid receptors (MRs). The present study was designed to test whether the mineralocorticoid action of glucocorticoid corticosterone on renal MR is involved in the development of diabetes-associated hypertension by measuring the alterations of renal 11beta-HSD2.

METHOD

We measured the mean systolic blood pressure, renal 11beta-HSD1, and mRNA levels in streptozotocin (STZ)-induced diabetic rats that received spironolactone, insulin, or no treatment, and in nondiabetic controls that received spironolactone.

RESULTS

Four weeks after an injection of STZ, the renal 11beta-HSD2 and mRNA levels were significantly lower in diabetic rats than in control rats, and the mean systolic blood pressure was 14.8% higher in diabetic rats than in controls. Subcutaneous injections of spironolactone into diabetic rats for three weeks partially reversed the decrease in renal 11beta-HSD2 activity and gene expression, and prevented the mean systolic blood pressure elevation. Spironolactone treatment for one week also resulted in a significant reduction in mean systolic blood pressure during the development of diabetic hypertension. However, treatment with STZ did not significantly decrease the renal 11beta-HSD1 activity and mRNA expression, and spironolactone treatment did not exert a significant effect on this enzyme in STZ-induced diabetic rats.

CONCLUSION

In the development of diabetes-induced hypertension, the effect of spironolactone on mean systolic blood pressure may be associated with the mineralocorticoid effects of corticosterone on renal MR, as well as an alteration of renal 11beta-HSD2 activity and its mRNA expression in insulin-dependent diabetic rats.

Effect of pyridoxine and insulin administration on brain glutamate dehydrogenase activity and blood glucose control in streptozotocin-induced diabetic rats

Blood glucose level and kinetic parameters of glutamate dehydrogenase (GDH) were measured in the cerebellum, brain stem and cerebral cortex of control, insulin treated, pyridoxine treated, pyridoxine and insulin treated and untreated streptozotocin-diabetic rats. The combined administration of insulin and pyridoxine was found to be better in controlling the hyperglycaemia. Insulin with pyridoxine treatment brought back the increased maximal velocity of GDH during diabetes to control state. Also, there was an increase in Michaelis-Menten constant. These results suggest that pyridoxine and insulin together serve a better control for diabetes.

Gene expression of 11beta-hydroxysteroid dehydrogenase type 1 and type 2 in the kidneys of insulin-dependent diabetic rats

The presence of 11beta-hydroxysteroid dehydrogenase (11beta-HSD) activity in the kidney has been suggested to be important in the regulation of glucocorticoid-induced disorders of electrolyte balance and the control of blood pressure. To assess the possible effect of 11beta-HSD isoforms in diabetes-related hypertension, we measured the mean systolic blood pressure and the 11beta-HSD activity and mRNA levels for both 11beta-HSD1 and 11beta-HSD2 in the kidney of streptozotocin (STZ)-diabetic female rats. Three weeks after injection of STZ (65 mg/kg), the mean systolic blood pressure of diabetic rats was elevated 13.6% above that of normal rats (P<.01). The renal 11beta-HSD2 activity and level of mRNA expression were significantly decreased in diabetic rats (P<.01). However, the treatment of rats with STZ did not decrease the levels of renal 11beta-HSD1 activity and mRNA expression in diabetic rats. Insulin administered subcutaneously to diabetic rats for 2 weeks completely reversed the decrease in renal 11beta-HSD2 activity and gene expression and prevented the elevation in blood pressure in the diabetic rat. These results indicate that alteration of renal 11beta-HSD2 activity and gene expression may be primarily responsible for the changes in blood pressure of STZ-diabetic rats after early treatment with insulin.

Poor glycemic control induces hypertension in diabetes mellitus

We conducted this study to test the hypothesis that hypertension is a primary consequence of poor glycemic control per se very early in insulin-dependent diabetes mellitus. Sprague-Dawley rats (n=15) were instrumented with artery and vein catheters, placed in metabolic cages, and sodium intake was clamped throughout the study. Mean arterial pressure was measured 24 h/d. After a precontrol period, streptozotocin (70 mg/kg IV) was administered, and 15 hours later a continuous intravenous infusion was begun at 4 U/rat per day. The insulin infusion was titrated on an individual rat basis to maintain good glycemic control, and after this 7-day control period, blood glucose, urinary sodium excretion, and mean arterial pressure were not different from precontrol values, averaging 8.8 +/- 0.6 mmol/L, 2.8 +/- 0.2 mmol/d, and 103 +/- 2 mm/Hg, respectively, for control days 5 through 7. Subsequently, a 4-day period of poor glycemic control was initiated by reducing the insulin infusion rate. Blood glucose, urinary sodium excretion, and mean arterial pressure began to increase on day 1; for diabetes days 3 and 4, they averaged 23.4 +/- 1.0 mmol/L, 3.6 +/- 0.1 mmol/d, and 110 +/- 2 mm Hg, respectively. All were significantly elevated. When insulin treatment was restored, all variables returned to control levels during the next 4 days. A second 4-day diabetic period yielded similar results. These results indicate that elevated blood pressure is a primary consequence of poor glycemic control in insulin-dependent diabetes, occurring before renal injury has had time to develop, and therefore, may be a factor contributing to the initiation of end-organ injury.

Autonomic dysfunction in short-term experimental diabetes

Previous data showed that diabetes induced by streptozotocin for 5 days causes changes in arterial pressure control and baroreflex regulation of heart rate in male Wistar rats. The impairment of baroreflex may be related to autonomic neuropathy as described by several investigators. The aim of this study was to identify autonomic changes in short-term experimental diabetes in rats (induced for 5 days with streptozotocin 65 mg IP). Intra-arterial blood pressure signals were obtained from 6 control group and 7 diabetic group rats and processed in a data acquisition system (CODAS, 1 kHz). Both vagal and sympathetic function were assessed through intravenous injections of methylatropine and propranolol. Streptozotocin induced hyperglycemia (18.9 +/- 1.8 versus 5.8 +/- 0.2 mmol/L) and reductions in mean arterial pressure (102 +/- 2 versus 117 +/- 3 mm Hg) and resting heart rate (298 +/- 14 versus 332 +/- 2 beats per minute). Sodium and potassium levels were not different between groups. The intrinsic heart rate was reduced in the diabetic group (302 +/- 10 versus 398 +/- 6 beats per minute). This group also exhibited depressed vagal and sympathetic tone (50% and 22%, respectively), reduction of vagal effect (42%), and no change in sympathetic effect. In conclusion, early autonomic dysfunction in short-term streptozotocin-induced diabetes seems to be related to changes in arterial pressure and baroreflex control.

Effect of administration of insulin on streptozotocin-induced diabetic hypertension in rat

We have reported that streptozotocin-induced insulin-dependent diabetes mellitus in 25% reduced renal mass rats is associated with low-renin, volume-expanded hypertension and that the development of the hypertension can be prevented with insulin. In this study we examined the effect of insulin after the animals had developed sustained hypertension. Normotensive 25% reduced renal mass rats were treated with streptozotocin and, as expected, developed insulin-dependent diabetes mellitus and hypertension. After 4 weeks of sustained hypertension, neutral protamine Hagedorn insulin (6 to 8 IU/d) was administered subcutaneously for 4 weeks. As expected, insulin treatment decreased plasma glucose and increased body weight gain relative to untreated diabetic rats. On the other hand, insulin treatment did not reverse the hypertension and albuminuria. It also did not normalize extracellular fluid volume and plasma renin activity. Furthermore, insulin treatment did not reverse the increase in plasma Na+,K(+)-ATPase inhibitory activity (determined by both radioimmunoassay and bioassay) and the inhibition of myocardial microsomal Na+,K(+)-ATPase activity observed in the untreated diabetic hypertensive rats. 5'-Nucleotidase, a membrane marker, was not different between insulin-treated and untreated diabetic rats. These results show that insulin, given as here described, does not reverse the insulin-dependent diabetes mellitus hypertension in 25% reduced renal mass rats once it is established, perhaps because it does not reverse the albuminuria, volume expansion, increase in endogenous digitalis-like substance, and inhibition of cardiovascular muscle cell Na+,K(+)-ATPase activity.

Blood pressure reduction in hypertensive-diabetic rats by the somatostatin analog MK-678

A hypotensive effect of an orally-administered cyclopeptide somatostatin analog, MK-678, has been demonstrated in a hypertensive diabetic rat model. Sustained blood pressure reduction failed to occur when the drug was administered to the spontaneously hypertensive rat. The mechanism of hypotension appears independent of effects on a variety of hormones including insulin, glucagon, growth hormone, and components of the renin-angiotensin system including renin activity, plasma angiotensin converting enzyme, and aldosterone.

Hypertension and diabetes

Diabetes mellitus and hypertension are both common diseases, especially with an increasingly aged population. Hypertension accelerates the development of diabetic retinopathy, nephropathy, and peripheral vascular disease in the diabetic patient. Diabetes represents a type of premature aging and hypertension in the diabetic patient is characterized by many of the same pathophysiologic properties seen in the elderly hypertensive patient.

Systemic hypertension in diabetes mellitus

Hypertension occurs more frequently in diabetics and markedly exacerbates the vascular morbidity and mortality resulting from this metabolic disorder. However, the etiology of hypertension in diabetics remains poorly understood. Like aging persons, diabetics have increased systemic resistance and a probable reduction in baroreceptor sensitivity. They also have an expanded total body sodium pool and a tendency to lower levels of plasma renin activity. Some of these factors suggest that a subtle calcium deficiency could also be of etiologic importance.

Blood pressure and metabolic effects of streptozotocin in Wistar-Kyoto and spontaneously hypertensive rats

There are conflicting reports regarding the action of streptozotocin (STZ) on blood pressure (BP) in rats. This study investigated the BP, metabolic and hormonal effects of increasing doses of STZ in Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR), with consideration to methodological aspects. Indirect tail-cuff systolic BP measured in a conscious state was mildly elevated after 2 to 4 weeks and remained so in severely diabetic, emaciated WKY, whereas there were no changes in the SHR. Four and 20 weeks after STZ administration, systolic, mean and diastolic BPs measured in a conscious state with an arterial catheter were unchanged in the diabetic WKY and were decreased in the diabetic SHR. Thus, the changes in BP depended on the method used. Dose-dependent increases in blood glucose were similarly evident under conscious and ether-anesthetized conditions. Triglycerides were increased, and blood insulin and thyroxine levels were decreased in both strains. Between-strain comparisons revealed that the hypoinsulinemic response was similar, but the hyperglycemic and hypertriglyceridemic responses were greater in the SHR. The findings provide a data base for further investigation on STZ diabetes. In addition, the results suggest a different BP and metabolic susceptivity to STZ treatment in the SHR and WKY.

Does so-called streptozocin hypertension exist in rats?

Although the existence of so-called streptozocin hypertension seems well established, some reports have indicated that no rise in blood pressure (BP) occurred after streptozocin treatments. To ascertain the streptozocin-induced BP response, normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were treated with streptozocin, 40 to 45 and 35 mg/kg i.v., respectively, and BP was determined directly and indirectly every week for 3 to 4 weeks. Direct mean BP was determined without anesthesia or restraint through a cannula inserted into the rat's abdominal aorta. Indirect BP was determined at the tail without anesthesia after prewarming the rat in a holder. Compared with control values, indirect BP increased significantly in diabetic WKY 2 weeks after streptozocin treatment. In contrast, direct BP of these rats decreased, compared with control values. Indirect BP of diabetic SHR was as high as that of the controls, whereas direct BP of diabetic SHR decreased significantly 1 week after the treatment and thereafter, compared with control values. These discrepancies between the direct and indirect BP values may be caused by severe emaciation of diabetic rats. Extra pressure in the cuff may be necessary to occlude the bloodstream. These results indicate that under these conditions the value of BP obtained by the direct measurement is more reliable than that by the indirect one; therefore, we concluded that so-called streptozocin hypertension does not exist.

Cerebral cholinergic control of rat arterial blood pressure in streptozotocin-induced diabetes

Rats were made diabetic with a single intravenous injection of streptozotocin (STZ; 40 mg/Kg). Buffer treated animals were used as controls. Experiments were performed 7 and 14 days thereafter. One week diabetic rats (plasma glucose = 3.34 +/- 0.58 mg/ml), compared with control animals (plasma glucose = 0.94 +/- 0.33 mg/ml), showed higher (P less than 0.05), more prolonged and dose-dependent pressor and bradycardic responses to intracerebroventricular (icv) injection of carbachol (125, 250 and 500 ng), together with a significantly lower bradycardia after icv injection of physostigmine (1.25, 2.5 and 5 mcg). The pressor response to icv injection of physostigmine (1.25 mcg) was significantly reduced in diabetic rats. Pressor and bradycardic responses induced by angiotensin II (100 and 200 ng, icv) did not show any differences between control and diabetic animals, thus ruling out an impairment of peripheral nerve conduction. Diabetic rats exhibited higher content of acetylcholine (Ach) in the striatum (123.8 +/- 3.09 nmoles/g) and in the hypothalamus (45.7 +/- 1.31 nmoles/g). Three weeks diabetic animals (plasma glucose = 2.76 +/- 0.23 mg/ml) had neither different cardiovascular responsiveness to icv injection of muscarinic agonists nor changes in hypothalamus and striatum Ach content. Data strongly suggest that STZ-induced diabetes temporarily alters cerebral acetylcholine control of cardiovascular apparatus.

Diabetic control and the renin-angiotensin system, catecholamines, and blood pressure

Diabetic ketoacidosis is usually associated with marked secondary hyperaldosteronism. Plasma levels of renin, angiotensin II, and aldosterone are markedly raised before treatment in most patients, with values falling rapidly toward normal as metabolic control is restored. In a few patients, mostly those with long-term complications of diabetes, plasma levels of renin, angiotensin II, and aldosterone before treatment remain within the normal range. In moderately hyperglycemic patients who have glycosuria but not ketonuria, plasma levels of all three substances are significantly higher than when control is improved. Occasionally, moderately hyperglycemic patients have mild secondary hyperaldosteronism. Improved metabolic control in such patients causes a rise in plasma volume and a rise in total exchangeable sodium, the latter to levels significantly above normal. Plasma catecholamine levels are markedly elevated in diabetic ketoacidosis, probably as a consequence of the ketoacidotic state. In nonketotic patients with moderate hyperglycemia, basal plasma norepinephrine levels are normal; catecholamine responses to exercise may be exaggerated, however. Epidemiological and animal studies suggest a relationship between blood pressure and blood glucose levels. There are few clinical studies of the effects of altering metabolic control of diabetes on blood pressure, and this is an important area for further study.

Hypertension in experimental diabetes mellitus. Renin-prostaglandin interaction

To investigate mechanisms involved in the high incidence of hypertension in diabetes mellitus, the relationship between renin-angiotensin production and renal prostaglandin E2 synthesis was studied in rats 1 week after diabetes mellitus had been induced by streptozotocin injection. The diabetic rats became hypertensive, although plasma renin activity did not increase despite the plasma volume contraction resulting from polyuria and natriuresis. Subcutaneous insulin injection resulted in a marked increase in plasma renin activity, while more rigid control of diabetes mellitus achieved by constant insulin infusion decreased blood pressure. Cortical renin content and renin release as well as papillary prostaglandin E2 synthesis in vitro were significantly lower in diabetic rats than in nondiabetic controls. Isoproterenol and prostaglandin E2 stimulated renin release in controls, while diabetic rats responded only to isoproterenol. Insulin infusion by pump reversed these abnormalities. An additive effect of a maximum dose of isoproterenol (10(-5) M) and prostaglandin E2 (10(-4) M) on renin release was observed in nondiabetic controls and in diabetic rats treated with insulin pump, but not in untreated diabetic rats. The results suggest that 1) renal renin release and prostaglandin E2 synthesis in diabetes mellitus are insulin dependent, 2) inappropriately lower plasma renin activity in diabetes mellitus may be attributed to a diminished renal renin pool and a lack of renin release in response to renal prostaglandin E2, the synthesis of which is also impaired in diabetes, prostaglandin E2-induced renin release may operate independently from isoproterenol-induced renin release, and impaired renal prostaglandin E2 synthesis may contribute to the development of hypertension in the face of an unchanged prohypertensive renin-angiotensin II system.

The behavioral effects of the calcium agonist Bay K 8644 in the mouse: antagonism by the calcium antagonist nifedipine

Mice injected with the calcium agonist BAY K 8644 (2-4 mg/kg, i.p.) displayed profound behavioral changes including ataxia, decreased motor activity, Straub tail, arched back, limb clonus and tonus, and an increased sensitivity to auditory stimulation. BAY K 8644 significantly impaired rotorod performance in mice with an ED50 of 0.8 mg/kg. The behavioral effects of BAY K 8644 were antagonized by nifedipine, but not by the non-dihydropyridine calcium channel antagonist verapamil or the alpha-adrenoceptor antagonist prazosin. Further, the actions of BAY K 8644 were not mimicked by the alpha-adrenoceptor agonist methoxamine at doses up to 4.5 mg/kg. These observations, coupled with the findings that BAY K 8644 is a potent, competitive inhibitor of [3H]nitrendipine binding to the dihydropyridine binding site in mouse brain (Ki = 7.0 X 10(-9) M), suggests that BAY K 8644 may produce its behavioral actions via an interaction with the DHP binding site, which has been linked to the control of calcium flux across membranes in peripheral tissues.

Morphometric studies on retinal microangiopathy and myocardiopathy in hypertensive rats (SHR) with induced diabetes

In experiments of 4 and 8 months duration, blood pressure (BP) and heart rate (HR) were measured periodically in streptozotocin-diabetic (D) rats of normotensive (WKY) and hypertensive (SHR) strains and in corresponding controls (C). BP and HR of D were lower than those of C. In both experiments body length, heart weight and the heart weight/body length ratio (W/L) of D were reduced at autopsy. The number of myocardial cell nuclei per test area was increased in D, indicating a reduced myocardial cell size. No specific change was found at any time in the coronary arteries of rats from any of the groups examined. At 4 months, the retinal capillary basement membrane (BM) of the SHR strain was significantly thicker than that of the WKY strain. At 8 months the BM of D rats was significantly thicker than that of C, but there was no difference between strains. Our results demonstrate that long-term diabetes promotes BM thickening in both normotensive and hypertensive animals. Hypertension does not seem to potentiate diabetes in inducing microangiopathy. Diabetes, however, by causing a lower heart weight and by reducing the heart rate may influence negatively the development of hypertension in the SHR strain. It also appears that there is no direct relationship between increased retinal capillary BM thickness and myocardiopathy.

Chronic sucrose ingestion induces mild hypertension and tachycardia in rats

As a means for increasing sympathetic activity, male weanling rats were given 8% sucrose solution to drink instead of water. After 5 weeks, systolic pressures measured with a tail-cuff method became appreciably elevated, and the elevation was verified when phasic pressures were later recorded directly from femoral catheters. Successful induction of sympathetic overactivity was considered a likely explanation because sucrose-ingesting rats, compared with untreated controls, had faster heart rates and larger hypotensive responses to alpha-adrenergic blockade with phentolamine. Upon graded electrical stimulation of the ventromedial hypothalamus under urethane anesthesia, resulting pressor and sympathetic nerve responses were also larger in sucrose-treated rats. By contrast, pressor responses to injections of norepinephrine or tyramine were unaffected, thereby indicating that cardiovascular sensitivity had not been enhanced by sucrose ingestion. During intravenous glucose tolerance tests, increases in plasma insulin were consistently lower in sucrose-treated than control rats even though corresponding increases in plasma glucose were just transiently higher. These results support the interpretation that chronic sucrose ingestion inhibits pancreatic insulin secretion and elevates blood pressure by stimulating the ventromedial hypothalamus to increase sympathetic activity.