Microneurographic analysis of thyrotropin-releasing hormone effects on sympathetic outflow in human muscle nerves.

In order to clarify the human sympathetic response to thyrotropin-releasing hormone (TRH), microneurographic analysis was performed in 12 healthy subjects. Multi-unit vasoconstrictive impulses of muscle nerve sympathetic activity (MSA) were directly recorded from tibial nerve fascicles together with ECG and blood pressure. Subcutaneous injection of saline (1 ml) had no effect on blood pressure and neurograms of MSA. Following s.c. injection of TRH (0.5 mg, 1 ml), however, a significant increase in heart rate and blood pressure occurred 2 min after injection; the change was statistically correlated with increased MSA, and disappeared after about 10 min. The observation suggests that TRH-activated vasoconstrictive impulses of MSA increase the peripheral vascular resistance and elevate blood pressure. We speculate that TRH directly activates vasomotor centers or higher autonomic centers rather than peripheral organs, because the parallel increase of blood pressure and MSA after TRH injection counteracts the effect of baroreflex mechanism.