Nitric oxide buffers effects of angiotensin II on arterial baroreflex control of heart rate in conscious lambs

Abstract

Previously, we showed that both nitric oxide (NO) and angiotensin II (ANG II) modulate the arterial baroreflex control of heart rate soon after birth. To test the hypothesis that NO buffers the effects of ANG II through activation of AT1 receptors early in life, three experiments were carried out at intervals of 48 h and in random order in two age groups of conscious, chronically instrumented lambs: ∼one week (N=9) and ∼six weeks (N=11): The arterial baroreflex was assessed before and after administration of the selective AT1 receptor antagonist, ZD 7155, and vehicle (experiment one), ZD 7155 and L‐NAME (experiment two), or vehicle alone (experiment three). There were no effects on any of the parameters governing the arterial baroreflex at six weeks after ZD 7155 with or without L‐NAME or vehicle. In contrast, after ZD 7155 at one week there was a decrease in the heart rate range, an increase in minimum heart rate, and an increase in maximum gain. After L‐NAME, in the presence of ZD 7155, these changes were exacerbated at one‐ but not six weeks. These findings support the postulate that there is an age‐dependent interaction between ANG II and NO in modulating the arterial baroreflex control of heart rate.