Response Characteristics of Pulmocutaneous Arterial Baroreceptors in the Toad, Bufo Marinus

1. Response characteristics of baroreceptors with receptive fields in the pulmocutaneous artery (p.c.a.) were determined in pithed toads by applying pressure steps, ramps, sine waves, and volume infusions into the vascularly isolated and perfused p.c.a. 2. The baroreceptors exhibited phasic and tonic discharge thresholds (30.3 +/- 2.3 and 36.2 +/- 2.8 mmHg respectively) which were above mean arterial pressure values reported for conscious undisturbed toads. They are comparable, in this respect, to non-myelinated mammalian arterial baroreceptors. 3. The maximum-phasic and minimum-adapted discharge frequencies of the p.c.a. baroreceptors were low (30 and 2-3 spikes s-1, respectively), but resembled those reported for mammalian non-myelinated baroreceptors when an adjustment was made for the difference in the temperature of mammalian and amphibian preparations. 4. The sensitivity of the baroreceptor discharge frequency to pressure (delta F/delta P) was estimated from pressure-step and pressure-ramp stimuli. Both estimates were greater than those reported for mammalian systemic arterial baroreceptors after the values were normalized to the maximum discharge frequency of the receptors. delta F/delta P and saturation discharge frequency values estimated from ramp stimuli increased with the dP/dt of pressure ramps. 5. The diameter of the p.c.a. was measured by sonomicrometry in toads anaesthetized with urethane. The diameter pulsation was 7.7 +/- 0.3% of the mean diameter (3.5 +/- 0.1 mm) at a mean pulse pressure of 22 +/- 1 mmHg, and Peterson's pressure-strain modulus was calculated to be 4.0 X 10(5) +/- 0.3 X 10(5) dyn cm-2, which suggests that the p.c.a. is highly compliant, and in this respect is comparable to the pulmonary artery, but not to systemic arteries, in mammals. Baroreceptor discharge began near the point of peak dynamic compliance (dV/dP) and continued as dV/dP decreased. Increasing the rate of infusion reduced the peak dV/dP, but increased the baroreceptor discharge frequency. 6. The response to sinusoidal oscillating pressure stimuli was distorted by rectification. Increasing the frequency of sinusoidal stimulation over the range 0.05-1.0 cycles s-1 reduced the number of spikes per cycle, but increased the mean discharge frequency.