THE EXERCISE PRESSER REFLEX IS ATTENUATED BY INTRATHECAL OXYTOCIN

We tested the hypothesis that oxytocin (Oxt) acts in the lumbar spinal cord to attenuate reflex presser (mean arterial pressure, MAP) and heart rate (HR) responses to static hindlimb contraction (i.e., the exercise presser reflex). Thus we compared MAP and HR responses to electrically stimulated hindlimb static contraction in the anesthetized cat before and after intrathecal injection of Oxt (30 pmol, n = 3; 300 pmol, n = 6; or 3 nmol, n = 6). The 300-pmol dose was most effective; it attenuated the presser response to static contraction by 39 +/- 10% but had no effect on HR. In three other cats, contraction-induced increases in MAP and HR were monitored before and after intrathecal injection of 300 pmol of Oxt + 300 nmol of the selective Oxt receptor antagonist [d(CH2)5(1),O-Me-Tyr(2),Thr(4),Tyr(9) Orn(8)]vasotocin. Pretreatment with the antagonist eliminated the effect of Oxt on MAP. In an additional 10 cats, increases in these same variables in response to static contraction were compared before and after intrathecal injection of the Oxt antagonist (30 nmol, n = 3 or 300 nmol, n = 7) into the lumbar spinal cord (L(1)-L(7)). Whereas 30 nmol of the Oxt antagonist had no effect, the 300-nmol dose augmented the contraction-induced presser and HR responses by 28 +/- 7 and 32 +/- 17%, respectively. These data imply that endogenous Oxt modulates the exercise presser reflex by its action on Oxt receptors in the lumbar spinal cord that can attenuate sensory nerve transmission from skeletal muscle.