Effects of pulse frequency on single-unit baroreceptor activity during sine-wave and natural pulses in dogs.

1. Previous studies using sine-wave pulses have shown that changes in pulse frequency do not alter single-unit activity per unit time. Since baroreceptor activity is phasic with most of the activity occurring in systole, we reasoned that the lack of effect of pulse frequency is due to the associated reciprocal change in duration of systole with sine-wave pulses. Therefore, we tested the hypothesis that change in frequency using natural arterial pulses alters baroreceptor unit activity per unit time. 2. In chloralose-anaesthetized dogs the isolated carotid sinus was connected to an electromagnetic pressure converter driven by a voltage generator that controlled pulse pressure and sine-wave pulse rate. To generate a natural pulse the converter was driven by the output from the pressure channel used to record arterial pressure. Systolic and diastolic pressures were maintained constant by a pressurized air source and an adjustable control on the voltage generator. The rate of natural pulses was decreased by electrical stimulation of the peripheral right vagus nerve which slowed the heart rate of the dog. 3. Decreases in sine-wave rate by approximately 73 pulses/min (from 156 +/- 2 to 83 +/- 1 pulses/min) at a mean pressure of 114 +/- 8 mmHg (n = 13) did not alter activity per unit time (28 +/- 3 vs. 29 +/- 3 spikes/s). 4. In contrast, decreases in natural pulse rate by an average of 78 pulses/min (from 182 +/- 7 to 104 +/- 15 pulses/min) at a comparable mean pressure (104 +/- 7 mmHg) (n = 9) decreased activity per unit time by 28% from 32 +/- 4 to 23 +/- 4 spikes/s (P less than 0.05). Similar findings were observed in five of six single units using an intact (normally perfused) carotid sinus preparation in dogs with heart block and cardiac pacing. 5. The results indicate that a change in natural pulse rate is an important determinant of single-unit baroreceptor activity.