Effects of nitroglycerin and nitroprusside on large and small coronary vessels in conscious dogs

The effects of nitroglycerin (NTG) and nitroprusside (NPR) were examined in conscious dogs on measurements of left circumflex coronary blood flow and coronary diameter and on calculations of late diastolic coronary resistance (LDCR) and left circumflex coronary internal cross-sectional area (CSA). The effects of infusions of NTG, 8 microgram/kg/Min for 7 minutes, and NPR, 2.5 microgram/kg/min for 7 minutes, were compared. These doses of NTG and NPR induced similar effects on mean arterial and left ventricular (LV) systolic and end-diastolic pressure, heart rate and LV dP/dt. However, NTG induced significantly greater (p less than 0.05) increases in CSA than NPR, while NPR induced significantly greater (p less than 0.05) reductions in LDCR than NTG. Just before cessation of infusion, CSA rose by 29.2 +/- 4.7% with NTG and by 22.7 +/- 3.9% with NPR, while LDCR fell by 8.8 +/- 3.3% with NTG and by 21.6 +/- 2.7% with NPR. Moreover, the effects of NTG on CSA were significantly more sustained than those for NPR. Thus, NTG and NPR in conscious dogs, in doses that exert similar general hemodynamic effects, elicit qualitatively similar but quantitatively different effects on small vs large coronary vessels. Whereas NTG induced significantly less dilation of small coronary vessels, it exerted significantly larger and longer lasting effects on large coronary arteries.