Relative contributions of vagal and cardiac sympathetic nerves to the reflex bradycardia induced by a pressor stimulus in the conscious rabbit: comparison of 'steady state' and 'ramp' methods.

The bradycardic response to a pressor stimulus, phenylephrine, was studied simultaneously in conscious rabbits by two different methods. The 'steady state' method, in which bradycardia was measured at the peak of each pressor stimulus, demonstrated the existence of two groups of animals, in which the maximal heart periods were 867 (s.e.m. = 49) and 563 (s.e.m. = 34) ms and the slopes of the MAP-HP relationship were 24.6 (s.e.m. = 1.6) and 8.1 (s.e.m. = 0.7) ms/mmHg, respectively. The difference in baroreflex sensitivity in the two groups was abolished by sympathetic nerve blockade with guanethidine (10 mg/kg) but not by vagal blockade. The 'ramp method' which measures bradycardia during the rapid phase of MAP rise after phenylephrine did not detect any difference in response of the two groups of rabbits. Guanethidine did not alter the slope of the MAP-HP relationship in either group of rabbits when this was assessed by the 'ramp' method. These findings demonstrate that the 'steady state' method can detect changes in both vagal and sympathetic activity, while the 'ramp' method measures only vagally induced bradycardia. It is concluded that some rabbits may have a genetic ability to activate baroreflex pathways mediating cardiac sympathetic inhibition in addition to vagal stimulation in response to a pressor stimulus.