Effects of p-chlorophenylalanine on ventilatory control in goats

The effects of tryptophan hydroxylase inhibition with p-chlorophenylalanine (PCPA; 100 mg/kg iv) on ventilatory control were studied in awake goats. Ventilation, CO2 production, and blood gases were measured 16-24 h after PCPA at rest and during mild exercise in normoxia and at rest in hypoxia and hypercapnia. PCPA increased ventilation 36% at rest, predominantly through an effect on respiratory frequency, and decreased arterial PCO2 (PaCO2) 6.5 Torr. Ventilatory gain in exercise (delta VI/deltaVCO2) was increased 20% by PCPA thereby maintaining PaCO2 at its new resting value. Hypoxia (fractional inspired O2 concentration = 0.12) had little effect on ventilation or PaCO2 at rest, either on control or on PCPA test days. Ventilatory sensitivity to CO2 at rest (delta VI/delta PaCO2) was unaffected by PCPA. Bilateral carotid body denervation (CBX) was performed in the animals, and experiments were repeated 3 mo after the first administration of PCPA. CBX alone decreased ventilation 29% and increased PaCO2 9.4 Torr. Administration of PCPA increased ventilation 35%, decreased PaCO2 by 10.2 Torr at rest, and increased ventilatory gain in exercise 26%. Thus carotid bodies are not necessary for the ventilatory response to PCPA. Furthermore, the primary neural pathways associated with exercise or hypercapnia are not specifically affected by inhibition of serotonin metabolism via PCPA.