Effect of low temperature on tracheal smooth muscle contractile and relaxing responses evoked by electrical field stimulation

OBJECTIVE: The aim of this work was to evaluate the effect of low temperature (LT) on the contractile and relaxing responses of rat tracheas (RTs) after electrical field stimulation (EFS). METHODS: Voltage-dependent (10-60 V, 40 Hz) and frequency-dependent (0.1-60 Hz, 40 V) response curves were constructed at 37 and 18 degrees C after the activation of tracheal intramural nerves with a Grass S88 stimulator. The EFS that produced half of the maximum contractile response (ES50) at 37 or 18 degrees C was determined and considered as the dependent variable. The relaxation of pre-contracted RTs (EFS; 5 Hz, 40 V) to sodium nitroprusside (SNP; 1 x 10(-7) - 1 x 10(-3) M) isoproterenol (ISP; 1 x 10(-9) - 1 x 10(-5) M) and to 20 mM potassium chloride (KCl) after low-K+ inhibition of the Na+/K+ pump at 18 and 37 degrees C were determined. RESULTS: We found that the tracheal responses elicited by EFS at 37 and 18 degrees C were completely blocked with 1 microM atropine. LT slightly increases the voltage-dependent ES50, from 33.7 +/- 4.0 to 37.8 +/- 4.8 V, n = 5 but decreases the frequency-dependent ES50 from 19.3 +/- 4.3 to 1.0 +/- 0.28 Hz, n = 5, p < 0.05. Relaxing responses to SNP, ISP and KCl at 37 degrees C correspond to 43.5 +/- 6, 36.7 +/- 12 and 12.1 +/- 1.5 respectively. No significant tracheal relaxations were elicited at 18 degrees C. Our results indicate that in RTs, LT enhances tracheal sensitivity to EFS and decreases it in response to bronchorelaxants. The LT-dependent enhanced contractile response is observed only after a low frequency stimulation range (0.1-20 Hz), that is very close to the frequency of vagal stimuli required for inducing bronchoconstriction in vivo. Furthermore, LT abolishes the sensitivity of RTs to exogenously added bronchorelaxants (NO and ISP). In addition, LT appears to decrease the Na(+)-K+ pump activity; this effect has been associated with increased tracheal hyperreactivity in vitro. ACH appears to be the main endogenous neurotransmitter involved on neurally mediated contractile responses at 37 and 18 degrees C. CONCLUSION: Low temperature enhances the contractile response of rat tracheas in response to endogenous ACH release. The effect of LT is limited to frequencies below 20 Hz, which are within the physiological range required for bronchoconstriction. Furthermore, LT severely impairs the sensitivity of RTs to relaxant stimuli, either of endogenous of exogenous origin.

Differential antagonism by amiloride and pirenzepine of the muscarinic receptors of rat tracheal smooth muscle

Amiloride (AM) is a well known potassium sparing diuretic. The effects of AM at the cellular level include blockade of Na+/H+ exchange in several tissues and inhibition of passive sodium flux in epithelial cells. In this study we have explored the interactions of amiloride with muscarinic receptors, using isolated rat tracheal rings and compared its effects to those of the muscarinic receptor subtype-selective antagonist pirenzepine (PZ). The results obtained demonstrate the ability of AM (100 uM to 1mM) to inhibit the ACh induced rat tracheall contractions. The inhibition resulted in the reduction of the Emax values of ACh in this preparation, and the apparent Ki for AM was of 478 uM. This effect was also observed in a sodium-free choline medium, indicating that it is independent from sodium transport mechanisms sensitive to AM. In contrast to AM, PZ displayed a surmountable type of antagonism with a pA2 value of 6.52. The results demonstrate a differential antagonism by AM and PZ of the muscarinic receptors present in the smooth muscle of the rat trachea