The effects of prostacyclin (PGI
2 ) were compared with those of prostaglandin E
1 (PGE
1) on the cardiac hemodynamics (coronary blood flow, systemic blood pressure, cardiac output and max dp/dt of the left ventricle) in closed-chest dogs. Moreover, the effects of PGI
2 were compared with those of PGE
1 on the concentrations of the coronary arterial and myocardial cyclic adenosine 3': 5'-monophosphate (cyclic AMP) and cyclic guanosin 3': 5'-monophosphate (cyclic GMP) in open-chest dogs. The intraventricular and intravenous injection of 0.1 to 4.0 μg/kg of PGI
2 increased the coronary blood flow and decreased the mean systemic blood pressure. Comparison of dose response curves for PGI
2 and PGE
1 in relation to the coronary blood flow indicated that PGI
2 was 4 times as potent as PGE
1 . On the other hand, the effect of PGI
2 on the systemic blood pressure was twice as potent as PGE
1 . The concentration of the coronary arterial cyclic AMP was significantly increased by the administration of PGI
2 and PGE
1 (control group: PGI
2 group was 0.201±0.022 vs. 0.264±0.017 pmoles/mg tissue, p<0.05; control: PGE
1 was 0.201±0.022 vs. 0.286±0.027 pmoles/mg tissue, p<0.05). The concentrations of the coronary arterial cyclic GMP were not significantly changed by the administration of PGI
2 and PGE
1, nor were those of myocardial cyclic AMP and cyclic GMP. The changes in cyclic nucleotide levels and cardiac hemodynamics induced by PGI
2 were qualitatively similar to those induced by PGE
1, but the change caused by PGI
2 was greater than PGE
1 in the coronary and systemic hemodynamics.
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