Effects of opiates during baroreceptor and ergoreceptor induced changes in blood pressure

Various opioids were used to investigate the role they might play in the cardiovascular responses to fatiguing isometric contractions. Changes in blood pressure were measured in cats anaesthetised with alpha-chloralose. Fatiguing isometric contractions of the hind limb muscles (ergoreceptor activation) were generated using a microprocessor controlled stimulator (50 Hz, 0.2 ms, 200-800 mV). Baroreceptor inactivation was elicited by carotid artery occlusion. Muscle contraction caused an increase in mean arterial pressure of 51 (SEM 12) mm Hg and carotid occlusion an increase of 56(9) mm Hg above resting levels in control conditions. Injection of dynorphin (0.5-5.0 micrograms.5 microliters-1) into the cerebral aqueduct just rostral to the 4th ventricle eliminated the pressor response to muscular contraction (mean arterial pressure at rest, 80-118 mm Hg: on fatigue, 72-129 mm Hg) but did not affect the pressor response to carotid occlusion in the same cats. Similarly, injections of met-enkephalin (1-100 micrograms.5 microliters-1) or beta-endorphin (10-100 micrograms.5 microliters-1) eliminated the ergoreceptor induced changes in mean arterial pressure during isometric contractions but had no effect on the changes caused by carotid occlusion. Pressor responses to nerve crush were not eliminated. These results support the suggestion that a catecholaminergic-opioidergic pathway in part mediates the cardiovascular responses to ergoreceptor afferent but probably not baroreceptor afferent input.