[Effect of intraventricular injection of substance P on pulmonary and carotid arterial pressure in rabbits].

In the present investigation changes of pulmonary and carotid arterial pressure in response to injection of substance P(SP) into the 4th ventricle of rabbits were studied. The results were as follows: (1) Intraventricular (ivt.) injection of SP could induce either an increase or a decrease in pulmonary arterial pressure as well as carotid arterial pressor response and bradycardia. (2) Bilateral cervical vagotomy could initiate a definite pulmonary pressor response and a marked decrease in the bradycardiac response to injection of SP in those rabbits originally showing a depressor response of pulmonary artery to SP. (3) Pretreatment by i.c.v. and i.v. phentolamine or alpha 1 blocker, prazosin, could block both the SP-induced pulmonary and carotid pressor responses. (4) alpha 2 blocker, yohimbine, or naloxone could enhance the pressor responses in the both arteries. (5) Propranolol had no effect on the pressor responses in both arteries. (6) The cardiovascular responses to SP could be blocked by the SP blocker (D-Pro2.D-Trp7.9)-SP. It is assumed that increase of SP in the brain may induce an increase in both the pulmonary and carotid arterial pressures and bradycardia by activating the SP receptors. The central mechanism responsible for the SP- induced pressor response involves the participation of adrenergic alpha 1 receptor activities, while the central adrenergic alpha 2 receptor system as well as the endorphin system exerts an inhibitory modulation on the central SP-induced pressor pathway. It appears that SP, catecholamine and opiate substance are all involved in the regulation of blood pressure by brain stem.