Cardiac but not pulmonary receptors mediate depressor response to IV phenyl biguanide in conscious rabbits

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Phenyl biguanide (PBG) stimulates pulmonary and cardiac receptors in the cat and rabbit. Previous reports have suggested that pulmonary receptors mediate the reflex respiratory responses and cardiac receptors mediate the reflex hypotension and bradycardia. Using selective denervation of the lung (LDX) and intrapericardial procaine to block cardiac nerves (CDX), we investigated the specific role of pulmonary and cardiac receptors in the reflex response to PBG infusion (60 micrograms/kg iv) in the conscious rabbit. Breathing frequency, arterial pressure, heart rate, and renal sympathetic nerve activity (RSNA) were recorded before and after LDX and CDX. Before LDX and CDX, PBG infusion produced tachypnea, hypotension, and bradycardia. This was accompanied by a withdrawal of RSNA to a level not different from that evoked by ganglionic blockade. These responses were preserved after LDX but were abolished after CDX with intrapericardial procaine. Intrapericardial infusion of PBG produced no response. These results indicate that in conscious rabbits both the respiratory and cardiovascular responses to PBG infusion are mediated by cardiac receptors not accessible from the epicardial surface. Furthermore, the reflex hypotension is mediated largely by withdrawal of sympathetic nerve activity resulting in a decreased peripheral resistance.