Sympathoinhibitory effects of central nifedipine in spontaneously hypertensive rats on high versus regular sodium intake

Long-acting calcium channel blocker, azelnidipine, increases endothelial nitric oxide synthase in the brain and inhibits sympathetic nerve activity

Nitric oxide (NO) in the central nervous system inhibits sympathetic nerve activity, thereby decreasing blood pressure. It is unknown, however, whether orally administered antihypertensive treatment alters NO synthase (NOS) expression, particularly in the brain, and how changes in NOS expression affects sympathetic nerve activity. Azelnidipine, a recently developed long-acting dihydropyridine calcium channel blocker, does not cause baroreflex-induced tachycardia. The aim of the present study was to determine whether antihypertensive treatment with azelnidipine alters endothelial NOS (eNOS), neuronal NOS (nNOS), or inducible NOS (iNOS) expression in the brain, and how changes in NOS affect sympathetic nerve activity. Azelnidipine (20 mg/kg/day) or hydralazine (20 mg/kg/day) was orally administered for 30 days in stroke-prone spontaneously hypertensive rats (SHRSP). Blood pressure and heart rate were measured by the tail cuff method. Urinary norepinephrine excretion was measured as a marker of sympathetic nerve activity. Western blot analysis was performed to examine eNOS, nNOS, or iNOS expression levels in the brain (cortex, cerebellum, hypothalamus, and the brain stem), heart, and aorta. The extent of blood pressure reduction was similar between the two groups. Heart rate increased in the hydralazine-treated group but did not change in the azelnidipine-treated group. Urinary norepinephrine excretion was significantly increased only in the hydralazine-treated group. Treatment with azelnidipine significantly increased eNOS expression levels in the brain, heart, and aorta, but did not alter nNOS or iNOS expression levels. Treatment with hydralazine did not change any of the NOS expression levels. These results suggest that antihypertensive treatment with azelnidipine attenuates reflex-induced sympathetic activation and enhances eNOS expression levels in the brain as well as in the heart and aorta.

Role of voltage-sensitive calcium-channels in nitric oxide-mediated vasodilation in spontaneously hypertensive rats

This study demonstrates that the vasodilator potencies of nitric oxide (NO) donors such as sodium nitroprusside are increased in conscious Spontaneously Hypertensive (SH) as compared to Wistar Kyoto (WKY) rats. For example, the NO donors do not dilate hindlimb resistance arteries in WKY rats whereas they elicit pronounced vasodilator responses in SH rats. This study also demonstrates that the NO-mediated vasodilator responses in WKY and SH rats were markedly diminished after blockade of voltage-sensitive Ca2+-channels (CaVS2+-channels) with nifedipine, diltiazem or verapamil. These findings suggest that NO dilates resistance arteries in vivo via direct and/or hyperpolarization-induced closure of CaVS2+-channels and that the increased potency of NO in SH rats may be due to the augmented CaVS2+-channel activity reported in this strain.

Sympatho-excitatory responses to once-daily dihydropyridines in young versus older hypertensive patients: amlodipine versus felodipine extended release

BACKGROUND

Once-daily dihydropyridines exert both indirect sympatho-excitatory and direct central sympatho-inhibitory effects. Age may affect this balance by influencing blood pressure (BP) or renin responses.

METHODS

We evaluated BP, sympathetic and cardiac responses after the first dose and after 8 weeks of treatment with placebo, amlodipine 5 mg/day or felodipine extended release (ER) 5 mg/day in 29 young (22-50 years) versus 37 older (60-77 years) hypertensive patients, using a double-blind, parallel group design.

RESULTS

In the young group, neither dihydropyridine dose decreased BP after the first dose and both caused decreases by 5-10 mmHg after chronic treatment. In the older group, felodipine ER decreased BP rapidly and amlodipine more gradually, and after chronic treatment, systolic BP decreased by 20-25 mmHg. Felodipine ER increased the heart rate by 5-10 bpm after the first dose in both age groups and caused persistent increases in the cardiac index (by 0.2 l/min per square metre) and the ejection fraction only in the older group. Amlodipine did not affect cardiac function in the young, and with chronic dosing decreased the heart rate by 3-5 bpm and the cardiac index by 0.2 l/min per square metre in the older group. In the young hypertensive patients, both dihydropyridines increased plasma norepinephrine (NE) after chronic dosing, with little effect after the first dose. In contrast, in the older group felodipine ER increased plasma NE after the first dose but not with chronic dosing, whereas amlodipine had no effect after the first dose, and after chronic dosing tended to decrease plasma NE.

CONCLUSION

We conclude that age is a major determinant not only of the BP but also of the cardiac and sympathetic responses to once-daily dihydropyridines.

Effects of low-dose nifedipine GITS on sympathetic activity in young and older patients with hypertension

BACKGROUND

Dihydropyridines have both sympathoexcitatory and sympathoinhibitory effects. To date, the latter have been characterized only in animals. During chronic treatment with long-acting dihydropyridines, sympathoexcitatory effects mediated via the arterial baroreflex are unlikely. However, increases in plasma angiotensin II in response to dihydropyridines could contribute to increases in sympathetic activity during chronic treatment. Such increases may be less in older than in young patients.

METHODS

We evaluated the effects of 4 weeks of treatment with low-dose nifedipine gastrointestinal therapeutic system (GITS; 20 mg/day) compared with placebo on muscle sympathetic nerve activity and plasma noradrenaline, in relation to changes in plasma renin activity and plasma angiotensin II and blood pressure in young and older patients with mild hypertension.

RESULTS

Nifedipine GITS decreased systolic and diastolic blood pressures significantly, by 10 +/- 3 mmHg and 7 +/- 2 mmHg respectively, in older patients (age 67 +/- 2 years), but not in younger patients (age 45 +/- 2 years) (decreases of 1 +/- 3 mmHg and 1 +/- 2 mmHg, respectively). Nifedipine GITS caused only minor changes in plasma renin activity and plasma angiotensin II in young and older patients. Compared with changes in response to placebo (-5.7 +/- 2.4 bursts/min), sympathetic activity was increased significantly by nifedipine GITS in the young patients (2.0 +/- 1.7 bursts/min; P < 0.05), but not in older patients (5.4 +/- 1.3 bursts/min by placebo compared with 4.1 +/- 3.5 bursts/min by nifedipine GITS).

CONCLUSION

We conclude that age-related differences in the response of muscle sympathetic nerve activity (and plasma noradrenaline) to low-dose nifedipine GITS in patients with mild hypertension are unlikely to be mediated by plasma angiotensin II. An increase in sympathetic activity may contribute to the absent blood pressure response in young patients with hypertension.

Sympathoinhibitory and depressor effects of amlodipine in spontaneously hypertensive rats

The authors examined whether central actions contribute to the hypotensive effects of peripherally administered amlodipine, a lipophilic dihydropyridine with slow onset and long duration of action. After 5 to 6 weeks of high (8%, H-Na) or regular (0.6%, R-Na) salt intake, changes in renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP), and heart rate (HR) were recorded at rest and in response to intravenous (iv) and intracerebroventricular (icv) injection, and prolonged iv infusion of amlodipine, in conscious spontaneously hypertensive rats (SHR). Iv injection of amlodipine at 50 to 100 microg/kg decreased MAP but increased RSNA and HR in a dose-related manner. In contrast, icv injection of amlodipine at 10 to 50 microg/kg caused parallel decreases in MAP, RSNA, and HR. Iv infusion of amlodipine at 50 microg/kg per hour for 3 hours followed by 100 microg/kg per hour for 2 hours also decreased in parallel RSNA, MAP, and HR. Maximal decreases in RSNA, MAP, and HR in response to icv injection and iv infusion were significantly larger in SHR on H-Na versus R-Na. All responses lasted at least 1 hour following iv and icv injection, or after the termination of iv infusion of amlodipine. These data suggest that in SHR during prolonged iv infusion, amlodipine appears to cross the blood-brain barrier, block brain l-type Ca2+ channels, and decrease sympathetic outflow and thereby BP. Central actions may prevail during iv infusion of amlodipine at low rates, and the decrease in BP is associated with sympathoinhibition. High salt intake markedly enhances its sympathoinhibitory action, likely through central mechanisms.

Calcium channel antagonists in the treatment of hypertension

Calcium channel antagonists are widely used antihypertensive agents. Their popularity among primary care physicians is not only due to their blood pressure-lowering effects, but also because they appear to be effective regardless of the age or ethnic background of the patients. The first available calcium channel antagonists utilized immediate-release formulations which, although effective in patients with angina pectoris, were not approved by the US FDA for use in hypertension. When long-acting once-daily formulations were approved in this indication, the short-acting preparations--which had by then become generic and inexpensive--retained some residual unapproved use for hypertension. An observational case-controlled trial, based on such usage, noted that these agents were associated with a greater risk of myocardial infarctions than conventional agents such as diuretics and beta-adrenoceptor antagonists. Further case-controlled trials showed, in fact, that the dangers of calcium channel antagonists were confined to the short-acting agents and that approved long-acting agents were at least as well tolerated and effective as other antihypertensive drugs. Cardiovascular outcomes during treatment with calcium channel antagonists have been examined in randomized, controlled trials. Compared with placebo, the calcium channel antagonists clearly prevented strokes and other cardiovascular events and reduced mortality. The effects of these agents on survival and clinical outcomes were similar to those with other antihypertensive drugs. There is a slight tendency for the calcium channel antagonists to be more effective than other drug types in preventing stroke, but slightly less effective in preventing coronary events. These observations extend to high-risk patients with hypertension including those with diabetes mellitus. Even so, patients with evidence of nephropathy should not receive monotherapy with calcium channel antagonists. Such patients are optimally treated with angiotensin receptor antagonists or ACE inhibitors, although addition of other drugs, including calcium channel antagonists, is often required to achieve the tight blood pressure control necessary to provide adequate renal protection. Calcium channel antagonists have a highly acceptable tolerability profile and careful reviews of available data have shown that their use is not associated with increased bleeding or promotion of tumor formation. It is now recognized that reduction of blood pressure in patients with hypertension to levels often <130/85 mm Hg should be undertaken in presence of other cardiovascular risk factors or evidence of end organ damage. Because of this important concept, calcium channel antagonists, like the other antihypertensive drug classes, are progressively being prescribed less often as monotherapy, but more typically as part of combination regimens.

Central sympathoinhibitory effects of calcium channel blockers

It is generally assumed that the arterial vasodilation induced by inhibition of Ca(2+) influx into vascular smooth muscle cells represents the main mechanism for the hypotensive effect of dihydropyridine calcium channel blockers. Increases in sympathetic tone have been related to activation of the arterial baroreflex by rapid lowering of blood pressure. This review highlights new findings in two areas. First, in animal studies, direct central administration of dihydropyridines such as nifedipine or amlodipine lowers sympathetic nerve activity and thereby blood pressure. Peripheral administration of nifedipine or amlodipine at low rates appears to result in gradual accumulation of drug in the central nervous system, and also causes lowering of sympathetic nerve activity and thereby lowering of blood pressure (rather than by arterial vasodilation). Second, in hypertensive humans treated with long-acting dihydropyridines and presumably little activation of the arterial baroreflex, some studies have demonstrated lowering of sympathetic activity (as assessed by plasma norepinephrine), but others reported increases (as assessed by plasma norepinephrine or microneurography). This sympathoexcitatory response may be due to activation of the renin-angiotensin system, particularly at higher doses.

Sympathoinhibitory and depressor responses to long-term infusion of nifedipine in spontaneously hypertensive rats on high-salt diet

Short-term (by hour) intracerebroventricular (i.c.v.) or i.v. infusion of nifedipine at low rates evokes parallel decreases in renal sympathetic nerve activity (RSNA) and blood pressure (BP) in spontaneously hypertensive rats (SHR). In the present study, effects of long-term administration of nifedipine on BP and control of sympathetic tone were examined in SHR on a high-salt (8%) diet. From 6 to 8 weeks of age, for 2 weeks concomitant with taking a high-salt diet, rats were also treated with subcutaneous infusion of nifedipine at 10, 50, or 100 microg/kg/h or vehicle solvent as control using osmotic minipumps. At the end of the 2-week treatment period, mean arterial pressure (MAP), heart rate (HR), and RSNA at rest and in response to air-jet stress, i.c.v. injection of the alpha-adrenoceptor agonist guanabenz (25 microg), and i.v. injection of the ganglionic blocker hexamethonium were recorded in conscious rats. In rats on nifedipine 50 or 100 microg/kg/h, resting MAP was significantly lower (136+/-4 or 130+/-4 vs. 145+/-2 mm Hg in control rats, p < 0.05 for both), the sympathoinhibitory and depressor responses to i.c.v. guanabenz were significantly decreased, and the absolute decreases in MAP in response to i.v. injection of hexamethonium were significantly smaller. Sympathoexcitatory and pressor responses to air-jet stress, however, were not affected by nifedipine. Infusion of nifedipine at the three rates for 2 weeks caused concentrations of plasma nifedipine in a dose-related manner. Nifedipine was not detected in tissues of rats treated with 10 microg/kg/h nifedipine but was present in brain and other tissues of rats treated with nifedipine at the two higher rates. Thus in SHR on high-salt intake long-term treatment with nifedipine at 50 or 100 microg/kg/h decreased resting BP and the sympathetic component in BP control. In addition to possible peripheral effects, long-term administration of nifedipine may also act centrally to decrease sympathetic activity and BP, likely by increasing activity in central pathways involving sympathoinhibition, but not in pathways involving sympathoexcitation as evaluated by air-stress.