HINDQUARTER SYMPATHETIC TONE INDUCED BY SMALL BLOOD-LOSS IN CONSCIOUSRATS

We have designated the hindquarter (or terminal aortic) vascular resis tance (HQR) of the rat as hindquarter compensator (HC) because it incr eases after such hypotensive interventions as pentobarbital anesthesia and nitrate administration, presumably due to reflex excitation of re gional vasoconstrictor fibers. The aim of the present study was to obs erve whether the HC mechanism is also mobilized in response to hemorrh age. Rats were implanted with a 1.5 or 2 mm diameter electromagnetic f low probe at the terminal aorta for measurement of hindquarter flow (H QF). An indwelling catheter was placed in the right common carotid art ery to measure arterial pressure(AP) and withdraw blood. Experiments m ere performed in conscious rats two or three days after implantation. HQR was calculated by dividing AP by HQF. About 10 min after withdrawi ng blood (0.3 ml/100 g body weight), ganglionic blockade with hexameth onium bromide significantly decreased HQR, which indicated a mobilizat ion of the KC mechanism, a change not observed with superior mesenteri c resistance. A quantitatively similar change mas observed in HQR afte r withdrawing double the amount of blood, i.e., 0.6 ml/100 g body weig ht, suggesting that the HC mechanism is activated almost fully by the relatively small amount of blood loss of 0.3 ml/100 g body weight. Cop yright (C) 1997 by the Tohoku University Medical Press.