European Journal of Pharmacology, 250 (1993) 133-139 © 1993 Elsevier Science Publishers B.V. All rights reserved 0014-2999/93/$06.00 EJP 53408 Effects of indapamide on contractile responses and 45Ca2+ movements in various isolated blood vessels Marcela Del Rio, Teresa Chulia, Pilar Gonzalez and Teresa Tejerina * Department of Pharmacology, School of Medicine, UniversidadComplutense, 28040Madrid, Spain Received 15 September 1993, accepted 17 September 1993 The effects of indapamide on contractile responses in various isolated artery rings and on spontaneous mechanical activity in portal vein segments were investigated. Arteries used were: rabbit aorta, mesenteric (fifth branch), femoral and basilar, and sheep coronary arteries. 45Ca2+ uptake was also analysed in rabbit mesenteric arteries. Indapamide (10-1°-3 × 10 -4 M) produced a concentration-dependent inhibition of the contractile responses induced by high K ÷ (80 mM), 5-hydroxytryptamine (10 -5 M), and noradrenaline (10 -6 or 10 -4 M) in all the arteries studied. The inhibitory effect was greater in mesenteric (fifth branch) the IC50 values being 9.2 + 3.0 × 10 -6 and 5.5 + 4.0 × 10 -8 M for depression of high K ÷- and agonist-induced contractions, respectively. Indapamide inhibited in a concentration-dependent manner the contractile responses elicited by the addition of Ca 2÷ (1-5 mM) to Ca2+-free high K ÷ solution as well as the spontaneous mechanical activity of rat portal vein. Indapamide also reduced the 45Ca2+ uptake in rabbit mesenteric arteries stimulated by noradrenaline (10 -4 M) or by high K ÷ (80 mM) without affecting the Ca 2+ influx in resting tissues. Our results indicate that indapamide blocked both depolarization- and noradrenaline-induced Ca 2÷ influx while it did not modify passive Ca z+ entry. Peripheral resistance vessels were demonstrated to be the arteries most sensitive to indapamide vascular effects. Indapamide; Conductance vessel; Resistance vessel; 45Ca 2+ movement; (Rabbit); (Sheep). 1. Introduction Indapamide is an antihypertensive agent which has been shown to an exert effective control of hyperten- sion in experimental animals (Kyncl et al., 1975; Finch et al., 1977) and in man (Abbou, 1985; Acchiardo and Skoutakis, 1983). The nature of its antihypertensive effect is still under discussion. It has been proposed that indapamide decreases blood pressure via a dual mechanism: limited diuretic action combined with a direct effect on vascular reactivity (Campbel, 1983; Caruso et al., 1983). Its efficacy in patients with renal insufficiency and with renal failure supports the thesis of a primarily vascular effect (Santoro et al., 1982). It has been demonstrated that indapamide inhibits vascu- lar smooth muscle contractile response to various pres- sor agents and electrical stimulation in vitro (Finch et al., 1977; Uhlich et al., 1977; Kyncl et al., 1975; Usui et al., 1978). In the presence of indapamide, adrenaline, angiotensin, tyramine, prostaglandin F2~ and nicotine elicit a decreased vasoconstrictor action. Furthermore, the relaxing effect shown by indapamide on contrac- tions induced by prostaglandin F2, does not appear to Corresponding author. Tel. 34/1/3941476, fax 34/1/3941463. be associated with ganglionic blockade or decreased noradrenaline from adrenergic nerve terminals but rather with a direct action on vascular smooth muscle (Usui et al., 1978; Burgess et al., 1981). Mironneau and co-workers, working on rat portal vein and myo- metrium preparations, have shown (Mironneau and Gargouil, 1979; Mironneau et al., 1986; Mironneau, 1988) that indapamide acts primarily on the plasma membrane of smooth muscle cells by reducing the inward Ca 2+ current. The aim of the present study was to determine the effects of indapamide on the contractile responses of different isolated arteries as well as to compare the effectiveness of the drug in certain blood vessels. To find out more about the indapamide mechanism of action, transmembrane calcium uptake was also anal- ysed in mesenteric arteries (fifth branch). 2. Materials and methods 2.1. General procedure New Zealand white rabbits of both sexes, weighing 2.5-3 kg, were killed by exsanguination from the com- mon carotids. The thoracic aorta, femoral, mesenteric