Selectivity of dihydropyridines for cardiac L-type and sympathetic N-type Ca²⁺ channels

Abstract

The blocking effects of cilnidipine and other dihydropyridines on L-type cardiac Ca²⁺ channels (I_Ca,L) and N-type sympathetic Ca²⁺ channel currents (I_Ca,N) were studied using a whole-cell patch-clamp technique. At −80 mV, cilnidipine had little inhibitory effect below concentrations of 1 μM on I_Ca,L (IC₅₀ value; 17 μM). However, 1 μM cilnidipine strongly shifted the steady-state inactivation curve of I_Ca,L toward negative potentials without changing the current–voltage relationship. Each action of cilnidipine was characterized by a high affinity for the inactivated channel in preference to the resting channel. The IC₅₀ values of dihydropyridines for I_Ca,L were in the range between 0.01 and 10 μM, and those for I_Ca,N were between 3 and 30 μM. Cilnidipine had the strongest affinity for I_Ca,N among the dihydropyridines tested. These results suggest that cilnidipine did not cause hypotension-evoked tachycardia deficiency by depression of cardiac L-type channels but by sympathetic N-type channels blockade.

Introduction

Dihydropyridine-type Ca²⁺ channel antagonists have been widely used in the treatment of arterial hypertension. Their pharmacological and therapeutic properties were, to date, thought to be attributed to the blockade of Ca²⁺ influx through L-type Ca²⁺ channels, and not to interaction with other Ca²⁺ channel subtypes. However, recent electrophysiological data have revealed that some second-generation dihydropyridine derivatives (cilnidipine, nimodipine, amlodipine) have a blocking potency for N-type as well as L-type Ca²⁺ channels (Furukawa et al., 1997; Uneyama et al., 1997) and discussion has started about the possible involvement of non-L-type actions of dihydropyridines in their therapeutic features. Oral administration of cilnidipine, for instance, attenuates stress-induced hypertension which is not effectively treated with clinically available dihydropyridines (Saihara et al., 1993; Hosono et al., 1995b). We have already a wide variety of data on the pattern of selectivity of the dihydropyridines for L-type Ca²⁺ channels with different pharmacological characteristics (Wei et al., 1988), but we have two little information about most dihydropyridines concerning their ability to cause N-type Ca²⁺ channel blockade and their therapeutic potential.

A fast-acting dihydropyridine such as nifedipine causes a baroreceptor-mediated reflex increase in sympathetic tone and activates neurohormonal systems such as the adrenergic system and the renin–angiotensin system. Increased plasma levels of these neurohormones result in tachycardia, increased myocardial contractility and stroke volume, to increase the workload of the heart as well as an increase in the myocardial oxygen demand (Kimichi and Lewis, 1991). The Secondary Prevention Reinfarction Israel Nifedipine Trial 2 (SPRINT 2) study reported that early administration of nifedipine increases the risk of mortality in patients with suspected acute myocardial infarction (Goldbourt et al., 1993). In contrast, a slow-acting dihydropyridine such as amlodipine can, in a way, overcome the clinical disadvantage of the fast-acting dihydropyridine, namely, the attenuation of the sympathetic nerve hyperactivation, by slowing its rapid hypotensive effects (Haria and Wagstaff, 1995).

Cilnidipine (FRC-8653) is an antihypertensive dihydropyridine known to possess an antagonistic effect on L- and N-type Ca²⁺ channels (Yoshimoto et al., 1991). The N-type Ca²⁺ channel-blocking action of cilnidipine has, in particular, been extensively examined in various neuronal cells (Fujii et al., 1997; Uneyama et al., 1997, Uneyama et al., 1998). In in vitro experiments, cilnidipine blocks catecholamine secretion from vessel walls elicited by electrical stimulation (Nakashima et al., 1991) or that from nerve growth factor (NGF)-differentiated PC12 cells elicited by high K⁺ stimulation (Uneyama et al., 1998). Oral administration of cilnidipine reduced blood pressure without increasing the heart rate or blood catecholamine levels (Hosono et al., 1995a). This reduction in hypotension-induced baroreflex activity is thought to be due to attenuation of sympathetic nerve activation by N-type Ca²⁺ channel blockade. However, it is not clear whether this might be caused simply by indirect N-type blocking action or by a cardiac depressant action, such as elicited by diltiazem (Walsh, 1987; Michalewicz and Messerli, 1997), because the direct action of cilnidipine on cardiac L-type Ca²⁺ channels has not been investigated. In the present experiments, we examined the effects of cilnidipine on cardiac L-type Ca²⁺ channel currents and compared these with the effects of other clinically available dihydropyridines, using the conventional whole-cell patch-clamp technique. At the same time, we also evaluated the inhibitory effects of these dihydropyridines on the sympathetic N-type Ca²⁺ channel currents to clarify their Ca²⁺ channel characteristics for cardiac L- and the N-type channels, and discuss the possible merit of dihydropyridine-induced blockade of N-type Ca²⁺ channels in hypertension therapy.