Discovering the sympathomodulatory properties of new calcium channel blockers

The study by Inomata et al.[1], published in the present issue of the Journal of Hypertension, can count as background on four ‘solid’ sets of evidence. First, the finding that sympathetic overdrive represents a hallmark of the essential hypertensive state, documented via the assessment of human adrenergic function throughout the measurement of haemodynamic [heart rate (HR)], biochemical (plasma norepinephrine and/or its urinary metabolites), neurophysiological (efferent postganglionic muscle sympathetic nerve traffic), neurofunctional (systemic and regional norepinephrine spillover and clearance) as well as power spectral variables [2,3]. Second, the evidence that potentiated sympathetic cardiovascular influences concur, together with the blood pressure (BP) overload and other humoral factors, at the development as well as at the progression of cardiac (left ventricular hypertrophy and left ventricular diastolic dysfunction), vascular (arterial and arteriolar structural and functional remodelling and hypertrophy) and renal (renal insufficiency and failure) target organ damage, thereby augmenting the already elevated cardiovascular risk profile of a given hypertensive patient [2–4]. Third, the recognition that sympathetic overdrive is an independent determinant of cardiovascular risk and of cardiovascular-related fatal and nonfatal events in a variety of disease, such as congestive heart failure, renal failure, life-threatening cardiac arrhythmias, obstructive pulmonary disease and acute stroke [5–11]. Finally, the therapeutic need that modulation of the sympathetic overdrive represents a goal of antihypertensive pharmacological or nonpharmacological interventions aimed at reducing the cardiovascular risk not only by lowering elevated BP values, but also by reducing sympathetic cardiovascular influences [2,3,12]. There is, however, another key antecedent to the study by Inomata et al.[1], namely the finding that antihypertensive drugs may display a nonhomogeneous behaviour regarding their effects on sympathetic neural function. Indeed, whereas thiazide diuretics, vasodilating compounds and alpha-adrenergic blockers have been shown to exacerbate the already elevated sympathetic cardiovascular drive of the essential hypertensive patient [12], beta-blockers, central sympatholytic agents, angiotensin-converting enzyme inhibitors, angiotensin II receptor blockers as well as mineralcorticoid receptor antagonists are capable of reducing (but not of fully normalizing) sympathetic activity [12]. The heterogeneity of the adrenergic effects of antihypertensive drugs is detectable when comparison is made not only between drugs belonging to different pharmacologic classes, but also between compounds within the same class. This is specifically true for calcium channel blockers, because whereas short-acting ‘classical’ compounds exert marked sympathoexcitatory effects, long-acting ‘classical’ drugs may have more favourable effects, resulting in an overall neutral effect on the adrenergic cardiovascular function [12]. The only exception to the already described general behaviour is the calcium channel blocker cilnidipine of the dihydropyridine group, which, by exerting marked inhibitory effects not only on L-type but also on N-type voltage-dependent calcium channels (particularly those located in the central nervous system), decreases the release of norepinephrine from adrenergic nerve endings and also exerts potent central sympathoinhibitory effects [13–15]. In the above-mentioned scenario of information, the study by Inomata et al.[1] reports new data on the effects of azelnidipine vs. amlodipine (both calcium channel blockers of the dihydropiridine group without any effect on N-type calcium channels) on BP (sphygmomanometric and noninvasive tonometric monitoring) as well as on three different indices of adrenergic drive, that is, HR, low-frequency and high-frequency ratio of the HR signal (power spectral bands analysis) and direct microneurographic recording of efferent postganglionic muscle sympathetic nerve traffic in the peroneal nerve. The results of the study, which was performed according to a prospective, randomized, open-label crossover design, provide evidence that for similar BP-lowering effects, azelnidipine markedly reduces the tachycardic and sympathoexcitatory responses (as assessed directly via the microneurographic technique and indirectly at cardiac level via the power spectral analysis of the R–R interval) detected during prolonged treatment with amlodipine. Indeed, the HR and muscle sympathetic nerve traffic values recorded during prolonged treatment with azelnidipine were by 30–35% less than the one recorded during amlodipine treatment and virtually superimposable to the ones seen in the pretreatment condition [1]. The intriguing results of the study performed by the Japanese investigators [1] raise a number of questions, which can receive, however, only a partial answer from the research's findings. The first question is whether the data collected can allow to conclude that azelnidipine exerts a ‘true’ sympathoinhibition or rather a ‘simple’ sympathomodulation. The difference between the two words is not only semantic. With the term ‘sympathoinhibition’, we refer to the results of a nonpharmacological or pharmacological intervention which allows to significantly reduce the predrug values of sympathetic activity, thereby showing clearcut decrease in the sympathetic outflow. An example of sympathoinhibition is the one described with cilnidipine which effectively reduces baseline sympathetic drive, as documented in different published studies by the decrease of the low-frequency component of the HR or SBP signal as well as by the reduction in circulating plasma norepinephrine levels [13–15]. In contrast, with the term ‘sympathomodulation’, we refer to the results of a nonpharmacological or pharmacological intervention which allows to maintain the predrug values of sympathetic activity, thereby preventing the sympathoexcitation related to the drug-induced BP fall without causing, however, any further reduction compared with the predrug sympathetic values. This is the case, according to the results of the study by Inomata et al.[1], for azelnidipine which maintains virtually unaltered muscle sympathetic nerve firing rate throughout the treatment weeks despite the drug-induced BP fall. The different behaviour between cilnidipine and azelnidipine implies that the two drugs may have a different potency in reducing sympathetic drive. They may also mean that cilnidipine may display direct sympathoinihbitory effects, by blocking the central nervous system N-type channels, which azelnidipine does not necessarily have [16]. The impact of the difference in the effects of a given drug on adrenergic drive may be relevant, as shown by the potential favourable consequences of a sustained sympathoinhibition, as summarized in the list below. Potential favourable effects of a calcium channel blocker with ‘true’ central sympathoinhibitory effects are as follows: sustained and more homogeneous BP reduction during the 24-h period; decrease in 24-h BP variability; lack of tachycardia, possibly HR-lowering effects; sustained coronary and renal vasodilatation; improvement in insulin sensitivity; pronounced favourable effects on the regression of cardiac and renal end-organ damage. The considerations made so far allow a final comment on the mechanisms responsible for the sympathomodulatory effects of azelnidipine. Indeed, the authors of the present study [1] tested the impact of the two drugs treatment on baroreflex sensitivity, as assessed either via the power spectral analysis of the spontaneous HR changes or via the evaluation of the spontaneous muscle sympathetic nerve traffic changes plotted against the concomitant DBP changes. In both instances, however, the sensitivity of the baroreflex was unaffected by the drug, indicating that the sympathomodulation was unrelated to the reflex mechanisms. The mechanism(s) of the azelnidipine-related sympathomodulation thus remain to be defined. An action of the drug on other reflexogenic areas involved in modulation of the central sympathetic outflow, such as cardiopulmonary volume sensitive receptors [17], cannot be excluded, however. ACKNOWLEDGEMENTS Conflicts of interest None declared.