Baroreflexes and cardiovascular regulation in hypertension.

The primary purpose of the arterial baroreflex is to keep blood pressure close to a particular set point over a relatively short period of time. The rapid resetting of arterial baroreceptor afferents toward any sustained new level of blood pressure ensures that the reflex acts as an effective buffer of short-term blood pressure fluctuations that accompany daily life but also ensures that arterial baroreflexes play little role in setting the long-term level of blood pressure. Nevertheless, the minimization of blood pressure variability by baroreflex mechanisms is important as studies suggest that a diminished baroreflex is an independent risk factor or sudden death after myocardial infarction. In hypertensive humans and animals, the baroreflex control of heart rate is diminished. Using the steady-state method for assessment of the cardiac baroreflex in rats, we have shown that the change in baroreflex sensitivity is due to a reduction in the vagal range. Although the cardiac sympathetic component of the baroreflex is normal, the level of cardiac sympathetic activity is enhanced, particularly in young hypertensive rats. We have shown that there is a stronger inverse relationship between vagal heart rate range and levels of cardiac hypertrophy than with other variables, such as blood pressure, hypertension, or indexes of vascular hypertrophy. Treatments that reduce cardiac hypertrophy restore cardiac vagal function. Centrally acting antihypertensive agents increase the sensitivity of vagal baroreceptor heart rate reflexes, mainly through an action on central alpha 2-adrenoceptors. They also reduce cardiac sympathetic activity and diminish cardiac sympathetic baroreflexes through a non-alpha 2-adrenoceptor, possibly an imidazoline receptor mechanism. Both of these effects are beneficial in hypertension, where cardiac sympathetic function is enhanced and vagal activity is reduced. Thus, these actions would be expected to cause a desirable reduction in blood pressure variability. The effect of hypertension on baroreflex control of sympathetic vasomotor function is less clear. Studies have shown diminished, normal, and enhanced sympathetic vasomotor baroreflex control. Basal renal sympathetic drive, however, appears to be increased in human essential hypertension. Our studies in conscious rabbits have shown that rilmenidine reduces renal sympathetic baroreflex function. Rilmenidine acts principally at the level of the rostral ventrolateral medullary imidazoline receptors to markedly reduce the basal renal sympathetic nerve activity and the maximum response to transient fluctuations in blood pressure. Thus, in addition to their antihypertensive actions, centrally acting agents, such as rilmenidine, reduce cardiac and renal sympathetic baroreflex responses and increase cardiac vagal baroreflex sensitivity. This provides an ideal profile of action for the restoration of baroreflex function in addition to reversal of cardiac and vascular hypertrophy in hypertension.