Many studies indicate that cardiovascular events, such as myocardial infarction, stroke, and sudden death, often occur in the morning. 1–5 At this time of day, over-activation of the sympathetic nerve may increase peripheral vascular resistance and trigger these cardiovascular events. Suppression of this sympathetic nervous system over-activation may therefore prevent cardiovascular events. Recently, an unexpected rise in blood pressure (BP) in the morning (known as the morning surge), due probably to sympathetic over-activation in the morning and inhibited by the α-blocker doxazosin, 6,7 has been targeted in the management of hypertension.
Cilnidipine is a long-acting, 8,9 unique calcium antagonist of the 1,4-dihydro-pyridine type; it has a blocking action against N-type calcium channels in addition to the L-type calcium channel. N-type calcium channels are widely distributed in neurons, including sympathetic neurons. 10–13
For neurotransmission at a terminal neuron, an increase in Ca2+ concentration via the voltage dependent N-type calcium channel is believed to be essential. Cilnidipine may therefore suppress the morning surge by its blocking action against the N-type calcium channel. We study below the effects of cilnidipine on the circadian variations of BP, heart rate (HR), and activity of the autonomic nervous system, by monitoring the ambulatory BP, HR, and the power-spectrum of R-R intervals for 24 hours in essential hypertensive patients. Bedtime dosing of cilnidipine may be better than morning dosing of cilnidipine in suppressing the morning surge; the effects on these parameters of morning and bedtime dosing with cilnidipine were compared in an open, randomized crossover study.
Seventeen untreated essential hypertensive patients gave informed advance consent to take part in our study. The protocol was approved by the ethical committee of our institution. One of the patients chose to drop out during the protocol. In 2 patients, data from the 24-hour ambulatory BP monitoring was not suitable for analysis because of incomplete recording. Medication non-compliance was observed in 1 patient. Data from 13 of these patients (age, 55.2 ± 2.6 years; 12 men and 1 woman), who met all our criteria and completed the protocol, were finally included in the study.
We designed an open, randomized crossover study (Fig. 1). After the drug-free observation period (≥4 weeks), patients who had hypertension (diastolic BP of 90 to 109 and /or systolic BP of 140 to 179) were assigned at random to either of 2 treatment groups.
On each visit to the outpatient clinic, repeated sphygmomanometric BP measurements were performed after the patient had undergone a 5-minute rest in the sitting position. During the observation period, secondary forms of hypertension were ruled out, and the average BP levels at the last 2 visits during the observation period was taken as the casual BP. The degrees of hypertension were determined from the casual BPs based on the Seventh Report of the Joint National Committee on Detection, Evaluation, and Treatment of High BP. 14
In 1 of the 2 groups, morning dosing with cilnidipine began with an initial dosage of 5 mg once daily. The dose of cilnidipine was increased until BP at the outpatient clinic reached optimal values (less than 140 mm Hg in systolic BP and less than 90 mm Hg in diastolic BP) or until a dose of 20 mg was reached. For each patient in the group, the dose at this time was administered for 8 weeks. Thereafter, the same dose was administered at bedtime for 8 weeks. In the other group, bedtime dosing with cilnidipine began with the same initial dose. The dose was increased until either the casual BP became optimal or a dose of 20 mg was reached; for each patient the dose at this time was maintained for 8 weeks. Thereafter, the same dose was administered in the morning for 8 weeks. On the last day of the trial, the ambulatory BP (ABPM) and R-R interval were measured for 24 hours.
Ambulatory Blood Pressure
Blood pressure was monitored every 30 minutes during the waking period, defined as daytime, and every 60 minutes during the sleeping period, defined as nighttime. An ABPM device was used (TM-2425, A&D Co., Tokyo, Japan). The maximum value of the moving average BP of 2 successive measurements, from 1 hour before waking to 2 hours after, was taken as the maximum BP in the early morning. The minimum value of the moving average BP of 2 successive measurements during the nighttime was taken as the minimum BP in the nighttime.
Power-Spectrum Analysis of the R-R Interval
The R-R intervals of the electrocardiogram were recorded by our ABPM device (TM-2425, A&D Co., Tokyo, Japan) and the power spectrum of the HR was analyzed every 5 minutes during the electrocardiogram recording. Ectopic beats or artifacts were automatically excluded, although all subjects had normal sinus rhythm. Systolic BP measurements greater than 280 mm Hg or less than 70 mm Hg were excluded from the data. The frequency range 0.05 to 0.15 Hz was taken as the low-frequency (LF) component; this is an index of parasympathetic and of sympathetic nerve activity. The frequency range 0.15 to 0.40 Hz was taken as the high-frequency (HF) component, and reflects parasympathetic nerve activity. The LF/HF ratio was calculated as an index of sympathetic activity.
Results are expressed as mean ± SEM. The 3 regimes (drug-free observation period; morning dosing; evening dosing) were compared using Friedman's test, and subset analysis using the Wilcoxon t test with Bonferroni correction post hoc test for P < 0.05.
Baseline characteristics of the 13 essential hypertensive patients are shown in Table 1.
Effects of Morning and Bedtime Dosing with Cilnidipine on Ambulatory Blood Pressure and Heart Rate
Administration of cilnidipine began with morning dosing in 7 patients and bedtime dosing in 6 patients. The average final dose of cilnidipine was 11.2 ± 1.5 mg/d. Bedtime dosing with cilnidipine reduced the average systolic and diastolic BPs during both daytime and nighttime, and consequently over 24 hours overall (P < 0.05). Morning dosing reduced the average systolic and diastolic BPs over 24 hours and in the daytime and the systolic BP during nighttime. The reduction in the average nighttime diastolic BP with morning dosing was not significant. The average heart rate did not differ among the 3 different regimes (Table 2).
Both the morning dosing and the bedtime dosing with cilnidipine significantly reduced the maximum systolic BP in the early morning. The minimum systolic and diastolic BPs during the nighttime were significantly reduced by bedtime dosing, but not by morning dosing (Table 3).
During the control period, the average systolic BP started to increase 1 hour before awakening. Both morning dosing and bedtime dosing with cilnidipine significantly reduced the average of systolic BP after awakening (Fig. 2A).
Power Spectrum Analysis
The average LF, HF, and LF/HF ratio during daytime and nighttime were similar across the drug-free observation period, the morning dosing regimen, and the bedtime dosing regimen (Table 2). For the morning dosing regimen, the average LF/HF ratio between awakening and 1 hour later was lower than during the observation period (Fig. 2B). For the bedtime dosing regimen, the average LF/HF ratio between 1 and 2 hours after awakening was lower than during the observation period. The averages of LF or HF in the early morning were not different among the 3 regimes (data not shown).
We believe that this is the first randomized, crossover study to compare the effects of morning and bedtime dosing of cilnidipine. Several studies have demonstrated the effects of cilnidipine on 24-hour BP and HR in comparison with other calcium antagonists. Cilnidipine was effective as a once-daily antihypertensive agent and caused less reflex tachycardia than nisoldipine 15 and nifedipine. 16 Cilnidipine is a favorable antihypertensive drug for avoiding sympathetic over-activation and acts to counter vasodilation, which occurs with the administration of short-acting calcium antagonists. 17
The averaged final dosage of cilnidipine in this study (11.2 mg/d) is considered efficient to reduce BP as seen in the previous studies. 16,18 Ishii et al 18 reported that significant reduction of BP (20 mm Hg or more decrease in systolic BP, or 10 mm Hg or more decrease in diastolic BP) was achieved with cilnidipine at averaged dosage of 9.7 mg once daily in 87% of the patients (39/45) with essential hypertension compared with placebo-control.
The difference in clinical effects between cilnidipine and other dihydropyridine is thought to be due to the difference in the blocking potency for N-type calcium channels. Against others, cilnidipine has the smallest half-maximal inhibitory concentration (IC50) for N-type calcium channels and the highest ratio of IC50 for L-type/N-type channels (21 for cilnidipine and 0.008–0.43 for other dihydropyridine). 8 These results indicate that, of these dihydropyridine, cilnidipine has the greatest blocking potency for N-type cardiac calcium channels and the highest selectivity for N-type cardiac calcium channels.
N-type calcium channels are widely distributed in neurons, including sympathetic neurons. 10–13 Activation of N-type calcium channels causes the release of catecholamines from sympathetic nerve endings. 19 For neurotransmission at terminal neurons, increase of the calcium concentration via the voltage-dependent N-type calcium channel is thought to be essential.
In this study, we take cardiac autonomic nervous activity, using data from the power spectrum analysis of R-R intervals, as a measure of systemic autonomic nervous activity. Sakata et al assessed the effects of amlodipine and cilnidipine, both of which have a blocking action on N-type calcium channel, 20,21 on cardiac sympathetic activity in patients with mild essential hypertension, using 123I-metaiodobenzylguanidine (MIBG) scintigraphy. 22 They concluded that the suppressive effect of cilnidipine on cardiac sympathetic overactivity was greater than that of amlodipine. Molderings et al 23 reported that an N-type calcium channel controlled sympathetic neurotransmission in the atrium of the human heart. Our results suggest that cilnidipine probably inhibits the N-type calcium channel situated on the sympathetic terminal neuron of the sinus node.
Considerable evidence now exists of sympathetic neurotransmission through presynaptic N-type calcium channels in vascular tissue and the antihypertensive effect of N-type calcium channel blockage. 24–26 Omega-conotoxin GVIA, which is a specific N-type calcium channel blocker, markedly suppresses [3H]norepinephrine release by stimulating periarterial nerves. 25 Intravenous administration of omega-conotoxin GVIA has a potent hypotensive effect in conscious spontaneously hypertensive rats. 27
In consequence, cilnidipine was expected to suppress the activation of sympathetic nerves and the rise in BP induced by other stresses. Hosono et al 28 reported that cilnidipine—but not other dihydropyridine-derivative calcium antagonists—reduced the plasma norepinephrine concentration and the pressor response induced by acute cold stress in spontaneously hypertensive rats. In clinical studies, cilnidipine also suppresses hypertensive responses to several stresses such as cold pressor stress 29 and the “white-coat” effect 30; these are mainly mediated by sympathetic activation, 31,32 probably via inhibitions of N-type calcium channels.
The morning surge in BP has recently been targeted in antihypertensive therapy. Millar-Craig et al 33 studied circadian variations in BP by continuous intra-arterial BP recording in 20 hypertensive and 5 normotensive ambulant subjects. BP was highest at midmorning and then fell progressively to its lowest value at 3 am, after which it rose again, before awakening. The significance of these results regarding therapeutic management of hypertension was emphasized. The importance of the sympathetic nervous system in the circadian rhythm of BP is suggested by diurnal rhythms of plasma catecholamine levels and power spectrum analysis of heart rate variability. 34–36 Cardiovascular events such as myocardial infarction, stroke, and sudden death often occur in the morning. 1–5 Over-activation of the sympathetic nerve increases peripheral vascular resistance and may trigger such events in the morning. In this regard inhibition of the N-type calcium channel is expected to suppress the morning surge as α-blocker, doxazosin, suppresses the morning rise in BP and reduces sympathetic nervous activity. 6,7
The present study has demonstrated that both morning and bedtime dosing of cilnidipine reduce the maximum systolic BP in the early morning and suppress the morning rise in BP. These hypotensive effects of cilnidipine on morning BP were accompanied by only partial inhibition of the morning increase in LF/HF ratio. The effect of cilnidipine (by inhibiting the N-type calcium channel) on the activation of the sympathetic nervous system during the morning might be canceled out by the counter regulatory system that acts against reduction of BP by inhibiting the L-type calcium channel.
Minami et al 16 also reported the effects of cilnidipine on the activity of the autonomic nervous system in essential hypertensive patients. There are several differences between their study and ours. In their study, cilnidipine significantly increased the average of the LF/HF ratio, which did not change in our study. A relatively high dose and a long period of cilnidipine treatment (average dose: 6.4 and 11.2 mg/d, period: 4 and 8 weeks, in their study and in our study respectively) may be needed to suppress sympathetic nervous activity.
Although the effect of cilnidipine on diastolic BP during the night differed between morning dosing and nighttime dosing, the effects of cilnidipine on BP, HR, and the autonomic nervous system for the morning dosing regimen were essentially the same as for the bedtime dosing regimen. Pharmacokinetic studies following repeated oral administration of cilnidipine (p.o. 10 mg/d) for 1 week has shown that the Tmax was 3.0 hours and the half-life of the α-phase was 1.1 hours. 37 However, cilnidipine has a long-lasting hypotensive effect when it is medicated p.o. once daily to hypertensive patients. The in vitro study of Yoshimoto et al showed that the inhibitory effect of cilnidipine on the contractility of rat mesenteric artery persisted for more than 7 hours after its removal from the perfusate. 9 This characteristic is probably responsible for the long-lasting effect of cilnidipine on BP, HR, and the autonomic nervous system.
Beneficial effects of cilnidipine on regional hemodynamics, and cardiac and renal functions, 16,22,38–44 which were observed in clinical and experimental studies, might also occur related to the blocking action against N-type calcium channels in addition to the L-type calcium channel. Recently, structure-activity relations have been reported for each type of calcium channel blocker. 45 Further development of novel N-type channel blockers will clarify the function of the N-type channel in regulating heart rate and vascular tone via the activity of the autonomic system.
There are several limitations to our study. As stated previously, we used data extracted from the power spectrum of variation in R-R intervals to measure the activity of the autonomic nervous system, rather than variation in BP. Although direct recording of muscle sympathetic nerve activity or direct measurement of intra-arterial pressure 46,47 is not suitable for evaluation of the circadian variation of the autonomic nervous system, it proves valuable in evaluating the effects of cilnidipine on autonomic nerve activity at several times of day.
We did not use a 3-way randomized crossover method, but started from the control period in all patients. Because a small but significant decrease in average daytime BP (mean decrease in systolic BP and diastolic BP were −0.8 and −1.0 mm Hg) at repeat recording was reported by Palatini et al, 48 our results might be influenced by the choice of control period. The hypotensive effect of cilnidipine was clear in each treatment period, however (see Table 2). Consequently the effect of this choice should not be important in the present study.
Our results indicate that cilnidipine administered once daily is an efficient antihypertensive drug regardless of the time of dosing, without reflex tachycardia and with no increase in sympathetic nervous activity, but rather with inhibition of sympathetic nervous system activation in the morning.
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