Open-loop analysis of the pulmocutaneous baroreflex in the toad Bufo marinus

In the conscious toad, Bufo marinus, heart rate responses to manipulations of arterial pressure by vascular occlusion confirm that receptors sensitive to changes in intravascular pressure are located in the pulmocutaneous arteries. The cardiovascular responses to electrical stimulation of the central end of the laryngeal branch of the vagosympathetic trunk indicate that their afferents lie within this nerve. Inflation of an isolated pulmocutaneous sac in conscious toads in which the contralateral laryngeal nerve was cut resulted in a reduction of mean aortic pressure, with an associated bradycardia in some animals. Maximum open-loop gain for the relationship between sac pressure and aortic pressure was 0.35 +/- 0.06 (n = 5), resulting in a predicted maximum closed-loop gain of 0.25 +/- 0.03. Maximum gain occurred at 4.0-6.0 kPa, which was higher than the normal range of mean pulmocutaneous pressures in conscious undisturbed B. marinus, suggesting that the baroreceptors may be more effective in compensating for a rise than for a fall in pressure. This functional characteristic of the receptors suggests that not only may they modulate mean pressure in both the systemic and pulmocutaneous circuits, but that they may be particularly important in protecting the lung vasculature from excessive hydrostatic pressures.