What best describes the mechanism of action of ephedrine?

The mechanism of action of ephedrine is best described by its ability to both act directly on adrenergic receptors and indirectly stimulate the release of norepinephrine (NE). This dual mechanism makes ephedrine particularly effective as a bronchodilator and vasopressor.

When ephedrine binds directly to alpha and beta adrenergic receptors, it produces effects such as increased heart rate (from beta 1 receptor stimulation) and bronchodilation (from beta 2 receptor stimulation). Additionally, its indirect action enhances the release of norepinephrine from sympathetic nerve endings. This means that, in addition to activating adrenergic receptors, ephedrine increases the overall concentration of NE in the synaptic cleft, leading to prolonged physiological effects.

The other options focus primarily on direct actions or indirect mechanisms alone, which do not encompass the full scope of ephedrine's pharmacological activity. This complexity of action is what enables ephedrine to be used in various clinical scenarios, making its mechanism of action particularly significant in understanding its therapeutic applications.