[Sodium nitroprusside and inhibition of coronary vasoconstriction in isolated hearts of guinea pigs].

Recent data suggest that nitrate compounds, beside determining vasodilatation by cGMP-mediated Ca++ uptake in the sarcoplasmic reticulum, may also inhibit vasoconstriction by reducing phosphatidyl inositol bisphosphate hydrolysis, a major step in the biochemical pathway responsible for vasoconstriction induced by a variety of agents including angiotensin II (AII). We assessed the inhibitory effects of Na-nitroprusside (NP) on the coronary resistances induced by increasing doses of AII in Krebs perfused (basal pressure: 2.5-3 KPa) Guinea-pig isolated hearts. AII bolus injections (5 to 100 ng) were given before and 10 min after a 7.6.10(-8) to 7.6.10(-6) M infusion of NP for 10 min. AII produced a highly reproducible, linearly dose-related, increase in perfusion pressure (PP) (5 ng: 4.4 +/- 1.16; 10 ng: 8.1 +/- 2.1; 25 ng: 18.7 +/- 3.06; 50 ng: 23.6 +/- 2.31; 100 ng: 30 +/- 1.37 mmHg) that was persistently and dose-dependently inhibited by NP. Well after (greater than 10 min) full recovery from the initial NP-induced vasodilatation and drop in PP its inhibitory effect was observed, varying from 22.1% at the maximal AII and minimal NP does and 90.6% at the minimal AII and maximal NP doses. This study demonstrates that, apart from causing the well-known immediate vasodilatory response by stimulation of cGMP synthesis, NP also causes delayed inhibition of AII-induced coronary vasoconstriction, possibly by preventing phosphatidyl inositol hydrolysis.