Effect of substance P on venous tone in conscious rats

Substance P is an undecapeptide that belongs to the mammalian tachykinin family. We investigated the effects of substance P on venous tone in conscious rats through measurement of mean circulatory filling pressure. The effects of substance P (10 and 30 nmol/kg, IV) and the vehicle (0.9% NaCl) on mean arterial pressure, heart rate, and mean circulatory filling pressure were examined in 2 groups each of conscious rats: rats without or with ganglionic blockade through pretreatment with mecamylamine (10 mg/kg, IV) and norepinephrine (4 μg/kg/min, IV). In the unblocked rats, both doses of substance P reduced mean arterial pressure (-19 ± 1 and -35 ± 2 mm Hg) and increased heart rate (+113 ± 14 and +115 ± 23 beats/min). The high dose of substance P also decreased mean circulatory filling pressure (-1.7 ± 0.3 mm Hg). In the ganglionic-blocked rats, both doses of substance P decreased mean arterial pressure (-48 ± 6 and -73 ± 6 mm Hg) and mean circulatory filling pressure (-2.9 ± 0.4 and -4.2 ± 0.8 mm Hg) but did not affect HR. In conclusion, substance P is a vasodilator peptide that has a prominent venodilator action.