Arterial baroreflex control of heart rate in the horse, pig, and calf.

The study investigates the response of heart rate to changes in arterial pressure in horses, pigs, and calves, and how the parasympathetic nervous system influences these responses. It is observed that varying degrees of parasympathetic and sympathetic nervous system activities modulate the heart rate in response to arterial pressure changes in different ways among the three species.

Objective of the Study

• The primary objective of the study was to understand the responses of heart rate to stepwise changes in mean arterial pressure in horses, pigs, and calves. The research sought to elucidate the role of the parasympathetic nervous system in modulating these responses.

Methodology

• The researchers induced changes in mean arterial pressure using methoxamine and sodium nitroprusside.
• The baroreflex sensitivities (defined as the change in heart rate per unit change in mean arterial pressure) were determined for both decreasing and increasing arterial pressure.
• The nervous system’s effect was assessed by blocking the parasympathetic nervous system with atropine and observing the resultant changes in baroreflex sensitivity.

Key Findings

• The steady-state arterial baroreflex sensitivities varied among the species, showing different sensitivities to decreasing and increasing arterial pressure.
• Blocking the parasympathetic nervous system resulted in a decrease in the reflex sensitivity to reducing arterial pressure in horses and pigs, whereas there was no significant difference in sensitivity for the calves.
• The sensitivity to increasing arterial pressure also diminished in pigs after blocking their parasympathetic nervous system.
• Horses and calves exhibited a similar response to slight artery pressure increases both before and after parasympathetic nervous system blockage.
• The research suggests that changes in heart rate due to variations in arterial pressure are influenced by a combination of parasympathetic and sympathetic nervous system activities in pigs and for decreasing arterial pressure in horses. For increasing arterial pressure in horses and calves, the response is predominantly driven by the parasympathetic nervous system. In contrast, in calves, the sympathetic nervous system primarily drives the response to decreasing arterial pressure.

Comparative Analysis

• The study concludes with comparison to observations in other mammalian species to position these findings in a broader biological context. However, the specifics of this comparison are not mentioned in the abstract.