What substance's concentration influences the amount of norepinephrine released from sympathetic nerve terminals the most?

The concentration of extracellular Ca2+ is crucial for the release of norepinephrine from sympathetic nerve terminals due to its role in the process of neurotransmitter release. When an action potential reaches the nerve terminal, voltage-gated calcium channels open, allowing calcium ions to flow into the neuron. The influx of Ca2+ triggers a cascade of events that lead to the fusion of neurotransmitter-containing vesicles with the presynaptic membrane, resulting in the release of norepinephrine into the synaptic cleft.

An increase in extracellular Ca2+ concentration directly enhances this process, resulting in a greater amount of norepinephrine being released. This is fundamental to the function of sympathetic nerves, which regulate various physiological responses, such as the "fight or flight" response.

While extracellular Na+ and K+ concentrations are important for generating action potentials, they do not directly influence the neurotransmitter release mechanism in the same way that Ca2+ does. Intracellular Ca2+ concentration is also critical, but its levels are primarily affected by the influx of extracellular Ca2+ and the release from intracellular stores. Thus, while intracellular Ca2+ is important, the direct influence of extracellular Ca2+ on neurotransmitter release makes it the correct answer in this context.