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.


OBJECTIVE
To compare NTG-induced changes in capacitance and conductance.


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.
Chihara
Eiichi
E
Department of Medicine, University of Calgary and Foothills Hospital, Calgary, Alberta.
Manyari
Dante E
DE
Isaac
Debra L
DL
Tyberg
John V
JV
D000704:Analysis of Variance; D000818:Animals; D001810:Blood Volume / Q000187:drug effects* / Q000502:physiology; D002302:Cardiac Output / Q000187:drug effects; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005260:Female; D015637:Gated Blood-Pool Imaging; D006439:Hemodynamics / Q000187:drug effects; D007262:Infusions, Intravenous; D007422:Intestines / Q000098:blood supply*; D008297:Male; D023421:Models, Animal; D005996:Nitroglycerin / Q000494:pharmacology*; D011336:Probability; D012016:Reference Values; D012039:Regional Blood Flow / Q000187:drug effects; D014655:Vascular Resistance / Q000187:drug effects
D005996:Nitroglycerin