Question

a reduction in extracellular ca²⁺ levels would lead to
multiple choice
increased force of cardiac muscle contraction due to increased interactions between actin and myosin.
decreased force of cardiac muscle contraction due to a lack of release of ca²⁺ from the sarcoplasmic reticulum.
heart block due to depolarization of the sarcolemma.
heart block due to hyperpolarization of the sarcolemma.

Explanation:

Calcium ions ($Ca^{2+}$) play a crucial role in cardiac - muscle contraction. Extracellular $Ca^{2+}$ influences the release of $Ca^{2+}$ from the sarcoplasmic reticulum. A reduction in extracellular $Ca^{2+}$ levels leads to decreased release of $Ca^{2+}$ from the sarcoplasmic reticulum, which in turn decreases the force of cardiac - muscle contraction as there are fewer $Ca^{2+}$ ions available to facilitate the interaction between actin and myosin.

Answer:

decreased force of cardiac muscle contraction due to a lack of release of $Ca^{2+}$ from the sarcoplasmic reticulum.