TRH-induced tachyphylaxis: present in the smooth muscle but not in the cardiorespiratory effects

The development of tachyphylaxis to the pressor, tachycardiac, ventilatory and smooth muscle effects of thyrotropin-releasing hormone (TRH) was studied. Subsequent administrations of increasing doses of TRH (0.1-1000 nmol/kg intracerebroventricularly), at 20 min. intervals in urethane-anesthetized rats dose-dependently increased the mean arterial blood pressure, heart rate and minute ventilation volume. At all doses the effect on mean arterial blood pressure was apparent at least 20 min. after each injection whereas at low doses (0.1-1 nmol/kg) the duration of the effects on heart rate or minute ventilation volume were shorter. The cardiorespiratory effects of TRH 10 nmol/kg intracerebroventricularly did not subside completely within an observation period of 1 hr. However, a repeated administration of TRH at 1 hr resulted in the levels of mean arterial blood pressure, heart rate and minute ventilation volume not statistically different from the peak levels of the first injection. While the cardiorespiratory effects of TRH were long-lasting, the TRH induced inhibition of the contractions of the rat duodenal smooth muscle lasted only 12-18 sec. The TRH-treated duodenal preparation responded normally to noradrenaline but remained unresponsive to further administrations of TRH until washed extensively. When the medium after cessation of the TRH response was transferred into another chamber with an untreated preparation, a full response was resulted. Inhibitors of TRH metabolism, bacitracin, EDTA and iodoacetamide, did not prolong nor enhance the response. Likewise, a stable TRH-analogue MK-771 did not produce any longer response than TRH. It is concluded that no tachyphylaxis was produced to the neurally mediated pressor, tachycardiac or ventilatory effects of TRH.(ABSTRACT TRUNCATED AT 250 WORDS)