Modulation of baroreceptor activity by ionic and paracrine mechanisms: an overview.

1. The primary mechanism of activation of baroreceptors is mechanical deformation during vascular stretch. In addition, baroreceptor activity is modulated by ionic mechanisms and by neurohumoral and paracrine factors that act directly on the nerve endings. 2. Ionic mechanisms play a major role in causing baroreceptor activity to decline during a sustained increase in arterial pressure (adaptation) and in the suppression of activity that occurs after pressure returns to basal levels (post-excitatory depression). Activation of a 4-aminopyridine-sensitive K+ channel contributes to adaptation, whereas activation of an electrogenic sodium pump is responsible for post-excitatory depression. 3. Factors released from vascular endothelium exert powerful effects on baroreceptor sensitivity. Prostacyclin increases baroreceptor sensitivity and contributes to baroreceptor activation during vascular stretch. Nitric oxide, endothelin and oxygen-derived free radicals suppress baroreceptor activity particularly at high levels of arterial pressure. The sympathetic neurotransmitter norepinephrine modulates baroreceptor activity: a) indirectly through its vasoconstrictor action, b) directly by binding to alpha-adrenergic receptors on the nerve endings, and c)through release of a cyclooxygenase metabolite, possibly prostacyclin, from endothelium. 4. Endothelial dysfunction contributes to baroreceptor impairment in atherosclerosis and in chronic hypertension. Loss of the excitatory influence of prostacyclin and increased formation of free radicals and possibly endothelin contribute to the baroreceptor dysfunction. Platelets aggregating at sites of endothelial damage in the carotid sinus release a stable diffusible factor that impairs baroreceptor sensitivity. 5. Therapeutic interventions may alter baroreceptor sensitivity through paracrine mechanisms. Treatment of hypertension or atherosclerosis may improve baroreceptor sensitivity by restoring endothelial function. Antiplatelet agents may enhance baroreceptor sensitivity. Antidepressant agents may decrease baroreceptor sensitivity by inhibiting prostacyclin and/or stimulating nitric oxide formation, which may contribute to dysregulation of the circulation in patients treated for depression.