Static muscular contraction elicits a pressor reflex in the chicken

Static muscular contraction has been shown to increase arterial blood pressure and heart rate in humans and other mammals. It is not clear, however, whether birds exhibit a similar response to this maneuver. Therefore, we designed these experiments to determine if the chicken exhibits a cardiovascular response to static muscular contraction and if the observed responses are evoked through a reflex involving muscle afferents. Static contraction of the gastrocnemius muscle was evoked by electrically stimulating the sciatic nerve at 1.5-3.0 times motor threshold (30-40 Hz; 0.025 ms) in 13 chloralose-anesthetized cockerels. We measured arterial blood pressure and muscle tension before and during static contraction and calculated mean arterial pressure and heart rate from the arterial pressure trace. We found that static contraction of the gastrocnemius muscle increased mean arterial pressure from 71 ± 4 to 95 ± 4 mmHg (P < 0.05) and increased heart rate from 304 ± 8 to 345 ± 10 beats/min (P < 0.05). Furthermore, we found that stimulation of the sciatic nerve after paralysis of the birds with vecuronium bromide or stimulation of the cut peripheral end of the sciatic nerve (using the same stimulation parameters described above) evoked no change in mean arterial pressure or heart rate. We conclude that static muscular contraction of the gastrocnemius muscle in the chicken elicits a pressor response and that this response is due to a reflex arising from the contracting muscles.