| Abstrakt: |
The heart rate (HR) response to stepwise changes in mean arterial pressure (MAP) produced by methoxamine and sodium nitroprusside was studied in conscious horses, pigs, and calves. The respective steady-state arterial baroreflex sensitivities (delta HR/delta MAP expressed as beats min-1 . mm of Hg-1) were -1.20, -1.21, and -0.39 for decreasing MAP and -0.08, -0.91, and -0.56 for increasing MAP. After parasympathetic nervous system (PNS) blockade with atropine, the reflex sensitivity to decreasing MAP in horses and the pigs decreased to -0.62 and -0.65, respectively (P less than 0.05), whereas the sensitivity in calves was not significantly different from the calves' base-line sensitivity (P greater than 0.05). In pigs, the sensitivity to increasing MAP decreased to -0.43 after PNS blockade (P less than 0.05). Horses and calves had a response to small increases in MAP similar to that observed in the absence of PNS blockade. There was no additional change in HR with larger increases in MAP in these 2 species. These responses indicate that varying degrees of mixed PNS and sympathetic nervous system activity changes mediate the baroreflex change in HR seen for increasing and decreasing MAP in pigs and for decreasing MAP in horses. Predominantly PNS activity is responsible for the response to increasing MAP in the horses and calves, whereas predominantly sympathetic nervous system activity is responsible for the response to decreasing MAP in calves. Comparisons were made between the observations in these 3 species and other mammalian species. |
