Insulin induces airway smooth muscle contraction

Abstract

Background and purpose: Recently, the use of inhaled insulin formulations for the treatment of type I and type II diabetes has been approved in Europe and in the United States. For regular use, it is critical that airway function remains unimpaired in response to insulin exposure.

Experimental approach: We investigated the effects of insulin on airway smooth muscle (ASM) contraction and contractile prostaglandin (PG) production, using guinea-pig open-ring tracheal smooth muscle preparations.

Key results: It was found that insulin (1 nM-1 microM) induced a concentration-dependent contraction that was insensitive to epithelium removal. These sustained contractions were susceptible to inhibitors of cyclooxygenase (indomethacin, 3 microM), Rho-kinase (Y-27632, 1 microM) and p42/44 MAP kinase (PD-98059, 30 microM and U-0126, 3 microM), but not of PI-3-kinase (LY-294002,10 microM). In addition, insulin significantly increased PGF(2alpha)-production which was inhibited by indomethacin, but not Y-27632. Moreover, the FP-receptor antagonist AL-8810 (10 microM) and the EP(1)-receptor antagonist AH-6809 (10 microM) strongly reduced insulin-induced contractions, supporting a pivotal role for contractile prostaglandins.

Conclusions and implications: Collectively, the results show that insulin induces guinea-pig ASM contraction presumably through the production of contractile prostaglandins, which in turn are dependent on Rho-kinase for their contractile effects. The data suggest that administration of insulin as an aerosol could result in some acute adverse effects on ASM function.

Figure 1

Insulin induces airway smooth muscle contraction. Guinea-pig tracheal rings were mounted for isometric recording and exposed to increasing concentrations of insulin. Data shown represent means and s.e.m. (vertical lines) of three experiments, each performed in duplicate, obtained using three different animals. ^*P<0.05 compared to basal (inherent) tone.

Figure 2

Insulin-induced contractions are sustained in nature. Responses shown (0.1 (squares) and 1 (circles) μM insulin) are corrected for inherent myogenic tone, which amounted to 0.23±0.02 g on average. Data shown represent means and s.e.m. (vertical lines) of three experiments, each performed in duplicate, obtained using three different animals.

Figure 3

Insulin-induced contractions are independent of the epithelium. Responses shown (1 μM insulin) represent means and s.e.m. (vertical lines) of three experiments, each performed in duplicate, obtained using three different animals. ^*P<0.05. Basal tone amounted to 0.18±0.06 g (−epithelium) and 0.24±0.07 g (+epithelium).

Figure 4

Intracellular signalling associated with insulin-induced airway smooth muscle contraction. Insulin (1 μM)-induced contractions were recorded in the absence and presence of LY-294002 (10 μM), PD 98059 (30 μM), U-0126 (3 μM) or Y-27632 (1 μM). Responses shown in (b) are corrected for inherent myogenic tone and are derived from the data shown in (a). All data shown represent means and s.e.m. (vertical lines) of five to six experiments, each performed in duplicate, obtained using five to six different animals. ^*P<0.05; ^***P<0.001 compared to control. ^#P<0.05 compared to the control insulin response.

Figure 5

Maximal histamine-induced contractions are independent of Rho-kinase (a), p42/44-MAPK (b) and PI-3-kinase (c). Data represent means and s.e.m. (vertical lines) of five to seven experiments, each performed in duplicate, obtained using five to seven different animals. Maximal histamine-induced contraction amounted to 1.53±0.1 g on average.

Figure 6

Insulin induces a cyclooxygenase-dependent release of PGF_2α, which is insensitive to Rho-kinase inhibition. Responses shown represent PGF_2α-production by single tracheal rings. Data represent means and s.e.m. (vertical lines) of four experiments, obtained using two different animals. ^**P<0.01, ^***P<0.001 compared to basal. ⁺⁺P<0.01 compared to control insulin response. ^#P<0.05 compared to control basal.

Figure 7

Role of contractile prostaglandin receptor stimulation in insulin-induced airway smooth muscle contraction. Insulin (1 μM)-induced contractions were recorded in the absence and presence of indomethacin (3 μM), the FP-receptor antagonist AL-8810 (10 μM) or the EP₁-receptor antagonist AH-6809 (10 μM). Responses shown in (b) are corrected for inherent myogenic tone and are derived from the data shown in (a). All data shown represent means and s.e.m. (vertical lines) of three experiments. ^*P<0.05; ^**P<0.01; ^***P<0.001 compared to control. ^##P<0.01; ^###P<0.001 compared to the control insulin response.