Vagal and sympathetic components of the heart rate range and gain of the baroreceptor-heart rate reflex in conscious rats.

Previous studies in conscious rats have examined the relationship between mean arterial pressure (MAP) and heart rate (HR) only during relatively small or unidirectional changes in blood pressure. We have now examined this relationship more fully in conscious Sprague-Dawley rats using graded bolus i.v. doses of phenylephrine and nitroprusside to alter MAP over a range of 60-160 mm Hg with a view to determining the contribution made by the vagus and sympathetic (after atenolol or methylatropine, respectively). In 25 rats the relationship of HR to MAP followed a sigmoidal rather than a linear function (P less than 0.001) with clear upper and lower HR plateaus. The HR range estimated from a logistic equation was 217 +/- 7 b/min while the average gain between the inflection points was 4.1 +/- 0.2 b/min/mm Hg (1.8 times greater than from a linear fit). The vagus makes a greater contribution to the HR range than the sympathetic (61 vs 39%, respectively) while the converse is the case for the gain (46% vagus, 63% sympathetic). In the presence of both blocking drugs, changes to HR were less than 7% of control. These results suggest that the baroreceptor-HR reflex in the conscious rat is best characterised by a sigmoid curve with approximately equal contributions from both the cardiac vagus and the sympathetic nerves.