Effect of clonidine and naloxone on the pressor response during contraction of cat hind-limb muscles.

The possible involvement of an adrenergic-endorphin system in the mediation of the pressor response to isometric muscular contraction was studied in cats. Fatiguing contractions of the gastrocnemius and plantaris muscles caused an increase in the mean arterial blood pressure by 35 to 70 mmHg. Intravenous infusion (30 micrograms X kg-1) as well as intracisternal injection (2.5 micrograms) of clonidine-HCl eliminated the pressor response to muscular contraction. In both sets of experiments, the mean blood pressure remained at the resting level throughout the duration of the isometric contraction. Injection of naloxone (0.5 mumol X litre-1) into the cisterna magna did not alter the resting blood pressure and did not affect the rise in mean arterial pressure during muscle contractions. Intracisternal injection of naloxone (0.5 mumol X litre-1) prior to an intracisternal injection of clonidine (2.5 micrograms) did not alter the resting blood pressure but effectively antagonised the anti-pressor effects of clonidine during fatiguing isometric contractions. These data may indicate that activation of muscle "ergoreceptor" afferents (group III and IV fibres) during muscular contraction may cause an increase in the arterial blood pressure by interfering with an inhibitory adrenergic-endorphinergic pathway in the brainstem.