Forskolin reverses tachyphylaxis to the bronchodilator effects of salbutamol: An in-vitro study on isolated guinea-pig trachea

The objective of this study was to assess the relaxant responses of salbuta mol, a beta(2) agonist, and forskolin, an activator of adenylate cyclase, a nd the possible role of forskolin in reversing tachyphylaxis to salbutamol. The in-vitro bronchodilator effects of salbutamol and forskolin (10(-9)-10( -5) M) were tested on isolated guinea-pig tracheal rings precontracted with carbachol (10(-7) M). Both salbutamol and forskolin elicited concentration -dependent relaxation. Potency (EC50; the dose resulting in 50% relaxation) was determined from cumulative concentration-response curves. Salbutamol w as more potent than forskolin in relaxing the tracheal preparations (-log m olar EC50 7.68 +/- 0.14 and 6.3 +/- 0.17, respectively). Reproducible relax ant responses to salbutamol could be elicited after 24 h incubation in Kreb s solution. Tachyphylaxis to the relaxant effects of salbutamol was experim entally induced incubation (24 h) of the preparations in Krebs solution con taining salbutamol (10(-6), 3 x 10(-6) or 10(-5) M). This pretreatment of t he tissues resulted in a significant reduction in the potency of salbutamol . The potency of salbutamol was reduced to 6.85 +/- 0.2, 6.8 +/- 0.1 and 5. 9 +/- 0.27 after 24 h incubation with salbutamol 10(-6), 3 x 10(-6) or 10(- 5) M, respectively. The potency of salbutamol was increased from 7.35 +/- 0 .2 to 7.76 +/- 0.28 by addition of forskolin (3 x 10(-7) M) under control c onditions. Moreover, forskolin (3 x 10(-7) M) reversed the development of t achyphylaxis to salbutamol-induced relaxation in tissues pretreated with sa lbutamol. The potency of salbutamol was increased to 7.29 +/- 0.41, 7.37 +/ - 0.17 and 7.23 +/- 0.35 after the addition of forskolin (3 x 10(-7) M) to preparations pre-incubated (24 h) with salbutamol 10(-6), 3 x 10(-6) or 10( -5) M respectively. These results show that in guinea-pig tracheal ring preparations, forskolin shares with salbutamol the ability to relax airway smooth muscle and produ ces an apparent reversal of tachyphylaxis to the bronchodilator effects of salbutamol, particularly in the low concentration range. This effect could provide an alternative therapy for long term use, particularly with high do ses of beta(2) agonists in bronchial asthma.