Direct effects of nitroglycerin on the resistance, exchange and capacitance functions of the canine intestinal vasculature

In the isolated dog ileum perfused with constant flow, nitroglycerin (NTG) produced dose-dependent arteriolar and venous dilatation. Experiments were performed in 22 ileum preparations to advance the study on the pre- and postcapillary resistances, functional capillary pressure, capillary filtration coefficient (CFC) and vascular compliance. At venous pressure (Pv) of 0, 10 and 20 mm Hg, and intra-arterial infusion of NTG in a submaximal dose (30 microgram/min) reduced the total and precapillary resistances, but did not change the postcapillary resistance and functional capillary pressure. This agent slightly increased the CFC calculated from a Pv range of 0 to 10 mm Hg and significantly increased the CFC value calculated from a Pv change of 0 to 20 mm Hg. At Pv = 0 mm Hg, NTG infusion caused a regional volume pooling by 1.2 +/- 0.4 mg/100 g. The vascular compliance in terms of delta blood volume/delta Pv was also significantly increased after NTG. The myogenic responses (reduction in CFC and increase in precapillary resistance) to an elevation of Pv were essentially identical before and after NTG infusion. The results suggest the NTG at the dose used acts primarily on the arterioles, precapillary sphincters and postcapillary capacitance vessels. The postcapillary resistance vessels (mainly the venules) are little affected. Although the resistance and exchange vessels are dilated by NTG, the myogenic constriction of these vessels upon Pv rises is not significantly altered.