Which of the following substances is known for its effects on heart muscles?

Cardiac glycosides are a group of compounds that have a well-established effect on heart muscles, primarily increasing the force of contraction of the heart (positive inotropic effect) and influencing heart rate. They work by inhibiting the sodium-potassium ATPase pump, which leads to an increased intracellular concentration of sodium and subsequently affects calcium levels. This modulation of calcium improves the contractility of cardiac muscle cells, making cardiac glycosides significant in the treatment of conditions like heart failure and certain types of arrhythmias.

The other substances mentioned do not have the same specific impact on heart muscle. Harpagoside has anti-inflammatory and analgesic properties and is often derived from devil's claw; however, it does not primarily affect heart muscle function. Saponins are primarily known for their role in affecting cholesterol absorption and may have some immune-modulating effects, but they do not directly influence heart muscle contraction. Glucosinolates are primarily associated with defense mechanisms in plants and have been studied more for their potential anti-cancer properties rather than any direct effects on cardiac muscle. Thus, cardiac glycosides are distinguished clearly for their targeted action on heart muscles.