Complex sentence: One theory is that the drug causes vasoconstriction (narrowing of the blood vessels) in salivary glands, decreasing salivary flow, and this is thought to be due to the activation of alpha-adrenergic receptors in these glands.
Split into simpler sentences: One theory is that the drug causes vasoconstriction (narrowing of the blood vessels) in salivary glands, decreasing salivary flow. This constriction is thought to be due to the activation of alpha-adrenergic receptors in these glands.