Nitric oxide modulates arterial baroreflex control of heart rate in conscious lambs in an age-dependent manner

Experiments were carried out in conscious chronically instrumented lambs ag ed 1 (n = 6) and 6 wk (n = 5) to evaluate the arterial baroreflex control o f heart rate (HR) during postnatal maturation and to investigate any modula tory role of endogenously produced nitric oxide (NO). Before and after intr avenous administration of 20 mg/kg of the L-arginine analog N-G-nitro-L-arg inine methyl ester (L-NAME), the arterial baroreflex was assessed by measur ing HR responses to increases and decreases in systolic arterial pressure a chieved by intravenous administration of phenylephrine and sodium nitroprus side. The HR range over which the baroreflex operates and minimum HR as wel l as maximum gain were greater at 1 than at 6 wk of age. These age differen ces were abolished in the presence of L-NAME, which decreased the HR range and gain of the arterial baroreflex control of HR at 1 but not at 6 wk of a ge. These data provide new information that age-dependent effects of the ar terial baroreflex appear to result from effects of endogenously produced NO .