Use of exercise challenge to investigate possible tolerance to beta-adrenoceptor stimulation in asthma.

The effect of prolonged salbutamol administration on beta-adrenoceptor function in asthma has been examined. Six adult patients received salbutamol tablets (16 mg daily) for between 4 and 20 weeks and six adolescents received salbutamol aerosol (800 microgram daily) for 2--5 weeks. Before and after the treatment period the acute bronchodilator response to inhaled salbutamol and the ability of inhaled salbutamol to protect against exercise-induced asthma were examined. Lymphocyte beta-adrenoceptor function was also measured in the patients on tablet therapy. Inhaled salbutamol was less effective in protecting against exercise-induced asthma at the end of the treatment period in the patients who had received tablet therapy, but otherwise there was no significant change in beta-receptor function of either airways or lymphocytes. This apparent loss of efficacy of inhaled salbutamol in the prevention of exercise-induced asthma in some subjects, even when its acute bronchodilator effect is preserved, might reflect differences in the susceptibility of different beta-adrenoceptors to desensitization after prolonged stimulation: its clinical importance remains uncertain.