[Effects of morphine on the control of the cardiovascular system by the carotid-sinus-reflex and by the carotid chemoreflex].

The effects of morphine (2 mg/kg i.v.) on the carotid baroreceptor control and the carotid chemoreceptor control of the circulation were examined in 12 chronically instrumented dogs. Bilateral carotid occlusion in conscious dogs with spontaneous ventilation increased heart rate (decrease in cardiac cycle length by 19 +/- 2%; mean +/- S.E.M.), mean arterial pressure (38 +/- 3%) and mean iliac resistance (33 +/- 11%). Morphine, however, failed to change these reactions significantly. Chemoreceptor stimulation, accomplished by intracarotid injections of nicotine (4-8 micrograms), in conscious dogs with spontaneous ventilation elicited complex effects on heart rate and mean iliac resistance--due to secondary stimulation of pulmonary inflation reflexes. Chemoreceptor stimulation in conscious dogs with ventilation controlled increased cardiac cycle length (246 +/- 17%) and elicited intense iliac vasoconstriction, i.e. mean iliac resistance rose by 288 +/- 25%. Morphine significantly (p less than 0.01) attenuated the increase in cardiac cycle length and in iliac resistance with chemoreceptor stimulation (42 +/- 10% and 64 +/- 9%, respectively). Thus, morphine interferes with the carotid chemoreceptor control of the circulation while the efficacy of the carotid-sinus-reflex seems to be well maintained.