Control of blood pressure mediated by baroreflex changes of heart rate in the chicken embryo (Gallus gallus)

Pharmacological manipulation of peripheral resistance via sodium nitropruss ide and phenylephrine was used to study baroreflex function over the latter two-thirds of incubation in embryonic chickens. From day 9 to day 19 of in cubation, there is a positive linear relation between heart rate and blood pressure, indicating the feedforward action of arterial pressure on heart r ate. A reciprocal relationship between blood pressure and heart rate became pronounced during the last 3 days of incubation. For the purpose of the st udy, gain of the baroreflex was calculated as maximal gain (only those embr yos that demonstrated the response) or average gain (all embryos). Maximal gain increased progressively from 13 +/- 7 beats min(-1).kPa(-1) at 18 days to 105 +/- 83 beats min(-1).kPa(-1) in 2-day-old hatchlings. The percentag e of embryos older than 18 days with baroreflex responses increased from 33 % on day 19 to 56% on day 21, indicating that baroreflex regulation begins late in incubation (similar to 90% incubation time), and the gain of this r eflex exhibits a maturation over the final 3 days of incubation.