Selective pressor enhancement by monoamine oxidase inhibitors in conscious rats.

To compare the cardiovascular effects of chronic monoamine oxidase (MAO) A and B inhibition, rats were given s.c. injections of saline clorgyline or l-deprenyl daily for 3 weeks. Indwelling vascular catheters and a Doppler flow probe were implanted chronically to allow subsequent recording of femoral pressure, heart rate and iliac blood flow before and during the treatment while the rats were awake. On days 7 and 21, average femoral pressure were significantly higher in rats treated with either saline or l-deprenyl than in those treated with clorgyline. Pressor responses elicited by injecting graded doses of phenylephrine, angiotensin or tyramine i.v. were always accompanied by bradycardia and reduced iliac flow. Magnitude of all responses was unaltered in control rats treated with the saline vehicle. In rats treated with l-deprenyl responses to tyramine were enhanced slightly, but in those treated with clorgyline enhancement was more pronounced and included not only responses to tyramine but also those to phenylephrine and angiotensin. Because clorgyline inhibits MAO A whereas l-deprenyl inhibits MAO B our findings imply that enhanced pressor responsiveness depends on inhibition of MAO A rather than MAO B.