Intra-arterial injection of Mesobuthus tamulus venom elicits cardiorespiratory reflexes involving perivascular afferents

Role of perivascular afferents for the cardiorespiratory alterations produced by Mesobuthus tamulus (BT) envenomation was examined in urethane-anaesthetized male rats. Blood pressure (BP), respiratory rate (RR) and heart rate (HR) were recorded after injecting BT venom/saline in the distal end of femoral artery for 60 min. In addition, paw oedema was also determined. Injection of venom produced an immediate (within 2 s) increase in RR followed by a decrease and finally a sustained increase up to 60 min. BP was increased (within 10 s) by 30-50%, which gradually declined but remained above the initial level up to 60 min. The bradycardiac response was late to occur (after 50 s) and the peak response was seen between 10 and 50 min, which remained at that level. There was oedema in the ipsilateral hind paw (venom injected side) as compared to contralateral side and saline control group. The oedema and cardiorespiratory changes were maximal at 1.0 mg/kg of venom. Pretreatment with indomethacin significantly attenuated the venom-induced responses and also blocked the paw oedema. Present experiments reveal that BT venom in a segment of an artery produces oedema by involving prostaglandins to sensitize the nociceptors present in perivascular tissues to evoke the cardiorespiratory reflexes.