Vascular reactivity, adrenergic mechanisms, and arteriolar resistance in hypertension

Hypertension, both essential and experimental is characterised by increased vascular resistance. This is in part structural, due to increases in the wall-to-lumen ratio secondary to vascular hypertrophy; however, functional increases in responses to neurohumoral stimuli also occur. The latter may be a consequence of hypertrophy or if a selective increase is observed, an indication of a primary underlying hypertensive mechanism. In essential hypertension, enhanced responsiveness to both angiotensin II and alpha-adrenoceptor agonists are seen. However, in some experimental models, more selective increased responses have been reported. The mechanism of alpha-adrenoceptor-mediated vascular smooth muscle contraction is reviewed and the role of inositol phosphates and diacylglycerol as second messengers is discussed. The precise mechanism of increased vascular responses in hypertension is still uncertain.