How do cardiotonic drugs primarily affect the heart muscle?

Cardiotonic drugs primarily enhance the contractility of the heart muscle by increasing intracellular calcium levels. When these drugs are administered, they facilitate the influx of calcium into the cardiac myocytes, which is essential for muscle contraction. This increase in calcium enhances the force of each contraction, making the heart more efficient in pumping blood, which is especially important in conditions where the heart's pumping ability is compromised, such as heart failure.

The other answer choices involve different physiological systems. Blocking the renin-angiotensin system is relevant to controlling blood pressure and fluid balance but does not directly improve the contractility of the heart muscle. Similarly, blocking the sympathetic nervous system can reduce heart rate and contractility, whereas blocking the parasympathetic influence primarily affects the heart rate without directly enhancing the contraction strength. Therefore, when considering the primary effect of cardiotonic drugs, the increase in intracellular calcium levels is the fundamental mechanism that directly impacts the heart's ability to contract effectively.