Diltiazem (hydrochloride)

Diltiazem hydrochloride is a Ca2+ influx inhibitor (slow channel blocker or calcium antagonist).

Benzothiazepine Ca2+ antagonist diltiazem hydrochloride interacts with transmembrane segments IIIS6 and IVS6 in the α1 subunit of L-type Ca2+ channels[1]. Diltiazem causes a dose-dependent inhibiton of contractions as well as Ca2+ influx stimulated by alpha adrenoceptor activation and high-K+ depolarization. Diltiazem is roughly equally potent in inhibiting contractions induced by high-K+ and a low concentration of norepinephrine (NE)[2]. Diltiazem also inhibits the Na-dependent Ca-efflux from heart mitochondria. Both the (+)-optical isomers of the cis- and trans-forms of diltiazem inhibit Na-Ca exchange activity with comparable potency (IC50 of 10-20 μM)[3].

Diltiazem produces a noncompetitive inhibition of Ca2+-induced contractions of depolarized rabbit aorta. Furthermore, there is a lack of parallelism between the smooth muscle effects of removal of [Ca2+]ex and of addition of diltiazem[2]. Diltiazem improves the cardiac microcirculation and function in an experimental model of hyperthyroidism in rats. The treatment of hyperthyroid rats with losartan diltiazem (4.7±0.7%; P < 0.001) significantly reduces the percentage of fibrosis areas in the left ventricle [4]. In conscious spontaneously hypertensive rats (SHR), diltiazem dose-dependently decreases the blood pressure and increases the heart rate after intravenous administration (0.03--1 mg/kg). Oral administration of diltiazem (100 mg/kg) also reduces the blood pressure of SHR[5].

References:
[1]. Kraus RL, et al. Molecular mechanism of diltiazem interaction with L-type Ca2+ channels. J Biol Chem. 1998 Oct 16;273(42):27205-12.
[2]. van Breemen C, et al. The mechanism of inhibitory action of diltiazem on vascular smooth muscle contractility. J Pharmacol Exp Ther. 1981 Aug;218(2):459-63.
[3]. Chiesi M, et al. Stereospecific action of diltiazem on the mitochondrial Na-Ca exchange system and on sarcolemmal Ca-channels. Biochem Pharmacol. 1987 Sep 1;36(17):2735-40.
[4]. Freitas F, et al. Cardiac microvascular rarefaction in hyperthyroid rats is reversed by losartan, diltiazem, and propranolol. Fundam Clin Pharmacol. 2015 Feb;29(1):31-40.
[5]. Sato M, et al. Hypotensive effects of diltiazem hydrochloride in the normotensive, spontaneously hypertensive and renal hypertensive rats (author's transl). Nihon Yakurigaku Zasshi. 1979 Mar;75(2):99-106.