Effect of obstructive sleep apnea on the response to hypertension therapy.

Obstructive sleep apnea (OSA) often precedes cardiovascular disease, partly due to treatment resistant hypertension. The nocturnal apneas of OSA trigger increased sympathetic nervous discharge during both sleep and wakefulness. Apneas also trigger cardiac release of the endogenous diuretic atrial natriuretic peptide. We hypothesized that treatment of the excess sympathetic nervous activity of OSA with a ß1 blocker would lower 24 h blood pressure (BP) more than diuretic therapy. Subjects with OSA associated hypertension received 2 weeks of placebo followed by the ß1 blocker nebivolol or hydrochlorothiazide (HCTZ) for 6 weeks in a blinded crossover study. BP, baroreflex sensitivity (BRS), heart rate variability (HRV), arterial reactivity, and stiffness were measured after placebo and each treatment. The ß1 blocker lowered clinic BP by -11/-8 mmHg, more than the -3/-1 effect of HCTZ (P < 0.01). The ß1 blocker lowered 24 h diastolic blood pressure (DBP) more than HCTZ. Although given at bedtime, neither drug increased BP dipping. Nebivolol increased HRV in the high-frequency band. Nebivolol did not alter BRS while HCTZ significantly diminished BRS compared to nebivolol (P < 0.01). Nebivolol increased flow-mediated brachial artery dilation when compared to HCTZ and slowed pulse wave velocity, indicating a decrease in arterial stiffness. Diuretic therapy failed to lower BP in OSA subjects and this might account for the frequent association of OSA with treatment resistant hypertension. However, blockade of the excess sympathetic nervous activity of OSA with a ß1 blocker lowered both clinic and 24 h DBP.

Chronic ß2 adrenergic agonist, but not exercise, improves glucose handling in older type 2 diabetic mice.

Insulin resistant type 2 diabetes mellitus in the obese elderly has become a worldwide epidemic. While exercise can prevent the onset of diabetes in young subjects its role in older diabetic people is less clear. Exercise stimulates the release of the ß(2)-agonist epinephrine more in the young. Although epinephrine and ß(2)-agonist drugs cause acute insulin resistance, their chronic effect on insulin sensitivity is unclear. We fed C57BL/6 mice a high fat diet to induce diabetes. These overweight animals became very insulin resistant. Exhaustive treadmill exercise 5 days a week for 8 weeks had no effect on their diabetes, nor did the ß(2)-blocking drug ICI 118551. In contrast, exercise combined with the ß(2)-agonist salbutamol (albuterol) had a beneficial effect on both glucose tolerance and insulin sensitivity after 4 and 8 weeks of exercise. The effect was durable and persisted 5 weeks after exercise and ß(2)-agonist had stopped. To test whether ß(2)-agonist alone was effective, the animals that had received ß(2)-blockade were then given ß(2)-agonist. Their response to a glucose challenge improved but their response to insulin was not significantly altered. The ß(2)-agonists are commonly used to treat asthma and asthmatics have an increased incidence of obesity and type 2 diabetes. Although ß(2)-agonists cause acute hyperglycemia, chronic treatment improves insulin sensitivity, probably by improving muscle glucose uptake.

Epinephrine and the metabolic syndrome.

Epinephrine is the prototypical stress hormone. Its stimulation of all α and ß adrenergic receptors elicits short-term systolic hypertension, hyperglycemia, and other aspects of the metabolic syndrome. Acute epinephrine infusion increases cardiac output and induces insulin resistance, but removal of the adrenal medulla has no consistent effect on blood pressure. Epinephrine is the most effective endogenous agonist at the ß2 receptor. Transgenic mice that cannot make epinephrine and mice that lack the ß2 receptor become hypertensive during exercise, presumably owing to the absence of ß2-mediated vasodilatation. Epinephrine-deficient mice also have cardiac remodeling and poor cardiac responses to stress, but do not develop resting hypertension. Mice that cannot make epinephrine have a normal metabolism on a regular 14% fat diet but become hyperglycemic and insulin resistant when they eat a high fat diet. Vigorous exercise prevents diabetes in young mice and humans that overeat. However, exercise is a less effective treatment in older type 2 human diabetics and had no effect on glucose or insulin responses in older, diabetic mice. Sensitivity of the ß2 receptor falls sharply with advancing age, and adrenal epinephrine release also decreases. However, treatment of older diabetic mice with a ß2 adrenergic agonist improved insulin sensitivity, indicating that ß2 subsensitivity can be overcome pharmacologically. Recent studies show that over the long term, epinephrine prevents hypertension during stress and improves glucose tolerance. The hyperglycemic influence of epinephrine is short-lived. Chronic administration of epinephrine and other ß2 agonists improves cellular glucose uptake and metabolism. Overall, epinephrine counteracts the metabolic syndrome.

Cardiovascular Regulation in Obstructive Sleep Apnea.

The majority of patients with obstructive sleep apnea (OSA) suffer from hypertension as a complication of both the metabolic syndrome and OSA. In animal studies, intermittent hypoxia that simulates changes seen in OSA leads to chemoreceptor and chromaffin cell stimulation of sympathetic nerve activity, endothelial damage and impaired blood pressure modulation. Human studies reveal activation of sympathetic nerves, endothelial damage and exaggerated pressor responses to sympathetic neurotransmitters and endothelin. Although treatment of the OSA normalizes sympathetic nerve responses, it only lowers blood pressure modestly. Agents that block the consequences of sympathetic over activity, such as ß1 blockers and angiotensin antagonists have effectively lowered blood pressure. Diuretics have been less successful. Treatment of hypertensive patients with OSA usually requires consideration of both increased sympathetic nerve activity and the metabolic syndrome.

Endogenous epinephrine protects against obesity induced insulin resistance.

Epinephrine (E) is a hormone released from the adrenal medulla in response to low blood sugar and other stresses. E and related ß2-adrenergic agonists are used to treat asthma, but a side effect is high blood sugar. C57BL/6 mice prone to overfeeding induced type II diabetes had the PNMT gene knocked out to prevent E synthesis. These E deficient mice were very similar to control animals on a 14% fat diet. On a 40.6% fat diet they gained 20 to 33% more weight than control animals and increased their blood glucose response to a glucose tolerance test because they became resistant to insulin. Although the short term effect of ß2-agonists such as E is to raise blood glucose, some long acting ß2-agonists improve muscle glucose uptake. Endogenous E protects against overfeeding induced diabetes. Since adrenal E release can be impaired with aging and diabetes, endogenous E may help prevent adult onset diabetes.

Cardiovascular responses to electrical stimulation of sympathetic nerves in the pithed mouse.

The pithed rat model has been used extensively to study peripheral cardiovascular responses to electrical stimulation of the sympathetic nervous system, as pithing eliminates central and reflex effects. However, since the transgenic mouse has become a standard and economical model organism, an electrically stimulated pithed mouse would facilitate a variety of studies. We have developed surgical techniques, drug doses and stimulation parameters for an electrically stimulated pithed mouse to study peripheral sympathetic nerve effects on blood pressure. Similar to the pithed rat, the pithed mouse showed voltage and frequency-dependent blood pressure responses to a pulsed train of electrical stimuli. In addition, alpha-adrenergic stimulation with phenylephrine gave a marked systolic pressor response, while the beta2 agonist salbutamol lowered diastolic blood pressure. Furthermore, pithed transgenic mice unable to synthesize catecholamines in adrenergic cells displayed smaller pressor responses than pithed control mice. In summary, the electrically stimulated pithed mouse can be used to study peripheral effects of the sympathetic system on cardiovascular dynamics unencumbered by central responses.

Epinephrine regulation of hemodynamics in catecholamine knockouts and the pithed mouse.

Phenylethanolamine N-methyltransferase (PNMT) catalyzes synthesis of epinephrine (E) and is present in the brain, heart, and adrenal. E is a neurotransmitter and important hormone; however, its role in regulating cardiovascular dynamics is still unclear. We generated an E-deficient mouse model by knocking out the PNMT gene. The PNMT KO mouse had normal resting blood pressure, while treadmill exercise caused hypertension, suggesting an impaired response to stress in the absence of the stress hormone E. As PNMT occurs at a lower concentration in many extra-adrenal tissues including the brain, we set up a pithed mouse model to study the peripheral effects of E on cardiovascular dynamics, using pithing to eliminate central and reflex effects. The pithed mouse requires different surgical techniques and stimulation voltages than rats, and showed voltage- and frequency-dependent blood pressure responses to electrical stimuli. Stimulation with the alpha-adrenergic agonist phenylephrine gave a marked systolic pressor response, while the beta2 agonist salbutamol lowered diastolic blood pressure. The pithed PNMT KO mouse had an exaggerated blood pressure response to salbutamol, suggesting beta2 receptor supersensitivity. A targeted KO of tyrosine hydroxylase in PNMT-producing cells produced a mouse deficient in catecholamines in the adrenal. These targeted KO mice displayed significantly smaller pressor responses than pithed control mice. We find that E release during stress prevents an excessive increase in blood pressure.

L-NAME raises systolic blood pressure in the pithed rat by a direct adrenal epinephrine releasing action.

It is generally thought that inhibition of nitric oxide synthase leads to blood pressure elevation largely through reduction in vascular levels of the vasodilator nitric oxide. However, there are several reports suggesting that NO synthase inhibitors cause adrenal epinephrine (E) release by both central and peripheral mechanisms. We investigated the role of adrenal E in the pressor effects of the nitric oxide synthase inhibitor L-NAME in the pithed rat to help distinguish central from peripherally mediated actions. L-NAME (10 mg/kg) raised both systolic and diastolic BP by about 30 mm Hg (P < .01) in the absence of exogenous electrical stimulation of sympathetic nerves. During stimulation at 10 V and frequencies of 1 or 2 Hz, systolic BP was about 70 mm Hg higher in L-NAME treated rats than in drug free stimulated rats. This enhancement of systolic BP by L-NAME was less pronounced at 5 or 10 Hz stimulation frequencies. Following these types of electrical stimulations of pithed rats, both plasma norepinephrine (NE) and E levels were dramatically elevated above resting plasma levels. L-NAME pretreatment of these electrically stimulated rats increased plasma E levels by an additional 60% and decreased NE by 18%. Acute adrenalectomy dramatically reduced plasma E levels and abolished the ability of L-NAME to enhance the pressor effect of sympathetic stimulation. In contrast, acute adrenalectomy of unstimulated pithed rats did not significantly reduce the pressor response to L-NAME. We conclude that adrenal E release may mediate much of the systolic pressor response of L-NAME in the stimulated pithed rat, but the magnitude of this effect varies with stimulation frequency. Since pithing disrupts central pathways, this induction of adrenal E release by L-NAME is a peripheral effect.

Selective peripheral regulation of noradrenaline and adrenaline release by nitric oxide.

1. Nitric oxide (NO) has complex effects on the sympathoadrenal and cardiovascular systems and may act at both central and peripheral loci. Nitric oxide appears to act directly on blood vessels and indirectly by modulating the sympathoadrenal system. In the present study, we investigated the contribution of catecholamine release from peripheral vascular and adrenal sympathetic nerves to the cardiovascular effects of the NO synthesis inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 10 mg/kg). Our experiments were performed in pithed vagotomized rats to remove the influence of central and baroreflex pathways. 2. Spinal cord stimulations for 30 s periods at 1, 2, 5 and 10 Hz using pulses of 1 msec at 10 V caused marked increases in plasma adrenaline and noradrenaline. N(G)-Nitro-L-arginine methyl ester did not alter resting plasma catecholamine concentrations. However, L-NAME generally more than doubled stimulation-evoked release of adrenaline while reducing the extent of noradrenaline release relative to vehicle (saline)-treated controls. 3. N(G)-Nitro-L-arginine methyl ester significantly enhanced the vasopressor responses to spinal cord stimulation. The alpha1-adrenoceptor antagonist prazosin (0.2 mg/kg) reduced the pressor responses of electrically stimulated L-NAME-treated rats to levels below those of vehicle-treated control rats. 4. In the absence of electrical stimulation, L-NAME raised the blood pressure of pithed rats without altering plasma catecholamines and the pressor effect was briefly attenuated by L-arginine, but was unaffected by prazosin. 5. We conclude that the augmented pressor response to sympathetic stimulation in L-NAME-treated pithed rats is due largely to enhanced adrenal adrenaline release mediated by a peripheral mechanism. Stimulation of alpha(1)-adrenoceptors plays a major role in the pressor response to electrical stimulation of L-NAME-treated rats, but this is not due to L-NAME augmentation of noradrenaline release from vascular sympathetic nerves.

The pressor effect of NO synthase inhibition correlates to pre-existing systolic BP in the rat.

A number of studies have found that the vasopressor effect of nitric oxide (NO) synthase inhibition is small following treatment with hypotensive agents but is enhanced after hypertensive agents, and have implicated NO in the mechanism of action of these drugs. We investigated the hypothesis that the rate of vascular NO synthesis is directly related to blood pressure. The vasopressor effect of 10 mg/kg of L-nitro-L-arginine methyl ester (L-NAME) was studied in relation to changes in BP induced by a variety of treatments in both pentobarbital sodium anesthetized and pithed rats. BP reductions were induced by blood withdrawal, surgery and pithing. BP increases were made by injecting 10 and 15 microg/kg boluses of phenylephrine or by injecting 5% albumin solution. Pithing decreased baseline BP and attenuated the vasopressor effect of L-NAME while phenylephrine increased both BP levels and the hypertensive effect of L-NAME. Volume expansion with 5% albumin solution increased both BP and the vasopressor effect of L-NAME. Both surgery (abdominal incision) and withdrawal of 1 ml blood reduced BP and attenuated the pressor effect of L-NAME. When the results of all these studies were combined, systolic BP was found to correlate strongly with the vasopressor effect of L-NAME (R2 = 0.73, P < 0.0001). Diastolic BP correlated less well with L-NAME (R2 = 0.36, P < 0.0003). The results suggest that shear stress generated by blood flow during the systole releases NO, and lowers BP. The pressor effect of NO synthase inhibition is closely related to pre-existing systolic BP.