Calcium channel blocker in patients with chronic kidney disease

The voltage-dependent calcium channels are localized in the plasma membrane and are essential for the release of neurotransmitters and hormones [21]. These channels are classified into L-, P/Q-, N-, R-, and T-type subtypes based on their pharmacological and electrophysiological properties. Molecular biological analysis has shown that calcium channels are composed of α1, α2/δ, β, and γ subunits [22], among which α1 subunits are the most important in defining channel properties (Fig. 1). The α1 subunits are encoded by CACNA1 gene family consisting of 10 genes. These α1 subunit genes were cloned and classified into the following three subfamilies based on their sequence similarity: Ca_v1.x, Ca_v2.x, and Ca_v3.x (Fig. 2).

In the kidney, several calcium channels are expressed, including Ca²⁺_V1.2 (α1C), Ca²⁺_V1.3 (α1D), Ca²⁺_V2.1 (α1A), Ca²⁺_V2.2 (α1B), Ca²⁺_V3.1(α1G), and Ca²⁺_V3.2 (α1H). They are classified according to function into L-type (Ca²⁺_V1.2, Ca²⁺_V1.3), P/Q-type (Ca²⁺_V2.1), N-Type (Ca²⁺_V2.2), and T-type (Ca²⁺_V3.1, Ca²⁺_V3.2) Ca²⁺ channels [8, 23, 24]. Figure 3 and Table 1 shows the localization of calcium channels in the kidney.

First, the following basic researches have been reported on rats and mice. L-type calcium channel, Ca²⁺_V1.2, is expressed in cells of the distal tubules as well as in the outer and inner medullary collecting ducts of a rat kidney by immunohistochemistry [25]. Andreasen et al. examined the nephron localization of Ca²⁺_V3.1, T-type calcium channel in rats, and found it to be expressed in the distal convoluted tubules and collecting duct [26]. On the other hand, Poulsen et al. reported that T-type calcium channels, Ca²⁺_V3.1 and Ca²⁺_V3.2, are expressed in the efferent arterioles of a mouse kidney [27]. Few reports on the localization of calcium channels have explicitly mentioned humans. Hansen et al. reported that L-type (Ca²⁺_V1.2), P/Q-type (Ca²⁺_V2.1), and T-type (Ca²⁺_V3.1, Ca²⁺_V3.2) calcium channels are expressed in the human renal artery and dissected intrarenal blood vessels by PCR analysis [28]. Hansen et al. also reported that Ca²⁺_V2.1, a P/Q-type calcium channel, contributes functionally to renal blood vessel contraction in humans [29].

Fan et al. demonstrated N-type (Ca²⁺_v2.2) calcium channels’ immunoreactivity in rat renal vascular walls, possibly nerves in the adventitia, and in the distal tubules and podocytes [30]. They also reported that the N-type calcium channel in cultured podocytes was involved in producing reactive oxygen species by angiotensin-II [30]. Previously, we reported that Ca²⁺_v2.2 was localized in the glomerulus including podocytes and in distal tubular cells of mouse by immunohistochemical study [31].

The renal microvasculature comprises two major categories: the glomerular capillary and peritubular capillary (PTC). When the arterial blood flow enters the kidney, it gradually shifts to the smaller arteries and enters the glomerulus capillaries through the afferent arterioles. Post-glomerular blood flow enters the peritubular capillaries (PTCs) that feed the tubules.

L-type calcium channel blockade does not reduce the intraglomerular pressure because the L-type calcium channels are present only in the afferent arterioles. Their blockade mainly dilates the afferent arterioles in a rat kidney [32]. On the other hand, T-type calcium channels are present in both the afferent and efferent arterioles of the rat kidney. Therefore, T-type calcium channel blockade dilates both arterioles and reduces the glomerular capillary pressure, drawing attention to the role of T-type channels in renal protection [33, 34]. However, Thuesen et al. conducted experiments using T-type CC knockout mice and found that deficiency of Ca_V3.1 increased renal blood flow (RBF) and deficiency of Ca_V3.2 increased glomerular filtration rate (GFR), suggesting that Ca_V3.1 is involved in afferent tone and Ca_V3.2 influences efferent dilatation [35]. N-type calcium channels are present at the synaptic nerve endings of both the afferent and efferent arterioles [36]. Cilnidipine, an N/L-type calcium channel blocker, ameliorates glomerular hypertension by dilating both the afferent and efferent arterioles in the canine kidney [23]. Since L-type CCBs act on afferent arterioles, cilnidipine improves glomerular hypertension by blocking N-type calcium channels. The reduction of glomerular pressure is one of the important factors in reducing proteinuria in hypertensive patients [37]. To improve glomerular hypertension, controlling the arteriolar resistance, especially in the efferent arterioles, is highly effective [38].

Although there are several studies referred to the localization of calcium channels in the blood vessels as above, there exist few reports referred to the localization in glomeruli. Three types of calcium channels, L-type [39], P/Q-type [40], and T-type [39], have been reported to localize in the mesangial cells. Previous reports showed that glomerular podocytes express N-type calcium channels [30, 41].

Konda et al. reported that cilnidipine, an N/L-type CCB, reduces BP and improves glomerular sclerosis in Dahl salt-sensitive rats [42]. Fan et al. also reported cilnidipine suppressed more proteinuria level than amlodipine by inhibiting N-type calcium channel-dependent podocyte injury in spontaneously hypertensive rats/ND mcr-cp (SHR/ND) [30]. Cilnidipine significantly prevented the podocyte injury assessed by desmin staining and restored the glomerular podocin and nephrin expression compared with amlodipine in SHR/ND rats.

We employed mice lacking the N-type calcium channel α1 subunit gene (Ca_v2.2^−/−) to generate db/db (diabetic) and Ca_v2.2^−/− double mutant mice [31]. In our study, albuminuria and glomerular histological changes were significantly alleviated in diabetic Ca_v2.2^−/− mice (Fig. 4). In cultured podocytes, depolarization-dependent calcium responses were decreased by ω-conotoxin, a Ca_v2.2-specific inhibitor. Furthermore, the reduction of nephrin by transforming growth factor-β (TGF-β) in podocytes was abolished with ω-conotoxin. These results suggest that Ca_v2.2 inhibition exerts renoprotective effects against the progression of diabetic nephropathy, partly by protecting podocytes.

CCBs are widely-used antihypertensive drugs and a large body of human evidence supports their pleiotropic effect. L-type CCB is the most widely prescribed drug worldwide and has the longest history. Recently, however, the renal protective effects of new types of T-type and N-type CCBs have received more attention [43, 44].

Clinical studies reported that L-/T-type CCB (benidipine) is superior to treatment with L-type CCB (amlodipine) in decreasing urinary albumin excretion and plasma aldosterone level [45‐47]. In the JATOS trial, a large-scale clinical trial evaluating the optimal target BP in elderly hypertensive patients, the renal subanalysis showed that efonidipine, T-/L-type CCB, improved eGFR in patients with proteinuria [48].

N-/L-type CCBs have also been reported to have renal protective effects. Konoshita et al. reported that cilnidipine, an N-/L-type CCB, leads to less activation of RAAS in hypertensive patients than does amlodipine, and that UAE with cilnidipine administration was also significantly lower than that with amlodipine [49]. The CARTER study reported that cilnidipine reduced proteinuria in patients with CKD who were already treated with RAAS inhibitors [50].

There are several reports referred to the localization in the tubules. Brunette et al. reported that L-, P/Q- and T-type calcium channels exist in the luminal side of the distal tubules in rabbits, because their channel antagonist, including diltiazem, ω-conotoxin, and mibefradil, decreased Ca²⁺ transport in the absence of sodium [51]. Andreasen et al. showed the nephron localization of Ca²⁺_V3.1, T-type calcium channels, in the inner medullary collecting ducts, distal collecting ducts and collecting tubules, especially in the apical sites [26]. Sugano et al. investigated the effect of the stereoselective T-type calcium channel blocker R(-)-efonidipine and CKD progression in spontaneously hypertensive rats that had undergone subtotal nephrectomy [52]. They showed that T-type calcium channel blockade has renal protective actions that depend not on hemodynamic changes and on the inhibition of Rho-kinase activity, tubulointerstitial fibrosis, and epithelial-mesenchymal transitions. Baylis et al. reported that T-/L-type CCBs, mibefradil, resulted in superior nephroprotection including amelioration of proteinuria and glomerular injury, to traditional L-type CCBs, amlodipine, in deoxycorticosterone acetate (DOCA)-salt rats which are the models of high glomerular BP and rapidly developing kidney damage [53]. In contrast, L-type CCBs, amlodipine, failed to improve renal injury. They suggest that amelioration of tubulointerstitial injury may contribute to the nonhemodynamic effect of T-type CCBs. However, the relationship between calcium channels and renal tubules is not fully investigated. Further studies are needed to address these questions.