What is the primary action of GTN in treating heart conditions?

The primary action of GTN (glyceryl trinitrate) in treating heart conditions is that it causes venous and arterial vasodilation. This is significant because vasodilation leads to a reduction in blood pressure, which decreases the workload on the heart. By vasodilating the veins, GTN allows for more compliant venous return, reducing the volume of blood that the heart must pump. Additionally, arterial vasodilation decreases the resistance against which the heart must work, making it easier for the heart to pump blood.

This dual effect of venous and arterial dilation plays a crucial role in the management of conditions such as angina pectoris, where the heart muscle is deprived of oxygen due to narrowed coronary arteries. By improving blood flow and reducing myocardial oxygen demand, GTN effectively alleviates symptoms and improves the heart's efficiency during episodes of ischemia.

The other options do not accurately reflect the primary mechanism of action of GTN in the context of treating heart conditions. Increasing heart rate is contrary to the desired outcomes in angina, and reducing blood viscosity or enhancing oxygen uptake in cells do not encompass the primary therapeutic effects of GTN.