Comparative effects of nitroglycerin on intestinal vascular capacitance and conductance.

Background: Nitroglycerin (NTG) dilates capacitance veins and resistance arterioles, but its relative effects on veins and arterioles are not known.

Objectives: To compare NTG-induced changes in capacitance and conductance.

Animals And Methods: Aortic, left ventricular and portal venous (P(port)) pressures, portal flow and relative changes in intestinal blood volume (IBV) ((99m)technetium blood-pool scintigraphy) were measured in seven isoflurane-anesthetized, splenectomized dogs. Changes in intestinal vascular capacitance and conductance (mean portal flow/[mean aortic pressure - mean P(port)]) were determined when NTG was continuously administered (0.8 to 150 microg/kg/min) into a jugular vein. Pressure-volume (ie, P(port)-IBV) curves were defined by impeding portal flow, and capacitance was defined as the IBV at P(port)=7.5 mmHg.

Results: At lower doses, NTG increased capacitance without increasing conductance, but conductance increased considerably with little further increase in capacitance at higher doses. Dose-response analysis revealed that the half-maximum capacitance effect was achieved at an NTG infusion rate of 3.5 microg/kg/min, whereas a rate of 35 microg/kg/min was required for the half-maximum conductance effect.

Conclusions: At lower doses, NTG dilates capacitance vessels primarily, and that effect approaches its maximum before significant dilation of conductance vessels is manifest. However, at higher doses, the increase in conductance is substantial with little additional effect on capacitance.