Differential effects of chlorpromazine on two vasoconstrictor tones in the rat

Hindquarter compensator tone (HCT) is referred to as the sympathetic vasoconstrictor tone to the hindquarters of the rat induced by such hypotensive interventions as pentobarbital anesthesia, nitrate administration and blood loss. The aim of this study is to observe whether chlorpromazine (CPZ) injected intravenously in rats (0.5 mg/kg) inhibits the following two different kinds of vasoconstrictor tone: HCT induced by pentobarbital and the renal tone which is normally present in the conscious state. Rats were implanted with an electromagnetic flow probe around the terminal aorta or the left renal artery. The right common carotid was cannulated for mean arterial pressure (AP). Regional peripheral resistance (hind-quarter resistance [HQR] or renal resistance [RR]) was calculated as AP divided by regional flow. In rats under pentobarbital anesthesia, after CPZ, ganglionic blockade with hexamethonium bromide (25 mg/kg) did not decrease HQR. However, in conscious rats after CPZ, blockade decreased RR significantly. These findings indicate that CPZ inhibits HCT almost completely but scarcely decreases the renal tone and further suggest that HCT and the renal tone are generated by different mechanisms.