Propofol inhibits medullary pressor mechanisms in cats

Propofol may cause hypotension and the mechanism is complex. The present study was designed to determine the direct actions of propofol in medulla of cats.

Mean systematic arterial pressure (MSAP), heart rate (HR) and cardiac contractility (dp/dt) were compared before and after administration of propofol the femoral vein (2, 3, or 4 mg.kg-1), vertebral artery (1 mg.kg-1) or the lateral cerebral ventricle (0.5 mg.kg-1) in eight anaesthetized cats. To study the direct effect of propofol in medulla, pressor areas of the dorsomedial medulla (DM) and rostral ventrolateral medulla (RVLM), or the depressor area of the caudal ventrolateral medulla (CVLM) were first identified with electrical stimuli and then confirmed by pressure microinjection of glutamate (Glu, 0.25M, 30 nl) via a multibarrel-micropipette in 28 cats. One hour later, propofol (0.001%, 50 nl) was microinjected at the same site. Electrical stimulation and Glu were applied again to compare changes of SAP, HR and dp/dt with that of the control.

Propofol dose-dependently decreased SAP, HR and cardiac contractility. The percent increase of MSAP induced by Glu were reduced by propofol in DM (59 +/- 3% vs 13 +/- 2%, n = 11, P < 0.01) or in RVLM (56 +/- 4% vs 18 +/- 2%, n = 9, P < 0.01). In CVLM, propofol slightly but not significantly increased depressor responses elicited by Glu (-27 +/- 2% vs -33 +/- 3%, n = 5, P > 0.05).

Our results show that propofol principally inhibits the vasomotor mechanism in the dorsomedial and ventrolateral medulla to effect its hypotensive actions.