Baroreflex actions of substance P microinjected into the nucleus tractus solitarii in rat: a consequence of local distortion.

Biologically active substance P (SP) (1000 ng in 0.1 microliter saline) microinjected into the nucleus tractus solitarii (NTS) of 25 rats did not affect arterial pressure, heart rate, or the baroreceptor reflex. However, microinjection of saline alone in volumes greater than 0.3 microliter consistently elicited hypotension and bradycardia followed occasionally by transient hypertension. These data suggest that previously reported cardiovascular effects of SP microinjected into the NTS resulted from local distortion.