Electric muscle stimulation in the hind leg of the spontaneously hypertensive rat induces a long-lasting fall in blood pressure

The influence of prolonged low-frequency, low-intensity electric stimulation of the gastrocnemius muscle or of the biceps femoris muscle on blood pressure and heart rate was investigated in unanaesthetized, spontaneously hypertensive rats (SHR). In both groups, elevations of blood pressure and heart rate were elicited during the 60 min of muscle stimulation. After cessation of the stimulation, a depressor response developed within 60 min. Thirty to sixty minutes post-stimulation the fall in blood pressure was 19 ± 3 and 17 ± 4 mmHg, respectively (mean ± SE) compared with controls. In both groups, the depressor response lasted for over 5 h. In addition, the gastrocnemius-stimulated animals also developed a post-stimulatory bradycardia. In one group of SHR the sciatic nerve was anaesthetized with bupivacaine. The arousal response during stimulation was similar to that in the other groups, but after termination of stimulation blood pressure returned to the control level without any further drop. To investigate further the neurotransmitters involved, one group of gastrocnemius-stimulated SHR was given naloxone by infusion during the stimulation. A modest post-stimulatory blood pressure fall also occurred in this group, but it lasted only 90 min. Another SHR group was pre-treated with parachlorophenylalanine, a serotonin synthesis blocker, which completely abolished the post-stimulatory depressor response. These results indicate that prolonged muscle stimulation gives rise to a post-stimulatory long-lasting drop in blood pressure and that this response is mediated by somatic nerve afferents. Involvement of the endorphin and serotonin systems is also suggested.