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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.

R1551-R1559, 2003. First published February 20, 2003 10.1152/ajpregu.00519.2002Avian intrapulmonary chemoreceptors (IPC) are vagal respiratory afferents that are inhibited by high lung PCO 2 and excited by low lung PCO 2. Previous work suggests that increased CO 2 inhibits IPC by acidifying intracellular pH (pHi) and that pH i is determined by a kinetic balance between the rate of intracellular carbonic anhydrase-catalyzed CO 2 hydration/dehydration and transmembrane extrusion of acids and/or bases by various exchangers. Here, the role of amiloride-sensitive Na ϩ /H ϩ exchange (NHE) in the IPC CO2 response was tested by recording single-unit action potentials from IPC in anesthetized ducks, Anas platyrhynchos. For each of the IPC tested, blockade of the NHE using dimethyl amiloride (DMA) elicited a marked (Ͼ50%) dose-dependent decrease in mean IPC discharge (P Ͻ 0.05), suggesting that NHE is important for pH i regulation and CO2 transduction in IPC. In addition, activation of the NHE using 12-O-tetradecanoylphorbol 13-acetate stimulated six of the seven IPC tested, although the overall effect was not statistically significantly (P ϭ 0.07). Taken together, these findings suggest that CO 2 transduction in IPC is dependent on transmembrane NHE although it is likely to be much slower than carbonic anhydrase-catalyzed hydration-dehydration of CO2. carbon dioxide chemosensitivity; intracellular pH regulation; respiratory control; neuron; acid; base BIRDS HAVE INTRAPULMONARY chemoreceptors (IPC) that monitor lung PCO 2 and exert reflex effects on the pattern of breathing (9,15,16,36,37,43,45). These IPC have afferent axons in the vagus nerves, cell bodies in the nodose ganglia, and sensory endings in the parabronchial tissue of the lungs (9, 25, 34). The action potential discharge of IPCs responds to both rapidly changing (i.e., phasic) and sustained (or tonic) levels of intrapulmonary PCO 2 , thereby encoding information about the temporal relationships between ventilation, perfusion, and metabolism (4,9,15,16,19,24,34). IPC sensory feedback helps terminate inspiration by sensing CO 2 washout from the lung, helps maintain arterial homeostasis in response to moderate inspired hypercapnia (35,37,45), and helps adjust breathing to metabolic demands (4,9,19,24,50).IPC are unusual respiratory chemoreceptors because their action potential discharge rate is inversely proportional to intrapulmonary PCO 2 . Low PCO 2 stimulates IPC firing, and high PCO 2 inhibits firing (9, 15, 16); thus the IPC response is backward compared with that of traditional respiratory chemoreceptors like the carotid bodies (18, 29), many presumptive central chemoreceptors (40), and snail pneumostome ganglia chemoreceptors (13,14). Many presumptive chemoreceptor neurons located in the mammalian medullary raphe, however, have also been shown to be inhibited by high PCO 2 (41, 54), as are reptilian IPC, mammalian pulmonary stretch receptors, and mammalian laryngeal CO 2 chemoreceptors (10,32,39,44). Thus the inverse CO 2 response (discharge rate inhi...

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