Baroreflex control of regional capacitance and blood flow distribution with or without alpha-adrenergic blockade

Regional blood volumes (Vb), unstressed volumes (V0), blood flow distribution, venous compliances (Cv), venous resistances (Rv), and time constants of drainage (tau v) were determined in dogs anesthetized with alpha-chloralose at carotid sinus pressures (Pcs) of 50 and 200 mmHg and dosed with alpha-adrenergic or ganglionic blockade at a Pcs of 50 mmHg. Vb was measured in each region from indicator dilution curves and mean transit times. V0 was extrapolated from the pressure-volume curves. Pcs of 50 and 200 mmHg were maintained in random order. With a decrease in Pcs, arterial pressure increased from 58.7 +/- 4.1 to 104.6 +/- 6.4 mmHg (P < 0.01), peripheral fractional blood flow decreased from 69.8 +/- 3.8 to 55.8 +/- 3.9% (P < 0.001), splanchnic Vb decreased from 28.3 +/- 1.9 to 19.3 +/- 1.2 ml/kg (P < 0.01), and splanchnic V0 decreased from 19.6 +/- 1.4 to 6.3 +/- 2.1 ml/kg (P < 0.001). Splanchnic Rv and tau v also decreased, whereas splanchnic Cv increased. Phentolamine at low Pcs only partially reversed the decrease in splanchnic capacitance, whereas hexamethonium completely abolished it. In conclusion, changes in splanchnic Rv and blood flow distribution are important components of the carotid sinus reflex, and alpha-adrenergic receptor activation is only partially responsible for the changes in vascular capacitance by the baroreceptor reflex.