Stimulation of group III and IV afferent nerves from the hindlimb by thromboxane A2

These experiments were designed to test the hypothesis that the TxA2 mimetic, U46,619, would stimulate group III and IV afferent nerve endings from the hindlimb of the anesthetized cat. Nerve impulses were recorded from the dorsal rootlets of the L7-S1 segments of the spinal cord, and afferent units identified by measurement of conduction velocities, mechanical probing of hindlimb muscles, and local injection of chemical stimulants (capsaicin and bradykinin). Five of the 15 group III fibers were stimulated by U46,619 (2-10 micrograms injected into the abdominal aorta; mean baseline impulse frequency increasing from 7.3 (+/- 3.2) impulses/s to 16.0 (+/- 3.1)), while 7 of the 12 group IV fibers responded to U46,619 (impulse frequency increasing from 4.3 (+/- 3.2) to 8.8 (+/- 3.6)). The average latency for the response (20-30 s) did not differ between the two groups of afferent fibers. We conclude that group III and IV afferent fibers originating from the skeletal muscle of the hindlimb are stimulated by TxA2 and that the release of TxA2 in skeletal muscle could evoke cardiorespiratory reflexes known to be activated by stimulation of these afferent nerves.