The linked roles of nitric oxide, aldose reductase and, (Na+,K+)-ATPase in the slowing of nerve conduction in the streptozotocin diabetic rat.

Metabolic and vascular factors have been invoked in the pathogenesis of diabetic neuropathy but their interrelationships are poorly understood. Both aldose reductase inhibitors and vasodilators improve nerve conduction velocity, blood flow, and (Na+,K+)-ATPase activity in the streptozotocin diabetic rat, implying a metabolic-vascular interaction. NADPH is an obligate cofactor for both aldose reductase and nitric oxide synthase such that activation of aldose reductase by hyperglycemia could limit nitric oxide synthesis by cofactor competition, producing vasoconstriction, ischemia, and slowing of nerve conduction. In accordance with this construct, N-nitro-L-arginine methyl ester, a competitive inhibitor of nitric oxide synthase reversed the increased nerve conduction velocity afforded by aldose reductase inhibitor treatment in the acutely diabetic rat without affecting the attendant correction of nerve sorbitol and myo-inositol. With prolonged administration, N-nitro-L-arginine methyl ester fully reproduced the nerve conduction slowing and (Na+,K+)-ATPase impairment characteristic of diabetes. Thus the aldose reductase-inhibitor-sensitive component of conduction slowing and the reduced (Na+,K+)-ATPase activity in the diabetic rat may reflect in part impaired nitric oxide activity, thus comprising a dual metabolic-ischemic pathogenesis.

Metabolic and hemodynamic effects of moxonidine in the Zucker diabetic fatty rat model of type 2 diabetes.

We studied the effect of moxonidine, an imidazoline ligand, on metabolic and hemodynamic parameters in Zucker diabetic fatty rats, a model of type 2 diabetes. In one group (metabolic group), 8-week-old rats were started on a diet containing either moxonidine (3 or 10 mg x kg(-1) x day(-1)) or vehicle for 4 weeks. Body weight and food intake were monitored daily, plasma insulin and glucose were monitored weekly, and an oral glucose tolerance test (OGTT) was performed at study's end. In another group of rats (hemodynamic group), radio frequency transmitters were implanted 1 week before starting the diet, and mean blood pressure, heart rate, and motor activity were continuously monitored at baseline and for 4 weeks after beginning drug exposure. Moxonidine (10 mg x kg(-1) x day(-1)) significantly decreased elevated glucose levels and prevented the decrease in plasma insulin noted in vehicle-treated or pair-fed groups. Moxonidine also decreased fasting glucose (3 and 10 mg x kg(-1) x day(-1)) and prevented the decrease in fasting insulin (10 mg x kg(-1) x day(-1)) compared with vehicle. Fasting glucose at 10 mg x kg(-1) x day(-1) was equivalent to lean littermates. Both doses significantly increased glucose disposal and the insulin secretory response during the OGTT. Moxonidine lowered daily mean arterial pressure compared with both baseline values and vehicle and decreased daily heart rates. Motor activity was unaffected, except for an increase in the 10 mg x kg(-1) x day(-1) group during low activity periods. Moxonidine did not significantly affect body weight, fluid intake, or urine volume, but the 10 mg x kg(-1) x day(-1) dose reduced urinary protein excretion compared with vehicle-treated animals. These results demonstrate that, in an animal model of type 2 diabetes, the antihypertensive agent moxonidine induces a beneficial effect on abnormal glucose metabolism and renal protein excretion at doses that are effective in lowering arterial blood pressures and heart rate.

Alterations in the secretion of atrial natriuretic factor in atria from aged rats.

We measured plasma atrial natriuretic factor levels and atrial natriuretic factor secretion by isolated left atria from aging rats to determine the secretory response to stretch and adrenergic stimulation. Systolic arterial pressure and right atrial pressure were measured in vivo. Twenty-four hours later, atria were removed and studied in vitro in a perifusion system. After removal, stabilization at 0.7 g tension, and equilibration for 65 minutes, atria were stretched by increasing external tension for 20 minutes. After reequilibration atria were perifused with phenylephrine, 10(-5) M, for an additional 30 minutes. Right atrial pressure was not different between young (3 months) and aged (16-24 months) rats. Aged rats had higher plasma atrial natriuretic factor levels (52 +/- 8 versus 21 +/- 6 pmol/l; p less than 0.05) than young rats. Basal atrial natriuretic factor secretory rate in vitro was greater in atria from aged rats than young rats (875 +/- 35 versus 402 +/- 22 pg/min; p less than 0.05). Atria from aged rats had an increased response to phenylephrine compared with young rats (1,687 +/- 143 versus 788 +/- 113 pg/min; p less than 0.05) when means were adjusted for basal secretory rate. The secretory response to stretch was less than that of young rats (673 +/- 37 versus 773 +/- 27 pg/min), although this difference was not significant (p = 0.07). Atrial natriuretic factor secretion in response to adrenergic stimulation is increased with aging, and these secretory responses may contribute to increased plasma levels that occur during aging. In contrast to increased adrenergic responses, atrial natriuretic factor secretion after external stretch is not increased in aging rats.

Sustained hypertension induced by orally administered nitro-L-arginine.

To study the hemodynamic and metabolic effects of chronic inhibition of endothelium-derived nitric oxide, we treated conscious rats with an oral solution of N omega-nitro-L-arginine (LNA), an inhibitor of nitric oxide production by endothelial cells. After 3 days of treatment with 2.74 mM LNA, rats had higher blood pressures (136 +/- 5 versus 113 +/- 3 mm Hg, p < 0.0005) than did the control animals. This effect was maintained through 7 days of treatment (142 +/- 6 versus 109 +/- 4 mm Hg, p < 0.0005) and in three animals for 35 days (167 +/- 7 mm Hg). The blood pressure rise was dose dependent. The hypertensive effect of oral LNA was not enhanced by the administration of 20 mg intraperitoneal LNA and was prevented by pretreatment with L-arginine, although L-arginine also caused a transient but significant increase in urinary sodium excretion. When LNA treatment was discontinued, blood pressure fell gradually, with an effective biological half-life of 4.2 days. Metabolic balance studies did not identify differences in sodium or potassium balance between treated and control animals. Plasma renin activity was lower in LNA-treated animals, and aldosterone concentrations tended to be lower. In contrast, atrial natriuretic factor levels and serum electrolyte concentrations were unchanged after 7 days of treatment with LNA. These data support the premise that endothelium-derived nitric oxide plays an important role in basal hemodynamic homeostasis. Oral administration of LNA may serve as a model of chronic nitric oxide-deficient hypertension and allow for the future study of endothelium dependence in hypertension.

Sustained saline-induced secretion of atrial natriuretic hormone is not maintained by atrial stretch.

To determine the relationship between changes in right and left atrial pressures and changes in plasma levels of immunoreactive atrial natriuretic hormone (ANH), 11 normal men were studied during rapid infusion of 1 L 150 mmol/L NaCl. Right atrial pressure, pulmonary capillary wedge pressure, and peripheral plasma ANH levels were measured serially for 30 min in 6 men and for 90 min in 5 men. There were significant increases in right atrial pressure at 15 and 30 min [4.8 +/- 0.4 (+/- SE) vs. 8.9 +/- 0.3 and 6.5 +/- 0.4 mm Hg; P less than 0.001] and in pulmonary capillary wedge pressure at the same time intervals [8.5 +/- 0.6 vs. 13.6 +/- 0.8 (P less than 0.001) and 10.6 +/- 0.6 mm Hg (P less than 0.01)]. Plasma ANH increased significantly at 30 min (11.5 +/- 2.4 vs. 20.6 +/- 3.0 pmol/L; P less than 0.001). Regression analysis revealed no correlation between the increase in plasma ANH at 30 min and the increase in either right atrial or pulmonary capillary wedge pressure at 15 min (r = 0.46; P = 0.16 for right atrial pressure; r = 0.02; P = 0.96 for pulmonary capillary wedge pressure). In the 5 men studied for 90 min, right atrial and pulmonary capillary wedge pressures returned to basal values by 45 min. In contrast, plasma ANH levels remained significantly elevated at all sampling times from 30-90 min (P less than 0.001); the peak value occurred at 75 min. We conclude that ANH secretion persists after saline infusion and that the cause of this prolonged secretion is not atrial stretch.

Corticoid regulation of atrial natriuretic factor secretion and gene expression.

This study investigated the acute effects of glucocorticoids and mineralocorticoids on atrial natriuretic factor (ANF) biosynthesis in vivo. Groups of male Sprague-Dawley rats were injected with 1 mg dexamethasone (Dex), 10 mg deoxycorticosterone acetate (DOCA) or vehicle alone. Different groups were studied after periods of 30 min to 8 h. Plasma and left atrial ANF concentrations and ANF mRNA levels were measured 2-8 h after corticoid injection. From 30 min to 2 h after injection, ANF mRNA was analyzed by quantitative and qualitative assessments. There was a two- to threefold increase in plasma levels of ANF in Dex-treated rats compared with controls at all time periods (P < 0.05). Although ANF plasma levels increased over time following DOCA treatment, they were not significantly different from control values. Dex treatment also increased normalized ANF mRNA levels 77% above control levels during the first 4 h after injection (P < 0.05). Thereafter there was a return of mRNA levels to that seen in controls. There was no qualitative difference in the ANF mRNA at any time as assessed by Northern hybridization. In contrast, DOCA increased ANF mRNA levels only after 8 h (P < 0.05). No significant changes in left atrial ANF content were noted during this study. In a separate study, Dex was administered to isolated left atria in vitro in a superfusion system. Superfusion with 2 x 10(-5) M Dex produced a 40% increase in ANF secretory rate within 20 min (P = 0.036). We conclude that Dex induces a direct rapid increase in ANF mRNA levels and ANF secretion in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased sensitivity to vasoconstrictors in aortic rings after acute exposure to nitric oxide.

Nitric oxide (NO) has been postulated as a regulator of vascular reactivity, and the current study tested the hypothesis that NO-induced decreased sensitivity to vasoconstrictors persists following removal of NO. Endothelium-denuded segments of rat aorta were incubated 2-4 h at 37 degrees C with the NO donor S-nitroso-N-acetylpenicillamine (SNAP). Incubation produced rightward shifts in concentration response curves for phenylephrine [i.e., half-maximum effective concentration (EC50; in microM): control = 0.016, NO = 0.14], aluminum fluoride (i.e., EC50 in mM: control = 1.66, NO = 2.29), and KCl (i.e., EC50 in mM: control = 5.9, NO = 23.9). Similar shifts were seen for two other NO donors. The SNAP-induced shift was not attenuated by a guanylyl cyclase inhibitor, LY-83583 (10 microM) and was not mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate (100 microM). It was attenuated by 1,4-naphthoquinone (50 microM), an inhibitor of endogenous mono-ADP ribosyltransferases. NO incubation increased cGMP content (4.6 +/- 0.8 vs. 1.5 +/- 0.15 pmol/mg protein), an increase unaffected by 1,4-naphthoquinone (3.3 +/- 1.0 pmol/mg protein) but prevented by LY-83583 (1.6 +/- 0.36 pmol/mg protein). ADP ribosylation of three proteins was observed in membranes from HEK 293 cells: 88,66, and 38 kDa. ADP ribosylation of the 38-kDa protein was stimulated in a concentration-dependent manner by NO but was not decreased by 1,4-naphthoquinone. In conclusion, NO produces a long-lasting inhibition of vascular contractility by both a cGMP-dependent and -independent mechanism. Based on the observations of 1,4-naphthoquinone, we conclude that the cGMP-independent mechanism is not stimulation of endogenous ADP ribosylation but some other covalent modification in the pathway that mediates contraction.