14 August 1997, Volume 121, Issue 8, Pages 1627 – 1636

Article

Study of the in vivo and in vitro cardiovascular effects of a hydralazine-like vasodilator agent (HPS-10) in normotensive rats Francisco Orallo1 Departamento de Farmacología, Facultad de Farmacia, Universidad de Santiago de Compostela, Campus Universitario Sur, E-15706 Santiago de Compostela (La Coruña), España     1Author for correspondence

Keywords

Conscious and anaesthetized normotensive rats;   rat aorta;   HPS-10;   rat atria;   hydrazinopyridazines;   vasodilator agent;   hypotension;   bradycardia;   whole-cell clamped rat aortic myocytes

Abstract

1   In this work, the cardiovascular effects of HPS-10, a new vasodilator agent, were studied in rats. 2   In conscious normotensive rats, oral administration of HPS-10 (4 – 9 mg kg-1) produced a dose-related and long-lasting fall in systolic arterial blood pressure (ED30 of 5.32 mg kg-1), accompanied by an increase in heart rate (ED30 of 8.43 mg kg-1). This tachycardia was totally inhibited by pretreatment with (±)-propranolol (10 mg kg-1, p.o.). 3   In anaesthetized normotensive rats, HPS-10 (0.3 – 0.6 mg kg-1, i.v.) produced a gradual, dose-dependent and sustained decrease in systolic, diastolic and mean arterial pressure (MAP) (ED30 for MAP of 0.41 mg kg-1, i.v.), accompanied by a significant bradycardia at high doses (>0.4 mg kg-1; ED20 of 0.61 mg kg-1, i.v.). HPS-10 (0.5 mg kg-1, i.v.) did not modify the positive chronotropic effects induced by intravenous administration of noradrenaline (NA; 5 µg kg-1), angiotensin II (AII; 0.2 µg kg-1) and nicotine (200 µg kg-1) but markedly inhibited the hypertensive response produced by these agents. 4   In rat isolated rubbed aorta, HPS-10 (0.1 – 1 mM) non-competitively and with almost equal effectiveness antagonized the contractions induced by NA, AII (in normal Krebs solution) and Ca2+ (in depolarizing Ca2+-free high-K+ 50 mM solution). In the experiments in Ca2+-free medium, HPS-10 (1 mM) considerably inhibited the contractions induced by NA, AII and caffeine in rat aorta. 5   Furthermore, in the studies with radioactive Ca2+, HPS-10 (1 mM) did not modify the basal uptake of 45Ca2+ but strongly decreased the influx of 45Ca2+ induced by NA, AII and K+ in rat aortic rings. 6   In rat isolated atria, HPS-10 (1 mM) produced a positive inotropic/negative chronotropic effect. 7   HPS-10 (0.3 mM) significantly inhibited the sustained and transient Ba2+ inward current (IBa) recorded in whole-cell clamped rat aortic myocytes. 8   These results indicate that the non-selective vasorelaxant effects of HPS-10 in rat aortic rings can be attributed to transmembrane Ca2+-antagonist activity and an intracellular action on smooth muscle cells. The direct vasodilator action of HPS-10 observed in rat isolated aorta may be responsible for the HPS-10 hypotensive activity in anaesthetized normotensive rats.

Received 20 January, 97; Revised 23 April, 97; Accepted 19 May, 97

^© Macmillan Publishers Ltd 1997