T-1032, a novel specific phosphodiesterase type 5 inhibitor, increases venous compliance in anesthetized rats

Nitric oxide (NO) donors including organic nitrates dilate capacitance vessels. As inhibition of phosphodiesterase type 5 results in the accumulation of guanosine 3'5'-cyclic monophosphate (cGMP), specific phosphodiesterase type 5 inhibitors are expected to have a vasodilator property similar to that of NO donors. To test this hypothesis, we examined the effect of methyl2-(4-aminophenyl)-1,2-dihydro-1-oxo-7-(2-pyridinylmethoxy)-4-(3,4,5-trimethoxyphenyl)-3-isoquinoline carboxylate sulfate (T-1032), a novel specific phosphodiesterase type 5 inhibitor, on mean arterial pressure and mean circulatory filling pressure (an index of venodilation) compared with that of nitroglycerin and diltiazem in mecamylamine- and noradrenaline-treated anesthetized rats. Intravenous infusion of T-1032 (0.1, 1, 10 microg/kg/min) dose-dependently decreased mean arterial pressure (-3.8+/-0.3%, -9.1+/-0.8%, -16.8+/-1.5% at doses of 0.1, 1 and 10 microg/kg/min, respectively) and mean circulatory filling pressure (-6.1+/-0.9%, -12.5+/-0.7%, -18.6+/-3.0% at doses of 0.1, 1 and 10 microg/kg/min, respectively). The mean circulatory filling pressure-mean arterial pressure relationship revealed that T-1032 had a selective action on the mean circulatory filling pressure compared with diltiazem (10, 100 microg/kg/min) and a similar or more selective effect than nitroglycerin (0.3, 3 and 30 microg/kg/min). In the next study, we calculated venous compliance and unstressed volume from the mean circulatory filling pressure-volume relationship. Intravenous infusion of T-1032 (3 microg/kg/min) increased venous compliance (3.35+/-0.40 in T-1032 vs. 2.31+/-0.15 ml/kg/mm Hg in vehicle, P<0.05) without changing the unstressed volume (37.2+/-2.80 in T-1032 vs. 42.6+/-2.37 ml/kg in vehicle, P>0.05). It was concluded that T-1032 increased venous capacitance by increasing venous compliance, and that this selective phosphodiesterase type 5 inhibitor appeared to have a different vasodilator action from that of an NO donor and a Ca(2+) channel antagonist in that it had a selective action on the mean circulatory filling pressure.