Increased tracheal responsiveness to beta-adrenergic agonist and antagonist in ovalbumin-sensitized guinea pigs

Influence of epithelium on beta-adrenoceptor desensitization of guinea pig tracheal smooth muscle

The effect of tissue incubation with a beta2-agonist of denuded and intact epithelium trachea on the responsiveness to isoprenaline and beta-receptor blocked by propranolol (CR-1) was examined in this study. We examined the effect of epithelium removal on the beta-adrenoceptor desensitization resulting from incubation of guinea pig trachea in the beta-adrenergic agonist isoprenaline (10 microM). Desensitization was measured as the change in EC50, the concentration of beta-agonist that produced 50% relaxation of tracheal rings contracted with methacholine. As a second measure of desensitization, we measured the shift in EC50 resulting from incubation of tracheal rings with the beta-adrenoceptor antagonist propranolol (20 nM), expressed as CR-1 ([post-propranolol EC50/baseline EC50]-1). Initially, we measured desensitization immediately after incubation in isoprenaline; subsequently, we repeated the protocol and allowed a 30 min rest between the end of incubation and the measurement. The sensitivity of denuded epithelium trachea to isoprenaline and (CR-1) was significantly higher than that of intact epithelium only in non-incubated preparations (p<0.05 to p<0.001). Incubation to isoprenaline caused a significant reduction in the tracheal response to isoprenaline in both the denuded groups (p<0.005 for both cases) and intact epithelium groups (p<0.05 for both cases). Incubation to isoprenaline also caused a significant reduction in (CR-1) value in both the denuded groups (p<0.005 for group 2 and p<0.001 for group 4) and intact epithelium only in group 1 (p<0.05). However, the changes in EC50 due to tissue incubation with isoprenaline were significantly greater in denuded than intact epithelium trachea (p<0.05 for all cases) and for CR-1 value only in groups 1 and 2 (p<0.05). These results indicate decrease in both tracheal response to beta-agonist (tolerance) and CR-1 (due to incubation of tissues with isoprenaline), which were greater in denuded epithelium groups.

Tracheal responsiveness to both isoprenaline and beta2-adrenoreceptor blockade by propranolol in cigarette smoke exposed and sensitized guinea pigs

OBJECTIVE

Airway hyperresponsiveness is the main feature of asthma and also exists in cigarette smokers. In previous studies we have shown increased airway responsiveness to isoprenaline in asthmatic patients and smokers. In this study, tracheal responsiveness to isoprenaline and beta-adrenergic receptor blockade was investigated in animals exposed to cigarette smoke (AECS) with or without sensitization by ovalbumin (OA).

METHODOLOGY

Guinea pigs were exposed to cigarette smoke over a 3-month period with or without sensitization by injection and inhalation of OA. Tracheal responses in AECS, AECS + sensitized and control animals (n = 7 for each group) to isoprenaline in the absence and presence of 20 nmol/L propranolol were measured and EC(50) was established. The propranolol blockade (concentration ratio minus one (CR-1)) was calculated (post-propranolol EC(50)/EC(50)) - 1.

RESULTS

The tracheal response of AECS and AECS + sensitized guinea pigs to isoprenaline was significantly higher than that of control animals (EC(50): 4.24 +/- 0.54, 3.66 +/- 0.53 and 7.71 +/- 0.68.79 micromol for AECS, AECS + sensitized and control animals, respectively) (P < 0.001). There was no significant difference in EC(50) between AECS and AECS + sensitized. CR-1 was also significantly higher in the trachea of AECS and AECS + sensitized compared with controls (13.39 +/- 2.22 and 15.35 +/- 2.95 vs. 3.10 +/- 0.6, P < 0.05 in both cases). There was no significant difference in CR-1 between AECS and AECS + sensitized. There was a significant correlation between the tracheal response to isoprenaline (EC(50)) and CR-1 (r =-0.731, P < 0.001). There was no significant difference in tracheal maximum response to isoprenaline between the three groups of animals.

CONCLUSIONS

The results of this study indicate an increased tracheal response to a beta(2)-adrenergic stimulating drug and enhanced beta(2)-adrenergic blockade by propranolol in both AECS and AECS + sensitized. These results suggest similar increase in airway responsiveness to beta(2)-adrenergic agonists and beta(2)-receptor blockade in AECS and AECS + sensitized guinea pigs.