Viscoelastic behavior of "in situ" aortic wall during hemorrhagic hypotension

Viscoelastic and electrophysiological mechanisms have been implicated in resetting of baroreceptors in hypertension, but resetting in response to hypotension has been less exhaustively studied. To assess the importance of viscoelastic mechanisms in hypotension, we examined the behavior of the "in situ" aorta during hemorrhage. Fifteen minutes of hemorrhage in anesthetized Wistar rats produced stable hypotension (30 mmHg) and a progressive contraction of the mean aortic caliber (-93.8 +/- 18.0 microns, P < 0.05) compared with control measurements. Contraction was not altered by sinoaortic denervation, vagotomy, nephrectomy, adrenalectomy, hexamethonium (30 mg/kg), losartan (10 mg/kg), V1 antagonist (10 micrograms/kg), arterial pH and blood gas control, or indomethacin (3.0 mg/kg). Aortic contraction was greater in rats treated with N omega-nitro-L-arginine (-164.0 +/- 43.0 microns, P < 0.05) than in those treated with sodium nitroprusside (-54.1 +/- 7.5 microns, P < 0.05). The results indicate that aortic contraction is compatible with viscoelastic contraction and suggest that shortening of viscoelastic elements in series with baroreceptor endings increases stress at the baroreceptor membrane and contributes to the development of baroreceptor resetting to hypotension.