Possible mechanisms of abnormal norepinephrine sensitivity and reactivity of resistance vessels and the development of hypertension in spontaneously hypertensive rats. A hypothesis. This study examined structural and functional changes of mesenteric resistance vessels in early, developing, and established stages of hypertension development in spontaneously hypertensive rats in an attempt to identify possible mechanisms of the development and maintenance of hypertension. Our results suggest that the development of hypertension in spontaneously hypertensive rats may be caused by genetic structural and functional abnormalities of resistance vessels. Both abnormalities may be caused by hyperreactivity to norepinephrine through an altered signal transduction process, including the regulation of protein kinase C in smooth muscle cells of resistance vessels in spontaneously hypertensive rats. 