Which of the following agents or changes has a negative inotropic effect on the heart?

Question

Quizplus · Accepted Answer

[IV C]. A negative inotropic effect is one that decreases myocardial contractility. Contractility is the ability to develop tension at a fixed muscle length. Factors that decrease contractility are those that decrease the intracellular $\left[\mathrm{Ca}^{2+}\right]$. Increasing heart rate increases intracellular $\left[\mathrm{Ca}^{2+}\right]$ because more $\mathrm{Ca}^{2+}$ ions enter the cell during the plateau of each action potential. Sympathetic stimulation and norepinephrine increase intracellular $\left[\mathrm{Ca}^{2+}\right]$ by increasing entry during the plateau and increasing the storage of $\mathrm{Ca}^{2+}$ by the sarcoplasmic reticulum (SR) [for later release]. Cardiac glycosides increase intracellular $\left[\mathrm{Ca}^{2+}\right]$ by inhibiting the $\mathrm{Na}^{+}-\mathrm{K}^{+}$pump, thereby inhibiting $\mathrm{Na}^{+}-\mathrm{Ca}^{2+}$ exchange $(a$ mechanism that pumps $\mathrm{Ca}^{2+}$ out of the cell). Acetylcholine (ACh) has a negative inotropic effect on the atria.

Which of the Following Agents or Changes Has a Negative